Experiments on mechanical properties of Cretaceous red sandstone after freeze-thaw process

The boreholes in frozen coal mines of western China mainly cut through the Cretaceous and Jurassic rock formations characterized as water-rich and weakly cemented strata. The rock mass in the frozen wall undergoes a complete freeze-thaw process under the coupling effect of temperature field and crustal stress field. In this paper, considering the coupling effect of the temperature field and the geostress field in the process of freezing and thawing, the Cretaceous water-rich and weakly-bonded red sandstone is selected to see its mechanical properties based on experiments. Meanwhile, the deterioration mechanism of Cretaceous weakly cemented red sandstone induced by freeze-thaw process as well as the influence of crustal stress during the freeze-thaw process are analyzed. During freeze-thaw process, the confining pressures are set to be 0, 2, 4, 6, 8 MPa separately, the freezing temperatures are -5, -10, -15 degrees C separately, and the thawing temperature is 20 degrees C for all samples. The results show that the Cretaceous weakly cemented red sandstone is very sensitive to freezing and thawing process because of its very poor cementation and low strength. After only one freeze-thaw cycle, the uniaxial compression strength decreases 28.39%. The crustal stress during the freeze-thaw process enhances the binding forces of pores and cracks of Cretaceous weakly cemented red sandstone, therefore the freezing fully develops to the secondary micro-cracks. Due to the further development of freezing effect, the frozen-heave force in the red sandstone increases, strengthening the damage of red sandstone. Therefore, the mechanical properties of red sandstone after melting are further reduced relative to the non-confined freezing and thawing condition. The experimental results in this paper is helpful for the design of shaft lining of borehole located in the frozen coal mines in western China.