Large Deformation Strain-Softening Parameters for Marine Clay from Vane Shear Tests

Strain-softening behavior of marine clay under large deformations plays a critical role in offshore geotechnical engineering, particularly in foundation installation and submarine landslide development. Traditional full-flow penetration tests (e.g., T-bar and ball penetrometers) face challenges such as incomplete backflow and initial soil disturbance, which limit their effectiveness in capturing this behavior. In contrast, the vane shear test, which allows continuous rotation and enables soil remolding, offers a promising alternative. This study presents a comprehensive investigation of the mechanisms in cyclic T-bar tests and vane shear tests using large-deformation finite-element analysis. Particular attention is paid to the influence of key strain-softening parameters [soil sensitivity (St) and ductility index (ξ95)] on test results. A novel framework is proposed to estimate strain-softening parameters from the torque–rotation response of 360° continuous vane shear tests. Simulations reveal that peak torque occurs at approximately 10° of vane rotation, corresponding to the formation of an annular failure surface. Validation against laboratory results shows strong agreement with numerical predictions. These findings enhance the interpretation of strain-softening behavior in structured soft clays and offer a practical approach for evaluating large deformation responses in offshore geotechnical applications.