This report presents a centrifuge modeling investigation into the performance of rocking footings supported by various ground improvement strategies with the aim of controlling their performance during earthquake shaking. The rocking footings investigated in this study are modeled after the shallow foundations supporting a two-span highway bridge at the crossing of Hwy 108 and Sanguinetti Road in Sonora, California, which also served as the archetype bridge in previous centrifuge studies on rocking footings. These earlier studies confirmed that under-designed foundations re-center reliably while dissipating energy. The present study focuses on the strategic application of ground improvement to preserve these beneficial traits of rocking footings while limiting settlement and residual rotation.
By developing a strategy to control the detrimental kinematics of rocking footings during foundation soil capacity mobilization and plastic hinge development, geotechnical engineering practice can move towards embracing a cost-effective solution in support of performance-based earthquake engineering (PBEE) of transportation infrastructure. Based on case histories and advice from geotechnical practitioners in California, ground improvement for the centrifuge testing program presented herein included two basic concepts, namely (1) different arrangements of vertically-oriented sand-cement columns and (2) varying depths of horizontally-oriented geogrids. Soil-cement mixing was selected as it is a well-known technique to reduce liquefaction hazard that is widely used in practice to improve ground around existing and new bridges, and has the potential to reduce settlement of shallow footings on loose-medium sand which is the focus of the present study. Geogrids were selected as they offer a simple solution feasible to integrate during site preparation below shallow footings during initial bridge construction. A baseline case was developed for a rocking footing on loose-medium sand without ground improvement by subjecting it to shaking at a level that caused it to settle and rotate more than would be tolerable by typical performance criteria (e.g., at the Life Safety level as outlined in Chapter 8 of ASCE 41-17). This baseline case was established reliably through repeated tests before being compared to other cases with a variety of ground improvement configurations. Foundation performance in all tests was assessed by computing settlement accumulation and cumulative rotation demand. In addition, correlation diagrams relating re-centering and energy dissipation were developed. The most promising ground improvement configurations were identified with minimal settlement and maximum energy dissipation and re-centering. The centrifuge test results have been organized in a manner allowing their seamless integration into the existing Foundation Rocking in Dynamic (FoRDy) database of rocking foundation tests. The expanded database will increase the engineering community’s receptiveness to adopting rocking foundations in design. Such a design reduces risk to infrastructure in the event of severe earthquakes, limiting potential service disruptions and costly repairs to transportation systems.
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