Abstract: Under the freezing-thawing cycles, three types of rocks (clastic-rock, mudstone, and sandstone) chosen from the protecting-slope and berm of the Qinghai-Tibet Railway were studied. Their failure processes and weight were observed. The results show that the severe longitudinal and circumferential cracks appeared on the outside of the clastic-rock specimen; and that the debris shedding caused the weight changes less than 1% during the period of 30 freezing-thawing cycles. In the crack growth process of mudstone specimen, a horizontal, circumferential crack initialized almost parallel to the bottom of the cylinder; the growth of the crack tip then tilted gradually toward the bottom to a degree such that the direction of the crack was almost perpendicular to the axel of the cylinder. Such tilt did not appear in the sandstone specimen, in which only a short-slim circumferential crack grew outside. However, both rocks had no reduction in weight because no debris was shed. Their ultrasonic velocity testing was made by the digital acoustic-waves-monitor (RSM-SY5). According to the changes of the ultrasonic velocity at different freezing-thawing cycles, elastic modulus, rigidity modulus, bulk modulus, and Poisson's ratio of the three types of rocks were calculated and analyzed. The results show that the ultrasonic velocity, three moluli and Poisson's ratios all decrease with the increase of the freezing-thawing cycles, especially the Poisson's ratios begin to be negative values after the tenth freezing-thawing cycle, disclosing the severe failures inside of the rocks. At last, by the uniaxial compression tests of the three types of rocks subjected to different cycles of freezing and thawing, of which the compressive strengths gradually decrease with the increase of the freezing and thawing cycles.

Key words: freezing-thawing cycles, ultrasonic velocity, rock, physico-mechanical properties