The Development of a Method Specification for Compaction of CDSM Spoil as Fill at San Pablo Dam

San Pablo Dam is a 170-foot-high, 1,200-foot-long earthfill embankment located in El Sobrante, approximately 10 miles from downtown Oakland, California, and it impounds a 38,600 acre-feet reservoir for customers in the East Bay Municipal Utility District’s (EBMUD’s) service area. As a result of updated studies, a seismic upgrade was designed and constructed between 2008 through 2010. Work included the removal of a previously-existing 140,000 cubic-yard downstream buttress and replacement with an enlarged 300,000 cubic-yard buttress founded on alluvial/colluvial materials that were improved in-place by Cement Deep Soil Mixing (CDSM) technology. The CDSM generates some amount of spoil material, which is comprised of soil-cement slurry that hardens into soil-cement. For the project, 160,000 cubic yards of CDSM spoil was processed on-site and reused as compacted fill for the lower portion of the new buttress.
Laboratory shear strength tests showed that processed CDSM spoil had excellent engineering properties for compacted fill. However, results from traditional compaction testing methods were found to be unrepresentative of field conditions. When compaction tests were completed in the lab, the material degraded in the compaction testing mold, which made the laboratory dry unit weight unrealistically high. The initial quality control tests showed that the fill materials failed to meet compaction requirements despite clear indications that compaction was being achieved, such as walk-out of the rolling sheepsfoot compaction equipment, difficulty in penetrating the surface with a geotechnical probe, and difficulty in excavating pits for sand cone testing. Also, the material’s low compacted dry unit weight (approximately 65 to 75 pcf) and high water content (approximately 35 to 45%) was outside the normal range for compacted materials. EBMUD had specified traditional laboratory compaction testing for earthwork quality control. However, ultimately, a method specification was used. This paper describes the various tests were performed in an effort to find methods that would be appropriate for material that exhibits breakdown during compaction. The method specification that was ultimately adopted required additional field inspection and required testing to control field moisture, but was consistent with geotechnical engineering principals and appropriate for use with this material. 14 pp., 20 references.