An associated elastic-viscoplastic constitutive model for sandstone involving shear-induced volumetric deformation

In contrast to hard rocks, weak rocks often have significant shear-induced volumetric contraction/dilation under short-term or long-term loadings, and this behavior necessitates the development of an unconventional constitutive model. As such, an elastic-viscoplastic constitutive model characterizing the mechanical behavior of weak rocks is proposed, and its validity is demonstrated by comparing the simulated and the actual deformational behaviors of weak sandstone under various stress paths. In order to distinguish the immediate and the time-dependent deformation, the procedures involving multi-stage loading-unloading and creep tests, together with controlled pure-shear stress path, have been undertaken to clarify the deformation behavior of sandstone. Through these experimental procedures, elastic and viscoplastic deformations can be effectively separated, and the coupling between shear stresses and volume stresses can also be identified. Consequently, it is found that the function form of yield surfaces can also be applied to plastic potentials and viscoplastic potentials so that the complexity of the constitutive model to be developed is significantly reduced. Furthermore, the parameters for the proposed constitutive model can be readily obtained by fitting experimental data. Through systematic comparisons of the predictions and the actual behaviors of the studied sandstone, it is found the proposed constitutive model is capable of describing the elastic, plastic, and viscoplastic deformational behaviors not only under pure shearing stress path that were used to calibrate parameters, but also under other stress paths.