Study on the Mechanism of Fracturing and Permeability Enhancement of Coal Body under Liquid Nitrogen Freeze-Thaw Cycles

Abstract To investigate the mechanical properties and fracture damage variations of deep coal and rock under liquid nitrogen freeze-thaw cycles, triaxial loading and seepage experiments were conducted on water-saturated freeze-thaw coal and rock. The results show the following:(1) As the number of freeze-thaw cycles increases, cracks gradually expand and form a crack network, leading to a decrease in the compressive strength and elastic modulus of the coal samples.(2) The wave velocity decreases with the increasing number of freeze-thaw cycles, reflecting a reduction in the coal's elastic modulus and integrity.(3) As the number of freeze-thaw cycles increases, the initial permeability, permeability at failure, and overall permeability of the sample increase, with the most significant increase observed after five freeze-thaw cycles.(4) As the number of freeze-thaw cycles increases, the coal becomes fragmented into granular particles, resulting in a denser overall structure. The number of generated pores and fractures increases, thereby enhancing permeability, and the extent of improvement is gradually increasing.