Constitutive modeling of principal stress rotation associated with sand under simple shear loading

An elasto-plastic constitutive model is proposed to account for the effect of principal stress rotation that commonly occurs and significantly influences soil behavior. The model is based on a concept of adding plastic strain increments obtained from two mobilized planes: a plane of maximum shear stress which swings as the principal stress rotates, and a horizontal plane which is spatially fixed. Models based on the plane of maximum shear stress alone are particularly sensitive to horizontal effective stress ratio K₀ (=σ′_x/σ′_y), whereas the proposed model gives a similar skeleton behavior for soils at the same density and mean effective stress, regardless of the value of K₀, as observed in laboratory tests. The model is firstly calibrated by data from drained and undrained monotonic and cyclic Direct Simple Shear (DSS) tests on K₀-consolidated specimens of Nakdong River sand in loose and dense states. The capability of the proposed model in modeling of the conventional liquefaction effects: initial shear stress condition α (=τ′_xy/σ′_y) with K_α corrections, and initial confining stress condition σ´_y with K_σ corrections, is also emphasized in this paper.