TABLE OF CONTENTS

INTRODUCTION

1. CONSIDERATIONS FOR DESIGN AND CONSTRUCTION

• Soil Modification
• Soil Stabilization
• Roadbed Modification (RBM)

2. TREATMENT OPTIONS FOR DESIGN AND CONSTRUCTION

• Determination of Site Conditions
• Bridging Over Soft Soils
• Surface Treatment
• Winterization Treatment
• Depth, Percentage and Type of Treatment

III. ECONOMIC CONSIDERATIONS

• Treatment vs. Other Stabilization Methods
• Excavation and Replacement of Soft Soils
• Air-Drying of Wet Soils
• Chemical Treatment of Wet Soils
• Improving Marginal Aggregate Bases
• Cost Comparisons: Baserock vs. Treated Subgrade
• Life Cycle Considerations

IV. TREATMENT PRODUCTS

• Quicklime (Dolomitic and High Calcium)
• Quicklime Plus (Quicklime / HAC Mix)
• Hi-Alkali Cement (HAC)
• Portland Cement (Type II or SS)
• Fly Ash (Class C & F)
• Chemical Analysis for Soil Treatment Products

5. QUALITY CONTROL DURING CONSTRUCTION

• Reagent Spread Rate
• Pulverization
• Mixed Efficiency
• Reagent Content
• Moisture Content
• Initial Mixing
• Re-Mixing
• Density
• Curing
• Weather Limitations

6. QUALIFICATIONS FOR STABILIZATION CONTRACTOR

• Pre-Qualification for Soil Stabilization Contractor
• Personnel
• Equipment

VII. LABORATORY TESTING OF TREATED SOILS

• Introduction
• Curing and Remolding
• Accelerated Curing with Heat (Optional)
• Compaction Test (ASTM D-1557)
• Atterberg Limit (ASTM D-4318)
• Expansion Test (ASTM D-3877)
• Unconfined Compression (ASTM D-5102)
• R-Value
• Conclusion

VIII. ENVIRONMENTAL CONSIDERATIONS

• Airborne Particles During Treatment
• Elevated pH of Treated Soil
• Leaching Effects of Treated Soil

9. CASE STUDIES

• Soil Stabilization of Residential Streets
• Pavement Recycling Using Treatment
• Lime Stabilized Base Just Gets Stronger
• Sulfate Study
• Treated San Francisco Bay Mud For Building Foundations

10. APPENDIX

1. GUIDE SPECIFICATIONS

• Lime Treatment Guide Specifications
• Cement Treatment Guide Specifications
• Type SS Cement (HAC)Treatment Guide Specifications
• RBM Guide Specifications

B. STANDARD TREATMENT LETTERS

• Standard Winterization Letter
• Standard Treatment Letter

C. SULFATE TESTING

This manual is a compilation of the field and laboratory practices of experienced engineers and contractors in the soil stabilization industry. The intended purpose of this manual is to bring clarification to the various products and procedures involved in the soil stabilization practice. The information is presented in summary form with detailed explanations and procedures following in the appendices. This manual not only gives guidance for routine stabilization projects, but also provides a format for the applications that require unique solutions, well within the capabilities of the experienced consultant and contractor.

It is our goal that users of this manual discover solutions to engineering and construction situations by gaining more understanding of the significant value engineering potential presented by the use of soil treatment.Users of this manual will discover the significant "value engineering" potential generated by the use of soil treatments to provide engineering and construction solutions that comprehensively address soil stabilization challenges.

The addition of reagents, such as lime and cement, into soils has been used for centuries throughout the world. The first known use of such soil treatment was by the Romans on the famous "Appian Way", still in use today. The modern version of lime stabilization is less than 40 years old, but considerable advances have been made in construction procedures during these past four decades. The progress of this method of stabilization is a result of the efforts of many engineers, contractors, and scientists summarized as follows:

• Basic and applied research by numerous state highway departments, governmental agencies, and universities;
• Education through worldwide publication of research studies and construction reports of actual lime stabilization projects; and
• Equipment manufacturers’ recognition of the potential of stabilization and development of equipment to meet the needs of the contractor. Stabilization equipment is 2-3 times more efficient.

However, Eeven in these modern times, however, considerable confusion seems to beexists as to the best products and the best procedures to use for specific soil conditions and the associated economics. We have found that there is very little literature that consolidates theoretical and practical ideas about soil treatments. This manual is tailored to the Northern California marketplace, where conditions vary greatly from project to project.

This manual can also be used as a training resource for civil contractors and engineers. As in most engineering and construction practices, the proper procedures have been refined through years of experience and proven methodss,. Likewise, this manual presents the best management practices for site stabilization and soil modification gained through experience.

The significance of soil properties and their behavior is a vital construction consideration. The impact made by site and grade preparation can range from a durable project, completed on schedule and budget, to a costly failure. There are two forms of in place soil treatment considered in this manual: (1) soil modification and (2) soil stabilization.

Soil modification and soil stabilization are different applications, which, although similar in technique, differ in purpose, design, and quantity of treatment required. The purpose of soil modification (the changing of soil behavior, principally through reduction in excess moisture), is to expedite construction. Stabilization is the improvement of a subgrade or subbase to withstand applied loads and /or to reduce shrink/swell potential. Although some stabilization inherently occurs in soil modification, the distinction is that soil modification is a construction expedient, whereas stabilization is part of the project design.

Modification is the changing of soil behavior principally through the reduction of excess moisture to expedite construction. Modification is commonly performed on subgrade and subbases in order to expedite compaction and subsequent paving. A wide range of problem soils can be modified with various treatment products to improve behavior. Included in this category are soils with high silt content where reduction of moisture sensitivity can be achieved. In addition to reducing excess moisture, the texture of clay soils can be modified using a few percent of lime, flocculating the clays into a sand-like material that can be easily worked. Unstable, fine-grained sands can also be cemented to form a stable platform. Modification may involve drying up construction sites and access roads regardless of the in-situ soil types. The common denominator for soil modification is the improvement of soil behavior, which permits other work to proceed without delay.

If drying is required for only a shallow depth (12"), the subgrade soils can normally be treated and worked in place. Caution should be exercised, however, in areas where water may have collected at greater depths. Such areas may require more treatment product mixed to greater depth (18") for effective bridging. The depth of modification needed to bridge a soft subgrade is generally equivalent to the depth required to stabilize the subgrade by excavation, placement of a geofabric, and backfilling with an aggregate material.

When water is encountered, an evaluation should be made to determine if water is infiltrating from an outside source. If the flow of water is continuous, then dewatering will be required prior to any treatment. Dewatering should to be at least 1 inch below the bottom of treatment to allow for "wicking". If it is determined that the water is only perched, then areas containing any standing water should be pumped prior to treatment.

Contractor ingenuity and experience can usually be relied upon for the appropriate technique to solve most job site variations. However, the services of a Geotechnical Engineer may be required on the more complex projects and are recommended on "first time" applications.

Soil modification is an effective and economical technique that expedites construction with generally modest engineering requirements. In most instances, soil modification with various treatment products will correct adverse conditions immediately and permit construction activities to proceed on schedule.

Soil stabilization is the construction of a load-bearing subgrade or subbase for a structural purpose. The engineered intent, based on a laboratory mix design, is to increase compressive strength and/or reduce shrink/swell potential. Although the actual construction methods and techniques are similar to those used in soil modification, there are significant differences that govern stabilization.

When treatment is for the purpose of soil stabilization to increase soil strength or lower plastic index, the consultant should develop a mix design by sampling and testing materials and percentages of reagent required. Soil samples should be obtained from actual site locations, taking into account the variable soil conditions. The mix design should be conducted well in advance of construction.

Laboratory technicians normally develop the mix design for a stabilization project (see Laboratory Testing section). Lab testing will vary depending on the intended purpose of the treatment. For pavement design purposes, tests should be conducted to determine improvements in the R-Value or Unconfined Compressive Strength with different percentages of reagent.

Soil stabilization has proven to be a substantial cost-saving method for pavement design. Treatment will increase the load-bearing capabilities of marginal subgrade materials, therefore decreasing the amount of aggregate base required for the pavement. The stabilization process has proved to be the structural and economic solution for correcting unexpected, poor pavement support conditions.

Roadbed modification is an in-place recycling method for pavements that have deteriorated beyond maintenance treatments. Pavements showing distresses such as base failure, stripping, rutting, transverse cracking, flushing and bleeding are candidates for re-construction using roadbed modification. Poor subbase is often a contributing factor to many pavement distresses. RMB increases the load carrying capacity of the base course, therefore, the pavement section may be constructed thinner. The thinner pavement section requires less material, saving our depleting virgin aggregates. In addition, less pavement layers requires less traffic control, saving both dollars and inconvenience to the driving public.

In terms of construction, roadbed modification is quite versatile. The project engineer is capable of adapting and handling any peculiarities that may be encountered, such as poor subgrade conditions, including pockets of clay. Options available to the engineer include re-pulverizing, increasing the percent of cement or increasing the depth of the pulverized section to accommodate any problem areas.

The existing surface is pulverized full depth, usually six to eight inches. The pulverized material is relayed and Portland cement is added, at a rate of 2% by weight, to the pulverized base and surfacing material. Each project is evaluated to determine if virgin aggregates are required in the modified section. Processing of the roadbed modification includes mixing all materials to a specified depth, compaction of the mixture, and finishing of the completed base. The roadmixing machine is a cross shaft type and is capable of providing a uniform homogeneous mixture. It is also equipped to introduce water at the time of mixing. A metering device is used to control the correct quantity of water to produce a completed mixture with uniform moisture content. After the materials have been properly mixed, it is then bladed and compacted to 95% relative maximum density and is kept moist until a curing seal of MC-250 is applied at a rate of .20 gsy to .25 gsy. This is done within 24 hours after completion of the final rolling. The bituminous curing seal is a protection against drying and provides a continuous membrane over the surface. Sand blotter is then applied to the curing seal to allow traffic and construction equipment on the roadbed modified surface. Interstate projects are an exception, where traffic is not allowed on the modified roadbed until the first lift of plant-mix is placed. Typical production rate is 10,000 linear feet per day. The equipment required for this type of operation is readily available, which aids in keeping the cost of construction down.

Roadbed Modification provides a long-term solution to base failure, which is a contributing factor to many pavement distresses. The benefits of using this method to stabilize and improve the performance of the base include increasing the load-carrying capacity of the pavement structure, which increases the pavement life. It utilizes the existing pavement, eliminating the need and cost of transporting and disposal of the material. It is a cost- effective, versatile construction method that utilizes existing material and equipment.

To determine the required design and construction procedures, the Geotechnical Engineer must determine whether the project requirement is soil modification for constructability or soil stabilization to enhance the structural strength of the material requiring treatment.

• The first consideration is to determine the amount of area requiring treatment. The Geotechnical Engineer should be aware of the economic considerations in determining which form of treatment is best suited for the project. A rule of thumb in making this determination is that area requiring more than $6,000 of remediation or over two days of repair should be considered good candidates for treatment using additives.

• Another factor in considering the appropriate method is to determine the depth of treatment required. Mixing equipment can apply treatment to a depth of 18 inches. If conditions require depths of greater than 18 inches, the consultant should consider logistics and other equipment requirements. A larger surface area will be required at the site for a mixing table where the stabilization contractor can process material before it is used as fill. All areas requiring less than 18 inches of treatment can be mixed in place.

• The next consideration is to determine what type of product is appropriate for the soil types and conditions. A general rule is that the more clayey soils should be treated with quicklime products and the more granular materials should use products containing cement. For combination soils, excellent treatment alternatives would include products containing both lime and cement, such as HAC (see Treatment Products for details).

• The last factor in determining treatment options is the percentage of product necessary to meet project requirements. If the purpose of the treatment is to dry out the site, then the consultant can be flexible in his recommendations for percentage of product required. If the purpose of treatment is to enhance the strength or for other design considerations, then the engineer should qualify the mix design percentage with the appropriate laboratory testing.

Once the Geotechnical Engineer has determined that treatment is the best option for the project, the next step is to evaluate the actual conditions of the site. The first determination is whether the treatment section is required to bridge over weaker subsurface soils or is a surface treatment only. If bridging is part of the project goal, the construction process will require more involvement from the Soils Engineer to determine the extent of subsurface conditions.

The involvement of the Geotechnical Engineer should include evaluating the severity of unstable conditions. This will lead to the determination of additional amounts or depth of treatment required. One of the crucial elements of the bridging process is the curing time of the treated section. It is critical to determine when the section has been fully cured in order for the bridge to hold the loading of construction equipment.

The Geotechnical Engineer should monitor the curing time by hand coring the treated section and visually evaluating the curing timeline. An evaluation should also be made of whether to wait for all areas of the site to be completely cured or to allow the contractor to proceed with the next step in the grading operation, allowing for special consideration around tender areas. Please note that accessing tender areas too quickly may result in failure of the treated section. This occurs when equipment loads are transferred through the treated section into the soft, underlying soils.

Treating unstable surface soils (upper 18 inches) with a full depth process does not require the same level of involvement by the Geotechnical Engineer. Curing of this type of treatment is not as critical because bridging of the underlying soils is not required. The treatment will continue to cure during the course of the construction process. The Geotechnical Engineer should evaluate construction activity for excessive channel loading over treated sections. If a section begins to fatigue from excessive loading, the Geotechnical Engineer may require the contractor to protect these isolated areas until the structural section is placed over the treatment area.

Treatment for winterization is commonly used to allow access to construction sites during the rainy season. Lime, cement, or composite products allow native soils to become more impervious to winter conditions. This type of treatment provides resistance to soil rutting, although the prolonged exposure to construction traffic will cause the surface to unravel. A prudent protection in traffic areas would be to place a thin section of baserock over treated soils.

12" For most surface treatment applications

6" – 12" Roadbed Modification, Surface Treatments

18" For severe or bridging treatments at surface elevation

18" – 30’+ For deep treatment using mixing table

3 – 6 % Soil Stabilization

2 – 5 % Roadbed Modification (RBM)

3 – 5 % Cement Treated Base (CTB)

2 – 4 % Soil Modification

4 - 8 % Pond Liners

3 – 5 % Soil Stabilization

2 – 4 % Soil Modification

4 – 8 % Bay Mud

4 – 6 % Heavy Clays / Pond Liners

In comparing which of the following repair methods to use for stabilizing saturated soils, the engineer should consider which approach is the most economical based on time and material. The three most common approaches would be to excavate and replace saturated soil with geofabrics and imported fill, air dry material by continually turning wet soils with a disc or other equipment, or treat in-place material with reagents such as lime and/or cement.

The repair method of excavating soft soil to depths of 18 inches, placing some type of geofabric at the bottom of excavation and replacing it with a granular material such as baserock, is a quick and durable method of repair for isolated areas. This method also would be the most expensive repair based on a square foot price. This repair would have to consider off-haul costs, labor costs for fabric installation, and import costs of replacement material.

Repair costs using this method may run over $2 per square foot. When isolated areas begin to grow over 5,000 square feet, consideration should be given to other repairs.

