2022 – Nonlinear seismic sliding stability of the Mt Bold Dam Safety Upgrade – An approach for arch dams

Sam Lalli – Senior Engineer, SMEC Australia Pty. Ltd., Michael McKay – Associate Engineer, SMEC Australia Pty. Ltd. and Francisco Lopez – Chief Technical Principal, SMEC Australia Pty. Ltd.

Traditional sliding stability analyses of concrete gravity dams employs Factor of Safety (FoS) criteria to determine a dam’s resistance to sliding based on the principles and methodologies presented in guidelines such as the ANCOLD “Guidelines on Design Criteria for Concrete Gravity Dams”. However, for dams with complex geometry such as arch dams, the assessment of the instantaneous sliding FoS during extreme seismic events may produce misleading conclusions as important factors contributing to the dam’s global stability, such as 3D transfer of demand and wedging effects are ignored.

Based on the acceptance criterion for the Safety Evaluation Earthquake (SEE) event in the ANCOLD “Guidelines for Design of Dams and Appurtenant Structures for Earthquake”, by which “…Damage can be accepted, but there should be no uncontrolled release of water from the reservoir…”, the authors of this paper argue that displacements at the base of the dam using a nonlinear finite element analysis can be used as a better indicator of damage and potential risk of failure during and after an earthquake than the traditional FoS approach.

This novel seismic sliding stability methodology was utilised for the recent “Mt Bold Dam Safety Upgrade – Concept Design” to show that although the instantaneous FoS drops below 1.0 during an earthquake, the analysis of displacement at the base resulting from nonlinear analyses proved that the upgraded dam would pass the SEE case and meet the requirements of ANCOLD (2013).

This paper explains the details and development of the methodology adopted for the analysis of the Mt Bold Dam Safety Upgrade, the results of the modelling, presents a proposed damage indicator based on instantaneous and cumulative displacements at the base of the dam, and provides a comparison with the results of a traditional FoS sliding stability approach.