Seismic Evaluations – Understanding the Process for Informed Decision Making of Embankment Dams

By Nicholas Paull – Project Engineer

The dam safety community continues to move towards using more risk informed methods for evaluating dam safety. As it does so, dam owners may need to perform new studies or update outdated studies to support seismic risk analyses for the dam. Owners may express concern over what these new studies may entail as advancing earthquake science often results in increased demands on their dam that may require costly structural modifications or retrofits to address.

Better understanding of the process for performing seismic evaluations can provide insight for dam owners and consultants. These can guide the use of often limited resources to improve the safety of the facility. Seismic evaluations for dams are complex and require careful consideration of the regulatory requirements that continue to evolve the state of practice for earthquake science and seismic analysis and the owner’s appetite for risk.

Understanding the Goals of a Seismic Analysis
A primary purpose for updated seismic analyses is to improve understanding of the seismic risk relative to other potential risks (i.e., hydrologic). This improved understanding helps the owner determine where to prioritize resources to improve dam safety and maximize the operational performance of the facility. To achieve this end, seismic evaluations are generally completed in the following steps:

1. Information Gathering
2. Seismic Hazard Analyses
3. Seismic Response Analyses

These steps can be incrementally increased in complexity to address the unique concerns for a given dam and decrease the uncertainties that may inflate the estimated risk. The incremental increase in complexity can provide milestones for the owner to assess if the next level of analysis (i.e., more complex) is necessary for an improved understanding of the seismic risks or if seismic risks can be “ruled out” when compared to other dam safety risks.

Information Gathering
The first step in any seismic evaluation is to understand what is known, what is unknown, and what may be outdated. Reviewing original reports helps to thoroughly understand the dam’s history and potential assumptions that may have been made. A review of both paper and digital archives can help identify any data gaps in the project’s history and may result in improved understanding of the dam.

A construction photo can be worth a thousand words and thousands of dollars to a dam owner. When paired with inspection records, they may reveal changes in conditions that might not be shown on as-built records and can also provide some fun stories to tell at your next cocktail party. In one instance, review of construction records revealed that one dam’s embankment material was so wet during construction that a flamethrower was used, unsuccessfully, to dry it out.

Understanding seismic evaluations at a dam starts with knowledge of the dam from below the ground up. This includes regional, local, and site geology and the geological processes that formed the dam foundation long before the dam was built. While some dams may be founded on rock, others are founded on a variety of foundation soils. Understanding the types and nature of these foundation soils and their geologic depositional history has a big impact on the seismic response of the dam.

Seismic Hazard Analyses
Seismic hazard analysis (SHA) is used to estimate the seismic loading at the site considering nearby faulting. Like the simplified seismic analyses described above, SHA can increase in complexity. The best place to start is often with web-based hazard tools (such as those published by the United States Geological Survey) that provide a general model of significant fault sources. These initial tools can be used to understand a general range of seismic loading that can be expected from a SHA of the dam site.

SHA can be performed either deterministically (DSHA) or probabilistically (PSHA). While DSHA is often the more simplified standard when it comes to seismic loading for design, PSHA is required for risk evaluations, as PSHAs provide the recurrence interval for a given seismic load. Performing site-specific DSHA or PSHA studies requires detailed understanding of the key local and regional faults (referred to as seismic source characterization) and their relationship with the site. The result can be used to provide the basis for seismic demands (ground motions) for further seismic response analysis of the dam. For SHAs, fully documenting the considered seismic sources and modeling methods/assumptions used is a necessary step towards regulatory approval and provides a good basis for any future seismic studies.

Seismic Response Analyses

Seismic response analyses (SRAs) are models of how the structure will respond to seismic loading. Like SHAs, these can vary in complexity. The simplest SRAs include empirical case history estimates and the most complex SRAs are detailed computational models of the dam or associated structures. Case history data is often a good starting point to estimate the possible range of seismic responses based on the observed performance of other similar dams from historical earthquake events.

The next step up in complexity for SRAs is often to perform 1-D or 2-D analyses that merge seismic loading with the material response of the dam. For an embankment slope stability analysis, this is often performed using a combination of slope stability analyses (which assess the dam’s resistance to movement) and sliding-block type analyses (which assess the dam’s reaction to the seismic loading condition). While more involved than case history or empirical estimates, these analyses are often “decoupled” and do not fully capture the interaction between material behavior and seismic loading but are typically sufficient for most seismic evaluations.

In cases where a more detailed understanding of the dam’s seismic response is necessary, computational models can be used to “couple” the interaction between material behavior and seismic loading for more accurate estimates of the non-linear soil response, and therefore offer better estimates of the full structure response. As the most complex SRAs, detailed documentation of these analyses is critical to assess the potential deformations and inform a risk analysis for the site.

For a deeper dive into each of these steps or for help performing seismic analyses for your dam, GEI can help. For further information, contact GEI’s own seismic experts, Nick Paull, Ph.D. P.E., and Chris Krage Ph.D., P.E., who will be moderating sessions on “Seismic Evaluations of Embankment Dams” and “Site Response Analyses and Modification Considerations” at this year’s USSD conference in Charleston, SC this week.