Non-coaxiality behaviors and unified flow rule of soil–structure interfaces

Non-coaxiality is one of the key characteristics of soil–structure interfaces that broadly occur in geotechnical engineering. A series of interface tests between gravel and structure was performed to address and model the non-coaxiality behaviors of interfaces subjected to various 3D loadings. Non-coaxiality of the interface was identified during yielding phase subjected to 3D cycling of shear stress and can be captured by non-coaxiality angle. The non-coaxiality angle does not evolve due to 3D cyclic shearing and depends primarily on the magnitude and direction of current shear stress and the direction of the shear stress increment. The shear stress amplitude ratio, shear stress amplitude, shear stress rotation and initial shear stress significantly affect the magnitude and change pattern of the non-coaxiality angle, attributed to varied shear stress and shear stress increment vectors. A unified flow rule for interfaces was established based on 3D interface test analysis. The flow rule perfectly determines the flow direction of kinds of interfaces prior to and at the mobilisation of shear strength under diverse 2D and 3D loading conditions, such as arbitrary shear paths and differing shear stress amplitudes, shear stress amplitude ratios, initial shear stresses, normal stresses, shear strengths, and anisotropy characteristics.