Mitigation of Liquefaction Potential Using Rammed Aggregate Piers

Trestle Glen is a mixed-use, mixed-income, transit-oriented urban reuse development on 6,880 square meters (1.7 acres) that now offers 119 units of affordable housing along with commercial and light industrial options adjacent to the Colma Bay Area Rapid Transit (BART) Station in Colma (San Mateo County), California. Subsurface soil at the Trestle Glen site includes artificial fill over potentially liquefiable, loose- to medium-dense silty to clayey sands, as well as soft silts and low-plasticity clays, extending to depths of about 7.6 to 9.1 meters (25 to 30 feet) locally along the historic creek alignments in the southeastern portion of the site. Groundwater is approximately 3.4 meters (11 feet) below the ground surface in this portion of the site. This paper presents a case study of the use of impact rammed aggregate piers (RAPs) to mitigate potentially liquefiable soil. The RAP system is a proprietary ground improvement method that involves driving a 30-centimeter (12-inch) diameter hollow mandrel with a 40-centimeter (16-inch) diameter rammer foot to the design depth. The hole is then backfilled with open-graded aggregate that is vibrated in 0.3-meter (1-foot) lifts by a dynamic impact hammer. This paper includes the results of a pre- and post-ground improvement Cone Penetration Test (CPT) program implemented to evaluate the post-ground improvement liquefaction and seismic settlement potential.