MECHANICAL PROPERTIES OF PHRA WIHAN SANDSTONE UNDER SUBZERO TEMPERATURE

Abstract

The objective of this study is to examine the mechanical properties of sandstone under subzero temperature. This sandstone belongs to the Phra Wihan formation which is commonly found in the northeast of Thailand. The cylindrical shaped specimens with diameter of 54 mm are prepared by cutting machine to obtain the specified length and ratios (L/D = 0.5 for Brazilian tension test, L/D = 2.5 for uniaxial compression test and L/D = 2.0 for triaxial compression test). The tests are performed on dry and saturated rock samples under the temperature varying from 25 to -40°C. The cubical rock samples with nominal dimensions of 26´26´26 mm³ are also prepared to be investigated the effect of water expansion in pore spaces during freezing-thawing cycle. The results indicated that the tensile and compressive strength under dry and saturated conditions slightly increased (ranging from 60.9 to 184.1 MPa for compressive strength and 5.3 to 9.0 MPa for tensile strength) with decreasing of temperatures (between 25 and -40°C). This is due to the fact that the specimens under low temperature have a lower thermal energy than those under high temperature.  This allows the rock sample to absorb more mechanical energy and to develop higher stresses before the failure occurs. The effect of temperature on strength under saturated conditions is considerably greater than that under dry condition. After the temperature below -5°C, the increasing rate of tensile and compressive strength tend to get high as temperature decreases. This is because the strength of ice in pore space becomes high. The strength parameters, cohesion (c) and internal friction angle (f) based on Coulomb criterion, slightly increase with decreased temperatures. The “m” constant based on Hoek and Brown criterion decreases with decreasing temperature.  Under saturated condition, the adhesive force between ice and grain of sandstone can strengthen the rock strength. The temperature has a significantly effect on elasticity of saturated rock specimens when it was below freezing temperature. The strength parameters determined from the test results of dry and saturated rock samples under subzero temperatures can be applied to determine the mechanical stability of the rock mass during the excavation with ground freezing techniques.