Air-drying is the most economical method of repair if time is not a consideration and weather is favorable. Air-drying requires equipment to continually turn and expose material to the air and sun. This method should also be monitored to confirm the rate of drying.

The use of a disc is commonly used to air-dry material. Discing leaves material in larger clumps, which does not allow for air to reach into the smaller particle sizes of soil, accelerating the drying process. Continual discing is required to break up material into smaller sizing. An alternative would be to use pulverization equipment that operates more like a rototiller, immediately breaking down the particle size of soil and allowing for quicker drying times.

Chemical treatment of wet soils is a proven method for dry-outs and stabilization repair. The results of treatment are immediate, thus expediting the construction process. Treatment eliminates the need to off-haul unsuitable material and enables the treated material to become impervious to additional saturation due to rain.

The cost factor of using the chemical treatment alternative can be measured in quantitative costs as opposed to time and material costs of the other alternatives.

Reagents such as lime or cement can be effectively used to upgrade marginal aggregates by stabilizing the plastic fines that may exist in these aggregates and/or by improving the textural and workability properties of these aggregates.

Specific advantages due to the reaction between the fines of marginal aggregates and lime or cement include an alteration in the physical properties of the fine portion of the aggregate. This alteration results in strength improvement, modulus enhancement, durability and consistency improvement. This is most beneficial when a local borrow material can be used instead of using an import material. In turn, freight costs will be reduced.

The amount of lime or cement required to stabilize marginal aggregate bases is typically between 0.5 and 4.0 percent by weight of the total aggregate. This generally results in a fairly high lime or cement content by weight of the fines (- No. 40 sieve) fraction.

Standard mixing equipment is generally used to upgrade marginal aggregates through lime or cement treatment. These usually include a reagent spreader for proper distribution of the lime or cement and a source of moisture.

Ultimate cure strength of lime or cement treated soils is a time-dependent function. Unconfined compression strengths improve with age. Early strengths of 450 psi are common with older sections of over 1000 psi being found in some lime or cement soil mixtures.

Depending on the thickness of the treated section, the expected early strengths and the type of traffic expected, it is advisable to keep in mind that the early cure strengths take a little time to come up, depending on temperature, and it is prudent to keep heavy traffic off for a few days to protect the section.

If the subgrade is dry, the stabilization contractor will often flood these areas with water to avoid the possibility of this dryness pulling excess water from the treated material, retarding proper hydration of the lime and cement, and causing early failure of the section, such as longitudinal cracking or differential expansion along contact areas.

Usually, in heavy soils, two or more mixing passes will be made to ensure that adequate particle size is being made and that no pockets or streaks of reagents are nested in the treated area. Uniformity of mixing can be tested with a solution of phenolphalein sprayed on the sides of a hole dug into the mixed section. Variations in color would indicate non-uniform mixing.

The decision whether or not to use a certain structural layer within a pavement system is all too often based only on cost, when the decision should more properly be based on the life-cycle cost over a defined analysis period.

A first step in evaluating the life-cycle benefits offered by introducing a lime-stabilized subgrade layer can be made by using the well-established and widely used AASHTO (1986) Pavement Design Approach.

The AASHTO approach predicts the performance period at a specified level of serviceability. The performance life is determined in terms of the number of design axle load applications, usually 18,000 lb. single axle loads (N18). N18 is a function of the specified (allowable) level of serviceability loss change in psi, average annual subgrade modulus (MR), reliability and variance of the pavement (T and So), and pavement structure number (SN).

The measure of pavement performance in the 1986 AASHTO Design Guide is the Present Serviceability Index (PSI). The PSI value for a newly constructed pavement is typically about 4.2 and deteriorates with traffic as roughness, rutting and cracking accumulate.

The SN is the parameter that quantifies the contribution of the pavement structure to the resistance to serviceability deterioration under traffic. The SN is equal to the sum of the products of the layer coefficients and the layer thickness.

The value of the structural layer coefficient (a) is dependent on the resilient modulus of each layer. This is largely due to the fact that stiffer or high modulus pavement layers resist pavement roughness and deep layer rutting by better protection of deep layers from being overstressed by wheel-load induced stresses.

A2 and A3 values of granular base and subbase layers are a function of the resilient modulus (AASHTO T274) of these materials at the appropriate field stress states. The regression equation used to calculate a2 from the resilient modulus is as shown here: a2 (ABC) Equation: a2 (ABC) = 0.249 log ₁₀ E_B – 0.977.

The word "lime" is a much-abused term, often connoting any type of calcareous material. According to Webster, however, the precise definition refers only to quicklime (calcium oxide), which is a burned form of high purity limestone (calcium carbonate). There is an ample supply of Quicklime available, while the Hydrated lime (which is a fine powder created by reacting quicklime with a measured amount of water) is not locally available in commercial quantities for the soil stabilization market.

Therefore, our discussions will concentrate on quicklime. Lime stabilization embraces only the burned lime products – not pulverized limestone. There are only a few types of quicklime worthy of discussion concerning the Western North American region: Natividad Dolomitic quicklime (high magnesium) from Salinas, California and High Calcium quicklime from Apex, Nevada or Nelson, Arizona. Both types of lime are effective for soil stabilization and have been used interchangeably and successfully for many years. They are both ASTM C-977 qualified and are both Cal-Trans approved as equals in equal quantities. The quicklime reagent is particularly effective with clay-bearing soils and aggregates. Quicklime reacts both chemically and physically to yield quality building materials. Other quicklime should be tested on a case-by-case basis.

The most effective use of quicklime is with soil containing a large clay fraction. Clayey soils present a problem for construction because clay has a large surface area-to-mass ratio, much like a deck of cards lying on the table. The cards will readily move side-to-side. Water acts as a lubricant to allow the clay soil to move when wet.

The chemical reaction of quicklime with clayey soils is twofold. First, it agglomerates the fine clay particles into larger, coarse, friable particles (the size of silts and sands) through a phenomenon called "cation exchange". The cation exchange is the substitution of powerful calcium cations for sodium at the surface of the clay, reducing its affinity for water and flocculating the material into sand-like particles. Next, if sufficient lime has been added to the soil to saturate it with calcium ions, the high pH (12.4) that has been formed dissolves the clay liberating silicates and aluminates. Those silicates and aluminates react with the calcium and water in a pozzolanic reaction that produces cement products, adding substantially to the soil’s strength.

Quicklime reacts best with high plasticity soils containing clay. Soils with Plasticity Indices greater than 15, and clay contents greater than 10 percent, are routinely stabilized with lime. When soils having low P.I.s and marginal clay fractions are encountered, combinations of lime and flyash often perform well. In those cases, the flyash provides supplemental silicates to assist the pozzolanic reaction’s formation of calcium-silica-hydrates (CSH) and calcium-alumina-hydrates (CAH), the building blocks of stabilization.

Lime changes the physical characteristics of most clay soils as follows:

1. The Plasticity Index drops sharply, often transforming a highly plastic soil into a non-plastic material. This is generally due to the Liquid Limit decreasing and the Plastic Limit increasing.
2. The soil is flocculated and agglomerated, decreasing the soil binder content (minus No. 200 mesh particles) substantially.
3. Limes (and water) accelerates the disintegration (breaking up) of clay clods during mixing as a result of increasing the soil pH and the calcium cation exchange. As a result of 1 and 2 above, the soil becomes friable and can be readily worked.
4. Lime’s chemical reaction is highly exothermic, which aids in drying out wet soils quickly, thus speeding up compaction.
5. The shrinkage and swell characteristics of clay soils are markedly reduced.
6. After curing, unconfined compressive strength increases considerably – in some instances as much as fortyfold.
7. Load bearing values, as measured by various tests (CBR and R-value), increase substantially.
8. The tensile or flexural strength, as measured by various tests (Cohesionmeter, split tensile, etc.), increases markedly. Thus, the stabilized layer develops beam strength.
9. The lime stabilized layer forms a water-resistant barrier by impeding penetration of gravity water from above and capillary moisture from below. Thus, the layer becomes a firm "working table," shedding rainwater readily and remaining stable over mud, thereby minimizing construction delays.
10. A subgrade correctly stabilized with lime creates a permanent pozzolanic matrix that is non-reversible. Because of the soil’s calcium saturation, the pH beneath the finished surface remains elevated for years, enabling a self-healing phenomenon called "autogenous healing."

Quicklime Plus is a mixture of Quicklime (ASTM) and Type HAC Cement. The ratio can be custom blended from 1:2 to 2:1. This mixture works well in stabilizing clay and silty materials. There are solid economic reasons for the use of Quicklime Plus. First, both the cement and quicklime materials are produced from kilns. Rotary kilns are continuous in nature and the construction markets usage is not as consistent. Silos, such as those operated by Griffin Materials in Redwood City and Dixon, California, are a haven for both cement and quicklime products in overflow situations. The cement and quicklime are held separate until blended for use in soil stabilization. The most common blend is 1:1 for predictable results in a wide variety of soils. The second reason has to do with the practical application of the lime and then the cement. This would be a "two-spread" operation that includes the extra cost of handling and trucking of the two separate products and the variability of the spreader truck itself times two. Quicklime Plus is pre-blended at the silo for the highest quality control.

Because of the overflow pricing and the economics of storage, Quicklime Plus has the benefit of less cost than then other reagents. For several years, Quicklime Plus has been successfully used in the Bay Area for reducing the plastic limit and increasing the R-Value of a wide variety of soils. There has now been a significant amount of data collected by Geotechnical engineering firms to confirm the consistency of this product as a strength enhancement and certainly as a soil stabilizer.

HAC (Hi Alkali Cement) is a manufactured product produced over the past 20 years as a soil and aggregate enhancement. HAC is essentially a cement product with a proportional amount of lime and pozzolan. This product only refers to HAC produced at the Cemex Cement Plant in Davenport, California and originally was known as KD83.

HAC is an excellent product for use with combination materials such as sandy clays or silty gravel. HAC can be used for quick dry-out needs because of activation of its cement base and is also an excellent product for use in strength enhancement. For years, Geotechnical Engineers have been using HAC for design purposes instead of Type II cement. There has now been a significant amount of data collected by geotechnical engineering firms to confirm the consistency of this product as a strength enhancement for increased R-Value and CBR results on various soil types.

"Cement-stabilized soil," usually referred to as CSS, is a term used to describe a soil or aggregate that has been treated with a relatively small proportion of Portland cement. The objective of the treatment is to amend undesirable properties of problem soils or substandard materials so that they are suitable for use in construction. The amount of cement added to the soil is less than that required in producing a hardened mass (soil-cement) but is enough to improve the engineering properties of the soil. For the small quantities of cement generally used, CSS becomes caked or slightly hardened. It still functions essentially, however, as a soil or aggregate, although an improved one.

Laboratory and fieldwork on CSS indicate that the relatively small quantities of cement bind the soil grains together to form small conglomerate masses of new soil aggregates. In addition to the cementing reaction, the surface chemistry of clay particles, in clay soils or clay fraction of granular soils, is improved by a cation exchange phenomenon. As a result, the modified soils have lower plasticity (cohesiveness), lower volume change characteristics, and greater strength than untreated soils.

The degree of modification increases with greater amounts of cement. Therefore, for a given soil, a cement content can be selected that will provide a material meeting the specified level of stabilization, expressed in terms of plasticity, strength, or other criteria.

When using cement, the amount can vary depending on the degree of modification desired. Typical cement contents for various applications are shown below.

Roadbed Modification (RBM) 2-4%

Cement Treated Base (CTB) 3-5%

Pond Liners 6-8%

Cement-stabilized soils are usually classified into two groups according to the predominant grain size as follows:

Cement-stabilization of silt-clay soils (soils containing more than 35% silt and clay). The general objective is to improve soils that are otherwise unsuitable for use in subgrade or subbase layers. Specific objectives may be to decrease plasticity and volume change characteristics, to increase the bearing strength, or to provide a stable working platform on which pavement layers may be constructed.

Cement-stabilization of granular soils (soils containing less than 35% silt and clay). The usual objective is to alter substandard materials so that they will meet requirements specified for pavement base or subbase layers.

Cement modification improves the properties of certain silt-clay soils that are unsuitable for use in subgrade construction. The objectives may be to decrease the soil’s cohesiveness (plasticity), to decrease the volume change characteristics of an expansive clay, to increase the bearing strength of a weak soil, or to transform a wet, soft subgrade into a surface that will support construction equipment.

Cement has been used to improve bearing values of granular base and subbase materials, to prevent consolidation, and to produce a stable-working table as a subbase. With the rapid depletion of acceptable granular materials for use as bases and subbases, it becomes ever more important to conserve the remaining limited supply of acceptable materials. Submarginal granular materials, cement-modified to improve their bearing values and reduce their plasticity, will meet specifications for acceptable base and subbase materials. Consequently, the limited supply of acceptable materials can be conserved. The resulting product, however, is still primarily a granular base material with all the characteristics of that type of construction.

Specifications for pavement base and subbase course materials place limits on the amount of fines and the plasticity (cohesiveness or stickiness) of the fines in granular materials. Excessive fines can lead to loss of stability, susceptibility to frost action, and mud pumping under traffic loads.

In working with cement base treatments it is important to remember that the mixing and compaction process must be completed within two hours. This is due to the quick reaction of the cement.

In selecting methods for mix design development and testing, it is important to understand, to some extent, the nature and mechanisms of Class C fly ash stabilization. Unlike lime treatment, which is dependent on cation exchange, fly ash stabilizes by physical as well as chemical binding of soil particles. A high quality Class C fly ash contains all the compounds required for cementing action, including calcium oxides. Fly ash, however, contains a negligible percentage of calcium in the form of calcium hydroxide, while hydrated lime is essentially all calcium hydroxide. It is calcium hydroxide that will flocculate and agglomerate fatty clay soils through cation exchange. Therefore, it may be beneficial in some circumstances to take advantage of the capability of hydrated lime to quickly modify heavy clay soil – which can be accomplished with a 2% lime (by dry soil weight) pretreatment and a 12-hour mellowing period – followed by fly ash stabilization. [Note: Generally, successful lime/fly ash projects combine lime and fly ash at ratios between 1:2 and 1:3. The blends are soil specific and should be determined by laboratory testing.] This method optimizes the performance of each material. Fly ash is a pozzolan that incorporates additional silica into the hydration process. This process can improve strengths, particularly in lower P.I. soils. Some evidence also exists that the additional silica can reduce the potential for free lime to combine with potentially disruptive soil constituents such as soluble sulfates. Because fly ash is a byproduct of coal combustion, its quality is highly variable by source and over time. Care should be taken to perform all testing using the actual fly ash that will be applied to the project.

A number of factors are critically important to the control of the quality of treatments during construction. These factors are as follows:

The spread rate of reagents is determined in pounds of reagent per unit area of surface. Probably the most direct and simplest way to determine the actual field-spreading rate is to place a one square yard tray under the spreader, also known as the Pan Test, and measure the actual spreading rate as the weight of reagent on the square yard tray. The spreader operator should have all equipment necessary to determine the spread rate.

