Constitutive modeling of weak interlayer zone subject to unloading stress paths in underground excavation based on modified equivalent plastic work

A simple yet innovative constitutive model of the weak interlayer zone (WIZ) is proposed to represent the unloading mechanical response and the failure mechanism of WIZ under complex unloading stress paths in underground excavation based on credible experimental results. The established model employs a modified equivalent plastic work with obvious stress path independency as the hardening parameter to appropriately interpret the effect of stress paths on the unloading plastic strain hardening characteristics of WIZ. Moreover, the deterioration of mechanical parameters with the hardening parameter is simultaneously explored. Additionally, the modified equivalent plastic work hardening constitutive model has been created with the aid of the updated Mogi-Coulomb yield criterion, associated flow rule for tensile failure, and nonassociated flow rule for shear failure, in order to accurately characterize the non-linear properties and significant dilation throughout the entire unloading conditions. The capability and rationality of the established model were then investigated and verified through the finite difference numerical algorithm by comparing several numeric examples with the laboratory unloading triaxial test results and the field deformation monitoring data. The model was then used to simulate and analyze the stability of rock masses with WIZ during the large underground cavern excavations. Results show that the model can effectively characterize the stress redistribution, the large discontinuous deformation of upper and lower rock masses of WIZ, as well as the structural stress -induced failures. The study can offer a crucial scientific foundation for appropriate construction and support optimization of underground engineering affected by WIZ.