Design of deep excavations in fine-grained soils accounting for changes in pore water pressures

The behaviour of a deep excavation is especially influenced by the strength and deformability of the soil located in front of the retaining walls, below the bottom of the excavation. In fine-grained, low-permeability soils, it is usually assumed that the excavation is carried out in undrained conditions, resulting in a significant decrease of the pore water pressures. This effect is beneficial, because the effective stresses are kept nearly at their initial values, but has a transient nature: depending on the problem layout, on the soil permeability and on the rapidity of the construction process, the negative excess pore pressure may dissipate in a time span comparable to that needed to complete the construction. For design purposes, it is essential to consider reasonable values of the pore water pressures, to ensure a satisfactory performance of the construction avoiding at the same time excessive conservatism. This paper illustrates the results of a parametric study, in which finite-element analyses were carried out to evaluate the excess pore water pressures induced by excava-tions and to asses the effects of their variation with time. The mechanical behaviour of the clayey soils was described using a constitutive model based on hardening plasticity, that is reasonably accurate in simulating the generation of excess pore water pressures in undrained conditions. The results of this study are used to provide practical indications about the values of excess pore water pressures to be considered for design.