Pressure melting of pore ice in frozen rock under compression

The pressure-melting effect of ice can lead to changes in the relative content of ice and unfrozen water in frozen rocks, as well as the pore structure, which will have a substantial impact on the mechanical behavior of frozen rocks. In order to study the pressure-melting effect of frozen sandstone, uniaxial compression tests were conducted at varying freezing temperatures, while changes in electrical resistance were recorded. The phase composition in pores was tested by nuclear magnetic resonance (NMR). The results indicate that: (1) The electrical resistance of saturated frozen rocks decrease rapidly in microcrack-compaction (M-C) stage and slowly in elastic deformation (E-D) and microcrack propagation (M-P) stages with the increase of strain, which is very different from oven-dried rock. (2) The average reduction rate of electrical resistance in M-C stage rises at 0 °C ∼ -4 °C, and remains constant at −4 °C ∼ -20 °C. (3) At 0 °C ∼ −4 °C, the free, capillary and part of adsorbed water in the rock are frozen. At −4 °C ∼ -20 °C, adsorbed water is frozen. Curvature induced premelting severely depresses the freezing point of water in microcracks, while, the stress concentration effect in microcracks induces pressure melting of pore ice. Therefore, we propose: (1) The pressure-melting effect of frozen sandstone occurs mostly in the M-C stage. (2) At −2 °C, the pressure-melting effect causes the melting of ice frozen by free and capillary water, and at −4 °C ∼ -20 °C, this effect leads to the melting of ice frozen by adsorbed water, rising the unfrozen water content, which may alter the mechanical behavior of frozen rocks.