In most specifications, the efficiency of pulverization is determined based on the amount of material passing the one-inch sieve and the # 4 sieve. The processed material is dry sieved to determine the percent passing each sieve. Care must be taken to ensure that the plus # 4 material fraction is not actually agglomerated soil-lime mixture that can be easily broken down by a simple kneading action to pass the Number 4 sieve. Pulverization is critical to the success of all soil stabilization projects because, without intimate mixing of the chemical stabilizers and the soil, the necessary reactions will not take place.

The efficiency of field mixing is of critical importance, as the basic reactions cannot proceed successfully or optimally without this efficient mixing. A simple procedure for evaluating mixing efficiency is:

1. Secure a sample of the field mixed soil-lime material
2. Halve the sample
3. Prepare strength specimens for unconfined compressive strength testing from one portion
4. Completely remix the other portion of the field mixture to ensure almost 100 percent mixing
5. Prepare strength specimens from the remixed material
6. Cure both sets of strength specimens and test them
7. Calculate the mixing efficiency as field mixed strength / laboratory mixed strength

For mixed-in-place operations, mixing efficiencies normally range from 60 to 80 percent. In some types of soil reagent mixing operations lower values may be acceptable.

Phenolphthalein is a color-sensitive indicator of ph; and, since soil-lime mixtures demonstrate an elevated pH, the indicator can be successfully used to indicate the presence of lime. Use phenolphthalein by spraying it on the soil-lime mixture. If lime is present, a reddish-pink color develops.

Moisture content is determined by using conventional procedures such as oven drying and nuclear methods. When the nuclear density gauge is used, it is important to ensure calibration for the soil reagent mixture. It is important to note that initial testing will measure a higher moisture content with some soils. This content will continue to reduce while treatment cures (refer to Laboratory Testing section). Moisture is a critical component of every chemical stabilization process. Not only does it contribute to the hydration of calcium-based stabilizers, but it must also be present in order for the pozzolanic chemical reactions to take place. It is important that the soil moisture remain 3-5% over optimum during lime stabilization to ensure that the pozzolanic reaction proceeds to completion.

Several things are important during the mixing operation. Lime has a very high affinity for water and water is an important factor in any lime stabilization project. In order for the lime to function, it needs water. (This can be a disconcerting element to someone dealing with a dry-up and the stabilization contractor shows up with a water truck).

Water is normally added to the mixing operation through spray bars attached to the mixer. A water truck’s travel rate and the mixer operator controls the water flow rates.

Mixing currently can be accomplished to depths of 18 inches in a single lift. Depth gauges on the mixer tell the operator how deep he is mixing. Calibration can be checked by digging test holes in the mixed material alongside unmixed soil. If the material being mixed were not tightly compacted before mixing, consideration would have to be made to determine what additional depth should be added to compensate for the loose nature of the original soil. Also, after mixing, the soil will be "fluffed up" to some extent, belying its true volume.

It should be noted that on the initial mix the soil would appear very wet. This is because much of the water has not yet reacted with the lime and soil. In a very short time the soil will dry out considerably and more water will have to be added to attain the proper amount (3-5% over optimum) to achieve hydration of the lime and stabilization of the soil. This can be achieved in the second mixing pass.

After the first mixing pass has been completed, all areas where the mixer cannot reach, (such as around manholes, water risers, curbs, walls, and inside tight areas) should be pulled out with a blade, skiploader or other appropriate equipment to incorporate the surrounding material into the rest of the mix. After the initial mixing, the grade should be lightly rolled to minimize the quantity of water lost to evaporation during the curing period. Care must be taken to ensure that the stabilized area remains moist until the second mixing.

After the lime treated soil has been allowed to cure (the curing time being determined by the characteristics of the soil), it will be re-mixed to bring the moisture content to optimum levels and to achieve specified gradation requirements, indicating proper mixing. Normal requirements are 98% passing the 1inch and 60% passing the # 4 sieve, excluding rocks and other non-pulverizable material. Any clods in the mix should be broken up to determine if the lime has actually been mixed into them and to determine if the clods are the result of impingement by action of the mixer. Mixing will continue until gradation has been achieved.

After remixing is accomplished, the pullout materials will be replaced into their excavations in preparation for compaction. These excavations will have been continuously preconditioned with water prior to the lime-treated material being placed back into them.

To ensure proper spreading, theoretical and actual spread rate logs are maintained by the operator of the spreading equipment. Once the operator knows what the spread rate is to be, he can calculate how large an area a given amount of lime can cover. He uses the certified weight ticket from the truck that delivers the lime, computes the theoretical coverage he should get, and compares this later with his actual spread.

Modern state-of-the-art spreading equipment has variable speed drop boxes with vane feeders that can apply product to a high degree of accuracy. On-board foot meters will tell the operator how far he has traveled while actually spreading the lime. To calibrate his instrumentation, he will apply material over a pan of known area and weigh the contents. Once the equipment is calibrated, spreading commences and actual rates are generated and logged. Continuous monitoring of theoretical and actual spread rates assures a high degree of accuracy.

Conventional procedures, such as the use of a nuclear density gage, are used to determine in-situ density of compacted soil reagent mixtures. It is important to ensure that the proper moisture density relation for the soil reagent mixture is used in density control. The moisture density relation for soil-reagent mixtures may change with curing time and variation in reagent content. When soils are stabilized with lime, the unit weight will decrease and the optimum moisture content will increase. If a new moisture density curve is not calculated, final compaction will be very difficult to achieve. An example of this is when the soil reagent mixture is reworked at some later date following initial construction. The maximum dry density and optimum moisture content for the mixture will probably be different from the original mixture.

When working with cement treatment it becomes necessary to perform density testing immediately after the initial compaction effort has been made. The contractor has approximately 90 minutes to complete compaction of the treated soil before the reagent begins to cement.

After the initial mixing operation has been completed, the lime treated soil must be allowed to cure. This time allows the lime to completely react with the soil to form the cementatious compounds that take from a few hours to a few days to develop. Temperature, moisture and soil types are the main items that affect cure times. Normally, the lime is spread and initially mixed the first day, allowed to cure overnight, then remixed the following day prior to compacting.

If during this time rain is threatening, the treated area can be lightly compacted with a roller and will shed rainwater readily. Care should be exercised so as not to develop "Bird Baths" as the water will not soak in and water removal would require trenching or pumping.

It should be noted that during the cure period, the lime treated soil would go through a density and volume change. This is due to the flocculation of the clay particles caused by the lime and is a normal reaction predicted by the preliminary soil evaluation and testing.

Do not construct subgrade when weather conditions detrimentally affect the quality of the materials. Do not apply reagent unless the air temperature is at least 40°F in the shade and rising. Do not apply lime to soils that are frozen or contain frost. If the air temperature falls below 35°F in the shade, protect completed reagent-treated areas by approved methods against the detrimental effects of freezing. Remove and replace or recompact, as indicated, any damaged portion of the completed lime-treated area in accordance with this specification at no additional cost to the owner.

The business and processing of soil stabilization is a very specialized aspect of the construction industry. As the industry moves forward in the use of design level soil treatment, it has become imperative to the project success to pre-qualify these specialty contractors. Pre-qualification should be based on years of experience and the use of specialized and sophisticated equipment necessary to meet today design level soil treatments.

It should be noted, that the on site soils treatment requires the manufacturing of a consistent and predictable engineering element. As an analogy, the treatment process can be compared to the ready-mix plant operation. All ingredients are measured precisely for weight and quality. When blending the ingredients, strict quality control is necessary to meet specifications and product consistency.

The results of a stabilization project, by today standards, may be undermined if the contractor does not have the proper experience, equipment, chemicals, coordination, or engineering elements involved in the treatment process. Failure of any one of these elements may lead to inferior results. Some of the following results may occur from the use of an unqualified contractor or out dated equipment:

• If the spreader does not have on-board computers with ground sensing radar, then the reagent will not be spread evenly across the site, especially in a parking lot situation.

• If the mixer does not have on-board computers with automatic grade controls then the mixer will not produce an even mixing bottom.

• If the mixer does not have automatic water controls, then the water will not be evenly distributed. Having water in the mix is just as important as the reagent itself.

The overall quality control of the spreading, mixing, and compaction of the treated area may be undermined if the contractor does not have the experience or proper equipment. Modern equipment has the technical capability to overcome these quality control issues by taking over some of the functions that use to be handled by the equipment operator. The operator still has the important responsibility to orchestrate the intricate relationship between all these processing equipment.

Attached is a pre-qualification specification for a responsible Stabilization Contractor. The goal is to specify contractors that have experienced personnel, up to date equipment, and the proper management and support structure to respond to today’s project challenges in a timely and through manor.

Due to the nature of the soil stabilization operation required to perform the specific work, bids shall only be accepted from pre-qualified soil stabilization subcontractors. To apply for pre-qualification, the contractor or subcontractor shall submit a list of at least twenty- (20) soil stabilization projects undertaken in the last three- (3) years. This is to include ten- (10) lime treatment and ten- (10) cement treatment projects. At least five- (5) projects shall be of equivalent scope. No contractor shall be considered acceptable without a minimum of ten years experience in soil stabilization using lime or cement.

The soil stabilization contractor shall submit a list of personnel to be used on the project, outlining their experience in this type of work. The contractor shall submit a list of equipment owned by his organization to be utilized in performance of the work. The contractor shall submit a detailed description of work procedure. Pre-qualification documents shall be submitted no later than five days prior to bid opening.

Stabilization contractor shall have a representative on site with a minimum of 5 years experience in soil stabilization. Their function should also include coordinating with other contractors and site representatives. All personnel should be properly trained in the treatment process, quality control, and safety procedures.

Spreader: The spreader shall be equipped with on broad computers that control spread rates with ground sensing radar. The operator shall demonstrate that the computer is working and is calibrated to specifications.

Mixer: The mixer shall have on board computers with automatic grade and automatic water pump controls. A minimum equipment requirement is 500 horsepower and a minimum water pump rating of 450 gallons per minute.

Water Trucks: All water trucks, on the initial mix, shall be equipped with a push bar attachment and a minimum 4 inch hose connection. Do not allow the water truck water pump to operate. They are not able maintain a constant and controllable stream of water flow.

Compactors: When compacting treated sections greater then twelve (12) inches a sheep foot REX type compactor should be used capable of compacting the entire section to the project specification.

Materials: If lime is being used as the reagent then it should conform to the requirements in ASTM C977. Type II cement should conform to ASTM C150. Equivalent materials can be used as an alternative to these reagents, if pre-approved by the project engineer.

Handling, spreading, and mixing operations should be conducted in such a manner that a hazard is not presented to construction personnel or the public.

Generally, procedures for testing treated soils have followed the same guidelines as untreated soil. Therefore, there is little information about geotechnical laboratory testing on treated soils (soils treated with lime, cement or combinations of both) Due to the effects of the treatment on soil samples, it has been the experience of many labs that special procedures are required for treated material to reduce inconsistency in test results from lab to lab. This section attempts to standardize testing methods with lime or cement and soil. This manual is not intended to replace ASTM procedures, but addresses points not included in the ASTM. Where there is some overlap in procedures ASTM should be followed.

When it comes to working with treated material, in the lab environment, the goal should be to duplicate actual or anticipated field conditions. Within the controlled environment of the laboratory, samples are often prepared at two to three percent over optimum. Often the lab technician is not taking into consideration the effects of the reagent (lime or cement) will have on the moisture content of the sample.

Under field conditions, the operator mixes the lime with the soil that is either already wet or water is added to the mix to help hydrate the lime. The mixture is left to cure overnight then remixed and compacted the next day. The goal is to get the mixture as wet as possible while still meeting the relative compaction requirement. The more water that is used, the better the lime will react.

The lab should approach treating with admix the same way. This manual assumes the lab technician has experience with expansion tests on remolded samples and other tests, so therefore, does not go into many of the details about the testing. Refer to the appropriate standards for testing details.

The lab should add the required amount of lime as a percentage of the total dry weight of the sample. Scalping gravel depends upon what type of test is going to be run and the size of the sample to be tested. Per many ASTM standards the diameter of the sample should be six times the largest particle size. If gravel is to be scalped the lime should be calculated based on the total dry weight of the sample less the gravel. If the percentage of lime is based on the original total bulk weight with the gravel the lime should be added prior to removing the gravel.

Most of the time the type of soil to be treated is fat clay, which usually does not have a significant amount of gravel. Add water to the sample with a squirt bottle that has an adjustable nozzle. Some have found it easier to add the water by weight rather than volume. Some labs calculate the required water in cc’s, then poor it into the sample. It may be easier to convert 1cc to 1 gram of water and add it to the sample with a fine mist while on a scale. Mix the soil-lime sample with a long handle spoon while misting the sample. Continue misting and stirring until the sample is about 10-12% over optimum.

One method of calculating how much water to add is to use 3% over optimum as the baseline, then add 2% water for every percentage of lime added. Let the mix mellow in open air over night. The hydration will drive off and use up much of the excess moisture if it is left exposed to air. Check the moisture content the next day and adjust it (if needed) to within 2-4% over optimum and remold to the required relative compaction based on the lime treated curve. The idea is to give the soils as much water as possible (especially highly plastic clay) but not have mud when it comes time to compact.

We have loosened up on the moisture range because it is very time consuming to dry the soil back to the calculated wet weight required to give an exact percent moisture content. So if we get to within an acceptable moisture range we can compact it to the required density easily, within a pound. If desired you can calculate the required total wet weight of the soil-lime-mix, water and tare that would give the desired moisture content.

While mellowing the soil, check the weight occasionally and bag the soil once it has dried back to the calculated weight. Let it continue mellowing over night and do a final moisture check the next day. The 2-4% moisture range gives the lab quite a bit of latitude in fabricating their specimen but there will also be a lot of variability in the field so there is no reason to try to be more accurate unless stated otherwise by the project engineer. If a higher relative compaction is needed (95%) the amount of water over optimum may need to be reduced to no more than 2-3%.

The next day thoroughly stir the mix again until the coloring of the mix looks uniform. Adjust the moisture to within 2-4% over optimum and fabricate the sample to the desired relative compaction (+- 1 pcf). If the adjustment is more than 4% an additional couple of hours of curing is advised. If desired, the moisture content can be double-checked in an oven, but this adds a day to the turnaround time.

Mellowing the sample in an open container is new to us. There may be some unforeseen problems with this method. Further research needs to be done. The reason for this change is twofold. 1) By leaving it open to air the high moisture content will be reduced to a workable range. 2) This is what is done in the field. If mellowing in an enclosed airtight container is desired keep in mind that after mellowing the sample may need to be dried back to meet the required moisture content requested by the Engineer. For the R-value test some technicians prefer to mellow the soil-lime mixture in an enclosed container because the moisture is closer to the desired starting moisture content than it would be if mellowed open to air.

It has been found that, by not adding enough water to a highly plastic material, the lime may not be totally effective. The lime needs to have enough water so that full hydration can take place. But by doing this and then curing in an enclosed container, the soil many times needs to be dried back to meet the desired moisture content. The intent here is to reduce complexity while more closely duplicating what the contractor does and therefore making it easier on the lab technician. We believe this approach is more realistic and if all the labs are using the same methodology there will be far fewer problems associated with variability of test results due to different curing and testing procedures.

