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:
- 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.
- 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.
- 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.
- 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.