Experimental Study of the Effects of the Roughness, the Interlocking and the Material Mechanical Properties on the Apparent Cohesion of Unbonded Rock Joints in the Context of Dam Foundations

International gravity dam foundation design guidelines are suggesting to consider the rock-concrete foundation interface as unbonded and to use the Mohr-Coulomb criterion to assess the structure sliding safety factor. Engineers are invited to use the apparent cohesion with care but no consensus on the determination or on its influence parameters exists. Thus, an experimental study was developed and new insights on the apparent cohesion influence parameters (roughness, interlocking, material mechanical properties, contact type and normal load) were found. 64 direct shear tests were performed under CNL conditions at normal stresses similar to those found inside a large gravity dam foundation (from 100 to 1000 kPa). Results showed that higher the roughness is, higher is the apparent cohesion values. The degree of interlocking also plays a role on the apparent cohesion. Indeed, for the same roughness, a significant drop of the apparent cohesion was observed between the perfect and good interlocked joints. For the same roughness and interlocking conditions, it appeared that the apparent cohesion is also affected by the joint material mechanical properties. However, the influence of the material on the apparent cohesion is lower than that of the roughness or interlocking. An increase of the normal load also led to an increase of the apparent cohesion. These observations on the apparent cohesion influence parameters suggest that using the apparent cohesion in calculations or in numerical modeling for assessing the shear strength of rock joints may be realistic.