An easy remolding method is to compact the sample in a consolidation ring/cell or a "Dames & Moore" brass ring, (1 inch by 2.415 inches) for expansion tests, or a 6 inches brass liner for strength tests (unconfined compression). If a 1-inch high sample is needed, remolding in two lifts should be sufficient. Some technicians use as many as 4 lifts per inch. The goal is a uniform density along the full length of the sample. If a height of 5 inches or more is required (for unconfined compression) remolding in 1-inch lifts should be sufficient. Make sure to scarify thoroughly between lifts. Try to knead the soil rather than compressing it all at once in a press.

For a one- or two-inch tall sample use a tool with a flat round surface area of about 1/4^th of the sample area. Add the first lift and work it by hand until the final height is achieved. The last lift can be finished off in a press after most of the soil has been kneaded in by hand. For strength tests the height should be from 2 to 2.5 times the diameter. There are test procedures that use a sample height-to-diameter-ratio of one. This in our opinion should be avoided as it gives artificially high strengths. Remolding a sample to a height of 5 or more inches by hand is too much work. We recommend using a mold that will hold a 6-inch brass liner and a rod that is very similar to a Cal-Impact drop hammer or a proctor hammer. You can get much more energy from this kind of device. There are many different and clever remolding tools and molds, which work just fine. When you deliver a blow, work the rod in a circular motion so as to knead the soil as much as possible.

Allow the sample to cure in a compacted state for an additional 24 hours in the ring or liner while also in an airtight container. If a brass liner or stainless steel is used, cap and tape the ends and put the sample in a cooler with a small amount of water at the bottom. If you have a wet room the cooler is not needed. It is desirable to keep the moisture in at this point. Two things of importance occur at this stage. The lime has a cementation quality/affect and the sample is able to rebound, which would have shown up as swell if put directly into the expansion device. It has been found over the years that the height of the remolded sample increases a small amount after compaction. This phenomenon is called rebound.

There has been some discussion as to weather the remolded sample should be left in a wet room open to air or kept in the remolding liner or other container. We think it is better to lock in the moisture at this point rather than leaving it exposed to atmosphere. Here are the reasons why: 1) Since not every lab has a wet room we propose that the samples would be better stored in an airtight container whether that be a liner or plastic bag. 2) The compacted soil out in the field will be exposed to atmosphere at the surface only. So for the most part it is also confined and will be completely confined once the road or structure is built on it.

There is a Caltrans Specification (Test 373) that requires the sample to be cured in an oven for seven days at 110⁰F. ASTM D-5102 also has a note regarding heat curing. It is as follows: "When accelerated curing conditions are necessary to expedite the curing process for simulating long-term field conditions, curing temperatures in excess of 48.9 degrees centigrade (120⁰F) should be avoided. Research indicates that a temperature of 40.6⁰C (105⁰F) at various curing times is appropriate for accelerated curing without introducing pozzolanic reactive products that significantly differ from those expected during field curing. A damp cloth placed beneath the soil-lime mixture will help maintain humid conditions for curing and will prevent drying."

The compaction test should be the first test addressed since many of the tests will be based on a relative compaction of the lime treated material. The bulk sample should be air-dried so that it can be prepared per ASTM D-698 or D-1557. Once the sample has been broken down to pass the #4 sieve and the percentage of gravel has been determined, the technician can decide the appropriate mold size to use and the amount of lime can be calculated and added. The portion of the sample to be tested will be treated with lime. The portion of the sample that is not tested but will be calculated in the rock correction, if any, will not be treated with lime. The lime is added as a percentage of the dry weight of the sample. Use the sample minus the rock as the total sample weight that will be used in the calculation for adding the percentage of lime. If the original total weight of the sample is used in the calculation the lime must be added prior to the removal of gravel.

Once the lime is added enough water has to be added so that the lime will hydrate completely. About 10-12 percent over optimum will be sufficient. Allow the mix to mellow open to air until the sample has dried back to a good starting point (about 4% below optimum). At this point the compaction can be run as usual.

The plasticity index PI, (Liquid Limit – Plastic Limit = Plastic Index), is a measure of a soil’s cohesive properties and is indicative of the amount and nature of clay in the soil. Soils with a high PI may be difficult to work with in construction because of their instability and stickiness (plasticity), when wet.

High PI soils also have potential for detrimental volume changes during wetting and drying, which can lead subsequently to heaving or shrinking and cracking pavement. The PI is an important indicator of soil expansion characteristics (but is not a replacement for the accuracy of an expansion test). While other factors (shrinkage limit and colloid content) are also shown as indicators, the PI alone is often taken as a simple index. Experience has shown that soils with PI’s less than about 15 to 18 usually cause no problems; highly expansive soils will have much higher PI’s.

The substantial reduction of PI’s and increase in shrinkage limits indicates not only an improvement in the volume change characteristics but also modification of the soils into more stable and workable materials. In many cases, reducing the PI to a value in the range of 12 to 15 serves as the criteria for selecting a cement content.

The easiest way to treat the material for a PI test is to use Method-B, which is the dry preparation method. The wet preparation method will be discussed later. Air-dry the material at the gradation received. Break the soil down to pass the #4 sieve and scalp on the ¾" sieve. Recombine and mix the –3/4" x +#4 material with the -#4 soil. Put the air-dried soil into an airtight container to maintain the existing moisture content. Take a representative moisture sample and oven dry per ASTM D-2216.

The next day calculate the total dry weight of the sample (less the weight of any scalped gravel) using the moisture content taken from the bagged sample. Add the percentage of lime by dry weight. Thoroughly mix the lime into the sample. Split the sample down to a workable size. Sieve the material over a #40. Note that the lime was added to the sample well before sieving over the #40. This is to try to duplicate the way the lime will be added in the field. If the lime is added to the minus #40 sieve material the percentage of lime to be added will have to be calculated using the minus #40 material as the total sample.

The two methods may yield slightly different results. If the lime is added prior to removing the +#40 sieve material more lime may be left in the sample than removed since the sand and gravel may not hold as much lime as the finer material. Adding lime to the -#40 material while using that as the total sample may yield more conservative results in that there may be less lime. More research needs to be done. When in doubt do it like it will be done in the field.

Once you have at least 150 grams of -#40 sieve material you can add water until the soil is a few percent lower than its plastic limit. Take 30 grams out and set aside for the plastic limit. Add additional water to the remainder of the sample until it is a few percent lower than the estimated liquid limit. Let it mellow over night. Highly plastic soils may need to mellow for at least 48 hours. Method-A was developed because it was found that air-drying some soils would decrease the plasticity, which will reduce the plastic index. We have tested most of the soil types in California and have found so far that only organic Bay Mud is affected by air-drying. There may be other soils also affected such as volcanic soils.

If method A is to be used the soil cannot be dried back. Take moisture content of the soil at its field moisture. Determine the total dry weight and add the lime as a percentage of the total dry weight. The sample should be mellowed at this point for 24 hours. Wash the soil-lime mixture over a #40 sieve with distilled water. The wash water will have to be collected and reused so as not to accumulate too much water. When there is only sand retained on the #40 sieve the washing is complete. The slurry can be dried back to the liquid limit.

There are various methods of doing this, which will not be discussed here. Refer to ASTM. Many times with Bay Mud and adobe 100% of the sample will pass the #40 sieve, so washing will not be required. If this is the case after the lime has been added and mellowing is complete the sample can be brought right up to its plastic limit with distilled water. Take the 30-gram portion for the plastic limit then bring it up to just dry of the liquid limit and cure overnight.

A direct measure of the expansive properties of soils, as opposed to the index tests discussed previously, is afforded in the soaking and swelling portion of the California Bearing Ratio (CBR) test. In this test, a "percent swelling" value of 4 (roughly corresponding to a PI of 20) is an approximate borderline between expansive soils and those that would usually not be troublesome. Highly expansive soils will have much higher values than 4% will.

Small quantities of cement have a greater effect on reducing swell or expansion than they do on improving the index properties discussed in the previous section. Since the latter are only indices, the CBR swell test is a better, more direct measure of this soil property.

Cement treatment of the upper portion of the moisture-controlled subgrade not only prevents volume changes but produces an impermeable layer that will protect the soil below from seasonable variations in moisture content. The cement-treated layer also provides protection against subgrade drying between the time the subgrade is compacted and the subsequent layers of the pavement are placed.

The next step is where standardization is most needed. There are many different types of expansion tests. For example, the expansion index is being used more often. The expansion index test has problems when lime treating is introduced. The procedure is to remold the sample at 50% saturation, which is too dry for the lime. There is not enough moisture in the sample to allow the lime to sufficiently hydrate.

We suggest a simplified swell test. Remold the sample to the required moisture and density. Put the remolded sample under 125 psf in a dead weight consolidometer or other simplified device that would allow the height of the sample to increase while maintaining the load. The sample should still be in the ring. Inundate with tap water and let the sample swell until complete. Completion is obtained when no more swelling occurs. The Engineer would have to set the spec for the amount of expansion that is acceptable. He or she may also want to use a different surcharge or load. This of course will affect the amount of expansion that may occur.

If shrinkage is of concern the sample can be air-dried after it is removed from the expansion device. Once the sample is air-dried (3 days should be sufficient) measure the height and diameter with a digital caliper and determine the change in volume. Other more accurate methods such as the mercury-displacement-method or coating the sample in a thin layer of paraffin then using water displacement to measure volume change can be used but can be difficult, dangerous and/or messy. Record the weight of the sample then put it in the oven at the standard temperature (110 degrees centigrade +-5 degrees per ASTM D-2216). After 24 hours in the oven weigh the sample and measure the height and diameter for the final time. The height should be an average of three readings. Mark each reading point with a waterproof felt pen or crayon. Take two diameter measurements 90 degrees apart and mark those spots also.

You can plot 4 points representing 4 conditions on an X Y plot, X being percent moisture content, Y being percent volume change. The sample at the end of curing will be plotted as zero volume change. The sample just after being removed from the expansion device will represent the saturated / expanded point with its corresponding volume, moisture and density. The air-dried and oven-dried volumes and moisture contents will also be plotted. If the sample is weighed at each stage a density at each condition can be back calculated. You end up with a four-point shrink-swell curve. A straight line should be drawn from the starting point which is the percent moisture content at the end of curing and before saturating, up to the saturated /expanded point, then down to the air dried point ending at the oven dried point.

The percent expansion plus the percent of shrinkage will be the total shrink-swell. It adds more complexity to the expansion test but gives a good graphical representation of all conditions. As long as the technician gets a total wet weight and measurement of the height and diameter of the sample at the end of each condition and a final, total oven dry weight the moisture, density and volume can be calculated for each condition. If the lime did its job there will not be much volume change to plot, in which case the report can be presented in a tabulated format without any graphics.

If the expansion pressure is of concern an expansion pressure test can be run. This takes a pneumatic consolidometer, which is problematic because many pneumatic consolidometers are not designed to allow the sample to expand. This will be discussed in the next paragraph. It is not a problem at this point because no expansion will take place. Once the sample is set up and the dial indicator or LVDT, DCDT or digital dial indicator is zeroed, inundate the sample with tap water. As the sample tries to expand increase the normal load by adjusting the pressure upward very slowly so as not to overshoot and consolidate the sample. Keep an eye on the dial indicator while keeping it at zero by adjusting the pressure. This process will take the better part of a day and many times the final adjustments occur the next morning. Once equilibrium is reached (the point where the exact pressure has been achieved that keeps the sample at zero volume change), that is the expansion pressure. It may have to be converted to the desired engineering units based on your calibration chart for that consolidometer. The test can be ended at that point or continued to develop an expansion pressure curve.

If an expansion pressure curve is desired this is the point where some intervention by the technician must take place before any expansion can occur. Some pneumatic consolidometers do not allow the samples to expand. The cross bar on top of the sample has to be raised to allow the sample to expand. Raise the knurled or wing nuts about ¼ inch. This will give the sample some room to expand. The air regulator will keep a constant pressure on the sample even though the position of the platter is changing. Now the sample can push the platter down when expansion occurs. Or with the newer models push the cross bar up.

Once the expansion pressure has been determined reduce the expansion pressure by half and allow the sample to expand for, at the very least, 24 hours. Note the final dial reading the next day and then reduce the pressure by half again. Continue this process until an expansion pressure curve has been developed with the final point at almost a free swell, with only the weight of the top cap on the sample. The results can be plotted on an XY or log graph with X being the expansion pressure and Y being the change in height in inches or percent. A lime treated sample may not develop an expansion pressure so therefore the test may be discontinued and reported as non-expansive.

The UC test is also frequently used with lime and other substances such as cement. The method of mixing and curing is the same as described above. Mix in the percent lime by dry weight. Add water to 10-12% over optimum and allow the sample to mellow while exposed to air. Check the moisture content the next day and adjust to 2-4% over optimum and remold to the required relative compaction. The required moisture and density may differ depending on the specifications.

If a higher density is required (95% +) the amount of water over optimum may have to be reduced to less than 3% over optimum. The important part is the initial method of curing. The height to diameter ratio should be 2-2.5" to 1", (the height being 2 to 2.5 times the diameter). Remold in one-inch increments and scarify between lifts. Once the sample has been fabricated it is acceptable to leave the sample in the liner with caps on both ends, if a 7 to 28 day cure is required. This is assuming that the sample was remolded in a brass or stainless steel liner.

There has been some discussion as to whether the remolded sample should be left in a wet room open to air or kept in the remolding liner. We think it is better to lock in the moisture at this point rather than leaving it exposed to atmosphere. Here are the reasons why: 1) Since not every lab has a wet room we propose that the samples would be better stored in an airtight container, whether that be a liner or plastic bag. 2) The compacted soil out in the field will be exposed to atmosphere at the surface only. So for the most part it is also confined and will be completely confined once the road or structure is built on it.

Note: This procedure differs from ASTM D-5102 in that the mellow time has been increased and the amount of water to add has been discussed. It has been found that with very highly plastic clays more water and longer mellowing times are required. Otherwise we agree with the ASTM procedure and recommend using it with the above modifications. Much of the testing procedures are not addressed here that are discussed in ASTM.

The R-value test is the most common test performed on lime-soil mixtures. It is also the easiest sample to lime treat. Prepare the R-value sample per Caltrans Test 301. Most of the time the sample to be lime treated will be clay (for obvious reasons), in which case batching will not be required. When the clay has been air-dried break it down over a #4 sieve. Thoroughly mix the sample and take a 100+ gram sample to be put in the oven for a moisture check. Put the rest of the sample in an airtight container to lock in what residual moisture is still in the soil.

The next day mix the required amount of lime in as a percentage of the total dry weight of the sample. Mix the water in to the sample with a squirt bottled that is adjusted to give a fine mist. Continuously stir the sample with a long handle spoon while misting the sample, until the sample is approximately 10-12% over optimum. Allow the sample to mellow open to air until the sample dries back to a good starting point for the R-value test. At that point you can put the sample in an airtight container and allow it to mellow over night.

