CS3B - Concurrent Session 03B: Assessment of Aging Dams for Seismic Potential Failure Modes: A Delta Approach

The importance of seismic design considerations for embankment dams was not fully recognized prior to the poor performance of the Lower and Upper San Fernando dams during the 1971 San Fernando earthquake. The state of practice in design and construction of embankment dams has been evolving for over a hundred years with the invention of construction equipment and knowledge of dam design and performance. However, many embankment dams that provide critical flood risk management, water supply, and other services were not designed and constructed for current seismic design considerations. As a result, they may not perform adequately when subjected to strong ground motions. An innovative qualitative approach has been developed to assess embankment dams for seismic related potential failure modes (PFMs) to assist for additional investigations, analyses, and evaluations. This delta () approach is based on comparing the current state of practice for design and construction that effectively addresses the seismic related PFMs with the design considerations, features, and construction methods for the existing embankment dam subject to assessment. Several major factors are documented in this evaluation process with explanations based on the current state of practice regarding the design and construction of different zones of a dam and the mechanisms for potential failure modes. These factors include comparison of (1) the seismic loading, (2) seismic analysis methods, (3) foundation and borrow site investigations and testing, (4) geologic conditions, (5) foundation preparation, (6) embankment zoning, (7) embankment construction techniques, (8) design and performance of static/hydrologic PFMs, (9) available freeboard, (10) potential seismic related PFMs, and many other factors. Also, system-wide issues such as the ability of the system to effectively lower the reservoir and post-earthquake conditions of appurtenant structures and operating equipment are essential parts of the initial effectiveness assessment of the system. The larger the delta (or the difference with the current state of practice) in different assessment areas, the higher the potential for poor performance of the embankment dam during a major earthquake. The delta approach provides a useful framework for assessment of existing embankment dams for seismic PFMs and an effective tool to communicate the overall system performance in a transparent manner. It can be useful to develop strategy by differentiating issues that may benefit from seismic analyses than that would require qualitative assessment to initiate upgrade measures.