Abstract

Pile foundations are integral part of many civil engineering structures and pile foundations are routinely constructed in soft clays. The seismic behavior of pile foundations is a very complex problem with interactions between soils (solid skeleton, pore water, and pore air), piles, and superstructure. This complexity is further increased when weak soils such as soft clays surround the pile foundation. Seismic behavior of pile foundations in soft clays may be improved using ground improvement techniques such as Cement Deep Soil Mixing (CDSM). The seismic behavior of pile foundations in improved ground is, however, poorly understood.

A fully coupled finite element computer code, TeraDysac, and centrifuge model tests are used to study the influence of CDSM ground improvement around pile foundations in saturated soft clays during earthquakes. TeraDysac considers the coupled differential equations governing the behavior of the solid skeleton, pore water, and piles. Bounding surface elastoplastic constitutive models are used to simulate the stress-strain behavior of soils in TeraDysac. This presentation will first detail the theory and computational aspects behind the computer code TeraDysac and then present TeraDysac simulations of centrifuge model tests involving pile foundations in unimproved and CDSM improved soft clays. The prototype of the centrifuge models consisted of 10 m of soft clay underlain by 8 m of dense sand with single piles and pile groups and various extents of CDSM ground improvement around the piles. Centrifuge model tests and TeraDysac simulations show that while lateral stiffness of piles and pile groups in improved soil always increases, the seismic behavior of pile foundations is complicated and depends on the extent of the ground improvement around the piles.