What Does a Dam Structural Engineer DO, Anyways?

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If your exposure to the world of dams is limited to flownets or calculating the triangular distribution of water load in one of your undergraduate classes, maybe a childhood trip to the Hoover Dam, or recreation at your local reservoir, you might be wondering what exactly a dam structural engineer does. You are not alone! Many people wind up in this industry by chance.

The United States has more than 91,000 dams, with an average age around 62 years old. Dams serve a variety of purposes, including storing drinking water, flood control, recreation, and hydropower. Every four years, the American Society of Civil Engineers releases an infrastructure report card which aims to depict the condition and performance of American infrastructure, assigning letter grades based on the physical condition and needed investments for improvement. While the overall grade for America’s infrastructure was a “C-,” Dams received a solid “D.” Part of this is due to the fact that over the last 20 years, the number of high-hazard potential dams has more than doubled.

The hazard classification reflects the hazards downstream of the dam: the potential consequences if the dam were to fail, laid out in Figure 1. High hazard dams are dams that have the potential for economic loss as well as the potential for loss of life if the dam were to fail; these dams are held to the most stringent safety standards. The hazard classification can change over the lifetime of the dam. As populations continue to grow downstream, more dams have the potential to become high hazard structures.

The oldest dam in the U.S., Old Oaken Bucket Pond Dam in Massachusetts, was completed in 1640. The early 1900s ushered in an era of “big dam” building in America as demands for water storage and electricity increased. Dam construction increased during the Great Depression and spiked the decade after World War II was over. Dam construction peaked in the 1960s and decreased rapidly after the 1970s.

Gravity dams rely on their self-weight to maintain stability and retain the reservoir they impound. Gravity dams have been around for thousands of years. Many older gravity dams were constructed with large masonry blocks. Current construction practices utilize mass concrete, which relies on the material properties of the concrete; steel reinforcement is generally not used.

Arch dams significantly cut back on concrete material and rely on the compressive strength of concrete. Due to arch action, the arch efficiently transfers loads into the abutments and foundation utilizing its unique shape and geometry. Arch dams can be thin or thick, and some are double curvature, meaning they have curvature in the vertical and horizontal directions.

In a slab and buttress dam, an inclined reinforced concrete deck slab holds back the reservoir load. A series of corbels and buttresses on the downstream side support the upstream sloped slabs. Slab and buttress dams are often referred to as Ambursen dams. Neil Ambursen patented a reinforced concrete design for this system in the early 1900s. This technique was used to construct more than 100 slab and buttress dams in North America.

Embankment dams are typically comprised of compacted earthen material. Most have a central core composed of an impermeable material to stop water from seeping through the dam. This type of dam is well suited for sites with wide valleys and can be built on hard rock or softer soils.

When thinking of structural engineering, the various codes and standards that typically come to mind include ASCE 7, the International Building Code, AASHTO, the AISC steel manual, American Concrete Institute (ACI), as well as wood and timber standards. Concrete dam analysis relies on guidelines developed by the Bureau of Reclamation, Federal Energy Regulatory Commission (FERC), and U.S. Army Corps of Engineers. These guidelines rely on fundamental engineering principles and place a significant emphasis on engineering judgment when evaluating these structures.

The Bureau of Reclamation has Design of Small Dams as well as Best Practices and several engineering monographs. The FERC produces chapters pertaining to several types of dams and associated features and appurtenant structures, such as Chapter 3 which covers Gravity Dams and Chapter 11 which covers Arch Dams. The U.S. Army Corps of Engineers also publishes a variety of engineer manuals for the design and evaluation of dams. These guidelines lay out recommended factors of safety associated with key failure modes.

The gravity method of analysis is used to analyze concrete dams. This method assumes that the dam is a two-dimensional rigid block, and the sum of the forces and the sum of the moments equal zero. The foundation pressure distribution is assumed to be linear. The gravity method of analysis should be completed before proceeding on to more rigorous studies since it provides good estimates at low computational costs. In most cases, if the gravity method of analysis indicates that the dam is stable, no further analysis is necessary. The drawback to the gravity method of analysis is that the analysis does not consider dynamic behavior characteristics which can magnify the effects of earthquake ground motions in the upper section of the dam.

The usual load combination is the normal, everyday load the dam sees. This includes the weight of the concrete, the load of the reservoir and tailwater, and sediment. Some evaluations also include thermal evaluations that look at how the ambient and reservoir temperatures impact the overall behavior of the concrete. Another usual load combination may include the addition of an ice load for reservoirs in areas where the reservoir freezes.

The unusual load combination evaluates the same conditions as the usual load case, with an increase in the reservoir level to reflect the flood load. For high hazard dams, this is typically the Probable Maximum Flood (PMF). The PMF is the worst-case scenario flood and may be anywhere between a 10,000-year event to a million-year event. For dams having a low hazard potential, the project should be stable for floods up to and including the 100-year flood.
The extreme load combination evaluates the seismic event. For high hazard dams, this is the maximum credible earthquake and is typically around a 10,000-year event. Advancements in earthquake understanding over the last few decades have shown engineers that seismic hazards may be higher than originally thought when many of the dams in the U.S. were designed. Current seismic loads can be double or even triple what the dam was designed for.

The post-earthquake load case is the usual load case accounting for the damage that occurs during the earthquake.

