Ground Improvement: Difficulties in Measuring Success: Fid-Bau Portal

Ground Improvement: Difficulties in Measuring Success

Pegues, James C. / Jordan, Jacob A. / Xu, Yingwu

The site available for construction of new clean air environmental controls equipment at an existing electric power generating plant proved problematic, with 10.5 to 12 m (35 to 40 ft) of very loose, coarse granular fill underlain by 1.5 to 3 m (5 to 10 ft) of soft sediment, overlying competent residual soils and bedrock. While most structures would be supported by rock bearing drilled shafts, subsurface conditions presented challenges for shallow foundations, as well as seismic design requirements given a Site Class F designation. The need for ground improvement was identified early in the project to improve the seismic site class, reduce liquefaction potential, and reduce potential settlement of shallow foundations, while accommodating the installation of drilled shafts. Initially, compaction grouting was selected as the preferred alternative. A compaction grouting test program over a portion of the site was conducted, but the success of the grouting program was marginal and inconclusive at best. Alternate ground improvement methods were solicited in site improvement bids, and the use of 400 mm (16-in) diameter cast-in-place grouted elements (CGEs) was selected. CGEs were to be installed to the depth of bedrock on a spacing of 2.4 m (8 ft) center-to-center, “split-spaced” with CGEs that only went to the bottom of the granular fill, or a depth of about 12 m (40 ft) using reverse-auger techniques in an attempt to densify the granular fill between each element. A soil-cement load-transfer pad was constructed to distribute the fill and slab loads to the CGEs, and the site was raised to finished subgrade through the placement of compacted fill. CPT soundings, both conventional and seismic, were used to assess the improvement in soil density between the CGEs. While the ground improvement program was ultimately deemed successful, and the goals of the project were achieved, the interpretation of CPT soundings and assessment as to the effectiveness of the ground improvement program proved to be a difficult task.