Evaluation of the bearing capacity of shallow foundations on layered soils using finite element analyses

Abstract

The design of shallow foundations is based on a realistic and reliable determination of the response of the underlying soils. Such response is typical defined in terms of an ultimate threshold of loading-ultimate bearing capacity, and deformations-settlements associated with various levels of loading. The ultimate bearing capacity is defined as the pressure at which shear failure occurs in the soil strata supporting the foundation. The case of foundations on soft clays is considered as particularly problematic in this context, given that such soils are typically characterized by a low bearing capacity and relatively large settlements. In some applications ground improvement of the substratum to a given depth (remove-replace-compact) may provide a cheap and practical solution to other more costly and elaborate measures such as a radical change in the foundation system towards adopting rafts and-or deep support elements such as piles. In the present study we explore the improvement of the bearing capacity of soft clay deposits by the removal of a variable thickness of these material at the base of the foundation and replacing them with higher quality and-or compacted soil. In practice, this technique may be used under light structures and-or in road construction. The design and analysis tools currently-readily at the disposal of the practicing engineer are based on assumptions that the supporting subsurface is homogeneous and isotropic, which is clearly not the case of foundation soils treated through removal and replacement. The few methods which allow for layered subsurface conditions do not capture the breadth of soil types-characteristics and problem geometries. The in fact are only applicable for the cases of either purely cohesive two-layer soils (phi 1=phi 2=0), or the situation where the replacement layer is a sand or gravel (c 1=0 kPa) overlying the cohesive stratum (phi 2=0). Given the nature and complexity of the problem at hand, closed-from or analytical solutions are not practical-possible. As such