Evaluating pile installation and subsequent thixotropic and consolidation effects on setup by numerical simulation for full-scale pile load tests

During pile installation, stresses and void ratios in the surrounding soils change significantly, creating large displacements, large strains, soil disturbance, and development of excess pore-water pressures. The surrounding disturbed soil tends to regain its strength with time due to both consolidation and thixotropic effects. In this paper, the pile installation process and subsequent consolidation, thixotropy, and load tests conducted at different times after end of driving (EOD) were modeled for test piles at the Bayou Laccassine Bridge site, Louisiana. In the finite element (FE) model, the pile was considered as an elastic material and the anisotropic modified Cam-clay model (AMCCM) was used to describe the behavior of the surrounding clayey soils. Pile installation was modeled by applying prescribed radial and vertical displacements on the nodes at the soil-pile interface (volumetric cavity expansion), followed by vertical deformation to activate the soil-pile interface friction and simulate static load tests. The thixotropic effect was incorporated by applying a time-dependent reduction parameter, beta, which affects both interface friction and material properties. Results from the FE numerical simulation include the development of excess pore-water pressure during pile installation and its dissipation with time, the increase in effective lateral stress at the pile-soil interface, changes in stress state of the surrounding soil, and setup attributed to both the soil consolidation and thixotropy at different times. FE results are compared with measured values obtained from full-scale instrumented pile load tests, which show good agreement between measured and FE-predicted results.