Determining the shear strength of a discontinuity is one of the most difficult tasks while it is probably the most important property to measure.
A simple and often adequate assessment can be made with the tilt test. Two pieces of rock including the discontinuity are tilted while the angle of the discontinuity with the horizontal is measured (Fig. 1) with, for example, the inclinometer included in a geological compass.
The angle measured at the moment the top block moves is the ‘tilt-angle’. If no infill is present and the two blocks had fitting discontinuity surfaces, the tilt angle equals the small-scale roughness angle plus the angle of friction of the surface material.
If the discontinuity roughness is completely non-fitting, the tilt-angle equals the material friction only. Undisturbed infill material will seldom be present in this test, but if it is, the tilt-angle includes the influence of the undisturbed infill. If it is not possible to obtain a sample with undisturbed infill material, it is sometimes possible to scrape infill material from another still in situ discontinuity and place this between the blocks of the tilt test sample.
The thickness of the so-formed infill layer should be the same as in situ. The measured tilt-angle is then including the influence of remoulded infill material. Cohesion is not measured in the tilt test separately.
If present, either real or apparent, it will be included in the tilt-angle. Steps on discontinuity planes causing hanging of the blocks, for example, result in a very high tilt-angle (which may be up to 90°). Whether this is realistic for the friction of the in situ discontinuity has to be judged on the strength of the cohesion or asperities in relation to the stresses in the in situ rock mass. Stresses in the rock mass will generally be far higher than those used during the tilt test and may cause either shearing of real cohesion or shearing of asperities along the in situ discontinuity.
Clearly, the tilt-angle is only representative for small-scale roughness and low normal stresses as tilting meter-scale rock blocks is normally not possible for the average engineering geologist!
A more sophisticated methodology is to use classification systems to estimate friction or shear strength along a discontinuity. An example is the relation given in Eq. 1 (Barton 1971):
JRC is the joint roughness coefficient (a number, low for smooth planar surfaces rising with increasing roughness, estimated by visual comparison of the discontinuity surface to standard roughness graphs), JCS is the joint wall condition strength, σn is the normal stress on the discontinuity, and φr is the residual friction angle. If no other test is available, it is possible to use the tilt-angle of a non-fitting discontinuity without infill material as residual friction angle.
Another example of a classification system for estimating discontinuity shear strength is the ‘sliding criterion’. Based on back analyses of slope stability a sliding criterion was developed to easily estimate the shear strength of a discontinuity (Hack and Price 1995; Hack et al. 2002). The discontinuity is characterised following Table 2.
The roughness is characterised by visually estimating large (Fig. 2) and small-scale (Fig. 3) roughness by comparing to standard profiles and by establishing tactile roughness, infill material, and presence of karst.
The different factors for the different characteristics are multiplied and divided by an empirically established factor. This results in the so-called ‘sliding angle’:
The ‘sliding angle’ is comparable to the tilt test idea but on a larger scale. The sliding angle gives the maximum angle under which a block on a slope is stable. The ‘sliding criterion’ has been developed on slopes between 2 and 25 m high. The ‘sliding criterion’ applies for stresses that would occur in such slopes, hence, in the order of maximum 0.6 MPa.