Defect-induced stress field triggering extensive tensile fracture growth in uniaxial compression

Mechanics of rock failure in uniaxial compression remains a challenge since the wing cracks produced by pre-existing defects/cracks in uniaxial compression are shown to wrap around the initial defect effectively arresting their further growth. As a result, they cannot grow to the extent sufficient for splitting rock samples. This presentation proposes another mechanism of rock failure based on extensive fracture growth caused by zones of tensile stresses formed as parts of self-equilibrating stress field induced by defects distributed in rock (both pre-existing and produced in the process of loading). The fracture driven by localised tensile stresses grows avoiding the zones of compressive stresses thus forming areas of interruption or overlapping. These areas work as distributed bridges constricting the fracture opening. Fractures with constricted opening have the stress intensity factors increasing with fracture growth making the fracture growth unstable and capable to break the rock.

Due to the necessity to avoid the compression zones the growing fracture will deviate form the straight path. If the sample size is not sufficiently large as compared to the size of the compression zones these deviations can be seen as inclined in one direction making the fracture oblique. This can be passed for the conventional shear fracture, which in fact cannot play a role in the process, as the shear fractures do not grow in their own plane forming wings instead. Therefore, the fractures observed in rocks failed in uniaxial compression, in both splitting and oblique failure types are tensile fractures with constricted opening formed and driven by the stress field induced by distributed defects. Whether the resulting failure is splitting or oblique (“shear” failure) depends upon the ratio of the sample size to the compression zones dimensions. We term this dependence “the scale effect in failure type”.

The proposed concept will form a basis for developing models of rock failure in compression necessary for analysing and predicting large scale failures in the rock mass, especially during mining operations.