UC San Diego
Large scale passive force-displacement and dynamic earth pressure experiments and simulations
- Author(s): Wilson, Patrick Richard
- et al.
During strong earthquakes, foundation structures such as bridge abutments and pile 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, a 2.15 m high, 5.6 m long, and 2.9 m wide dense, 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 the 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-wall-backfill 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 interesting features 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