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During strong earthquakes, foundation structures such as bridge abutments andpile caps mobilize resistance due to passive earth pressure. Dynamic earth pressure can also increase the demand placed on retaining walls during earthquake excitation. Current uncertainty in the passive earth pressure load-displacement behavior and the evaluation of dynamic earth pressure during earthquake excitation motivates the large scale experimental and numerical investigation presented in this dissertation.In the experimental investigations,29293
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a 2.15 m high, 5.6 m long, and 2.9 m widedense, well-graded silty sand backfill is constructed behind a stiff vertical concrete wall inside a large soil container. First, the passive earth pressure load-displacement curve is recorded in two tests. From those tests, the peak passive resistance compares well with theoretical predictions. Using the test data, a calibrated finite element (FE) model is employed to produce additional load-displacement curves for a wider range of practical applications. A spring model is also developed for representing the passive resistance in dynamic simulations. Next, dynamic earth pressure is measured in 26 events, as the soil container-wallbackfill configuration is subjected to shake table excitations. With peak input accelerations up to about 0.6 g, the earth pressure resultant force remains close to the static level. Small wall movements coupled with the high backfill stiffness and strength contribute to this favorable response. At higher input acceleration levels, the backfill shear strength is further mobilized, resulting in significant dynamic earth pressure increases.
FE simulations support and demonstrate the experimental observations. Results show that accurate consideration of the retaining wall-backfill interaction may result in more realistic dynamic earth pressure predictions than the simplified analytical methods which are currently used in design.The unique combination of laboratory and large scale test data reveals interestingfeatures regarding backfill soil shear strength. For instance, although the tested backfill soil had only 7% silty fines, cohesion contributed significantly to the passive resistance and helped to limit dynamic earth pressure. The backfill friction angle in the plane strain test configuration was also found to be relatively high, contributing favorably to the response under both passive and dynamic earth pressure loading. Introduction and BackgroundAccurate prediction of lateral earth pressure is critical in the design of keyinfrastructure such as bridges,
bulkheads, spillways, ports, culverts, roadways, basements, and subway systems. Considering the limitless possible combinations of backfill soil composition and adjacent supporting structures, significant uncertainty remains in the prediction of earth pressure, particularly with the added complexity of earthquake loading. Specifically, experimental insight is needed in the areas of: i) the passive earth pressure force-displacement relationship, and ii) dynamic earth pressure.During strong earthquakes, foundation structures such as bridge abutments andpile caps mobilize resistance due to passive earth pressure. Theoretical peak passivepressure predictions such as the Coulomb (1776), Rankine (1857), and Log Spiral(Terzaghi et al. 1996) methods can contradict each other, varying by a factor of two ormore (Cole and Rollins 2006). Backfill soil strength is often only roughly characterized (e.g. Caltrans 2004, AASHTO 2007, CBSC 2007), further contributing to the error in predicted passive pressure. Additionally, the above theoretical predictions do not address the load-displacement behavior, which may be needed in order to include the passive resistance in design applications. Earthquake excitation may also increase loads on retaining walls due to dynamic earth pressure and inertial forces. In some strong earthquakes, retaining walls were heavily damaged (Fang et al. 2003, Gazetas et at. 2004). However, many retaining walls have performed well, even when earthquake loads were not considered in design (Seed and Whitman 1970, Gazetas et al. 2004, Al Atik and Sitar 2008). Additionally, experimental results and case history analyses often contradict the theoretical dynamic earth pressure theories (Koseki et al. 1998, Gazetas et al. 2004, Nakmura 2006, Al Atik and Sitar 2008). As a result, there is not a strong consensus on retaining wall seismic design methodology. The remainder of this chapter provides an introduction for the investigation of passive and dynamic earth pressures.