Strength Criteria Based on Shear Failure Planes and Test Verification on Loess

Experimental results on loess preliminarily verified shear failure planes under axisymmetric compression stresses as well as their corresponding isotropic strength criteria, which exhibit the inherent natural structure behavior. However, when loading is applied to loess, its natural structure can be destroyed. Induced anisotropy complicates the structural behavior of loess. On the basis of shear failure planes responding to Mohr-Coulomb, Drucker-Prager, and Matsuoka-Nakai criteria, the axisymmetric compression spatially mobilized plane (AC-SMP) was carried out according to the following principles: (1) shear stress is directly proportional to normal stress, (2) failure strength is constant under axisymmetric triaxial compression, and (3) as the location of SMP changes with the rotation of principal stress axes, the isotropic failure criterion of AC-SMP is proposed. Changes in strength properties of loess with vertical microfractures and horizontal isotropy were investigated by true triaxial tests of loess under loading the major principal stress in the vertical and horizontal directions, respectively. True triaxial test results indicate the isotropic failure criteria of AC-SMP in a single direction. Comparison of the strength law under loading in the vertical microfractures direction with that in the horizontal direction displays the anisotropic strength of the natural structure of loess.