Roughness Effects on the Shear Strength of Concrete and Rock Joints in Dams Based on Experimental Data

This paper investigates the effects of joint roughness on shear strength based on experimental data obtained from shear tests on 18 specimens presenting an unbonded contact interface, i.e., fully open with no bond, drilled from concrete, concrete–rock, and rock joints in existing dams. Original and practical procedures are developed to numerically process 3D profiles of laser-scanned interfaces of the tested contact specimens, and then evaluate the corresponding roughness parameters for the prediction of joint shear strength. Orientation-dependent joint roughness coefficient (JRC) and joint matching coefficient (JMC) are also proposed to investigate roughness effects as a function of shearing direction. The analysis of the experimental results confirmed that the contribution of roughness to the shear strength of a dam joint has to be considered. Furthermore, the results suggest that only little shear-induced degradation occurs along the contact specimens during the tests. Shear strength values from shear tests conducted on contact specimens are compared to predictions using an analytical equation adapted to include the orientation-dependent JRC and JMC. The results highlight the significant role of roughness and matching properties on the shear strength of unbonded natural joints, although the quality of the predictions was found to be sensitive to the selection of the residual friction angle \(\phi_{\text{r}}\) and the joint compressive strength JCS. It is also found that joint roughness and matching depends on shear direction which can significantly affect shear strength predictions. These results clearly illustrate the need for sensitivity analyses to evaluate the variation of joint shear strength should a potential dam sliding occur along a direction different from the one adopted during shear tests.