Abstract: Weathering is one of serious geological diseases that the Yungang grottos have been faced with. The changes of the temperature and water in the rock masses are the main causes of weathering，especially under the condition of freezing-thawing cycles. Therefore, to research the physico-mechanical characteristics under the condition of freezing-thawing cycles by laboratory test have important significance to the stability evaluation of the rock masses in Yungang grottos. The sandstone specimens taken from Yungang Grottos are separated into three groups：saturated, dry and control groups. Laboratory tests are conducted on saturated group and dry group which have experienced 35 cycles of freezing-thawing to simulate the weathering process of sandstone. Before freeze-thaw test and after every 5 cycles of freezing-thawing，the mass and volume of specimens are measured, the ultrasonic P-wave velocities of specimens are tested using ultrasonic testing instrument; and the uniaxial compression tests of sandstone specimens are performed by INSTRON－1346 rock mechanics test system. After freeze-thaw test, the microstructures of the sandstone specimens are seen by scanning electron microscope. Through above-mentioned tests,the failure forms of the specimens under different water contents and the changes of the mass and volume,ultrasonic velocities,the complete stress-strain curves,uniaxial compressive strengths,frost resisting coefficients and microstructures of the sandstone specimens after different freeze-thaw cycles are obtained. Consequently, the main physico-mechanical characteristics of sandstone under the condition of freezing-thawing cycles are summed up.

Key words: Yungang grottos, sandstone, freeze-thaw cycles, physico-mechanical characteristics