The shear strength of concrete-rock interfaces, used in reliability-based analyses of sliding stability of concrete dams, is commonly quantified using a simplified version of the Mohr-Coulomb shear strength criterion. This criterion is based on the assumption of a mean value driven process occurring according to the requirements imposed by perfectly-plastic failure theory. However, tests on cores with bonded interfaces exhibit semi-brittle post-peak behavior, which in combination with a possible spatial variation in cohesion implies that a progressive failure mechanism is a more correct description of the interface behavior. This has to be considered in sliding stability analyses in order to avoid overestimation of dam safety. Uncertainties between model and real behavior like the one described above can be taken into account by introducing a random variable representative of the specific uncertainty into the limit state function. A drawback with this technique is that the real shear strength of the interface is difficult to define since physical observations are not easy to acquire. Krounis and Johansson (2014), therefore, used a technique where numerical analyses were used in order to estimate the real behavior. The technique requires several numerical calculations and is extremely time-consuming. Another, less time-consuming, technique was used by Westberg Wilde and Johansson (2013). In their work, the progressive failure of the bonded interface was taken into account by simulating new distribution parameters for cohesion. This was done by means of an analytical procedure where the cohesive strength of the interface was considered to behave as a brittle parallel system. The stress state of the interface, however, was not considered, which may have significantly affected the results of the analyses. In this paper, a comparison between these two techniques for a hypothetical dam monolith is made in order to study the magnitude of the possible discrepancy between the two techniques. The benefits and limitations of each method are also discussed based on the results of the analyses.

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