Assessment of Aging Dams for Seismic Potential Failure Modes: A Delta Approach

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. The importance of seismic design considerations for embankment dams was not fully recognized prior to the near failure of the Lower San Fernando Dam and poor performance of the Upper San Fernando dam during the 1971 San Fernando earthquake. However, many embankment dams were designed and constructed prior to the current seismic design standards. A qualitative approach was developed to assess and screen embankment dams for seismic potential failure modes (PFMs). This delta (Δ) approach consists of comparing the historical design and construction methods originally used for the existing embankment dam being evaluated to the current state of practice. The term delta (Δ) refers to missing features, as well as design and construction deficiencies between historical practice and the modern state of practice for addressing seismic Potential Failure Modes (PFMs). Several factors are documented in this evaluation process with explanations based on the current state of practice for the design and construction of embankment dams and defenses against seismic PFMs. The larger the delta (or the difference with the current state of practice), the higher the potential for poor performance of the embankment dam during an earthquake. The results can be used to help assess and screen seismic PFMs of existing embankment dams for additional investigation and evaluation to reduce uncertainty and better characterize the performance risk and the need for interim risk-reduction measures. This delta (Δ) approach provides an effective tool to communicate the potential overall system performance deficiencies in a transparent manner.