2022 – The Limitations of Methods of Stability Analyses, and Potential Disconnect between Deterministic and Risk – based Acceptance Criteria for Concrete Gravity Dams.

Robin Fell – School of Civil and Environmental Engineering, UNSW Sydney and Richard Rodd – Richard Rodd and Associates

Our experience as peer reviewers for assessments of the safety and design of upgrades of a number of large concrete gravity dams in Australia has shown that there is sometimes
a disconnect between the deterministic acceptance criteria using ANCOLD (2013) Guideline on Design Criteria for Concrete Gravity Dams and assessment using ANCOLD (2003, 2022) Guidelines on Risk Assessment tolerable risk criteria. This is caused by factors such as reliance on the results of numerical analyses for 3D effects in risk analyses, and/ or allowance for load sharing between monoliths, and/ or for formation of breach mechanisms. As a result dams with 2D factors of safety not satisfying ANCOLD (2013) acceptance criteria can satisfy tolerable risk criteria even for dams with a large potential life loss. In some cases the risk assessments rely on 3D numerical analyses which fail to properly model foundation strengths and pore pressures / uplifts. Often these are elastic analyses so fail to allow for stress redistribution resulting from yielding.