Study on mechanical properties and damage constitutive model of frozen sandstone under unloading condition

In order to research the mechanical properties and damage characteristics of low-temperature soft rock under unloading, triaxial loading and unloading tests of frozen sandstone(temperature －5 ℃, －10 ℃ and －15 ℃) at different initial confining pressures and unloading rates were carried out by using GCTS low-temperature rock triaxial machine, which was compared with conventional loading tests. By obtaining the stress-strain curve during the whole experimental process, the mechanical properties and deformation characteristics of unloaded frozen sandstone were analyzed. The failure characteristics of sandstone were studied. The results show that:(1) The failure mode of unloaded frozen sandstone is a combination of shear failure and splitting failure, and the main failure mode is splitting failure. At high initial confining pressure and high unloading rate, the brittle characteristics of the rock sample are obvious, and the failure is more severe. (2) The peak strength of unloaded sandstone is generally lower(about 1/4 times) than that of conventional sandstone, but the residual strength is exactly the opposite, and both strength values are linearly positive correlated with the initial confining pressure. The elastic modulus of unloaded sandstone is about 1.5 times that of conventional sandstone, while the Poisson's ratio is 2-4 times. (3) The deformation and dilatation of frozen sandstone are obviously increased by unloading. The pre-peak stress strengthening effect and post-peak stress weakening effect of stress-strain curve are weakened, and the post-peak stress appears horizontal section. (4) Unloading affects the mechanical strength of frozen sandstone mainly by reducing cohesion and increasing internal friction angle. With the increase of unloading rate and the decrease of temperature, the peak strength and elastic modulus of frozen sandstone increase, while the deformation, Poisson's ratio and residual strength decrease. Based on the equal strain hypothesis of damage mechanics and Weibull distribution, the unloading damage constitutive model suitable for frozen sandstone is established, which can effectively reflect the lateral unloading mechanical characteristics. The research results can provide important reference for the study of mechanical properties of low temperature soft rock and its engineering application. © 2024 Academia Sinica. All rights reserved.