The origins of shear strength are reviewed in the context of direct shear testing of rock discontinuities. A method of analysing data to correct for dilation over different increments of horizontal displacement throughout a test is demonstrated and various forms of presenting data are discussed. It is argued that the corrected strength envelope for the test as a whole can be used to derive appropriate lower bound strength parameters for design. Additional strength may be allowable to account for roughness at the field scale, but as an alternative approach, it is recommended that the dilation-corrected strength be used directly for design with a lower factor of safety on shear strength than would normally be adopted.
The shear strength of natural rock discontinuities is often difficult to predict largely because of their roughness. In addition, different minerals have different frictional properties; furthermore h'trill or weathering can influence all other parameters including the effective roughness. The problems are compounded by the apparent effects of scale and normal stress level.
One option is to carry out direct shear tests on small samples even though it is generally recognised that these may be unrepresentative of the discontinuities in the field, particularly with respect to roughness. The recommended methods for testing published by ISRM (Brown, 1978) and by CANMET (Oyenge & Herget, 1977) emphasise the importance of careful monitoring of displacement data as well as loads, but give litfie guidance on interpretation of results.
Reported direct shear test data tend to show the same general trends of increasing shear strength with increasing roughness, especially at low normal stress levels, but often little consistency in the results achieved. Even for tests on artificial planar surfaces, consistent results are not always obtained and require skill for interpretation. For example, Nicholson (1994) reported a variation in friction angle of 12.5 degrees for saw-cut samples of the same sandstone tested by four different laboratories. He commented that even greater variability would be expected for naturally rough discontinuities.
It is apparent that currently many geotechnical engineers consider direct shear tests on rock discontinuities to be of limited use for providing reliable design parameters. A procedure for analysing and interpreting test results which accounts for sample-variable roughness that the data can be used with confidence is described in this paper.