Abstract:
Deep Soil Mixing (DSM) ground improvement was selected as the most suitable and cost-effective technique to mitigate the site liquefaction hazard for Agnews Campus Project located in the city of San Jose, California. The project includes construction of 17 new school buildings on an approximately 55- acre site that were constructed at approximately existing grades and supported in shallow foundations. Liquefaction analysis performed for the site showed presence of potentially liquefiable sand lenses at the site which could cause liquefaction induced ground settlements that will exceed the project settlement design criteria for buildings if constructed without ground improvement. DSM was constructed in a cellular/grid pattern to provide support for the shallow foundation systems during seismic events and transfer the bearing loads to competent soils at deeper depths to reduce the total and differential building settlements. A multi-shaft (5 feet, 4-axis) mixing rig was used to create interconnected soil-mixed elements formed by partially overlapping columns arranged in a grid pattern. DSM was designed to achieve a minimum average unconfined compressive strength (UCS) of 250 psi at 28-days. The average ground area treatment ratio was 40% and the average DSM depth was 42 feet. A total of 235, 000 cubic yards of DSM was installed. This paper will discuss the project background, subsurface conditions and design of DSM and present the construction, quality control and strength of DSM improved ground.