Search

This paper presents a numerical study on the short-term bearing capacity of preloaded footings on soft clay. In practice, this situation needs to be addressed when re-using an existing foundation for a new building or when extra load is to be transmitted to an existing foundation. Previous studies have estimated the bearing capacity in this situation, allowing simply for the increase in the soil undrained strength due to the increase in effective stresses imposed by the preload. However, field and laboratory observations corroborate that there is a continuous increase in the stiffness and strength soil properties with time, even after the dissipation of the excess pore water pressures generated during the initial loading is complete. This paper quantifies the short-term bearing capacity of existing foundations allowing for the occurrence of creep deformations and associated soil hardening. The problem is investigated by means of a series of plane-strain coupled finite element analyses of a 2 m wide rough strip footing, in which the foundation soil is modelled using an overstress based elastic-viscoplastic model. The study investigates the effect of the level of preload, the applied loading rate, the creep characteristics of the foundation soil (magnitude and creep law type) and the footing geometry. It is shown that the time dependent nature of soil (e.g. occurrence of creep) accounts for a significant increase in bearing capacity.

This paper presents a three-dimensional elastic viscoplastic constitutive model that is able to reproduce the time dependent behaviour of soils with isotach viscosity. This constitutive law is based on the overstress theory and incorporates some important features, namely: (i) a non-linear creep law with a limit for the amount of creep deformation under isotropic stress conditions; (ii) a flexible loading surface that is capable of reproducing a wide range of shapes in p ′ − J stress space and incorporates the Matsuoka–Nakai failure criterion in the deviatoric stress space and (iii) assumes that the viscoplastic scalar multiplier is constant on a given loading surface to ensure that critical state conditions are reached. A full description of the model, its governing equations and implementation in a finite element program are presented. The model is then used to simulate the results of laboratory tests, highlighting the model’s abilities and shortcomings in reproducing the time dependent behaviour of soft clays. Furthermore, the paper investigates the importance of accounting for creep non-linearity and the consequences of isotach viscosity on the critical state line.