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

PPT Presentation

Pdf Summary

The Agnews Campus Project involved seismic ground improvement beneath 18 school buildings on a 55-acre site in San Jose, California. From March to September 2019, approximately 235,000 cubic yards of deep soil mixing (DSM) were installed.<br /><br />Subsurface investigations identified loose to medium-dense silt and sand lenses with moderate-to-high liquefaction potential during the magnitude 7.0 maximum considered earthquake. Predicted liquefaction-induced settlements of 1–5 inches exceeded project limits of 2 inches total and 1 inch differential over 40 feet.<br /><br />To mitigate these hazards, contractors used two multi-shaft rigs, each with four 5-foot-diameter mixing shafts, to construct interconnected DSM wall grids. The grids were designed to reduce seismic stresses and strains, contain potentially liquefied soils, limit pore-pressure migration, and improve bearing capacity. Treatment extended to an average depth of 40 feet, with building-specific area replacement ratios of 43–47 percent.<br /><br />Quality control included core sampling and strength testing, wet grab sampling, and optical televiewer imaging. Core strength results met project requirements, though gravel and variable subsurface conditions affected some core recovery and test results. Wet-sample strengths and televiewer images indicated generally uniform mixing and confirmed that gravel did not compromise overall DSM quality. The project demonstrates the use of multi-shaft DSM grids to reduce liquefaction risk and both static and earthquake-induced settlement.

Keywords

Agnews Campus Project

seismic ground improvement

deep soil mixing

liquefaction mitigation

San Jose California

DSM wall grids

ground settlement

multi-shaft rigs

subsurface investigations

earthquake engineering