Soil conditions and bounds to suction during the installation of caisson foundations in sand

• Finite element modelling of the normalised seepage problem around a caisson foundation in sand.
• Effects of suction-induced seepage on soil resistance against caisson penetration in sand.
• Critical soil conditions during caisson installation.
• Upper and lower bounds to suction.

Suction installation of caisson foundations is widely adopted in the oil offshore industry. When such foundations are installed in sand, seepage conditions are known to play a pivotal role in the installation process. Pressure gradients 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 in the tip resistance. This transient loosening of soil around the caisson wall facilitates caisson penetration into the seabed. However, these effects must be controlled to avoid soil failure due to critical conditions such as piping or loss of soil shear strength, which may cause the installation procedure to fail due to instability of the soil plug trapped inside the caisson cavity. In this paper, we endeavour to study these effects based on the analysis of the normalised seepage problem, assuming the installation process to take place in homogeneous sand. We first investigate the effects of seepage conditions on soil resistance to caisson penetration with a particular focus on how frictional resistance and tip resistance are differently affected. We then consider modes of failure due to soil piping inside the caisson cavity and sliding of soil mass in a failure mechanism where the soil plug inside the caisson cavity is pushed upward. Based on this study, some insight is gained into the critical conditions for piping. These conditions evolve during the installation process as the penetration depth increases under an increasing suction. Upper and lower bounds are also estimated for the critical suction based on an assumed mode of failure using a simple mechanism of rigid blocks. By comparing these modes of failure we conclude that piping is not always the most critical condition. The critical mode of failure for a given soil may change during the installation process and this is highlighted by comparing the critical suction for piping to the suction upper and lower bounds related to shear failure.