Deep cavity flow mechanism of pipe penetration in clay

Abstract

The evolution of penetration resistance as a function of penetration depth of a pipe into a cohesive seabed is of practical importance, particularly in the areas of pipeline on-bottom stability assessment and T-bar penetrometer data interpretation. In the past, this subject was addressed primarily in a discontinuous manner by separating the penetration response into two broad regimes of shallow and deep penetrations followed by deriving plasticity solutions assuming a simplified "wished-in-place" configuration. In this manner, the effects of evolving seabed topology and the progressive transition from a shallow failure mechanism to a deep failure mechanism are neglected. This paper aims to provide greater insights into the transition zone, which is especially important for the interpretation of T-bar test data at shallow depths. In this study, the penetration response of a smooth pipe over a wide range of normalized clay strengths is numerically simulated. A deep cavity flow mechanism where the bearing capacity factor is 12% less than the conventional full-flow mechanism is identified and found to be operative up to a depth of 10 pipe diameters under a certain combination of material properties. An analysis method is proposed to predict the load-penetration response for a given set of clay strengths and pipe diameters.