Failure of Frozen Sandstone under Uniaxial Compression with Interpretation of Pressure Melting of Pore Ice from Electrical Resistance

Abstract

Understanding the failure process in frozen porous rock is of great importance to both geomorphology research and engineering design in cold regions. The mechanical behavior of frozen porous rock has its own particularity due to the existence of ice and unfrozen water in pores. To investigate the failure process in frozen rocks, uniaxial compression tests on oven-dried, air-dried, and water-saturated sandstones at 15°C and −20°C were conducted, and the electrical resistances of rock samples were monitored during compression, with which the pressure-melting effect of pore ice can be interpreted. Moreover, the phase composition of pores in sandstone before and after freezing was measured through the nuclear magnetic resonance (NMR) method. The results show that the initial water content of frozen rock changes the variation trend of electrical resistance, that is, the starting point of increasing stage in electrical resistance is shifted from the microcrack propagation stage in frozen oven-dried and air-dried samples to the stress peak in the frozen water-saturated sample. Also, the electrical resistance of frozen water-saturated rock decreases by 29.4% in the microcrack compaction stage, which is 7.4 times higher than that of frozen oven-dried rock and 4.3 times higher than that of frozen air-dried rock. Finally, 96% of pore water (free water, capillary water, and amounts of adsorbed water) is frozen in the saturated rock. The freezing point of water in microcracks is severely depressed by the strong capillary pressure, while the stress concentration effect in microcracks enlarges the applied pressure greatly and eventually induces pressure melting of pore ice. Based on the results and analysis, we suggest that the pressure-melting effect works primarily in the microcrack compaction stage and microcrack propagation stage and remolds the failure process of frozen saturated sandstone by altering the interaction between ice and pore (or microcrack).