The application of equal and unequal external stresses to large rectangular blocks with pre-drilled circular holes has been investigated by several researchers and has been also used to model boreholes or tunnels. The fracturing occurs on the two opposite sides of the hole and the resulting failure grew in the minimum field stress direction, orthogonal to the borehole axis. Similar failures are also common around tunnels and galleries in highly stressed rock.
Fracture and breaking stress around the opening in blocks of sandstone with pre-drilled circular holes were investigated through several tests series. First the compressive strength of the material was obtained by means of uniaxial compression test of cylindrical specimens of sandstone. 24 tests in specimens of St. Margarethens sandstone were tested under uniaxial compression stress, 19 of them with dimensions 200x200x50 mm and a circular 25 mm diameter cavity and 5 of them with dimensions 300x300x100 mm and 36 mm diameter cavity. Experiments were performed using different loading conditions. Some specimens were placed in direct contact with the loading platen and some specimens had an interface (an aluminium plate) between the loading platen and the specimen.
Theoretically the apparent strength of the rock adjacent to the unsupported cavity is two to three times the uniaxial compressive strength. Throughout the tests fracture phenomena at the edge of the hole was observed at a point close to the maximum force. Thus these results contradict the theory. Observation of other experiments and of the results of the tests can explain this discrepancy between the obtained and expected results.