ABSTRACT
It is commonly assumed that deep excavations carried out in fine-grained deposits, characterized by a low permeability, occur in undrained conditions. A restriction of drainage is generally beneficial for excavations, because it results in negative excess pore water pressure and therefore in a larger available soil strength. But this beneficial effect is only temporary, since the progressive dissipation of the excess pore water pressures has a detrimental effect on the behaviour of the excavation. On the other hand, once the bottom of the excavation has been reached, the additional forces in the retaining structures produced by the equalisation of the pore water pressures can be contrasted by the installation of supplementary structural elements, such as base slabs, additional props, etc.Therefore, the designer of a deep excavation in a fine-grained deposit needs to consider carefully the beneficial effects of the excess pore pressures generated during the excavation, and to evaluate the possible effects of their partial dissipation in relationship with the construction sequence and with the possibility of modifying the structural layout of the retaining system. Evidence of the detrimental effect of time on the behaviour of deep excavations in low-permeability soils is well documented in the scientific literature (e.g.: Koutsoftas et al. 2000, Rampello & Callisto 2008).
