Abstract: In the alpine mountainous area， the rock mass is often subjected to reciprocating freeze-thaw load under the action of water and temperature difference， resulting in rock mass rupture or mechanical property degradation. By carrying out the freeze-thaw cycle test of four cracked rocks in the range of -20~ 20 ℃， the types and causes of different lithological crack propagation were discussed， the whole process of crack propagation with the increase of freeze-thaw cycles was analyzed， and the crack propagation mechanism of limestone and red sandstone under 20 freeze-thaw cycles was studied by using fracture mechanics. The results show that the crack propagation of cracked rock can be divided into three types， namely: perforating the rock bridge along the tip of the upper crack， circumferential extension along the tip of the upper crack and no obvious crack damage； the horizontal bedding of the slate causes the crack to expand along the circumference， the weak joint developed by the limestone causes the crack to propagate toward opposite direction from the lower crack， the high strength and low porosity of the granite make it difficult to produce macroscopic cracks under the test conditions. Fracture mechanics reveals the cause of I-type crack propagation in limestone and red sandstone， indicating that the initial crack propagation direction is similar to that of the upper fracture， and crack length is related to the tensile strength and fracture toughness of the rock.

Key words: Freeze-thaw cycle, Fractured rock, Crack propagation, Fracture mechanics