Experimental Study on Directional Fracturing of Granite by High-Temperature Thermal Shock

Hard roofs are a major cause of mining accidents such as rock bursts. To address the issue of hard roofs which do not collapse in time with coal mining workings, we propose directional fracturing of hard rock by high-temperature thermal shock. Experiments were conducted using a large true triaxial testing machine and a homemade heating system working with thermal shock. The results indicated that high-temperature thermal shock caused directional cracking in granite when it was heated to 500–550 °C at the heating rate of 114 °C/min. The temperature and temperature gradients along the heating direction were larger than those in the vertical heating direction at the same distance from the center of the borehole. The temperature gradient within 1.5 cm of the borehole in the heating direction was as high as 6300 °C/m. Under bidirectional compression conditions, we discovered that the horizontal stress difference played a dominant role in the initiation location of directional cracking by thermal shock. During the tests, the minimum horizontal principal stress was kept constant at 10 MPa and the maximum horizontal principal stress gradually increased. When the horizontal stress difference was smaller than 5 MPa, thermal shock-induced cracking occurred in the heating direction. When the horizontal stress difference was 6–7 MPa, thermal shock-induced cracks were generated both along and perpendicular to the heating direction. The results suggested that high-temperature thermal shock can produce cracks perpendicular to the maximum horizontal principal stress direction within a range of horizontal stress differences. These outcomes provide an essential guide for manually controlling hard roofs.