Experimental Study On Rock Mechanical Properties Under Freezing Conditions

With the widely application of artificial ground freezing in the shaft construction, many projects have met the problems of boring through the rock layers of deep thick and water-rich .However, there are less theoretical research and practical experience relatively in this aspects. To carry out the tests of the artificial freezing soft rock and to study its relative physical-mechanical characteristics would have important directive meanings for the design of shaft design by freezing method.In this paper, with the background of shaft freezing project in Hujiahe coal mine, the testing research and theoretical derivation for artificial freezing rock have been explored. Two typical soft rocks that are coal-rock and sandstone were obtained from engineering sites of Hujiahe. The uniaxial compression tests and triaxial compression tests were conducted on the two types of soft rock at different frozen temperature and different confining pressures. The whole stress-strain curves of samples were drawled under the conditions of different temperature and confining pressures. The strength and deformation characteristics under these conditions were analyzed .And their identity and difference were compared either. The main job were as follows:(1) The deformation behaviors of freezing soft rock was studied. At the same confining pressure conditions, the maximum value of axial strains dropped in size as the temperature decreased, which proved that the sample were tend to brittle transformation gradually as temperature decreasing. Although the passion ratio decreased with temperature changes, but not so much. Under the condition of same temperature, the plastic characters were strengthened as the confining pressure increasing as well as axial strain and the whole stress-strain curves become flat. Based which mentioned before, the constitutive relations including temperature factor was derived.(2)The strength characteristics of freezing rock were studied. Under usual temperature, both coal—rock and sandstone had low compress strength and the strength of sandstone was relatively higher than coal—rock. Under the same low temperature conditions, both the strength (including the limit of compressive strength and residual strength) and elastic modulus (including the secant modulus and tangent modulus) increased with the confining pressure increasing and the limits between strength and residual strength was reduced either. Under the same confining pressure conditions, strength and elastic modulus reduced with temperature increased. The analysis of cohesive force and friction angle showed that the cohesion force would increase with temperature decreasing but the friction angle had little change, which indicated that the factor of temperature does not affect its frictional strength but its cohesive strength. Considered which mentioned before, a new non-liner failure criterion was carried out and its reasonableness was tested by way of samples.(3)The damage characteristics were studied either. Under uniaxial condition, the failure form diverted than a single shear face under triaxial condition. The failure face become sloppy with temperature decreasing. On the other side, the freezing rock would show dilatancy characteristics after peak. The dilatancy value would decrease with the confining pressure increasing.