Technical Papers and Articles

»Loss Prevention QA

From the Loss Prevention Seminar 2003

Soil Stabilization Using Chemical Treatment

Quality control is essential to assure that the final product will be adequate for its intended use. Additionally, it must assure that the contractor has performed work in accordance with the plans and specifications.

This quality control plan attempts to identify those control factors that are most important in soil stabilization construction with quicklime, quicklime/pozzolans, and cement products. Inspection and testing requirements, for each of those factors, will also be discussed.

Scope

Development of laboratory methods for soil stabilization will be of little value if the results of these methods cannot be successfully applied in the field. The success, at least in part, is more likely if some plan is available to assure the quality of the final product. The engineer or technician in the field encounters highly variable conditions such as climate, efficiency of equipment and soil type. These items can be a major impact during construction. Therefore, field personnel must be aware of those factors that control the quality of the final product. The use of quicklime, quicklime/pozzolan, and cement products in soil stabilization can present significant problems, unless the field engineer has some familiarity with those factors which must be controlled to assure that an investment of time and money will not be wasted.

The quality of stabilized mixtures, as produced and placed, must be monitored on a continuing basis to ensure a quality product. The general tests normally conducted on these materials are listed below in their order of importance or frequency of testing.

1. In-place density (ASTM D 1557, Cal Test 216, and AASHTO T 238-73)

2. Stabilizer content - (Quicklime, Quicklime/pozzolan, and cement products – ASTM)

3. Gradation (ASTM D 136-71)

4. Moisture content (ASTM D 2216-71)

Quicklime Treatment - Quality Control and Guide Specifications

Detailed procedures have been identified for soil-quicklime construction. The factors most important to control during construction are:

1. Lime content,
2. Moisture
3. Pulverization, gradation
4. Uniformity of mixing
5. Time sequence of operations
6. Compaction
7. Curing

Preparing Material

The material to be treated shall contain no rocks or solids other than soil clods larger than 4 inches in any dimension. Removing and disposing of said rocks or solids larger than 4 inches will be paid for as extra work.

Lime Content

Prior to placement of reagent, engineer or technician should determine the soil weight being used to determine spread rate. With this information, a determination can be made on the reagent weight per square foot. The amount of lime is a percentage by weight of the treated material’s PCF dry weight.

Spreading

When lime is applied to soil, the spread rate can be determined by placing a 3 square foot pan on the ground in front of spreader truck. After the lime has been spread, weigh the reagent in the pan to determine the rate of spread in pounds per square foot.

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. 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

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 sec-tion. 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 complies with the following requirements:

For Sieve Size = l", Percent Passing = 98 Min.
For Sieve Size = No. 4, Percent Passing = 60 Min.

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

Re-mixing

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 mixing of lime, unless otherwise permitted by the engineer.

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.

Compacting

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

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 compact to the required density throughout the layer.

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.

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

Areas inaccessible to rollers shall be compacted to the required compaction by other means satisfactory to 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-tire rollers. Minor indentations may remain in the surface of the finished material as long as no loose material remains in the indentations.

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.

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.

Other Considerations

The National Lime Association provides specifications for hydrated lime and information on storage and handling requirements. Field personnel should assure that the lime used in the treatment process has not been rendered non-reactive through improper storage and handling.