A nonlinear visco-elasto-plastic creep model for sandstone considering creep initiation conditions and experimental verification

The development of a robust constitutive model to accurately describe the creep behavior of rock is critical for predicting the time-dependent performance of rock engineering structures. To capture the instantaneous deformation of sandstone post-loading, an elastoplastic component that accounts for the compaction of internal microdefects was formulated. Additionally, a viscoelastic component incorporating a creep initiation stress threshold was proposed based on the Kelvin model, whereas a viscoplastic component was defined with dual stress and strain thresholds. These elements were integrated using a combination element method to develop a nonlinear visco-elasto-plastic (NVEP) model. The model creep equations were derived via Laplace transformation. Subsequently, uniaxial and triaxial creep experiments were conducted to determine the deformation characteristics, creep initiation stress threshold, and long-term strength (LTS) of sandstone. A systematic approach for determining the creep parameters of the NVEP model was also implemented. Validation through experimental results confirmed that the NVEP model provides a detailed and accurate representation of each creep phase, effectively capturing the deformation behavior of sandstone under varying stress conditions.