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Experimental Investigation on the Effects of Ambient Freeze–Thaw Cycling on Creep Mechanical Properties of Sandstone Under Step Loading

In order to study the creep mechanical properties of sandstone subjected to graded loading under the condition of freeze-thaw cycles, 15 freeze–thaw cycles were conducted with four groups of rock samples. The porosity variation of sandstone before and after the freeze–thaw cycles were obtained by using the nuclear magnetic resonance (NMR) technique. The results show that under the action of freeze–thaw cycles, the cohesion between particles inside the rock decreases, the pore structure deteriorates continuously, and the porosity of the rocks increases. During the graded loading creep test, the rock underwent macroscopic failure under low-level loading. Moreover, the fracture morphology showed tensile-shear failure characteristics, which caused obvious decreases in the long-term strength of the rock. The comprehensive analysis of the experimental data to establish the creep model and constitutive equation under the condition of the freeze-thaw cycle, and discuss the relationship between creep model parameters and freeze-thaw factors. The nonlinear creep mode can effectively reflect the graded loading creep law of frozen and thawed sandstone.