Experimental Study on Damage Characteristics of Freeze-thaw Slate Based on Nuclear Magnetic Resonance Technology

In order to investigate the evolution law of microscopic pore structure and macroscopic damage characteristics of weathered carbonaceous slate under freeze-thaw cycles, nuclear magnetic resonance testing was conducted on carbonaceous slate with different degrees of weathering in the surrounding rock of tunnels under construction in the Karakoram Mountains of the Qinghai Tibet Plateau after different freeze-thaw cycles. The evolution law of pore structure of freeze-thaw slate was quantitatively analyzed. Research shows that: (1) The saturated mass and volume of weathered slate show a trend of slowly increasing and then rapidly decreasing. After the occurrence of macroscopic damage, its damage propagation speed accelerates. Compared to strongly weathered slate, moderately weathered slate with dense and high strength needs to undergo more frequent freeze-thaw cycles to undergo macroscopic damage; (2) The nuclear magnetic resonance T₂ spectrum of weathered slate mainly presents two peaks, with a discontinuous third peak representing extremely large pores. Under the freeze-thaw cycle, the pore size gradually expands, the pores increase, the connectivity between medium and large pore sizes is enhanced, and the proportion of large pore sizes significantly increases; (3) Research has found that the porosity of slate will reach a critical value when subjected to freeze-thaw damage. The critical values of freeze-thaw damage porosity for moderately weathered slate and strongly weathered slate are 15.36% and 17.16%, respectively. When the pore structure expands and the local porosity increases close to the critical value, local macroscopic damage of rock block detachment occurs. The mechanism of slate freeze-thaw damage is analyzed from the perspective of critical porosity. The research results have strong reference value for the antifreeze design and construction of rock mass engineering with different degrees of weathering in cold regions.