A nonlinear elasto-viscoplastic model for clayed rock and its application to stability analysis of nuclear waste repository

The creep behavior is one of the most important topics on the stability analysis of rock mass in underground engineering. In order to characterize the creep deformation of clayey rock more accurately, this study proposes a nonlinear elasto-viscoplastic (EVP) model based on a new yield surface and a modified Mohr-Coulomb (MC) flow potential. First, the finite element (FE) formulation for the constitutive creep behavior is derived and a user subroutine to define the material's mechanical behavior (UMAT) is implemented in the commercial finite element package ABAQUS. The numerical simulation is validated against the triaxial creep tests of a cylinder, and the results show that the proposed model can effectively predict the creep responses of clayey rock and thus compensate for the deficiency of the creep model in ABAQUS. Then, the proposed model is used to study the long-term stability of clayey rock formation surrounding an underground experimental site. Based on the results from triaxial creep tests and in situ monitoring of the lining deformation, the time-dependent material creep parameters are obtained for rock by using the back analysis method. The predicted lining deformation is in good agreement with the field data with most of the relative errors less than 3.5%. The creep zone in the surrounding rock demonstrates that the horizontal scope is larger than the vertical one. The proposed model is proven to provide accurate prediction on the creep characteristics of clayey rock, showing a potential to assess the long-term stability and reliability of large-scale underground engineering structures.