Effect of discontinuity surface roughness on the strength of jointed rock samples

It is seldom possible that rock engineering structures found without joints, cracks, or discontinuities. Discontinuities are one of the most important phenomena that cause the mechanical anisotropy in rocks. This mechanical anisotropy would impose reduction of strength. Therefore the discontinuities and degree of their influence on the rocks must be studied carefully in order to define the behavior of rocks. The confining pressure, on the other hand, is an important parameter that controls the percentage effect of the rock discontinuities on the rock strength. The purpose of this study is evaluation of the effect of discontinuity surface roughness on strength of jointed rock samples at different confining pressures. For this purpose, an intensive laboratory investigation was undertaken to test jointed specimen at various discontinuity angles (β=0, 15, 30, 45, 60, 75 and 90 degrees) and confining pressures (0.5, 1.5, 2, 3.5 and 5 MPa). The rock discontinuities possess various roughness that were controlled by joint roughness coefficient (JRC=0, 7, 12 and 17). Fabricated artificial samples which included a rough discontinuity were prepared by Cement and Plaster of Paris. These specimens were used for simulation of mechanical anisotropy subjected to uniaxial and triaxial tests based on the ISRM suggested methods. The results of experimental tests indicated that, with an increase in discontinuity surface roughness and lateral pressure, the strength of the anisotropic samples increases. Also it is shown that the orientation of discontinuity influence on the strength of specimens and the maximum strength achieved at the specimen with β=25 to 50 degrees.