Effects of bond crushing on the settlements of shallow foundations on soft rocks

The paper presents a model capable of describing the behaviour of shallow foundations on soft rocks when subjected to eccentric loading. The model is based on the concept of a 'macroelement'. According to this concept, the foundation and the subsoil can be seen as a unique volume element. A mathematical relationship is established that links the force and moment acting on the foundation and the corresponding displacement and rotation, as if they were generalised stresses and strains, respectively. This relationship is based on the theory of hardening plasticity, with two hardening variables, both functions of the permanent generalised strains. It is assumed that the second of such variables, connected to the action of intergranular bonds, monotonically decreases as strains progress and bonds crush. It is shown that the model can reproduce quite well the observed displacement and rotation of a model circular foundation on a layer of calcarenite. A parametric study also highlights that the model can in principle predict the phenomenon of collapse, actually experienced by foundations on high-porosity rocks when a certain generalised stress or strain threshold is reached. It is suggested that such a phenomenon can take place when bonds are characterised by high strength that is rapidly decreasing with strain. It is finally shown that weathering-induced collapse under constant external loading can also, in principle, be modelled in this framework.