Use of Multi-Shaft Deep Soil Mixing for Seismic Mitigation At Agnews Campus Project-Paper

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The Agnews Campus Project in San Jose, California, used deep soil mixing (DSM) to reduce earthquake-related liquefaction and settlement risks beneath 18 new school buildings. Site investigations found potentially liquefiable sand and silt layers, mainly at depths of 22–33 feet, as well as compressible clay. Without ground improvement, predicted settlements could exceed the buildings’ limits of 2 inches total and 1 inch of differential settlement over 40 feet.<br /><br />A continuous grid of overlapping, cement-treated soil elements was selected to support shallow foundations and limit ground deformation. Four-axis, 5-foot-diameter mixing rigs installed the DSM to an average depth of about 40 feet. The grids had area replacement ratios of 43–47 percent, and the project installed approximately 235,000 cubic yards of DSM.<br /><br />Quality control included real-time monitoring of mixing operations, full-depth coring, strength testing, wet grab sampling, and optical televiewing. Testing of 1,308 core samples produced an average 28-day unconfined compressive strength of 441 psi. Although measured strengths varied and gravel sometimes interfered with core recovery, wet-sample results and televiewer images supported the conclusion that the treated soil was generally uniform. The specified strength criteria were met, and design analyses indicated that the improvement would keep earthquake-induced settlements within project limits while providing adequate support and stability for the buildings.

Keywords

deep soil mixing

DSM

liquefaction mitigation

earthquake settlement

Agnews Campus Project

San Jose California

ground improvement

shallow foundations

unconfined compressive strength

soil stabilization