A study was undertaken to investigate changes in the strength and failure pattern of otherwise standard rock core specimens containing regularly spaced discontinuities. The objective was to assess changes in the nature of rock failure resulting from these discontinuities.

Test core specimens were prepared from sandstone having dimensions of 44 and 110 mm for diameter and height respectively. The core specimens were cut at right angles to the longitudinal axis of the core producing composite specimens having 2, 3, 4 or 5 regularly-spaced surfaces. As well as testing specimens having dry, clean fracture surfaces, the test program also considered different infill materials and hence friction values using dry and oiled-impregnated paper.

The study found the strength of specimens decreased with the number of discontinuities. While the UCS strength of the intact rock specimen was 35.8 MPa, the strength of composite specimens having five segments was reduced by 50% to 17.9 MPa. This is despite the reduction in the slenderness ratio of each segment in the composite specimen that would usually result in an increase in strength. Less sensitive were changes in infill material having little discernible effect over for the limited range of friction surfaces investigated.

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