Stiffness reduction in soft Chicago clay during deep urban excavations

The nonlinearity and anisotropy are two key issues in evaluating the soil stiffness for the precise prediction of the displacement. Recent studies on the stress-strain behavior of Chicago clay, especially focused on the responses at small-strains, reveal the pattern of nonlinear stiffness degradation as the clay is stressed. Herein, we attempt to apply these observational responses of Chicago clay to an excavation problem. The finite element simulation of the excavation in the downtown Chicago area was conducted. At some observation locations, the simulations yield typical stress paths. Changes in nonlinearity and anisotropy were discussed by comparing computed and experimental stress paths. Even in the first few excavations, the reduction of shear stiffness could be significant in soft Chicago clay.