Effect of permeability variation with depth on soil resistance to Caisson penetration

Suction-induced seepage is pivotal to the installation of caisson foundations in sand. Indeed, Pressure gradient generated by the imposed suction inside the caisson cavity cause an overall reduction in the lateral soil pressure acting on the caisson wall as well as caisson tip resistance. This transient loosening of soil around the caisson wall helps caisson penetration into the seabed. However, these effects must be controlled to avoid soil failure due to critical condition such as piping which may cause the installation process to fail because of instability of the soil plug trapped inside the caisson cavity. Therefore, in order to have a safe installation process the soil conditions, especially soil resistance and critical condition to piping must be predicted and controlled during entire installation procedure.
In this paper, the effects of excess pore water pressure gradients due to applying suction on soil resistance in homogenous sandy soil and isotropic heterogeneous seabed profile is addressed. For this purpose a simple finite difference model is used to solve the normalised seepage problem. In order to apply the results to any size of the caisson, the problem dimensions are scaled with regard to the caisson radius. The results show that a predication of soil resistance based on constant permeability in the seabed profile with varying permeability by depth is non-conservative, due to an overestimated reduction in effective stress under suction-induced seepage.