2022 – Shear strength of bonded and unbonded concrete-rock contact for dam stability assessment – Statistical analysis of in-situ and laboratory tests data

Mahdi Zoorabadi – Technical Principal-Tunnels and Geotechnics, SMEC, Melbourne, Australia, Associate Professor UNSW Sydney, Australia and Francisco Lopez – Chief Technical Principal-Concrete Dams, SMEC, Melbourne, Australia

The shear strength of concrete-rock contacts is a fundamental parameter to determine the stability of concrete dams. For the bonded contacts, the shear strength is mobilised at very small shear displacements and depends on the adhesion between concrete and rock surface, the applied normal stress, and the contact roughness. On the other hand, the shear strength of unbonded contacts needs relatively longer shear displacements to be mobilised. In-situ and laboratory direct shear tests are the best methods to estimate the shear strength of concrete-rock contacts. However, these types of tests are not always conducted as part of the investigations of concrete dam projects. Consequently, dams engineers usually resort to published data from historical projects particularly for early design stages of new dams or for safety assessment of existing dams. However, only limited information is available in recent literature. This paper examines a wide range
of in-situ and laboratory direct shear test results that have been gathered from historical projects and published researches. It was found that unlike residual shear strength, the peak shear strength of concrete-rock contacts is scale dependent and correction factors have been developed for scaling laboratory scale tests. The paper also discusses the suitability of the residual shear strength or the combination of basic friction angle and dilation angle for unbonded contacts. Finally, the paper presents suggested correlations to estimate the peak and residual shear strength of the concrete-rock contacts based on statistical analysis of the gathered data, which can be readily use un the stability analyses of concrete dams, particularly as constitutive models for concrete-rock interfaces in nonlinear analysis of concrete dams.