Dongdong Chang

Seismic Design of Pile Foundations for Liquefaction Effects

Procedures for the seismic design of pile foundations for liquefaction effects are presented with emphasis on the conditions relevant to bridges. Two local subsystems for a bridge are discussed in detail: (1) pile groups in laterally spreading ground away from the abutments and (2) pile groups at the abutments where the restraining or “pinning” effects of the piles and bridge superstructure can be advantageous. The recommended design procedures involve equivalent static analyses using beam on nonlinear Winkler foundation models. Guidance for these design procedures was derived from a combination of dynamic centrifuge model tests and associated nonlinear dynamic finite element studies. The design procedures, their basis, and other issues for design of bridges for liquefaction effects are discussed.

FEM Analysis of Dynamic Soil-Pile- Structure Interaction in Liquefied and Laterally Spreading Ground

A two-dimensional nonlinear dynamic finite element (FE) model was developed and calibrated against dynamic centrifuge tests to study the behavior of soil-pile-structure systems in liquefied and laterally spreading ground during earthquakes. The centrifuge models included a simple structure supported on pile group. The soil profiles consisted of a gently sloping clay crust over liquefiable sand over dense sand. The FE model used an effective stress pressure dependent plasticity model for liquefiable soil and a total stress pressure independent plasticity model for clay, beam column elements for piles and structure, and interface springs that couple with the soil mesh for soil-structure interaction. The FE model was evaluated against recorded data for eight cases with same set of baseline parameters. Comparisons between analyses and experiments showed that the FE model was able to approximate the soil and structural responses and reproduce the lateral loads and bending moments on the piles reasonably well.

NONLINEAR FE ANALYSES OF SOIL-PILE INTERACTION IN LIQUEFYING SAND

Nonlinear dynamic analyses using the finite element (FE) method are compared to the results of dynamic centrifuge model tests of pile-supported structures in liquefying sand profiles. The FE models utilized soil spring elements that connect pile elements to one- or two-dimensional meshes of a soil profile. Development and implementation of the soil spring materials models in the OpenSees FE platform are described. Single element examples are used to illustrate the behavior of the soil and soil spring material models. Comparisons of FE analysis to centrifuge test data of pile-supported structures in liquefying sand profiles suggest that these FE methods can reasonably approximate the essential features of soil and structural response.