FREE BOUNDARY, FLUID FLOW, AND SEEPAGE FORCES IN EXCAVATIONS

The role of pore fluids in predicting the strength and deformation behavior of excavations is investigated in the context of nonlinear finite element analysis. It is shown that the imposition of a volume constraint exerts a significant influence on the structural response of excavations during construction. For a fully undrained condition, the constraint is viewed as that of incompressibility; for a fully drained condition under steady-state seepage, the volume constraint vanishes, but fluid flow gives rise to seepage forces that alter the mode of deformation of unsupported and braced cuts. Predictions of drained responses of unsupported and braced excavations are made using a program called DIG-DIRT, a nonlinear finite element code capable of dealing with free-surface flow and seepage forces for solid stress and deformation calculations. The same code is used to predict the undrained response of excavations from equivalent formulations based on mixed and penalty methods with selective reduced integration to deal effectively with the incompressibility constraint. Examples of drained and undrained response predictions are then discussed to obtain a better understanding of the role of pore fluids on the strength and deformation behavior of excavations.