Finite element analyses of HDR-4 excavation

Simulations of construction of a 40-ft-deep braced excavation in saturated clays in Chicago have been made using a coupled finite element formulation. Surface and subsurface ground movements, pore water pressures, and sheet-pile deflections are measured throughout construction at the site and are compared to results of the simulations at key stages of the excavation. The finite element simulations closely modeled all phases of construction including sheet-pile installation and the actual duration of construction. Computed sheet-pile deformations agreed quite well with those observed throughout all stages of excavation and bracing. Soil deformations, ground surface settlements, and pore pressures agreed reasonably well with observations until an incipient shear surface developed within the soil mass as the unsupported height of the wall reached as much as 19 ft. Parametric studies assess the importance of modeling the sheet-pile installation, the effects of different types of assumed constitutive responses, the impacts of assumed boundary conditions, and importance of closely modeling the actual construction process. The importance of the available resistance of the soil on the passive side of the excavation in controlling ground movements is demonstrated. Anisotropy is a factor that must be considered when evaluating potential ground movements associated with a proposed excavation through soft to medium clays.