Strength Variation of Rocks Surrounding Road Tunnel Entrance/Exit in High–Altitude Mountain Areas under Freeze–Thaw Cycles

The Duku Highway in the study area is located in the high-altitude mountainous region of Xinjiang, China, and it is affected by various environmental factors during construction, among which the influence of freeze–thaw cycles cannot be ignored. In order to study the effect of freeze–thaw cycles on the strength of tuff surrounding rock at high-altitude mountainous areas, uniaxial compressive strength and shear wave velocity tests with different numbers of freeze–thaw cycles were conducted on dry and saturated rock samples from the tunnel entrance of the Duku Highway. The test results showed that the tuff specimens condensed a thin layer of ice on the surface with raised freezing points during the freezing stage, but the thickness of the thin ice and the density of the freezing points did not change with the increase of the number of freeze–thaw cycles. Analysis of the test data showed that the wave velocity, uniaxial compressive strength, breaking strain, modulus of elasticity, and Poisson’s ratio of the rock decreased as the number of freeze–thaw cycles increased. We believe that the freeze–thaw cycles caused the deterioration of the rock strength. The reason for this phenomenon is that we believe that the freeze–thaw cycling action changed the rock internally and affected its density, which, in turn, caused the attenuation of strength, etc. However, there is a limit to the effect of freeze–thaw cycling on the strength of the surrounding rock. After exceeding the limit of the effect of freeze–thaw cycling, the strength parameters of the surrounding rock will no longer change with the increase of the number of freeze–thaw cycles. The results of this study can provide a theoretical basis for the prevention and control of the stability of tuff surroundings at high-altitude mountain tunnel openings.