If the sample is left open to air over night you will most likely have to add water back to the sample and allow it to cure for a couple of hours or more if you prefer. For the R-value test in particular some technicians prefer to mellow the sample in an enclosed container because the starting moisture content is usually very high. Caltrans addresses lime and cement treating in Test 301 and have some time restraints in regards to curing times, which should be followed. We do not believe this manual conflicts with Caltrans Test 301.

After the 24-hour mellow the test can be run. The R-value plug will be inundated with water when put in the expansion pressure rack over night, which will be the cure. Many times the soil-lime plug will develop an expansion pressure during this phase of the test. Do not be alarmed by this. The treated clay is now acting like silt. Silts are tested closer to optimum so therefore can build up an expansion pressure during the soaking phase. Silts can build up a substantial expansion pressure but when the pressure is released not much expansion takes place. So it can be quite deceiving. That is the problem with that part of the R-value test.

Clays, which can be very expansive usually, do not show much of an expansion pressure because they are tested way beyond optimum. But silt is closer to optimum, for the R-value test and can build up a substantial expansion pressure. You have to fully understand how the R-value test is run before you can interpret the expansion pressure results. As stated earlier, they can be very deceiving. We think that part of the test should be dropped. Highly plastic silts (MH) will show a very high expansion pressure. Believe the expansion pressure of an (MH), it is real! If expansion is of concern an expansion test should be run. The R-value expansion pressure is not reliable.

The tests mentioned above are the most commonly used tests with lime. The methods discussed have been used for many years and are easily accomplished by the technician. We have attempted to make the procedures technician-friendly so that they will be used. Test procedures that have been put together by someone who does not do a lot of testing will be difficult to achieve and will be modified or not used at all by the technician. The methods outlined are the easiest way to accomplish what is needed and the most representative. Most of the time adding just enough water to bring the soil up to the desired moisture content for remolding will work just fine. Occasionally it will not work on a highly plastic soil and more water, lime and curing times are needed. The open-air curing method may pose moisture uniformity problems and may have to be abandoned.

We recently ran into a problem with very highly plastic hard clay. After lime treating with as high as 8% lime, the soil still expanded. Two other reputable labs tested similar soil from the same site and showed no expansion. It was tested again with some modifications, which helped but did not completely stop the expansion. More water was added for the mellow and cured for a longer time. This reduced the expansion from 6.5-7.0% down to 4.5%. Which was still too high. After further investigation it was discovered that the two labs that did not show expansion processed the soil over a #10 sieve. The lab that showed expansion prepared their sample over a #4 sieve.

It was assumed that the finer soil mixed much better with the lime so the lime was completely affective. The soil was very hard after air-drying and the larger clods that did not break down were coated with the lime but the lime did not affect the inside of the clods. When the sample was remolded, the hard clods did not completely break down and by that time, the lime was mostly spent and did not have an effect. When the sample was soaked the affinity of the water to clay took over and expansion occurred. An attempt was made to duplicate what the other labs did. Three samples were prepared, One -#4 material, one -#10 and one -#20. The samples were all remolded to the same moisture and densities. They were mellowed and cured for the same period of time.

The results were very surprising. The -#4 material expanded 4.5%, the -#10 material expanded 1.2% and the -#20 material expanded 0.2%. The question of which clod size was most representative of field conditions remained. On a lime-treating job seven bulk samples were taken. The material was air-dried. In order to maintain the natural clod size the samples were not pulverized or washed. A short sieve analysis was performed so as not to break down the clods. The results were as follows: An average of 52% passed the #4 sieve and 32% passed the #10 sieve. Therefore, it is strongly suggested that all remolding of lime treated soils in the lab should be broken down to 100% passing the #4 sieve, not the #10 sieve.

Special care should be taken not to completely pulverize claystones. It has been suggested to make sure that no more than 32% passes the #10 sieve, but further research needs to be done. The way we have always processed soils for remolding is to roll a heavy steel rod over the clay by foot. This will break down the soil but will not pulverize it into a fine powder. The soil is not going to be pulverized into a fine powder out in the field. If we are getting expansion test results differing as much as 7% we need to take a close look at the procedures being used. It could be devastating to a project engineer getting zero percent expansion from the lab and actually getting 7% in the field. So it is imperative that we try to duplicate the final field condition of the soil, especially when claystone is involved.

Further research is under way into the affects of lime on different clay clod sizes in remolded samples. Good lab technicians have a tendency to want to improve on the testing procedures. If most people break the soil down over a #4 sieve they will use a #10. If the standard calls out for measurements to .001" they will measure to .0001". If a spec calls for weights to a 100^th of a pound they will weigh to 100^th of a gram. If most of the technicians remold a sample in one or two lifts per inch they will do it in four. This is commendable and desirable to find a meticulous caring technician. But it backfired in this case. And as mentioned at the beginning of this section we need to view the lab procedures for lime-treated soils from more of a common sense point of view. Since ASTM and other agencies have really not addressed much of what has been discussed here. We need to try to duplicate, what is done out in the field, in the lab. If you know that the equipment out in the field is not going to process the soil into a fine powder, then we shouldn’t do it in the lab!

Representative is the watchword here. The technology out in the field cannot match what we can do in the lab. So if you ever have a question regarding something that is not addressed in the test procedure, think about how it will be done out in the field. This can be very difficult for meticulous, conscientious technicians who are used to complete control of every phase of the testing. Common sense must take the place of having complete control.

Originally home to a long-range naval communication installation, the Chollas Heights site is now home to one of the largest military housing projects in California. This 75-acre residential community includes 412 housing units, a historical district, and a 25-acre nature preserve. The $50 million design and build project was managed and constructed by a joint venture between Keller Construction Company and Swinerton & Walberg Company.

Pavement Considerations

The redevelopment of this site required construction of approximately 400,000 square feet of residential streets; Up to four inches of hot mix asphalt (AC) and 15 inches of aggregate base course (AB) over a traditionally- prepared subgrade were specified for a typical pavement section. The original design was based on an anticipated pavement subgrade R-value of 20.

However, the pavement subgrade exposed at the completion of grading consisted of clay soils exhibiting low R-values that ranged from 5 to 21. Given the poor soil quality, the project geotechnical consultant, Robert Prater Associates, presented Keller/ Swinerton with a lime-based soil stabilization plan that would provide structural and economic benefits. The proposal for the redesigned pavement section retained the four inches of AC, specified only three inches of AB, and added 12 inches of lime-stabilized subgrade (LSS).

The original specification required the extensive export (or removal) and disposal of soil. The LSS method eliminated this requirement. The application of lime stabilization, which chemically alters the soil properties, produced the required structural support requirements. The elimination of soil removal costs, in concert with enhanced soil structural and pavement performance properties, convinced Keller/Swinerton to select the LSS method.

The chemical alteration of soil properties through soil stabilization begins when the pH of the soil-lime mixture is raised and ion exchange occurs in the expansive clay soils. A pozzolanic reaction occurs between the clay and the lime that results in long-term strength development and durability of the pavement subgrade.

Lime was added to the soil as predetermined by the engineer. Specialized equipment was used to spread lime at a uniform rate. After spreading, the lime slurry is uniformly mixed with the soil and compacted to the engineer's design specification.

The use of the lime stabilization process was instrumental in reducing expenses for labor, materials, and other supplies. According to Bill Lee, Senior Project Engineer, Keller/ Swinerton, the actual stabilization process required only eight working days--half the time required for traditional aggregate base course construction. Lee also stated that the lime stabilization process proved to be the structural and economic solution for correcting unexpected, poor pavement support conditions.

"The pavement sections were not expected to be heavy. The poor soil conditions put us in a situation where significantly higher expenses would be incurred to construct the pavement. Lime stabilization of the clay soils kept us from realizing these higher costs while providing the required structural support for the pavements," commented Lee.

The original aggregate base course was budgeted at $1.99/square foot. Actual costs using the LSS alternative were $1.34/square foot. This represented a cost differential of $0.65/ square foot, a 33% savings on a square foot basis. When the final project costs were compiled, the use of the lime stabilization process was responsible for project savings of almost one quarter million dollars.

Pavement Recycling Using Treatment

With economics as a top priority, city officials in Roseville, California reconstructed a very busy road using a new method of lime stabilization and in-place pulverization. After five years of hard usage, the 2.7-mile strip of Baseline Road is exceeding expectations in terms of strength and durability. Furthermore, they cite savings of up to 50% over more conventional methods of reconstruction.

Increased traffic levels during the past decade, particularly from heavy trucks, had put Baseline Road in desperate need of repair. But Roseville, a suburb of Sacramento, had a limited budget for the job. The city’s consulting engineer, G.W. Rodgers, president of Polycon Engineers, Inc., was asked to develop reconstruction plans using the most cost-efficient method.

"Baseline Road needed to be re-profiled’, Rodgers said. "It was very undulated, which caused very poor sight distance. However, we had to make sure we did not disturb housing developments all along the south side of the road."

Rodgers decided to use an in-place pulverization technique using existing road materials. In addition, they chose lime to stabilize the different elements found in the native soil. They needed stabilization materials that would be more cost-effective and resistant to cracking than Portland Cement. "We wanted to salvage whatever materials already existed in the road," Rodgers said, "rather than carry out waste materials, only to carry others back in. I’ve always believed in establishing the roadway upon what you’ve already got there, using the consolidation that has taken place over the years."

The first step in the reconstruction process was to grind up, or pulverize the existing 3-6 inches of asphalt and mix it with the underlying aggregate base and native materials into a homogenous mass. Pulverization was completed a full week before stabilization took place. During this time, local traffic was permitted on the road.

The native soil in the Roseville area varied from dense clay to white sand. In order to stabilize this mixture, the engineers had to combine lime with the dense clay and fly ash with the sandy soil. They chose fly ash over conventional Portland cement because it has similar properties but is less expensive.

The road needed to have a minimum compressive strength of 400 psi to be sufficient enough to handle increased traffic levels. Three percent lime, nine percent fly ash and water were mixed in to the homogeneous mass. This final mixing process was critical to ensure stabilization. "We ended up with a pavement of tremendous strength that has been resistant to cracking," Rodgers said. "The material proved to be stronger than we had originally thought."

From the outset of the Baseline project it was planned to test the properties of the final road and monitor its performance over a period of years. Typical concerns could be items such as incomplete mixing of the lime, causing localized soft spots, and shrinkage of the lime-stabilized base, resulting in cracking of the surface. Immediately after completion of the road, deflection tests were run by CHEC Engineering Consultants, using a Dynaflect device. Testing continued on a six-month basis for the next 3-5 years.

"This material has been tested every six months and has been climbing the strength curve ever since," according to Rodgers. Fred Barnett, Director of Public Works, was an advocate of in-place pulverization, and was in charge of keeping costs down for this project. "We estimate that by using this method we saved the city approximately $1 million in construction and planning costs," said Barnett. "By using existing materials, and the lime/fly ash combination, this road will have a longer life without needing repairs."

The combinations of lime and fly ash for road reconstruction is new to the State of California, but the Roseville project may prove to be a gateway to more of this type of reconstruction. "There have been similar projects in other states, such as Texas, but this is the first of its kind in California," said Coster. "An important aspect of this project was that we used the existing materials. Except for lime, fly ash and water, we neither hauled material in nor out, saving both time and money and providing the city with a very strong road that will last a long time.

In September of 1988, a section of Baseline Road was constructed using lime stabilized base (LSB) design techniques. CHEC Engineering Consultants has conducted a 3½-year performance evaluation of the materials and techniques used in this project. Preliminary laboratory testing verified that the subgrade soil and R-values range from 12 to 45, which compares to Dynamic CBR values ranging from approximately 5 to 25.

Further analysis showed the clay subgrade would be a good candidate for lime stabilization. The clay had a Unified Soil Classification of CH and when mixed with 3% lime and 9% fly ash produced a compressive strength of 650 psi per California Test Method 373.

The design procedure for this project utilized the compressive strength approach as published in the California Department of Transportation manuals. This procedure utilizes a laboratory determined compressive strength that determines a structural equivalency factor for determination of layer thickness calculation. Based on a 20-year design Traffic Index of 8.0, the final design section selected for this project was 6 inches of asphalt concrete (AC) over 12 inches of lime/fly ash stabilized base. The base was mixed in-place and compacted in normal construction procedures. A decision was made to construct the road with only 4-in. of AC over the LSB and hold back the final 2 inches of AC until after field test results were obtained.

A performance-monitoring program was implemented immediately after construction of the LSB, prior to placing the asphaltic concrete, as well as immediately after the final acceptance of the project. It was continued on a 6-month basis for the next 3½ years.

The primary program consisted of a visual inspection of the pavement and deflection measurement using a Dynaflect testing device. Deflections were obtained in both lanes at 200-ft. intervals throughout the 13,000-ft.-long project. Deflections measure the amount of bending that the pavement experiences under the load of a fully loaded truck wheel. These data provide an analysis of the possible fatigue life of the pavement based on the anticipated amount of traffic and the strain on the surfacing due to bending.

The mean Benkleman Beam deflection value on Baseline Road was measured at 0.007 inches, and the 80^th percentile value used for analyses was 0.010 inches. In addition to deflection evaluations, cores of the LSB were obtained for laboratory strength testing. The LSB was found to be 13 inches in thickness rather than 12 inches as designed. These cores, taken after 3½ years for analysis of the in-place compressive strength, were measured at 1,056 psi (average of three tests).

The greater the compressive strength determined for LSB in the laboratory, the greater the structural gravel equivalency used in the design. Based on the compressive strength results from materials on this road, we calculated that the section should tolerate a design Traffic Index (T.I.) of 9.5. Using the 80^th percentile deflection value as measured on the most recent testing, we back-calculated that the pavement section should tolerate a T.I. of 10.0 prior to substantial fatigue failure requiring rehabilitation. The above data suggests that the road designed on a 20-year T.I. of 8.0 have an "as constructed" T.I. expectation of 9.5. For the projected traffic on Baseline Road, a T.I. of 9.5 provides a calculated service life of over 30 years.

There has been no visible cracking to date and routine pavement maintenance has not been required. This, plus the compressive strength and deflection data, indicates that:

-The additional 2 inch AC as designed was not required.

-Future road maintenance will be minimal for at least 15 years.

-Major rehabilitation could be delayed until after 20 years of age.

Nearly all transportation agencies agree that "total life-cycle cost", which includes initial, maintenance and anticipated rehabilitation costs, are the proper approach for selection of the optimum pavement structural section for a project. Using these criteria, it is obvious that the type of construction used on Baseline Road will compete well with other structural systems on life cycle cost comparisons. For Baseline Road, there has been a significant initial cost saving by eliminating 2 inches of the originally designed asphalt surfacing thickness. This analysis of Baseline Road shows that lime-stabilized base provides a very good, serviceable, economical road with an optimistic future life.

