Mathematical methods to unloading creep constitutive model of rock mass under high stress and hydraulic pressure

Abstract

The main purpose of this paper is to study the unloading mechanical characteristics of rock mass unloading under high stress and high water pressure. An unloading creep test of deep sandstone was conducted under different initial stresses and hydraulic pressures by using the TFD-2000 rock triaxial rheological test system. Based on the experimental results, we can explore unloading creep properties and time-dependent deformation characteristics of rock mass. Deformation characteristics of rock masses during the attenuation and stable creep stages under different stresses and hydraulic pressure conditions were analyzed, and the variation laws of creep parameters in different creep stages were studied and fitted by a mathematical model. The applicability of the fitting results was discussed. A nonlinear unloading creep constitutive model and mathematicalmethods of rock mass under high stress and high hydraulic pressure was proposed based on element model theory. Parameters of this nonlinear creep constitutive model were recognized. Results demonstrated that the nonlinear deformation characteristics run through the entire creep stage. During the attenuation and stable creep stages, elastic deformation parameters generally present exponential functional variations with the increase in time and hydraulic pressure. In the accelerating creep stage, viscous deformation parameters also present an exponential functional variation. The test curve fits with the model curve obtained by the optimal numerical analysis software 1stOpt, which verifies the accuracy and applicability of the model. This study can provide theoretical references for accurate prediction of time-dependent deformation of deep rocks under high stresses and high hydraulic pressures.