In the last decade the attention and application of a reliability-based methodology for concrete dams has increased. A recent project aiming at bringing forth a reliability-based methodology for design and assessment of concrete dams founded on rock for conditions applicable in a Nordic climate has resulted in a “Probabilistic model code for concrete dams” (PMCD). The objective of Theme D was to estimate probability of failure of an existing concrete dam for sliding along the concrete/rock interface and sliding along a joint in the rock mass, using the PMCD. The dam analyzed is a 25 m high concrete gravity dam located in the north part of Sweden. Contributions from six authors were received and have been analysed in this summary along with a reference solution by the authors. The first assignment was to estimate the deterministic factor of safety. Although the definitions of the factor of safety were similar there was large differences in the results. For the probabilistic analysis, definition of limit state functions was straight forward and have been defined similarly. Variables in the probabilistic analysis were defined somewhat differently, e.g. for concrete density, friction angle and ice loads. The results of the probabilistic analysis of sliding along the interface for normal water levels were varying, although five of the results were within the range of β = 3.7-5. There was less variability for the flood load case and for sliding along the rock joint. There reason was considered to be mainly due to the different parameter definitions. Identification of the most important parameters was successful; although the exact sensitivity values varied (due to variation in parameters), the most important factors were identified. In the calculation of system reliability, the previously described differences were reflected. Bayesian updating proved to be a tricky task, where especially results of the updated standard deviation varied. One conclusion is, however, that the updating of the friction angle is rewarding in terms of increasing the safety index due to the reduction in epistemic uncertainties.
For a probabilistic methodology to be trustworthy it should produce stable and reproducible results. The conclusion is that the PMCD is successful as a guideline in this process, but that further development and more experience of practical use is necessary. More benchmarks of similar characteristics are thus believed to be a good way forward and a broader discussion among practitioners would also be beneficial in reaching a “consensus” on how to perform reliability-based assessments.
QC 20191030