Constitutive Modeling for Viscoelastic Viscoplastic Orthotropic Soft Soils

ABSTRACT Soft soil, prevalent in oceans and coastal regions, commonly exhibits low strength and high moisture level, and its stress‐strain relationship usually undergoes time‐dependent behavior subjected to long‐term loading. On the other hand, due to the directionality of gravity deposition and water transportation during soil formation, the mechanical properties of the soil show orthotropic characteristics. Therefore, it is necessary to develop a constitutive model, taking viscoelasticity, viscoplasticity, and orthotropy into account, to study the long‐term behavior of soft soils. First, with component models and Perzyna's viscoplastic theory, the expression for the total soil strain increment is derived. Then, the orthotropic yield criterion is deduced based on the Hill's yield criterion. After combining the flexibility matrix and the yield criterion, the constitutive equation in three‐dimensional tensor form is obtained. Subsequently, after deriving Jacobian matrices in both non‐yield and yield states, the proposed model is programmed in FORTRAN and imported into the material subroutine interface UMAT of the finite element software ABAQUS. Finally, the effectiveness of the proposed model is verified and some beneficial conclusions are drawn by analyzing the influences of elastic, viscous, plastic, and orthotropic parameters on the time‐dependent behavior of the soils. It can be concluded that the elastic modulus E 0 and viscous coefficient η 1 in the Merchant model, and elastic modulus E 1 and viscous coefficient η 2 in the Bingham model significantly affect the settlement of the soil, while the effects of the anisotropic parameters ξ , Poisson's ratio μ and friction angle φ are relatively small.