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

摘要

The behaviour of a deep excavation is especially influenced by the strength and deformabilityrnof the soil located in front of the retaining walls, below the bottom of the excavation. In fine-grained, lowpermeabilityrnsoils, it is usually assumed that the excavation is carried out in undrained conditions, resultingrnin a significant decrease of the pore water pressures. This effect is beneficial, because the effective stressesrnare kept nearly at their initial values, but has a transient nature: depending on the problem layout, on the soilrnpermeability and on the rapidity of the construction process, the negative excess pore pressure may dissipaternin a time span comparable to that needed to complete the construction. For design purposes, it is essential tornconsider reasonable values of the pore water pressures, to ensure a satisfactory performance of the constructionrnavoiding at the same time excessive conservatism. This paper illustrates the results of a parametric study, inwhichrnfinite-element analyses were carried out to evaluate the excess pore water pressures induced by excavations andrnto asses the effects of their variation with time. The mechanical behaviour of the clayey soils was describedrnusing a constitutive model based on hardening plasticity, that is reasonably accurate in simulating the generationrnof excess pore water pressures in undrained conditions. The results of this study are used to provide practicalrnindications about the values of excess pore water pressures to be considered for design.