1 INTRODUCTION It is commonly assumed that deep excavations car- ried out in fine-grained deposits, characterized by a low permeability, occur in undrained conditions. A restriction of drainage is generally beneficial for ex- cavations, 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 be- haviour 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 excava- tion in a fine-grained deposit needs to consider care- fully the beneficial effects of the excess pore pres- sures generated during the excavation, and to evaluate the possible effects of their partial dissipa- tion in relationship with the construction sequence and with the possibility of modifying the structural layout of the retaining system. Evidence of the det- rimental effect of time on the behaviour of deep ex- cavations in low-permeability soils is well docu- mented in the scientific literature (e.g.: Koutsoftas et al. 2000, Rampello & Callisto 2008). Values of pore water pressures larger than ex- pected may also occur as an effect of a partial or to- tal loss of the suction induced by the total stress re- lease below the bottom of the excavation. This effect may be induced quite rapidly by the local presence of coarser soil, or by the availability of free water at the excavation bottom. Similarly to the equalisation of the excess pore water pressures, a loss of suction produces an increase in the pore water pressures and hence has a unfavourable effect. From the above discussion it follows that the ac- tual values of the pore water pressures may be dif- ferent from those assumed in ideally undrained con- ditions and that this variability needs to be incorporated into a robust design procedure. The common practice of using the undrained soil strength S u to describe the soil behaviour in undrained conditions is for many aspects unsafe (Callisto 2011) and obviously is not adequate to in- corporate any change in the pore water pressure re- sulting from the phenomena described above: a pro- cedure based on effective stresses is in order. In principle, an effective-stress-based finite ele- ment analysis employing an adequate constitutive model may be able to evaluate the correct pore water Design of deep excavations in fine-grained soils accounting for changes in pore water pressures L. Callisto, F. Maltese & F. Bertoldo La Sapienza Università di Roma, Italy ABSTRACT: 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 per- meability 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 con- sider 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 exca- vations 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 pressure in undrained conditions. The results of this study are used to pro- vide practical indications about the values of excess pore water pressures to be considered for design.