Anisotropic critical state model for sand and evaluation of bearing capacity of plate anchors in sandy seabed

The fabric anisotropy and evolution of sand significantly affect its mechanical behavior, and thus play an important role in altering the bearing capacity of foundations (e.g., plate anchor) in sandy seabed. In this study, an elasto-plastic critical state model for sand considering fabric anisotropy and its evolution along with the non-coaxial plastic flow rule is developed within the framework of anisotropic critical state theory (ACST). The model is then implemented into the three-dimensional finite element program ABAQUS. By introducing the nonlocal plasticity theory, the mesh dependency caused by strain localization of sand is minimized. The predictive capacity of the proposed model is validated through the successful simulation of sand element tests subjected to various stress paths, and the centrifugal model tests on the pull-out behavior of the plate anchor in sand. The effects of fabric anisotropy on the pull-out responses of plate anchors in sandy seabed are investigated via parametric studies, which consider different levels of initial fabric anisotropy (sedimentation angle α₀ =0°, 45°, 90°). It is revealed that: (1) For a given stress level and relative density of the sandy bed, the peak pull-out resistance of the plate anchor increases with α₀. This is because the fabric of the soil along the sliding wedge (above the anchor) evolves faster at a higher α₀ value, leading to a larger peak friction angle. (2) Ignoring the effects of fabric anisotropy leads to significant overestimation (by up to 100%) of the peak pull-out capacity of the plate anchor in sand, because the isotropic model that well predicts the triaxial compression behavior of sand will overestimate the strength of sand under other loading paths (such as triaxial extension and simple shear). (3) The traditional limit equilibrium analysis method does not consider the fabric anisotropy, and can only reasonably predict the pull-out capacity of the anchor plate when the sedimentation angle is α₀=0°, but underestimates the scenarios of α₀=45°, 90°.