How Structural Engineers Maintain Dam Safety and Vitality

Maintaining the structural integrity of dams is a complex process that begins with a comprehensive analysis. Structural engineers employ various methods to assess stability and safety and determine the need for dam rehabilitation and repair.

The gravity method of analysis is generally the first step in the analysis process, which assumes that the dam is a two-dimensional rigid block and the driving and resisting forces acting on it are balanced. The foundation pressure distribution is considered to be linear. Engineers should complete the gravity method before proceeding to more rigorous studies since it provides reasonable estimates with low computational costs.

No further analysis is necessary if the gravity method indicates that the dam is stable. However, the gravity method does not account for dynamic behavior, such as earthquakes, that can be amplified in the upper section of the dam.

To ensure the safety of dams, structural engineers evaluate them under different load combinations:

1. Usual Load Combination: Includes the standard loads, such as the concrete’s weight, the reservoir and tailwater load, and sediment. Thermal evaluations consider temperature variations that can impact the concrete’s behavior.
2. Unusual Load Combination: Engineers evaluate these conditions with an increased reservoir level to simulate flood loads, typically the probable maximum flood (PMF) for high-hazard dams. A PMF event can be on the order of a one in 10,000-year event or even as high as a one in a million-year event, meaning that every year, there is a one in 10,000 chance of the PMF occurring or a one in a million chance. For low-hazard dams, engineers look to ensure stability for floods up to and including the 100-year flood.
3. Extreme Load Combination: Focuses on seismic events, considering the maximum credible earthquake for high-hazard dams, which typically range from a 2,000-year to a 10,000-year event.
4. Post-Earthquake Load Combination: Considers the damage that may occur during an earthquake to ensure the dam’s stability in the aftermath of the seismic event.

Understanding potential failure modes (PFM) is crucial for a structural engineer evaluating the behavior of concrete dams. PFMs are hypothetical chains of events that could lead to unsatisfactory dam performance. PFM analyses help identify key failure modes and the events that trigger them.

PFMs can be classified into three stages:

1. Initiating Event: The initiating event can include normal operations, material defects, floods, or earthquakes.
2. Progression: Engineers define the steps leading to the failure, such as increased loading on the dam or crack development.
3. Failure: The result may involve the dam’s structural failure or an uncontrolled, unintended reservoir release.

There are several PFMs that engineers assess:

1. Dam Internal Instability (Overstressing): This occurs when the dam’s concrete exceeds its allowable strength, leading to cracking or crushing. For instance, increased reservoir loads during a flood can trigger internal instability.

2. 2. Dam External Instability: Unlike internal instability, separation occurs at the dam/foundation interface, increasing uplift and potentially leading to failure.

3. Dam Overtopping: This happens when excess water flows over the dam crest, causing erosion at the toe and potentially leading to sliding or overturning failure.

A potential failure modes analysis (PFMA) workshop helps dam owners and experts identify PFMs and understand how dams function and could fail. It also informs dam safety programs and highlights areas that require attention. The PFMA process involves reviewing relevant information, including design reports and inspection data, to create a comprehensive surveillance and monitoring plan.

A dam safety surveillance and monitoring plan (DSSMP) outlines observations needed to evaluate the dam’s condition. Its primary goal is to detect design inadequacies or changes in behavior that could lead to catastrophic failure. Regardless of size or hazard level, every dam should have a DSSMP tied to identified failure modes. This plan is crucial for maintaining dam safety and can include monitoring instrumentation and threshold limits.

Structural engineers have various analysis options at their disposal, depending on the problem’s complexity. Hand calculations, spreadsheets, and Mathcad analyses are suitable for more straightforward tasks and serve as quality checks for more elaborate studies. For more complex scenarios, engineers use two-dimensional and three-dimensional finite element analyses. These models simulate real-world dam behavior, accounting for thermal and dynamic loading in addition to static loads.

Structural engineers play a critical role in maintaining and improving dam safety, helping these vital structures continue to provide essential services to our communities and the economy. Through structural analysis, evaluating PFMs, and developing surveillance and monitoring plans, these professionals contribute to the resilience of our nation’s dams. As we continue to address pressing infrastructure challenges, understanding the intricate work of dam structural engineers becomes increasingly important for safeguarding our quality of life.

The webcast on which this blog was based, Understanding the Dam Basics: The Role of the Structural Engineer in Concrete Dams, offers additional content about dam structural engineering work. You can participate in this free on-demand webcast and earn one professional development hour (PDH) and one Construction Manager Certification Institute (CMCI) credit.

To learn more about related dam safety topics, such as failure modes and risk mitigation, explore Gannett Fleming’s webcast library. One PDH is awarded for each webcast you complete.

• Dam and Levee Ownership – Risky Business.
• Dam Performance Monitoring: Maintaining Data Quality and Trust in an Industry of Exciting Technological Advances.
• Dam Breach Modeling: State of the Practice.