The effect of creep on the short-term bearing capacity of pre-loaded footings

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.