Numerical Study of the Effect of Preloading on Undrained Bearing Capacity

In practice the situation often arises where extra load needs to be added onto existing foundations. This can arise due to the installation of additional equipment in a building, or to the construction of new floors, etc. It also arises in a situation where old foundations are used for a new building, a practice likely to become increasingly popular in the future. If the foundation soil is clay, then its undrained strength would have increased with time due to the dissipation of excess pore water pressures generated during the initial construction of the foundations. The design problem is determining how large this increase in undrained strength is, since this will determine the amount of new load that can be applied. This paper provides guidelines to answering this question through a numerical study of preloaded surface footings on both soft and stiff clay foundations. Additional information resulting from this study addresses the footing failure mechanisms, as well as the effect of a surface crust in soft clay on both the bearing capacity and the failure mechanism.