Study on the Triaxial Unloading Creep Mechanical Properties and Damage Constitutive Model of Red Sandstone Containing a Single Ice-Filled Flaw

Abstract

The freezing method is an effective approach for constructing coal mine shafts through water-rich soft rock strata. The frozen wall produced by this technique stops the movement of water and offers temporary support, ensuring the safety and stability of the shaft working face. However, most frozen rock masses generally comprise ice-filled flaws and frozen intact rock. A frozen fissured rock mass undergoing the long-term effects of excavation unloading under in situ stress gradually accumulates damage or creep deformation over time; this damage is mainly responsible for the significant deformation, cracking and even instability of the frozen rock wall. To study the creep characteristics of ice-filled fissured rock masses under unloading conditions, a series of conventional triaxial compression tests and triaxial unloading creep tests were performed on ice-filled flawed red sandstone specimens from the Shilawusu mine in Northwest China using a self-developed DRTS-500 subzero rock triaxial testing system. In conjunction with the experimental data, the instantaneous strength and deformation characteristics of the rock specimens were analyzed, and their creep deformation characteristics and damage evolution characteristics were discussed. In this paper, based on the nonlinear creep characteristics of frozen rock, fractional calculus theory and damage theory, a new nonlinear creep damage model of frozen fissured red sandstone was defined in series with the improved Burgers model and elastoplastic damage model. The proposed creep model can reasonably describe the creep deformation of frozen fissured red sandstone. Classical elastoplastic mechanics and creep theory were used to derive the three-dimensional creep damage constitutive equation, and the results of creep experiments and simulation results of the creep damage constitutive model were very consistent in this study. The new creep model not only reflects the whole creep deformation process of frozen fissured red sandstone but also exhibits better performance than the classical Burger model and improved Nishihara model in describing the primary creep stage and accelerating creep stage. Therefore, the proposed nonlinear creep damage model is suitable for studying the mechanical creep properties of frozen fissured rock under unloading conditions. The results can provide an important reference for the long-term stability assessment of frozen rock walls of coal mine shafts.