UC San Diego

Considerable bridge-ground interaction effects are involved in evaluating the consequences of liquefaction-induced deformations. Due to seismic excitation, liquefied soil layers may result in substantial accumulated permanent deformation of sloping ground near the abutments. Ultimately, global response is dictated by the bridge-ground interaction as an integral system. Generally, a holistic assessment of such response requires a highly demanding full three-dimensional (3D) Finite Element (FE) model of the bridge and surrounding ground. As such, in order to capture a number of the salient involved mechanisms, this study focuses on the liquefaction-induced seismic response of integral bridge-ground systems motivated by details of actual existing bridge-ground configurations. In these 3D FE models, realistic multi-layer soil profiles are considered with interbedded liquefiable/non-liquefiable strata. Effect of the resulting liquefaction-induced ground deformation is explored. Attention is given to overall deformation of the bridge structure due to lateral spreading in the vicinity of the abutments. The derived insights indicate a need for such global analysis techniques, when addressing the potential hazard of liquefaction and its consequences.In order to reproduce the salient response characteristics of soils, three plasticity constitutive models were developed and implemented into the employed computational framework OpenSees including: (1) A pressure-dependent sand model with the Lade-Duncan failure criterion as the yield function to provide a more accurate representation of shear response for gravel, sand and silt, incorporating liquefaction effects, (2) A 3D model for simulating the strain softening behavior of soil materials such as sensitive clays, cemented, over-consolidated, very dense, or frozen soils among others, and (3) A practical 3D model for simulating the cyclic softening behavior of soil materials, as might emanate from pore-pressure build-up, among other stiffness and strength degradation mechanisms. An opportunity to investigate liquefaction-induced lateral spreading and its effects on sheet pile was permitted by availability of large sets of experimental data. The underlying mechanisms of ground failure and damage to sheet pile were further explored by FE numerical simulations of a series of experiments as follows: (1) A total of 17 centrifuge tests on a liquefiable sloping ground, and (2) A total of 11 centrifuge tests on a sheet pile retaining wall system supporting liquefiable soils. The overall measurements were reasonably captured by the conducted FE simulations, demonstrating that the employed constitutive models as well as the overall computational framework have the potential to realistically evaluate the performance of ground-structure systems when subjected to seismically-induced liquefaction. Overall, the primary findings may be summarized as: (1) Response is highly dependent on the bridge-ground system as an integral global entity. Connectivity provided by the bridge deck, soil profile variability along the bridge length, and geometric configuration of the slopes are all factors that can significantly influence the outcome, (2) The bridge structure and its foundations may exert a significant restraining effect on lateral ground deformations. Such restraining effects partially stem from the bridge-ground global connectivity characteristics, which can be of considerable influence, (3) Incorporation of strain softening where applicable, is an important consideration for a wide range of ground scenarios involving sensitive clays, cemented, over-consolidated, very dense, or frozen soils among others, and (4) Strength and stiffness degradation due to strain softening mechanisms might play a substantial role in terms of accumulated deformations and its effect on the resulting ground acceleration and extent of permanent displacement.