Experimental and Numerical Studies of Seismic Soil Pressures on Non-rigid Subsurface Structures
Reliable seismic design of earth retention structures and substructure components necessitates the accurate evaluation of magnitude and distribution of earthquake induced soil pressures. Many experimental, analytical, and numerical studies have been conducted to further our understanding of the complex of the soil-structure interaction (SSI) between an underground component and its surrounding soil, hereby considering the wide range of assumptions inherent to soil and structural properties (e.g., structural stiffness, embedment depth, and nonlinear behavior of the soil material). This thesis research aims to further our understanding of the distribution and magnitude of seismic soil pressures induced on sub-surface structures through a combination of experimental, analytical, and numerical studies.
Large-scale shake table tests performed on underground structures at the E-Defense facility in Miki, Japan serve as experimental platform for this research. A densely instrumented system of large-scale underground structures consisting of two vertical shafts, connected through a cut-and-cover tunnel, and two independent shield tunnels was installed in an 8 m-diameter laminar soil box. The system was subjected to step-sine sweeps and scaled ground motion records of the Kobe (1995) earthquake. The underground structures were embedded in a two-layer soil system. The specimen instrumentation consisted of over 800 sensors, including strain gauges, accelerometers, displacement transducers, bender elements, and pressure sensors. Experimental results, including a dynamic system characterization, site response analyses, pressure time histories, and seismic pressure distribution profiles provided critical benchmark data for the subsequent analytical and numerical sections of this research.
Dynamic soil pressure measurements along the vertical shafts served as reference data to examine the suitability of different analytical methods proposed in literature, such as traditional and recently developed limit-state and elastic approaches for both, cohesionless and cohesive (c-φ) soils, in predicting the seismically induced earth pressures on flexible underground structures. Comparative results suggest that, despite the unique shaft geometry, analytical methods accounting for the structural flexibility were able to closely predict the experimental soil pressures, while methodologies derived for rigid subsurface elements can only serve as rough preliminary estimate and should not be employed in performance-based geotechnical analyses. Moreover, none of the analytical solutions included in this thesis work consider the 3D effects of the system, the nonlinear soil-structure interaction, or elastic-plastic behavior of the soil material. These aspects might not be controlling parameters for the behavior of shallow embedded structures, however, their influence becomes significant for deep non-rigid underground structures and should be taken into account.
Results obtained from the shake-table experiments were then used to calibrate a three-dimensional, nonlinear, finite element (FE) model built in ABAQUS. Good agreement was observed between the computed results and experimental data which deemed the reliability of the FE model suitable for the subsequent parametric investigation. By preserving the configuration of the original test system, a selected set of parametric studies was performed, which focused on the (1) the effects of the soil parameters (i.e., cohesion, and friction angle) and (2) the structural flexibility/ stiffness of the subsurface elements.
In the last section of this research, a simplified 3D soil-structure model was developed and influences of the most important parameters on the seismic soil pressures, including the retained soil mechanical parameters, structure and its base flexibilities, amplitude of the ground motion’s acceleration, and geometry of the wall were examined through an extensive set of parametric studies. Results of these parametric studies, including the seismic soil pressure distribution and variations of seismic forces and their corresponding moments against each of the studied parameters were presented as normalized graphs and tables, which could be used in the preliminary analysis and design stage of deep embedded structures.