Penetration of a modified suction caisson in sand considering seepage effect

Abstract

The modified suction caisson (MSC) adds an external skirt surrounding the top of traditional suction caisson (also called internal caisson here) to increase bearing capacity and to limit its deflection, which can be used in offshore engineering. The seepage effect will change the effective stress of soil around the caisson sidewall during penetration in sand. Meanwhile, piping will occur when the required suction exceeds the critical suction, leading to penetration failure. Therefore. The reliable method to predict the required suction for MSC penetration is necessary by considering the seepage effect. This study explores the influence of length/diameter ratio (L/D) on seepage fields by finite element analyses. The variation of excess porewater pressures (EPPs) and hydraulic gradients (HGs) around the caisson sidewall are revealed. Moreover, the probability of a risk of piping and sand flow decreases as L/D increases under the same suction. Finally, the seepage fields are analyzed when the suction is generated within the internal caisson. The method of calculating the MSC penetration resistance considering the seepage effect is formulated in terms of the principle of static equilibrium. In addition, a new method for estimating the required suction is proposed and validated by model tests.