Pore Pressure Generation, Dissipation and Resolidification in a Saturated Subsoil

Abstract

An analysis of a saturated sandy subsoil behaviour due to cyclic loading is presented. Cyclic loads create cyclic shear strains in a subsoil, and subsequently cause pore pressure generation that leads to the reduction of the shear strength of a subsoil and, in an extreme case, to the failure of structures founded on such soils. A pore pressure generation is accompanied by a process of pore pressure dissipation which reduces a rate of pore pressure generation. The assumption of undrained behaviour (no dissipation effects) allows for obtaining the “lower bound estimate” of subsoil behaviour, since the pore pressure dissipation effects make the rate of pore pressure generation smaller, and then we need more loading cycles to get the onset of liquefaction. An analysis of the problem of simultaneous pore pressures generation and dissipation is very much complicated because the differential equations describing these phenomena are coupled. We have proposed a numerical procedure which allows for studying this problem in a relatively simple manner. The pore pressure generation in a subsoil is computed with the help of the compaction theory which is formulated in terms of the cyclic stress and strain amplitudes. The pore pressure dissipation is computed with the help of a simplified version of the Biot theory. The process of resolidification starts when the action of cyclic loadings is over. The excess pore pressures now freely dissipate down to the equilibrium. We can also study this behaviour. In our contribution we present the fundamentals of the compaction theory, numerical algorithm of dealing with boundary value problems, as well as some numerical examples regarding the behaviour of a subsoil beneath a breakwater.