Under high stress, location of microscopic discontinuities, their orientation, size, density and extent determine the failure mode. Microscopic discontinuities effect the mode of failure of rocks, while macro discontinuities predominantly effect the mode of fault of rock mass. The existence of microscopic discontinuities results in variation in the value of uniaxial compressive strength. However, the effect of these discontinuities is not considered when interpreting the strength of rock mass. For hard and brittle cylindrical rock samples, five distinct modes of failure were identified: simple extension, multiple extension, multiple fracturing, multiple shear and simple shear.

The simple extension mode (vertical splitting, axial cleavage) denotes a failure along a plane parallel to the direction of compression. Where two or more fractures run parallel to the long axis of the sample, with fracture perpendicular to that direction, multiple extension failure takes place. Multiple fracturing involves sample disintegration along many planes at various angles. This type of failure of the rock sample is often dynamic and violent, with a large amount of energy released. When fracturing takes place along two or more planes situated obliquely to the direction of compression, the mode is called multiple shear. The single shear failure involves one parallel shearing plane situated at an oblique angle to the direction of maximum compression.

To determine the effect of microfractures on the mode of failure a non-destructive testing technique was used to inspect samples prior to and after mechanical testing. The technique allows for visualisation of micro-cracks, bedding planes and bonded planes of weakness which have surface traces. Observations under ultraviolet light of rock fragments after mechanical testing helps to determine whether samples failed along microscopic discontinuities or along new, stress-induced failure planes. The existence of microscopic discontinuities requires that “scale effects” factor, i.e. strength reduction with increase of specimen size, is used for design purposes.

The non-destructive method of sample inspection, when used in conjunction with an analysis of the mode of failure and the rock strength, can assist in explaining the wide scatter of testing results obtained from the same rock sample population.