UC San Diego

This work details a centrifuge modeling investigation of the performance of rocking footings supported by various ground improvement strategies with the aim of controlling their performance before, during, and after earthquake shaking. The rocking footings investigated in this study are modeled after the shallow foundations supporting a two-span highway bridge (Hwy 108 at Sanguinetti Road) in Sonora, California which served as the archetypal bridge in previous centrifuge studies. Where these earlier studies proved that under-designed foundations re-center themselves reliably while dissipating energy, the present study focuses on the capacity of ground improvement to preserve these beneficial traits while also limiting the foundation’s settlement and residual rotation, even when it is relatively heavily loaded to begin with.

By developing a strategy to control the detrimental kinematics of rocking footings during plastic hinge development, geotechnical engineering practice can move towards embracing a natural, cost-effective solution and its implementation in performance based engineering (PBE) of transportation infrastructure. Based on case histories and advice from geotechnical practitioners in California, ground improvement for the centrifuge testing program revolved around soil-cement columns in medium dense Ottawa F-65 sand. Soil-cement mixing was selected as it is a well-known technique to reduce liquefaction hazard that is widely used in practice, and also has the potential to reduce settlement of shallow footings on loose-medium sand which is the focus of the present study.

A baseline case was first developed for a rocking footing without ground improvement by subjecting it to shaking at a level that caused it to settle and rotate much more than would be tolerable by any performance criteria. This baseline case was established reliably through repeated tests before being compared to other cases with a variety of soil-cement column configurations. Foundation performance in all tests was assessed based on settlement accumulation against cumulative rotation demand, and a correlation diagram relating re-centering and energy dissipation. Ideal ground improvement configurations were identified with minimal settlement and maximum energy dissipation and re-centering. The centrifuge testing results have been organized in a manner allowing their seamless integration into the existing FoRDy database of rocking foundation tests. Ultimately, the expanded database will increase the engineering community’s receptiveness to adopting the rocking foundation design. Such a design reduces risk to infrastructure in the event of severe earthquakes, limiting potential service disruptions and costly repairs to transportation systems.