In 1986, Jim Mitchell, Professor of Civil Engineering at the University of California at Berkeley, presented a paper in the Terzaghi Lecture Series published by the American Society of Civil Engineers (ASCE). This paper addressed several interesting and unique geotechnical engineering problems. One of these problems was the Stewart Avenue pavement failure in Las Vegas, Nevada. It was later determined that the Stewart Avenue problem was simply due to very poor workmanship in the mixing and watering process. This event is very unlikely to occur again due to todays stabilization quality control techniques.

A paper by Dal Hunter followed Mitchell’s report and also addressed the Stewart Avenue failure but with a more complete description of the chemical and mineralogical aspects. Sulfate induced problems in soils stabilized with calcium-based stabilizers such as lime have been documented since the late 1950's and have been very few in number. In each occurrence, quality control is at the root. The mechanism has been studied by a number of highly qualified cement chemists in an effort to understand and control sulfate attacks on Portland cement concrete structures.

An in-depth discussion of the complex reactions of sulfate-induced distress in stabilized soils is not within the scope of this technical memorandum. However, it is important to understand the fundamentals of sulfate-induced distress. To date there is much uncertainty as to whether "sulfate-induced distress" even exists or is this just a symptom of poor workmanship and quality control.

Basically four components are thought to be the culprits in sulfate-induced distress in stabilized soils; calcium, aluminum, water, and sulfates. Together in the right combination, these components will produce calcium-aluminate-sulfate-hydrate minerals with very large expansion potential. One of these minerals is called ettringite. This mineral holds very large quantities of water within its structure. During the formation of ettringite, very high swell pressures can develop and very large volume increases can and do occur.

The formation of ettringite and similar troublesome minerals can be prevented by achieving a proper balance between the supply of all of the following four components; calcium, aluminum, water, or sulfate. When lime and water for construction are added to clay, the lime supplies the calcium and the aluminum is released from the clay in the high pH system produced by lime and water.

There is no easy answer to the problem. Calcium is present when quicklime is used for soil stabilization. Soils containing clay are rich with aluminum, a basic structural unit of clay. Water is necessary for compaction and for stabilization reactions and is present within pavement structures during their service life. Unfortunately, the sulfates cannot be efficiently or economically removed from the soil.

A number of efforts have been made to control the reactions that result in the formation of the problematic expansive minerals. Some of these efforts have been successful. Some are successful but economically impractical.

Presently, the best approach when dealing with lime stabilization of clay with a significant soluble sulfate content is to force the formation of the deleterious minerals prior to compaction. If these minerals form during the mellowing period before placement and compaction, no damage will be done to the pavement. Fortunately, the expansive minerals do form relatively rapidly, as long as the sulfates are soluble, the aluminum is released from the clay and adequate water is available for the formation of the minerals. The keys to success are to force the expansive mineral ettringite to form prior to placement and compaction of the pavement layer by providing adequate mellowing time (time delay between application of the stabilizer and compaction of the stabilized soil) and adequate water.

Adequate mellowing time may be as little as 24 hours or as much as 7 days, depending on the level of soluble sulfates in the soil. An adequate amount of water is typically 3 to 5 percentage points above the optimum needed to achieve maximum density during compaction. Excess water should be applied during the mellowing period, and plentiful amounts of water should be applied to the surface of the stabilized layer during curing.

Water is the most important component of the equation. Adequate water must be supplied throughout the stabilization construction process to force formation of the ettringite prior to compaction. The worst scenario would be to compact a lime-treated, sulfate-bearing clay with too little water. This is especially a problem if quicklime is used, and too little water is used to completely hydrate the quicklime. If this were the case, water entering the soil subsequent to compaction would cause development of expansive minerals in the compacted layer and produce very high and very disruptive expansive pressures. Forensic studies showed inadequate water and poor construction techniques in many areas of Stewart Avenue. The result was post-construction heave when water ultimately reached the quicklime causing hydration of the quicklime and the ensuing expansive chemical reactions.

In an effort to assist you in recommending lime stabilization in sulfate-bearing clays, the following general recommendations are made.

If the total level of soluble sulfates is below 0.3%, or 3,000 parts per million (ppm), by weight of soil, then lime stabilization should not be of significant concern. The potential for a harmful reaction is low. However, good mix design and construction practices should be followed as usual.

Total soluble sulfate levels of between 0.3% (3,000 ppm) and 0.5% (5,000 ppm) are of moderate concern. Generally, these sulfate levels do not result in harmful disruption, but on occasion have caused localized distress. Localized distress is often due to seams of higher sulfate concentration not detected in testing. The potential for some localized distress is a "fact of life" with sulfate levels in this range.

When encountering sulfate levels in the range of 0.3% to 0.5%, it is imperative to follow good mix design and good construction techniques explicitly. Special attention must be given to using excess water during mixing, mellowing, and curing. Mixing water should be at least 3% to 5% above optimum for compaction.

The mellowing period should typically be at least 72 hours, but may need to be longer.

Total soluble sulfate levels between 0.5% (5,000 ppm) and 0.8% (8,000 ppm) represent moderate to high risk. These soils can and have been successfully treated but require very close attention to construction technique. Generally, the same mix design and construction guidelines as described for soils containing sulfate levels between 0.3% and 0.5% should be followed. However, before treating these soils with lime, laboratory testing to determine swell potential is recommended. This testing will not only establish the approximate amount of swell but also will help establish the required period of mellowing between mixing and compaction.

Total soluble sulfate levels of greater than 0.8% (8,000 ppm) are generally of high risk to stabilize with lime. In certain situations, such soils have been successfully treated. However, the risk is generally too high for routine work. If such soils are to be treated, it should only be done following laboratory testing and by an experienced contractor, well schooled in lime stabilization of high sulfate soils.

Treatment of such high sulfate soils requires proper mixing, mellowing, curing water contents of 3% to 5% above optimum for compaction, and may require an extended mellowing period of longer than 72 hours. The required mellowing period may be as long as 7 days, during which monitoring of density is recommended. Double application techniques (discussed below) may be effective in successfully treating high sulfate soils.

Soils with total soluble sulfate contents greater than 1.0% (10,000 ppm) generally are not suitable for lime stabilization because of the high risk of sulfate-induced disruption and failure.

In certain situations, a double application of lime is effective in reducing heave potential and in providing successful long-term stabilization. Double mixing is obviously more expensive, and, therefore, must be cost-effective. Double mixing uses one-half the required lime initially. The soil, excess water, and lime are then mixed followed by a mellowing period of from 72 hours to about 7 days. The purpose of the long mellowing period is to allow time for expansive reactions prior to compaction. Then the second lime treatment is applied. The other half of the required lime is used. The lime-soil mixture is then compacted. Double treatment does not mean twice the amount of lime. It means that the same amount of lime is added in two increments. This technique should be thoroughly evaluated through laboratory testing of site-specific soils to establish appropriate lime application amounts, mellowing times, etc, before proceeding with field construction.

The only "fool proof" way to know whether or not sulfates will be a problem is to test the soil for sulfates. This is done by sampling the soil at enough locations and at the appropriate depths to reasonably assess the level and extent of sulfates.

Quantitative sulfate testing requires the extraction of sulfates from the soil. This is done by solubilizing the sulfates in water, followed by quantitative measurement. Since sulfate salts, such as gypsum (calcium sulfate), have a specific level of solubility, the amount of sulfate extracted from the soil is determined by the type of sulfates present and amount of water added. Therefore, ten parts water to one part soil will result in more solubilized sulfates than three parts water to one part soil, especially at higher sulfate contents.

Experience has shown that an extraction protocol using ten parts water to one part soil is the best for evaluating potential problems resulting from sulfate reactions. This also allows better comparison with most of the test data developed in related research efforts to date. Note that the sulfate levels and associated treatment guidelines provided in this document are based on the ten parts water to one part soil testing ratio and may not be applicable to other water:soil ratios.

Sulfates soluble in water are measured in parts per million (ppm) and often expressed either in ppm or percent. 10,000 ppm are equivalent to 1.0%. Therefore, 3,000 ppm are equivalent to 0.3% and 5,000 ppm to 0.5%, etc. The soluble sulfate content should be reported on a dry soil basis to insure consistency of test results. Soluble sulfates should be extracted from the soil using ten parts distilled water to one part soil. Test method Tex-620-J (Appendix A) prepared by the Texas Department of Transportation is recommended.

Any of several quantitative methods (barium precipitation, ion chromatography, etc.) may be used effectively to measure the water solubilized sulfates. Again, the important thing to remember is that the water:soil ratio used in preparation of the solution will control the amount of sulfates solubilized and measured by any of these methods, and that the guidelines presented here are based on 10:1 extractions.

In testing for sulfates, it is important to remember that sulfates often are present in concentrated areas and may not be distributed uniformly. Seams or veins of sulfates are common. It is also important to realize that sulfates tend to concentrate at a certain depth below the surface of the soil. This depth of concentration is dependent on the climatic conditions of the area or region. In Texas, this depth is often three to six feet (about one to two meters) below the surface.

Sulfates typically are concentrated nearer the surface in drier, western regions. As we move eastward into wetter and more humid climates, the general rule is that sulfates, if present, tend to concentrate at lower depths.

Probably the most beneficial and reliable tool for assessing the presence and significance of sulfates within an area is the United States Department of Agriculture’s County Soils Report. A report is available for every county in the United States and can be obtained from the Soil Conservation Service, a County Agent or the State Land Grant University. The soils report provides an abundance of engineering information conveniently tabulated. There is also a discussion of each soil series within the county and a discussion of the soil profile. This discussion will generally identify the presence of gypsum and other sulfate salts and the depth of significant concentrations, if any. This is an extremely valuable investigative tool.

Keep in mind that it is very important not only to identify the presence of sulfates but also the depth of occurrence. For example, a soil may be essentially sulfate-free in the upper two or three feet (0.67 to 1.0 meters) but have sulfate concentrations at a depth of 6 feet (approximately 2 meters). In this case, sulfates would not be of concern during normal surface stabilization operations but could be of concern in cut and fill areas.

The best approach in checking for sulfates are to ask the county agent where sulfates typically occur and at what depth to expect significant concentrations. It is also wise to buy or check out a County Soil Report. You can locate the construction job of interest to you on the aerial photographs of the county in the back of the report. From these photos the soil series in the area can be identified. Pertinent information on each soil series is presented in the discussion section and in the tabulated agricultural and engineering data for each soil.

If sulfates are present and identified in the County Soils Report, a field-testing plan should be established with the geotechnical engineer. The frequency of testing depends on the level of sulfates present and the geological information for the region.

If total soluble sulfate levels are above 0.5%, tests to determine the degree of expansion that may occur should be performed. These tests require monitoring the vertical and circumference swells on compacted lime-soil cylinders. The cylinders are subjected to water by placing them on porous stones, surrounding them with absorptive towels, and allowing the sample to take on water for at least 30 days or until swell levels off. The measured circumference and vertical swells are then compared to criteria established by the engineer. If total soluble sulfate levels exceed 0.8%, this type of testing should be mandatory.

Presented is a case history of a project in which San Francisco "Bay Mud" (a highly compressible, weak, inherently expansive silty clay) was treated to provide the support for a 4-story reinforced concrete office building. Heavy buildings on sites underlain by the Bay Mud almost invariably employ driven piles for their foundations. A description is given of one site where, after chemically treating the Bay Mud, the use of ordinary shallow spread footings emerged as an alternate to the use of piles.

The project described in this paper is located in the City of Belmont, on the west side of San Francisco Bay, about 25 miles southeast of the City of San Francisco. The site is in a zone commonly described locally as "Reclaimed Bayland". Prior to the mid to late-19^th Century, it was a zone of land frequently inundated by shallow water at the highest tides, or when there were large quantities of water flowing in the Sacramento River system from snow melt in the Sierra Nevada, some 200 miles east of San Francisco.

As population and civilization came to the West Coast of America, the fringe lands of the Bay Area were diked off, first for agricultural use, later for the construction of buildings. In the Belmont area the major impetus for the latter activity developed in the post-1945 years. Prior to that, the fringe Baylands were either in a virgin condition, or they were cut off from the Bay waters by dikes, and thus they were subject to desiccation during the eight dry months of late spring, summer and autumn. In the post-1945 years, these same lands were in many cases raised in elevation by the placing of fills, typically 4 to 6 feet in thickness.

In many cases the fills were generated from spoil created during the development of nearby hilly terrain to the west. In many of these cases, the fill materials were simply haphazardly placed and spread over unprepared ground without the application of any form of quality control.

The site of our project was one such case. Approximately 11 acres in size, the site was flat in appearance, and had had fill placed on it at an estimated period of 25 to 30 years before we investigated it. The site drained badly, but was protected from all but very occasional flooding by a dike on its eastern boundary. Vegetation consisted of a thick growth of grasses and weeds.

The proposed construction consisted of a 4-story office building, surrounded by paving for vehicular parking, with interspersed landscaping.

The building was to be approximately 1 acre in plan area, i.e., 210 feet square. All structural elements, including floors, were to be reinforced concrete. The estimated dead load of the building, spread uniformly over its plan area, was 600 p.s.f. The anticipated reduced, or real live load, was 130 p.s.f., if its load could be uniformly spread. Column loads were expected to be of the order of 430 kips, imposed at regular spacings.

The San Francisco Bay is a wide, north trending depression that lies between crudely parallel, northwest trending mountain ranges. Draining into the Bay from the east is the Sacramento River. The waters of the Bay Area are connected to the Pacific Ocean by the Golden Gate Channel, which is the drowned valley of the ancestral Sacramento River established during a period of lower sea level. On the west side of the Bay is the San Francisco Peninsula, along which the San Andreas Rift Zone is located, and at the north end of which is the City of San Francisco.

The San Francisco Bay depression is thought to have come into existence within the last million years by a combination of warping and faulting. During its life the depression was nearly filled with sediments, some of marine origin, some derived from the surrounding hills. Associated with the melting of the Continental glaciers at the end of the Wisconsin glacial period (10,000 years ago), the sea level began to rise, leading to the flooding of the depression and the formation of the Bay as we know it today. It was during this period of rising sea level that extensive deposits of soft, black (wet), silty clay known as Younger Bay Mud were deposited. This layer of Bay Mud has had a significant impact on the works undertaken in the development of large areas of land around the fringes of the Bay.

Test borings made to a maximum depth of 105.8 feet indicated that the site had a surface fill layer of sandy and clayey silt, containing rubble and organic materials. The fill was underlain by a layer of soft, silty clay (Younger Bay Mud), ranging from 5 to 13 feet in thickness. Below the Bay Mud, to the depth explored, were silty and sandy clays, with lenses and thin layers of dense sand. These clays contained varying amounts of fine, medium and coarse gravel’s and rock fragments. The clays below the Bay Mud were in a stiff condition down to a depth of approximately 35 feet, below which they were in a very stiff-to-hard condition. Groundwater levels were measured as little as 5 feet below the existing ground surface at the time of the site investigation. A graphical representation of the soil profile is shown in Figure 2.

