Upper bound analysis for drag anchors in soft clay

Abstract

This study presents an upper bound plastic limit analysis for predicting drag anchor trajectory and load capacity. The shank and fluke of the anchor are idealized as simple plates. The failure mechanism involves the motion of the anchor about a center of rotation, the coordinates of which are systematically optimized to determine the minimum load at the shackle. For a given anchor orientation, the direction of the shackle force is varied to establish a relationship between the magnitude and direction of the shackle load. Coupling this relationship to the Neubecker-Randolph anchor line solution produces a unique solution for the magnitude and orientation of the shackle force. The anchor is then advanced a small increment about the optimum center of rotation and the process is repeated. The upper bound method (UBM) provides a practical means to determine the trajectory of the anchor and the anchor load capacity at any point in the trajectory. To better understand of the anchor behavior, extensive parameter studies were carried out varying the properties of the anchor, anchor line, and soil. The UBM show good agreement with six full-scale tests covering several different anchor types and centrifuge model tests.