Experimental study of the influence of water and temperature on the mechanical behavior of mudstone and sandstone

Understanding the influence of water and temperature on the mechanical behaviors of coal measure rocks is important for deep coal resource exploitation. Using an electro-hydraulic servo-controlled testing system (MTS816) with a self-designed thermostatic water tank, a series of water absorption experiments and uniaxial compression experiments were performed on mudstone and sandstone samples that were immersed in water under different temperature conditions (from 25 to 95 °C). The water absorption characteristics at different temperatures and the effect of water and temperature on the mechanical strength, deformation and failure mode of the samples under uniaxial compression were systematically analyzed. In addition, computerized tomography (CT) scanning was used to examine the microstructural changes in the mudstone and sandstone before and after water saturation at different water temperatures. The results from the water absorption tests show that the water content of the mudstone and sandstone samples kept increasing with immersion time until a saturated state was reached, with the trend generally following an exponential law. The higher water temperature allowed additional water absorption in the saturated mudstone, but less water absorption in the saturated sandstone. The mechanical tests suggest that the presence of water can significantly reduce the mechanical properties of the coal measure rocks. Decreases in the uniaxial compressive strength (UCS) of 76.0 and 38.9 % and the elastic modulus of 68.1 and 48.5 % were observed in the mudstone and sandstone, respectively, because of water saturation at room temperature. Moreover, the water-weakening effect was sensitive to water temperature, and as the water temperature increased from 25 to 95 °C, the UCS and elastic modulus decreased linearly in the saturated mudstone by 53.8 and 70.4 %, respectively, and increased linearly in the saturated sandstone by 21.3 and 20.2 %, respectively. The increasing water temperature also promoted a transition in the saturated mudstone from brittle to ductile behavior, but it had a negligible effect on the failure mode of the saturated sandstone. The CT scanning tests demonstrated that new fractures are produced inside the mudstone after water saturation and that the increasing temperature can exacerbate such water-induced damage. However, no obvious fractures were observed in the CT images of the sandstone at room temperature or at high water temperatures, and the water-induced damage in the sandstone appeared as the micro-fractures at a scale below the CT resolution.