Behaviors of drag embedment anchor in layered clay profiles

Abstract

Layered soil profiles can complicate the installation of an offshore foundation and affect its performance. The goal of this study is to further understand the drag trajectory and capacity of a drag embedded plate anchor in layered clay profiles. Drag embedment tests using a model anchor were performed in various test beds made of marine clay with different profiles of undrained shear strength. The trajectory and orientation of the anchor during drag embedment were captured by a magnetometer. This innovative instrument is capable to measure the six-degree-of-freedom movements for buried objects in soil. A prediction model based on the simplified plasticity theory was calibrated against experimental results. The calibrated model can well predict anchor capacity, trajectory and pitch in single-layer soil profiles, including the constant and the linearly increasing shear strength profiles. For the layered clay profiles with a stiff layer interbedded in soft clay, one key factor determining whether or not the anchor can penetrate into the stiff layer is the pitch of the fluke when approaching the stiff layer, and a flattened pitch may be favorable for penetration. The stiff layer depth and the mooring line thickness significantly affect the pitch angle. Moreover, a lower undrained shear strength ratio of the stiff layer over surrounding soft soil facilitates the anchor to penetrate into the stiff layer.