Experimental investigation of the effect of water and temperature on mechanical properties of rocks

Abstract

In mining or civil operation, geological challenge could be encountered when excessive groundwater is present in the field. Artificial ground freezing is one of the effective methods to control or mitigate the potential of groundwater inflow. However, uncertainties still remain in terms of understanding and predicting the behaviour of rocks under sub-zero temperature. The main aim of the research is to investigate effect of temperature and water on the mechanical properties of rock by conducting the cracked chevron notch Brazilian disc (CCNBD) standard fracture toughness test. The dimension of the samples (Sandstone and Basalt) was prepared based on ISRM suggested guidelines. The CCNBD test was conducted in four different temperatures (250C, -250C,-500C, and -750C) under dry and wet condition. The obtained fracture toughness values from the test were correlated with temperatures, so that the change in fracture toughness in each temperature could be observed. The experimental results determined for sandstone showed that the fracture toughness in both dry and wet conditions increases as temperature decreases. In addition, fracture toughness was found to be lower for wet samples at 250C relative to dry samples and such value was 0.44 √ and 0.55 √ respectively. The reduction in fracture toughness at 250C is possibly due to the weakening effect of water which causes the microcracks within the specimen to enlarge and extent and therefore resulting in a reduction of strength. However, when temperature drops below 00C, the strength of rocks become a combination of ice strength and rock strength. Hence the strength of ice as well as the strength of rock increases as temperature continuously decreases. Furthermore, another observation can be seen from the result of sandstone is that the temperature has a greater effect on wet samples as the increasing rate of fracture toughness is much faster than dry samples. Such increase rate for wet samples can fit into a logarithmic relation but it is only a linear relation for dry samples. In addition, the differences in strength between wet and dry sample can be up to 120% when temperature is at -750C. One of the main reasons is that since the ice content of wet samples is larger than in dry samples, the strength of ice enhances the rock strength significantly as temperature decreases. In addition, the result of basalt in dry condition showed similar results. The fracture toughness can increase up to 14% when temperature reaches -75 0C. When temperature decreases, the mineral grains within the rock specimen shrunk which generated a confining stress to the specimen and result in a higher compressive strength. To conclude, temperature has a positive impact on the strength of rocks and wet samples are more sensitive to the change in temperature.