The effect of groundwater flow on the stability and design of retained excavations

The groundwater flow pattern around the retaining structures of deep excavations affects the resultant water pressure, the active and passive earth pressure, and the piping and bulk heave potential. A published case history involving a hydraulic failure of a 15 m deep, temporary sheet piled excavation is analysed. The soil profile, assuming the same excavation geometry, is subsequently altered and analysed by numerical and simplified models of flow patterns. A detailed discussion of the results is used to develop generally applicable design concepts and to illustrate the limitations of simplifications. It is suggested that partial factors of safety should be applied to each load component and it is shown that the assumption of homogeneous, isotropic soil conditions may lead to unsafe designs. Furthermore, it is concluded that the hydrostatic boundary conditions, as well as the possibility of anisotropic permeability at depth and particularly below low permeability layers, must be considered in the design of deep excavations.