Comparison of experimental and empirical methods for estimating the shear strength of rock joints based on the statistical approach

One of the most common geological hazards in mining and civil projects is the occurrence of instability in the rock slopes. Rock slope behavior is mainly influenced by the presence of discontinuities in the rock masses. Therefore, the determination of shear strength parameters for these weak surfaces is of particular importance. There are various shear strength criteria for estimating the shear strength of discontinuities, which among them the Barton–Bandis criterion (B‒B criterion) and Mohr–Coulomb criterion (M‒C criterion) are the most widely used. Accurate estimation of shear strength properties does not only depend on the correct procedure of tests, but it requires an accurate and detailed explanation of test results. The uncertainty in the measured shear strength leads to many problems in the analyzing and designing of rock slopes. Therefore, to reduce the uncertainty and increase the accuracy of rock slopes analyzing, it is better to use probability methods. The probability distribution function can be assigned for each input parameter of the failure criterion. In this research, the results of the direct shear tests related to Azad pumped storage power plant project have been considered to estimate the shear strength properties of rock discontinuities. The results are divided into three groups based on the magnitude of the joint roughness coefficient (JRC). Shear strength for each group is estimated depending on the two most well-known criteria i.e., the B‒B criterion and the M‒C criterion. Further, the probability distribution functions (PDF) for each shear strength parameter are determined by @RISK software. Besides, the results of the empirical B‒B criterion are compared with the results of the direct shear tests for the three groups of rock joints. The confidence intervals associated with both criteria are estimated and compared together. The results showed that the overlapping of the confidence interval of the M‒C criterion with the confidence interval of B‒B criterion increases with increasing JRC of rock joint. The results show the B‒B criterion covers data of the direct shear test almost at low normal stress levels for different JRC groups too.