Effect of thermal shock and rapid unloading on mechanical rock properties

Laboratory tests have been conducted to investigate the effects of rapid cooling and rapid unloading on rock damage. Thermal shock experiments were conducted consisting of slow heating up to a temperature of 100°C followed by rapid cooling with a fan. Samples were examined after one cycle (relevant to deep core drilling) and five cycles (relevant to ventilation in deep underground mines). The rock damage due to thermal shock was analyzed using before and after P and S wave velocity, porosity, and tensile strength tests. To analyze the effects of rapid unloading, cores collected from depths of 1000-2000 meters were reloaded back to their in-situ stress levels while simultaneously measuring P and S wave velocity. The results from the thermal shock tests were very interesting and showed that crack growth occurred in some rock types subjected to thermal shock (Granite, Diabase with ore veins, KVS) while crack healing occurred in other rock types (Diabase without ore veins, Quartzite, Skarn). Within a rock type the results were consistently crack growth or crack healing. In an effort to understand this behavior, the thermal stresses induced in the rock samples during the shock tests were analyzed. Our hypothesis is that overall crack healing is expected when the amount of crack healing in the central parts of the rock sample (due to slow heating and rapid cooling) exceed the crack growth that is occurring near the surface of the samples (due to rapid cooling), and vise versa.