During the construction peak of the 1950s and 1960s, heavy construction traffic was ever present in Belmont, conveying materials to the construction site and dispersing excavated soil from sites with a surplus to sites with a deficiency of soil. Few voices objected to this, provided there was attention given to traffic safety, and perhaps a contribution made to the City’s exchequer to defray the cost of keeping the streets clean and repairing the damage done by the heavy vehicles.

Those days were long gone by the start of the current decade. Rises in energy costs provided a natural constraint that made site developers more concerned about creating surplus excavation than they had hitherto been. More importantly, whilst the local citizens recognized that construction materials would of necessity have to be taken to a site, they were most determined in their opposition to the removal of materials from a site unless one could demonstrate a need borne of emergency. This attitude was not codified in the laws or ordinances of the City, but was one that would undoubtedly surface during the open hearings, which would precede any project approval.

As a broad generalization, Bay Mud will consolidate one inch for each 10 feet of thickness of the layer for each 100 p.s.f. of surcharge load placed on it. Experience in the area suggested there would exist at the site approximately 10 feet of Bay Mud. Thus, a surface-supported building with an effective load of 730 p.s.f. would experience an average settlement of from 4 to 10 inches relative to the adjacent ground surface. This was totally unacceptable.

Alternative foundation schemes discussed at the pre-investigation stage revolved around two concepts:

1. To remove the Bay Mud layer in the building footprint and replace it with something else; or
2. To carry the foundation loads through the Bay Mud layer into the underlying stiff clays.

Within category (a) were considered:

1. Constructing a basement parking area. This had both advantages and disadvantages. Disadvantageously, it would require hauling away from the site a large volume of wet silty clay, foreseeably causing great consternation to the residents of Belmont. Provision was made for importing some granular material to the site, to make up the volume which would be lost when the treated Bay Mud was backfilled as compacted fill, and to provide a slightly mounded pad upon which to construct the building.

Table 1. Properties of Bay Mud

Properties Natural 4% Lime

Maximum Density (pcf)* 45.6** 86.8

Optimum Moisture (%)* 94.6** 23.9

Liquid Limit (%) 112.9 100.0
Plasticity Index (%) 67 46

During the bidding period, the purveyors of a CaO/MgO product known as "quicklime" requested consideration of their product as an alternative.

A typical chemical analysis of Cycle II Hydrated Lime is:

Calcium Oxide - 42%

Magnesium Oxide - 38%

Silica - 10 to 12%

Chloride - 5 to 9%

Much of the stated calcium is bound tightly with the silica, and the free calcium, as determined by the State of California Test No. 414, "Method of Test for Free Lime in Hydrated Lime" (ASTM Test Method C25, modified).

Accompanying the encouragement of the laboratory test results there was a willingness to consider the use of Cycle II Hydrated Lime because of other factors, namely:

1. It was readily available in the necessary quantities, whereas the 75% hydrated lime was scarce at the time.
2. It was much less expensive than the hydrated lime.
3. It contained sand-size silica bulk which would help make up the loss in volume when the treated Bay Mud was compacted.

Table 2. Properties of Treated Bay Mud

Properties 4% Lime 20% CaO/MgO

Maximum Density (pcf)* 86.8 107.8

Optimum Moisture (%)* 23.0 16.5

Liquid Limit (%) 100.0 56.6

Plasticity Index (%) 46 18

Consolidation tests on undisturbed samples of the Bay Mud, and on remolded samples of the treated Bay Mud revealed the treated soil was very much less compressible than the untreated soil. Figure 3 shows representative consolidation curves. One surprising property also revealed was that the treated Bay Mud had a significant amount of residual swelling capacity when saturated—an expansion pressure of 1,600 to 3,300 p.s.f.

It was assumed by us that the contractor would affect the treatment by excavating the soil in the building area with a dragline bucket, placing the soil to one side and mixing the CaO/MgO product into the soil before placing it back in the excavation as a compacted fill. We anticipated that control of the water table would be affected by the use of sump pumps in the excavation.

Instead, the contractor constructed a deep subdrain between the highway and the building area. He then spread a calculated quantity of the CaO/MgO product over the building area, simultaneously mixing it by a rotary blade assembly into the upper 18 inches of soil. The next day, the upper 12 inches of treated soil were removed and stockpiled outside the building area. This procedure was repeated on a daily basis, the treated soil being allowed to stand overnight for the calcium to react and displace the sodium ions.

Therefore, at the start of each day the construction equipment was operating over a skin of stabilized soil. There were a few close calls when the equipment started to puncture through the skin into the underlying soft Bay Mud. However, special tractors with extra-wide caterpillar tracks were able to keep operations moving, and the entire mixing and excavation sequence was completed in less than three weeks. The placement of the treated Bay Mud as a compacted fill was a normal operation that was also completed in less than three weeks.

1.01 Description -- This work consists of mixing in-place material, lime and water, and spreading and compacting the mixture to the lines, grades and dimensions shown on the plans and as specified in these specifications and the special provisions.

1.02 Materials -- In-place material on the roadbed shall be the native material or embankment.

Lime shall be either high calcium quicklime or dolomitic quicklime conforming to the definitions in ASTM Designation: C 51 & C 977. When sampled by the Engineer from the lime spreader or during the spreading operation, the sample of lime shall conform to the following requirements:

• High-calcium quicklime shall contain not less than 113 percent calcium hydroxide Ca (OH)2, as determined by California Test 414.

• Dolomitic quicklime shall contain not less than 57 percent calcium oxide, CaO, and not less than 95 percent combined calcium oxide, CaO, and magnesium oxide, Mgo, as determined by California Test 404.

• When dry sieved in a mechanical sieve shaker for 10 minutes + 30 seconds a 250-gram test sample of quicklime shall conform to the following grading requirements:

In addition to the above, the use of alternative lime products that are of equal quality and of the required characteristics for the purpose intended will be permitted, subject to the following requirements:

The burden of proof as to quality and suitability of alternatives shall be upon the contractor and/or Supplier who shall furnish test data and all information necessary as required by the Engineer. Written request for alternatives, accompanied by complete data as to the equality and suitability of the material shall be made in ample time to permit approval without delaying the work. The engineer shall be the sole judge as to the quality and suitability of alternatives and his/her decision shall be final.

The lime shall be protected from moisture until used and be sufficiently dry to flow freely when handled.

A Certificate of Compliance in accordance with Caltrans Specification 6-1.07 shall be furnished with each delivery of lime and shall be submitted to the Engineer with a certified copy of the weight of each delivery.

Water shall be clean and potable and free of organics, sulfates and other deleterious materials. It shall be added as needed during mixing and remixing operations, during compacting, during the curing period, and to keep the cured material moist until covered.

1.03 Preparing Material -- The material to be treated shall contain no rocks or solids other than soil clods larger than 2 1/2 inches in any dimension. Removing and disposing of said rocks or solids larger than 2 1/2 inches will be paid for as extra work.

1.04 Mixing – The percentage of lime to be added to the native material should be determined by the engineer. The amount of lime is a percentage by weight of the treated materials PCF dry weight.

The engineer will determine the depth of treatment. Lime shall be spread by equipment capable of uniformly distributing the required amount of lime for the full depth and width of treatment.

The spread lime shall be prevented from blowing by suitable means selected by the contractor. Hydrated lime may be mixed with the material in either a slurry or dry state at the option of the contractor. The spreading operations shall be conducted in such a manner that a hazard is not present to construction personnel or the public. All lime spread shall be thoroughly mixed into the soil the same day lime spreading operations are performed.

No traffic other than the mixing equipment or other related construction equipment will be allowed to pass over the spread lime until after completion of mixing. This includes the spreader, mixer, and water truck.

Mixing equipment shall be equipped with a visible depth indicator showing mixing depth, an odometer or footmeter to indicate travel speed and a controllable water additive system for regulating water added to the mixture.

Mixing equipment shall be of the type that can mix the full depth of the desired thickness and leave a relatively smooth bottom of the treated section. Mixing and re-mixing, regardless of equipment used, will continue until the material is uniformly mixed, free of streaks or pockets of lime, moisture is at approximately 3-5% over optimum and all material other than rock or aggregate previously treated with asphalt, lime or cement complies with the following requirements:

l" 98 Min.

No. 4 60 Min.

Non-uniformity of color reaction when the treated material, exclusive of one inch or larger clods, is tested with the standard phenolphthalein alcohol indicator, will be considered evidence of inadequate mixing.

Lime treated material shall not be mixed or spread while the atmospheric temperature is below 35° F or below 1.67 C.

The first and final mixings shall not be performed on the same day. The entire mixing operation shall be completed within 7 days of the initial spreading of lime, unless otherwise permitted by the engineer.

1.05 Spreading and Compacting -- The treated mixture shall be spread to the required width, grade and cross section. The maximum compacted thickness of a single layer may be any thickness the contractor can demonstrate to the Engineer that his equipment and method of operation will provide uniform distribution of the lime and the required compacted density throughout the layer.

The finished thickness of the lime treated material shall not vary more than 0.1 foot from the planned thickness at any point.

The lime treated soils shall be compacted to a relative compaction determined by the engineer.

The sample of lime-treated soil used for determining the maximum dry density will be a composite of 5 samples taken at random from the area to be tested and obtained after all mixing operations have been completed, but prior to initial compaction.

Initial compaction shall be performed by means of sheepsfoot or segmented wheel roller. Final rolling shall be by means of steel-tired or pneumatic-tired rollers.

Areas inaccessible to rollers shall be compacted to the required compaction by other means satisfactory to the engineer.

Final compaction shall be completed within 36 hours of final mixing.

The surface of the finished lime treated material shall be the grading plane and at any point shall not vary more than 0.08 foot above or below the grade established by the engineer. The exception is that when the lime treated material is to be covered by material, which is paid for by the cubic yard, the surface of the finished lime treated material shall not extend above the grade established by the engineer.

Before finish compaction, if the treated material is above the grade tolerance specified in this section, uncompacted excess material may be removed and used in areas inaccessible to mixing equipment. After finish compaction and trimming, excess material will be removed and disposed of. The trimmed and completed surface shall be rolled with steel or pneumatic-tired rollers. Minor indentations may remain in the surface of the finished material as long as no loose material remains in the indentations.

At the end of each day's work, a construction joint shall be made in thoroughly compacted material normal to the center line of the roadbed with a vertical face.

After a part-width section has been completed, the longitudinal joint against which additional material is to be placed shall be trimmed approximately 3 inches into treated material, to the neat line of the section, with a vertical edge. The material so trimmed shall be incorporated in the adjacent material to be treated.

An acceptable alternate to the above construction joints, if the treatment is performed with cross shaft rotary mixers, is to actually mix 3 inches into the previous day's work to assure a good bond to the adjacent work.

1.06 Curing -- The surface of each compacted layer of lime treated material shall be kept continually moist until covered by a subsequent layer of lime treated or other material or by applying a curing seal immediately following final trimming and rolling of the lime treated layer.

A curing seal will be required only for the top layer of lime treated material if treatment is to be exposed for more than 3 days. The curing seal shall consist of SS or CSS grade asphaltic emulsion and shall be furnished and applied in accordance with the provisions in Caltrans Specifications Section 94, "Asphaltic Emulsions".

Curing seal shall be applied at a rate between 0.10- and 0.20- gallon per square yard of surface, the exact rate to be determined by the Engineer. The curing seal shall be applied as soon as possible after the completion of final rolling and before the temperature falls below 35° F or 1.67 C.

No equipment or traffic shall be permitted on the lime-treated material during the first 3 days after applying the cure seal, unless otherwise permitted by the engineer.

1.01 Description -- This work consists of mixing in-place material, Portland cement and water, and spreading and compacting the mixture to the lines, grades and dimensions shown on the plans and as specified in these specifications and the special provisions.

The Cement shall be protected from moisture until used and be sufficiently dry to flow freely when handled.

Water shall be clean and potable and shall be added as needed during mixing and remixing operations, during compacting, during the curing period, and to keep the cured material moist until covered.

1.02 Preparing Material -- The material to be treated shall contain no rocks or solids other than soil clods larger than 2 1/2 inches in any dimension. Removing and disposing of said rocks or solids larger than 2 1/2 inches will be paid for as extra work.

1.03 Mixing – The percentage of cement to be added to the native material should be determined by the engineer. The amount of cement is a percentage by weight of the treated materials PCF dry weight.

The engineer will determine the depth of treatment. Cement shall be spread by equipment capable of uniformly distributing the required amount of cement for the full depth and width of treatment.

The spread lime shall be prevented from blowing by suitable means selected by the Contractor. Portland cement may be mixed with the material in a dry state. The spreading operations shall be conducted in such a manner that a hazard is not present to construction personnel or the public. All cement spread shall be thoroughly mixed into the soil the same day cement spreading operations are performed.

No traffic other then the mixing equipment or other related construction equipment will be allowed to pass over the spread cement until after completion of mixing. This is to include the spreader, mixer, and water truck.

Mixing equipment shall be equipped with a visible depth indicator showing mixing depth, an odometer or footmeter to indicate travel speed and a controllable water additive system for regulating water added to the mixture.

Mixing equipment shall be of the type that can mix the full depth of the desired thickness and leave a relatively smooth bottom of the treated section. Mixing and re-mixing, regardless of equipment used will continue until the material is uniformly mixed, free of streaks or pockets of lime, moisture is at approximately 2 percent over optimum and all material other than rock or aggregate previously treated with asphalt, lime or cement complies with the following requirements:

l" 98 Min.

No. 4 60 Min.

Non-uniformity of color reaction when the treated material, exclusive of one inch or larger clods, is tested with the standard phenolphthalein alcohol indicator, will be considered evidence of inadequate mixing.

Cement treated material shall not be mixed or spread while the atmospheric temperature is below 35° F or below 1.67 C.

The entire mixing operation shall be completed within 2 hours of the initial spreading of lime, unless otherwise permitted by the engineer.

1.04 Spreading and Compacting -- The treated mixture shall be spread to the required width, grade and cross section. The maximum compacted thickness of a single layer may be any thickness the contractor can demonstrate to the engineer that his equipment and method of operation will provide uniform distribution of the cement and the required compacted density throughout the layer.

The cement treated soils shall be compacted to a relative compaction determined by the engineer.

The sample of cement treated soil used for determining the maximum dry density will be a composite of 5 samples taken at random from the area to be tested and obtained after all mixing operations have been completed, but prior to initial compaction.

Initial compaction shall be performed by means of sheepsfoot or segmented wheel roller. Final rolling shall be by means of steel-tired or pneumatic-tired rollers.

Areas inaccessible to rollers shall be compacted to the required compaction by other means satisfactory to the engineer.

Final compaction shall be completed within 90 minutes of final mixing. Field density tests should be performed prior to final compaction effort. Contractor should be notified if compaction is not meeting project specification and will require additional effort.

After final compaction, the treated material above the grade tolerance specified can be trimmed. Any excess material can be used in areas where standard equipment was not able to mix properly. The trimmed and completed surface shall be rolled with steel or pneumatic tired rollers. Minor indentations may remain in the surface of the finished material as long as no loose material remains in the indentations.

