Time and stress path dependant performance of excava- tions in soft soils

The knowledge of the influences of the deformation of the excavation wall and the relief of stresses due to excavation on pore pressure development and therefore the resulting time dependant change of effective stresses is of fundamental importance in the design of excavations in soft soils. The state of the art demonstrates that there is a possibility of undrained failure in excavation, although the drained condition is most critical. The actual state of an excavation lies between the cases of drained and undrained conditions. Comparative calculations of well monitored excavations show that the initial condition can best and realistically be described with the effective stress analysis. The application of the effective stress analysis, however, requires the knowledge of the excess pore pressure, which is crucially affected by the stress path dependant deformations. Both the redistribution of the stresses due to the excavation and the resulting wall deformation affect the development of the excess pore pressure and thus the rate of change of the effective stress in an excavation in soft soils. The paper outlines the ongoing research work on this basic soil mechanics problem, namely the time dependant effective stress development in excavations in soft soils. Excess pore pressure, total stress development and movements in and around an excavation in soft soil has been studied with small- and large-scale model tests. Results from the model tests are presented with due consideration of the effects of wall support, surcharge load and construction stages of an excavation in soft soils and compared with a FEM analysis using advanced constitutive soil models.