FRACTURE PROPAGATION IN ROCK BY TRANSIENT COOLING

A technique for breaking rocks which goes back to antiquity involves heating of the rock and then suddenly cooling it with water. The rapid decrease in temperature at the surface leads to tensile stresses which cause cracks to initiate and propagate into the rock. At the present time we can dispense with the initial heating phase since low temperature liquids are available. However, to be competitive as a modern rock fracturing process, the thermal shock technique should enable cracks to be driven rapidly and in a controlled manner. Calculations made for a semi-infinite slab cooled on its face by liquid nitrogen indicate that crack velocities are limited by the poor heat transfer rates associated with film boiling. Experiments on 10 cm cubes of Solnhofen limestone with liquid nitrogen injected into a central 5 cm long by 1 cm dia hole generally confirm the predictions. Crack propagation rates were slow and crack directions somewhat unpredictable. These observations confirm the widely held opinion that thermal shock methods are unlikely to form the basis of a commercially-viable process for rock destruction unless new developments allow the surface heat transfer rates to be substantially increased. © 1979.