At the end of each day's work, a construction joint shall be made in thoroughly compacted material normal to the center line of the roadbed with a vertical face.

After a part-width section has been completed, the longitudinal joint against which additional material is to be placed shall be trimmed approximately 3 inches into treated material, to the neat line of the section, with a vertical edge. The material trimmed shall be incorporated in the adjacent material to be treated.

An acceptable alternate to the above construction joints, if the treatment is performed with cross-shaft rotary mixers, is to actually mix 3 inches into the previous day's work to assure a good bond to the adjacent work.

1.05 Curing -- The surface of each compacted layer of cement treated material shall be kept moist until covered by a subsequent layer of cement treated or other material for a period not to exceed 3 days or by applying a curing seal immediately following final trimming and rolling of the cement-treated layer.

A curing seal will be required only for the top layer of cement treated material if treatment is to be exposed for more then 3 days. The curing seal shall consist of SS or CSS grade asphaltic emulsion and shall be furnished and applied in accordance with the provisions in Caltrans Specifications Section 94, "Asphaltic Emulsions".

Curing seal shall be applied at a rate between 0.10- and 0.20-gallon per square yard of surface, the exact rate to be determined by the Engineer. The curing seal shall be applied as soon as possible after the completion of final rolling and before the temperature falls below 35° F or 1.67 C.

No equipment or traffic shall be permitted on the cement-treated material during the first 3 days after applying the cure seal, unless otherwise permitted by the engineer.

1.01 Description -- This work consists of mixing in-place material, Type SS cement or HAC and water, and spreading and compacting the mixture to the lines, grades and dimensions shown on the plans and as specified in these specifications and the special provisions.

The Type SS cement or HAC shall be protected from moisture until used and be sufficiently dry to flow freely when handled.

Water shall be clean and potable and shall be added as needed during mixing and remixing operations, during compacting, during the curing period, and to keep the cured material moist until covered.

1.02 Preparing Material -- The material to be treated shall contain no rocks or solids other than soil clods larger than 2 1/2 inches in any dimension. Removing and disposing of said rocks or solids larger than 2 1/2 inches will be paid for as extra work.

1.03 Mixing – The percentage of Type SS cement to be added to the native material should be determined by the engineer. The amount of Type SS cement is a percentage by weight of the treated materials PCF dry weight.

The engineer should determine the depth of treatment. Type SS cement or HAC shall be spread by equipment capable of uniformly distributing the required amount of Type SS cement for the full depth and width of treatment.

The spread cement shall be prevented from blowing by suitable means selected by the Contractor. Type SS cement may be mixed with the material in a dry state. The spreading operations shall be conducted in such a manner that a hazard is not present to construction personnel or the public. All Type SS cement spread shall be thoroughly mixed into the soil the same day Type SS cement spreading operations are performed.

No traffic other than the mixing equipment or other related construction equipment will be allowed to pass over the spread cement until after completion of mixing. This is to include the spreader, mixer, and water truck.

Mixing equipment shall be equipped with a visible depth indicator showing mixing depth, an odometer or footmeter to indicate travel speed and a controllable water additive system for regulating water added to the mixture.

Mixing equipment, shall be of the type that can mix the full depth of the desired thickness and leave a relatively smooth bottom of the treated section. Mixing and re-mixing, regardless of equipment used, will continue until the material is uniformly mixed, free of streaks or pockets of Type SS cement or HAC. Material should be place with moisture content of approximately 2 percent over optimum. All material should comply with the following requirements:

l" 98 Min.

No. 4 60 Min.

Non-uniformity of color reaction when the treated material, exclusive of one inch or larger clods, is tested with the standard phenolphthalein alcohol indicator, will be considered evidence of inadequate mixing.

Type SS cement treated material shall not be mixed or spread while the atmospheric temperature is below 35° F or below 1.67 C.

The entire mixing operation shall be completed within 2 hours of the initial spreading of cement, unless otherwise permitted by the engineer.

1.04 Spreading and Compacting -- The treated mixture shall be spread to the required width, grade and cross section. The maximum compacted thickness of a single layer may be any thickness the contractor can demonstrate to the engineer that his equipment and method of operation will provide uniform distribution of the cement and the required compacted density throughout the layer.

The finished thickness of the Type SS cement or HAC treated material shall not vary more than 0.1-foot from the planned thickness at any point.

The Type SS cement or HAC treated soils shall be compacted to a relative compaction determined by the engineer. Field density tests should be taken before final compaction effort is completed. Contract should be informed if compaction is not meeting project specification and will require more effort.

The sample of Type SS cement or HAC treated soil used for determining the maximum dry density will be a composite of 5 samples taken at random from the area to be tested and obtained after all mixing operations have been completed, but prior to initial compaction.

Initial compaction shall be performed by means of sheepsfoot or segmented wheel roller. Final rolling shall be by means of steel-tired or pneumatic-tired rollers.

Areas inaccessible to rollers shall be compacted to the required compaction by other means satisfactory to the Engineer.

Final compaction shall be completed within 90 minutes of mixing.

Before finish compaction, contractor can remove excess material and place in areas inaccessible to mixing equipment. After finish compaction and trimming, excess material will be removed and disposed of. The trimmed and completed surface shall be rolled with steel or pneumatic tired rollers. Minor indentations may remain in the surface of the finished material as long as no loose material remains in the indentations.

At the end of each day's work, a construction joint shall be made in thoroughly compacted material normal to the center line of the roadbed with a vertical face.

After a part-width section has been completed, the longitudinal joint against which additional material is to be placed shall be trimmed approximately 3 inches into treated material, to the neat line of the section, with a vertical edge. The material so trimmed shall be incorporated in the adjacent material to be treated.

An acceptable alternate to the above construction joints, if the treatment is performed with cross-shaft rotary mixers, is to actually mix 3 inches into the previous day's work to assure a good bond to the adjacent work.

1.05 Curing -- The surface of each compacted layer of Type SS cement treated material shall be kept moist until covered by a subsequent layer of treatment or other material for a period not to exceed 3 days or by applying a curing seal immediately following final trimming and rolling.

A curing seal will be required only for the top layer of lime treated material if treatment is to be exposed for more than 3 days. The curing seal shall consist of SS or CSS grade asphaltic emulsion and shall be furnished and applied in accordance with the provisions in Caltrans Specifications Section 94, "Asphaltic Emulsions".

Curing seal shall be applied at a rate between 0.10- and 0.20- gallon per square yard of surface, the exact rate to be determined by the engineer. The curing seal shall be applied as soon as possible after the completion of final rolling and before the temperature falls below 35° F or 1.67 C.

No equipment or traffic shall be permitted on the Type SS cement or HAC treated material during the first 3 days after applying the cure seal, unless otherwise permitted by the engineer.

1. – General. This work shall consist of scarifying, pulverizing, mixing, and compacting a mixture of existing base and surfacing material with reagent (cement, HAC, or quicklime) as shown on the plans and as hereinafter specified.

02.01 – General. Materials shall conform to the requirements specified in the following sections and subsections:

Portland Cement ______________________________ ASTM

Quicklime ___________________________________ ASTM

At the Contractor’s option, Portland cement may be either Type I, Type II, or HAC. The limitation on the amount of alkali in cements used for roadbed modification is hereby waived.

02.02 – Gradation Requirements. The existing bituminous pavement shall be loosened and pulverized to the extent it will conform to the following sieve sizes:

Sieve Sizes Percentage by Weight

Passing Sieve

3 Inch 100

2 Inch 95 - 100

03.01 – Weather Limitations. Processing for roadbed modification shall not be allowed while the atmospheric temperature is below thirty-five (35) degrees Fahrenheit, or when conditions indicate that the temperature will fall below thirty-five (35) degrees Fahrenheit for a sustained period of four (4) hours within twenty-four (24) hours after final compaction.

The pulverization operation shall not be allowed to proceed when current or anticipated weather conditions will not permit the processing operation to follow due to the temperature limitations specified above.

03.02 – Preparation of Roadbed. The existing bituminous pavement shall be pulverized full depth.

Once the bituminous pavement has been pulverized as specified, reagent shall be spread and the material shall be mixed with sufficient amount of the existing gravel base to obtain the required depth shown on the plans.

The pulverization operation shall be restricted to one-half (1/2) of the roadway width at a time. The pulverized material shall, at the Contractor’s option, be relayed by one (1) of the following methods and public traffic placed thereon at the end of each working day:

1. The unprocessed pulverized material shall be relayed and compacted firmly as determined by the Engineer.
2. The cement may be added, the material processed, spread and compacted, all in conformance with the requirements contained in this section for completion of the finished product.

No more of the existing roadway surface shall be pulverized in any working day than can be relayed as specified above in that working day. The length of one-half (1/2) of the roadway width that may be pulverized shall not exceed three (3) miles in advance of the completed roadbed modification process at the end of each working day. Only as much reagent shall be distributed on the pulverized surface material and base as can be mixed and compacted within the same working day. The finished base shall be sealed within twenty-four (24) hours after final laydown, and until sealed the base shall be kept moist as directed by the Engineer, at the Contractor’s expense.

03.03 – Proportioning. Reagent shall be added to the pulverized base and surfacing material at the rate of two (2) percent by weight. The calculated maximum density of the pulverized base and surface material will be used to determine the weight of the material in place. The reagent shall be added in a dry state and every precaution shall be taken to prevent blowing. The rate of reagent spread shall not vary more than ten (10) percent from the designated rate.

03.04 – Mixing. The roadmix machine shall be a cross-shaft type mixer capable of providing a uniform homogeneous mixture throughout the material to the depth indicated on the plans.

Reagent to be mixed with the pulverized base and surfacing material may be furnished in bulk. Reagent furnished in bulk shall be spread by mechanical equipment. The spreading equipment shall be calibrated so the average rate of spread upon the pulverized base and surfacing material ahead of the roadmixing operation will be determined by the Engineer.

The roadmixing machine shall have provisions for introducing water at the time of mixing through a metering device or by other approved methods. The water shall be applied by means of controls that will supply the correct quantity of water to produce a completed mixture with uniform moisture content. Leakage of water from equipment will not be permitted. Care shall be exercised to avoid the addition of any excessive water.

The resulting mixture shall be uniform and more than one (1) pass of the mixer through the material may be required. If equipment is used that requires more than one (1) pass of the mixer, at least one (1) pass shall be made before any water is added to the material.

03.05 – Compacting and Finishing. After the materials have been satisfactorily mixed, the mixture shall be bladed and compacted to ninety-five (95) percent relative maximum density. Compaction shall be accompanied by sufficient blading to eliminate all irregularities.

Rolling shall be accomplished with a two (2) axle tandem steel roller weighing not less that ten (10) tons, or single or dual drum vibratory roller and pneumatic-tired rollers. The sequence in which these rollers are used shall be as directed by the Engineer.

03.06 – Protection and Curing. The surface shall be kept moist at all times until the curing seal is applied. Watering equipment shall be of a type, which will apply moisture in a fog- or mist- type application, free of pressure on the surface being treated.

The completed cement treated pulverized base and surfacing material shall be covered with bituminous curing seal as protection against drying. The curing seal shall be applied as soon as possible but not later than twenty-four (24) hours after completion of final rolling. Curing seal shall be liquid asphalt, Type MC-250, applied at the rate shown on the plans or as directed by the Engineer.

At the time of application of the curing seal, the surface shall be tightly knit, free from all loose material, and shall contain moisture to prevent excessive penetration of the asphalt. If necessary to ensure this sufficient water to fill the surface voids shall be applied immediately before the asphalt is applied.

Equipment or traffic may be permitted on the reagent-treated base when approved by the Engineer. After curing seal has been applied a Sand Blotter can be applied, if necessary as determined by the Engineer.

Subject: Lime Treatment Recommendations

Location

City, California

Earthwork construction is proposed to commence in xxxxxxx of this year and there is a high potential that the native soil will be too wet for adequate compaction. In order to dry the material sufficiently for compaction, it is our recommendation that the soil be lime treated. The lime treatment will dry out the soil and may increase the soil strength as well as decrease any expansion potential that may exist at the site. We recommend that the lime treatment operation be performed as follows:

1. Quicklime should be added to the soil at a rate of 3% by weight. The depth of treatment should be 18 inches below the finished subgrade of the pads and roadways.
2. A chemical treatment subcontractor who is experienced in this aspect of construction and has the proper spreaders and mixers should perform the lime spreading and mixing. Hand spreading or tailgating of the lime and mixing with equipment other than soil pulverizers or specialized rototillers is not recommended.
3. The lime treatment operation should conform to the attached Guidelines and Specifications.
4. The lime may be mixed in one 18-inch lift. Compaction may occur in one 18-inch lift provided that the contractor can achieve the required compaction throughout the lift.
5. A representative of the Soils Engineer should be present during the entire lime treatment operation to observe the operation and test the compaction and depth of treatment.
6. For complete soil stabilization of the site a qualified Geotechnical Engineer should prepare a laboratory-based design to determine the optimum quantity of lime to apply to the site.

If you require further information, please do not hesitate to contact us at your convenience.

Sincerely,

Date

Mr/Ms. xxxxxxx

Big Client, Inc.

000 Unstable Court, Suite 1

Somewhere, California 95001

Dear Client:

In accordance with your request, this letter summarizes our supplemental earthwork recommendations for site access and pavement subgrade preparation on public streets at the California _____________ project located in ____________, California. Our firm previously performed a geotechnical investigation for the project and the results were presented in our report dated _________ __, 200_.

We understand that the recent heavy rains have saturated the existing street subgrade soils and rendered them difficult or impossible to grade or compact using conventional earthwork construction practices under current weather conditions.

Therefore, to facilitate construction, we recommend that the subgrade soils be processed using a lime-cement chemical additive commonly known as HAC (cement kiln dust). The addition and processing with the HAC will provide drying of the surface soils and stabilization of the subgrade. These beneficial affects occur primarily as a result of:

• Hydration: Cement and quicklime in HAC reacts and consumes water present within the subgrade soils;

• Flocculation: Quicklime combines with clay minerals within the soils, and, through cation exchange, permanently alters the structure of the clay minerals to create less plastic clay minerals; and

• Cementation: HAC contains cement components which hydrate and bind the soil particles together, plus the clay-lime reaction removes silica from the clay mineral lattice to form products similar to those of cement hydration.

In order to provide temporary access for the underground utility installation operations, the existing street subgrade should be treated with at least _ percent HAC to a depth of _ inches. Compaction of the treated subgrade should be sufficient to allow access of the construction equipment.

Subsequently, we recommend that treatment and preparation of the street subgrade for pavement construction consist of the following:

1. Once a 3 to 4 day window of dry weather is predicted, any ponded water on the subgrade should be removed by pumping, if necessary.

Should you have any questions or require additional information, please do not hesitate to contact us.

Sincerely,

This method describes how to determine the chloride and sulfate content in soil.

This method covers the calculation of chloride and sulfate ions in water to determine its suitability for concrete, sprinkling or similar uses. Interference and methods of treating them may be found in ASTM D512 (Method B) for chloride and ASTM D516 (Method A) for sulfate.