In testing rock specimens containing a single discontinuity, both the direct shear test and the triaxial compression test may be used to determine the strength and deformability characteristics of the discontinuity. However, in employing these two techniques, various investigators have indicated that there appears to be a discrepancy between the respective experimental results. In an attempt to investigate this disparity, a series of laboratory experiments were conducted using both the direct shear test and the triaxial compression test, to compare the shear strength characteristics of Penrith sandstone (PrS) and an artificial rock -like specimen (Quickrock or Qu) containing different types of a single discontinuity.
Before embarking on experimental results of this research, it is worth emphasizing the importance of this work with an example. Consider a cylindrical specimen containing a discontinuity that is cored from an underground rock formation. Due to the difficulties in preparing the specimen for triaxial compressive test to determine the shear strength of the joint, it is expected that the direct shear method should be used. The question is whether the latter method satisfies the condition from which the specimen has been sampled and to what extend the determined shear strength is the same as the one that exists in triaxial environment such as the underground opening. To offer an answer to these questions, one should find out whether it is possible to use triaxial and direct shear testing methods interchangeably. Consequently the comparison of results from triaxial and direct shear tests is an issue of significance. The experiments were conducted in order to assess the extend to which the direct shear tests would produce the same results with the triaxial tests when applying equivalent loading conditions. Use was made of a special triaxial cell, a detail description of which may be found in the work by BUZDAR (1968). The confining pressures employed for the triaxial tests ranged between 2.5 MPa and 15 MPa and were applied by a pressure intensifier apparatus that monitors during testing the volumetric changes with a resolution better than 0.1 ml. During the triaxial compression tests the rate of the axial strain was maintained constant at 0.25%/min. The direct shear tests were conducted in a special shear test apparatus capable of applying a maximum 1 MN normal load resulting in normal and shear stresses up to 50 MPa and 35 MPa, respectively (GHAROUNINIK, 1993). Quickrock was used as casting material for the direct shear apparatus, mainly because of its ability to gain strength at a fast rate (its uniaxial compressive strength after a few hours reaches 30 MPa and after a day is more than 50 MPa).
In order to involve all the possible forms of discontinuities, various types of roughness in clean and infilled surfaces have been used. In other words, an attempt has been made in this study to simulate the surfaces from the simplest to the most cumbersome situation, i.e. clean smooth and infilled rough surfaces, respectively.
