As dam safety professionals, we sometimes come across dam projects that do not meet performance requirements under seismic conditions. This prompts us to decide on what action to take and choose a remedial alternative to rectify the issue. Depending on the type of project and issue, the remedial alternative selection can be a daunting process. Where to start? How much repair do we want? What are our options? What are the risks?
These are some of the questions that go through our minds as we look at the project. I want to discuss the different options at hand and identify some of the alternatives that address common issues that earthen dams face under seismic evaluations.
The first step in the remedial selection process is to conduct an initial screening of the potential remediation alternatives. This allows us to eliminate the options that do not meet risk acceptance level and other critical requirements. This initial screening can be divided in two major groups: impact of failure and serviceability
Impact of failure: Evaluation of safety, economic, and environmental risk levels
In the event of the Maximum Credible Earthquake, uncontrolled release of the reservoir (at normal operation levels) must be avoided to prevent loss of life, loss of economic value, and/or impacts to the environment. To quantify these requirements, we must evaluate several post-earthquake conditions, including:
- Factor of safety for post-earthquake limit equilibrium compared to regional/local requirements
- Maximum loss of freeboard compared to regional/local requirements
- Maximum horizontal deformations (upstream and downstream) compared to regional/local requirements
- Probability of occurrence compared to economic valuation of impact consequences
Serviceability: Preservation of project purpose
Dam and reservoir projects typically have multiple purposes (e.g., power generation, water supply, fish and wildlife, flood control, etc.). These functions should be preserved unaltered after remediation.
The optimum remedial solution may include one or more methods of improvement. Different methods could be applied to either the upstream side, the downstream side, or both. In any case, the selected solution, or combination of solutions, should be the most cost-effective possible that still reduces the seismic hazard to an acceptable level. The initial screening provides a better idea of what type of remediation might be needed for the project.
Types of remedial alternatives
Remedial alternatives, for the purpose of this discussion, are classified into the following categories:
- No Action: Where we accept the risk of a dam failure in a design seismic event
- Minor Remediation: where we mitigate the risk of failure typically by taking some action, which could also be combined with improved operation and maintenance (O&M) protocols
- Major Remediation: where we modify part of the structure of the dam to stabilize it and address the risk
- Total Remediation: where the entirety of the embankment is included in the remedial action
The flowchart below summarizes these alternatives, and gives details on the major groups of remedial actions and some of commonly implemented solutions:
As we can see, there are multiple solutions to address our seismic hazard mitigation concerns. I will discuss the benefits and drawbacks of these remedial alternatives in order of cost and complexity.
Remedial alternatives and their benefits and drawbacks
1. No Action
There are no direct costs associated with this alternative, yet there are some considerations that must be evaluated before choosing it. For example, the probability of occurrence of a design seismic event capable of inducing failure is remote. However, a lower seismic event may induce localized failures (e.g., liquefaction, slope slides, loss of freeboard). Such failures may impact other structures that are critical to thoroughly address the risk (e.g., failure of spillways, relief wells, etc.), which in turn could result in events that can trigger dam failure, such as piping or overtopping.
Depending on the project location, there could be significant impacts to residents, the environment, and/or resources downstream of the dam. The benefits provided by the project would be lost and there would be future costs to repair or rebuild the dam after failure, legal claims, etc.
2. Minor Remediation
These alternatives are relatively less costly and easier to implement than the next category. They mostly aim to mitigate and control failure trigger mechanisms such as underseepage and through-seepage, and they work well in combination with simple O&M measures. For example, positive control of underseepage would eliminate the need of pressure relief systems along the downstream toe and, therefore, the danger of piping if the existing system is compromised by post-earthquake large deformations of the embankment. The most common minor remediations include cutoff walls and relief/pumping wells, as indicated in Figure 1.
3. Major Remediation
These alternatives are more costly and harder to implement than the previous category, but less costly and complex than total remediation projects. They aim to reinforce portions of the dam to minimize or eliminate the potential for large deformations caused by earthquakes. They typically include reinforcement of the embankment by means of piles or anchors. Reinforcement of the foundation by means of Soil Stabilization Techniques such as Dynamic Compaction, Vibrocompaction, Compaction Grouting, Jet Grouting, Soil Mixing, Stone Columns, Gravel Drains, etc., as well as beefing up the embankment by means of berms and buttresses.
4. Total Remediation
These alternatives are much more costly and harder to implement than the previous categories. Although the benefits are great, total remediation also has major drawbacks: the annual project benefits are completely lost. All project functions (e.g., power generation, recreation, water supply, fish and wildlife, flood control, water quality, navigation, etc.) are not maintained. In addition, the environmental impacts are significant. However, at times it is necessary, and the benefits outweigh the drawbacks.
Local California examples of total remediation include the Dam Removal and Dam Replacement for the Lower San Fernando Dam, California. Los Angeles Department of Water and Power (LADWP) undertook this project after it was heavily damaged by the San Fernando earthquake in 1971.
Key Takeaways
There is no single solution that is always correct to address our seismic hazard mitigation concerns. Any solution will have benefits and drawbacks, and multiple factors must be carefully considered during the evaluation process. An experienced dam safety engineer can help you navigate the selection process and find a solution that works for your specific needs.
