Treatment Profiles

»Deep Foundations

Deep Stabilization vs. Pile Driving
Griffin Soil performed on a project in which San Francisco “Bay Mud” (a highly compressible, weak, inherently expansive, silty clay) was chemically treated to provide the support for a 5-story reinforced concrete parking garage. Heavy buildings on sites underlain by Bay Mud almost invariably employ driven piles for their foundations. On this site, however, after chemically treating the Bay Mud, the use of ordinary shallow spread footings emerged as an alternative to the use of piles.

As a broad generalization, Bay Mud will consolidate 1 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 that approximately 10 feet of Bay Mud was present at the site. Thus, if this building was surface-supported, it would experience an average settlement of between 4 and 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. Remove the Bay Mud layer in the building footprint and replace it with something else; or

2. Carry the foundation loads through the Bay Mud layer into the underlying stiff clays.

The decision was made to utilize deep soil stabilization because of cost advantages over piles, quicker mobilization and completion time, and the fact that debris was encountered during the geotechnical investigation.