Laboratory shear tests of rock joints are the common tool to evaluate the shear strength. Their results are fundamental to assess the safety conditions of Rock Engineering projects. This paper will present the different types of tests that can be carried out, with a special focus in what concerns the stress paths applied through out the tests. The advantages and disadvantages of using the same joint sample to perform several slidings will be discussed, along with other issues related to the joint re-positioning and surface wear. The importance of the stress path of the normal stress prior to each sliding is an important factor to assure that all slidings are performed under conditions as approximate as possible.
Since early days of Rock Mechanics, the shear strength of rock discontinuities has been a major topic and some well known papers are frequently quoted (Barton & Choubey 1977, Bandis et al. 1983). Its relevance arises in Rock Engineering works where stresses are lowin comparison with the strength of intact rock. In these cases, the rock mass stability and the safety conditions are controlled by block movements defined along or by discontinuities (joints, bedding planes, shear zones, faults, and cleavage or foliation planes). Safety evaluation of blocky systems requires the estimation of the shear strength of the rock discontinuities, which can be determined by shear tests (Goodman 1989, Hoek 2007). Due to their duration and cost, it is common practice to perform laboratory shear tests on relatively small samples of rock joints, instead of in situ direct shear tests. As in Soil Mechanics, the basic principle of common rock joint direct shear tests is to subject a joint sample to various normal stresses and in each case to determine the shear stresses required to produce a certain shear displacement (ISRM 1974, ASTM 1995). This paper discusses the different possibilities to perform these tests.
Rock joint shear tests are often included in the geomechanical characterization of rock masses for large and relevant projects. These studies also include the assessment of the main joint sets and the shear tests should also direct their attention to the evaluation of the mechanical characteristics of such sets. The analysis of several groups of joint shear tests from different types of rocks showed that the variability of the results is quite important. Coefficients of variation higher than 30% are very common, and smaller values around 10% are only found for particular joint sets with very low roughness, such as schistosity or foliation planes (Muralha 1995). If possible, a statistically significant number of laboratory tests should be performed to estimate the shear parameters for a given joint set. A number around 20 usually offers a satisfactory compromise between statistical significance, sampling difficulties and costs.