Potential failure modes or PFMs are the hypothetical chain of events that could lead to unsatisfactory performance of the dam and are a crucial step in understanding the behavior of a concrete dam. The analyses structural engineers perform parallel the potential failure modes; think of the failure mode event tree as an outline for the evaluation of the problem.

In concrete dams, concrete overstressing and rotational and sliding stability are the basic failure mechanisms typically evaluated.

In mass concrete, when evaluating for concrete overstressing, the evaluation compares the computed stresses from the analysis against the allowable strength of the material. If the computed stress exceeds the capacity, then the concrete is assumed to crack or crush.

Rotational and sliding stability evaluates sliding stability of the dam along analysis planes based on minimum allowable factors of safety. Evaluations along the dam/foundation interface are often referred to as global stability. Analysis of sliding planes within the dam section evaluate internal stability. Depending on the foundation material, sliding planes within the foundation may also be evaluated.

Developing a potential failure mode consists of three specific parts:

During a potential failure modes analysis (PFMA) workshop, a group of people, including the dam owner and subject matter experts, brainstorm and identify all the PFMs for a dam. Once the team identifies the PFMs, the progression is developed for all relevant failure modes. Through this exercise, all participants gain a better understanding of how the dam functions and could fail. Dam owners can use the lessons learned to help shape their dam safety program.

Through the PFMA process, risk reduction measures are identified that can be implemented to help prevent the full progression of the failure mode. The PFMA process is set up to help identify unknown flaws in the dam. Surveillance and monitoring measures should be in place to prevent these failure modes from occurring. There is not a set number of failure modes one should identify on a project. Ideally, enough PFMs are identified to provide a full understanding of the threats to the safety of the project.

Several analysis options are available; the proper method of analysis depends on the problem at hand and the level of complexity needed.

It is good practice to start simple and increase complexity as needed. Typically, the initial step is hand calculations, utilizing spreadsheet or Mathcad analyses, which lend themselves well to the gravity method of analysis. It is important to understand the general behavior of the dam before delving into more sophisticated analysis options. With more complex analyses, it is easy to get lost in the multitude of parameters and the complexity of the modeling software. These simpler methods can serve as a quality check for more sophisticated studies.

The more sophisticated analysis options include two-dimensional and three-dimensional finite element analyses using software such as ANSYS or Abaqus. Finite element models allow for more complex studies, such as thermal and dynamic loading combined with static loads to better simulate actual behavior of the dam and understand the load paths associated with the entire dam-foundation-reservoir system. The need for 2D or 3D depends on a variety of things, such as changes in geometry or capturing material changes in the dam or foundation. A two-dimensional finite element model may be sufficient to capture the behavior of a gravity dam with no changes in geometry throughout its length, while a three-dimensional model may be required to accurately represent an arch dam which relies on its shape to achieve stability.

As discussed previously, the hazard classification of a dam can change over its lifetime.

The arch dam in Oklahoma in Figure 2 is located within the boundary of a wildlife refuge. When it was designed and constructed in 1936, there was no population downstream of the dam and it was designed for minimal seismic loading.

The owner completed a hazard classification study of seven dams in the refuge, which involved developing inundation mapping of the flood waters if the dams were to fail. This study showed a population of about 1,000 was at risk directly south of the dam. This raised the hazard classification to high, now requiring the most stringent safety standards. The understanding of seismicity has also improved in this area, resulting in the need for an updated analysis of record, as the maximum credible earthquake at this site is quite high. The engineering team performed a structural stability analysis of the dam using a 3D finite element model to evaluate the safety of the dam for the increased loads (Fig. 3).

The arch dam in Colorado in Figure 4 was originally constructed as a gravity dam in the late 1930s. An arch dam was constructed on top in 1959, raising the dam to over 200 feet tall. In this example, the foundation is the focus. The geology and the foundation are crucial for the performance of a concrete dam, especially an arch dam. For arch dams, one of the failure modes evaluated is rock block stability. Engineers must evaluate the stability of potential rock blocks in the abutment formed by the intersection of foundation discontinuities. This failure mode has the potential to develop during normal, flood, and earthquake loading conditions. In the development of the PFM, the thrust load from the dam and uplift pressures could dislodge a rock block resulting in a loss of arch support and failure of the dam.

Rock anchors were placed in the foundation in the late 1980s. The role of the structural engineer on this project was to model the dam with a three-dimensional finite element model and evaluate the magnitude and directionality of the thrust loads on the abutment for the various loading conditions to determine if this failure mode was viable or could be ruled out.

For the third example in Figure 5, a slab and buttress dam built in the 1930s had concern of corrosion of the steel reinforcement. Localized deterioration had routinely been observed during regular inspections, so the quality of the concrete was in question. Core samples were taken, and the concrete strengths and reinforcement conditions were evaluated.

Several failure modes associated with structural capacity of the reinforced concrete slabs and corbels were evaluated. Hand calculations were used to analyze the loads on the structure and calculate the nominal capacities of the individual members accounting for various degrees of deterioration. These results were used to determine a proper plan to address deterioration concerns and how to prioritize what regions to focus on first for repair and rehabilitation measures and enhance the service life of the dam.

From the geology of the foundation, the hydrology of the reservoir and the hydraulics of the spillway to the mechanics of the dam’s behavior, a structural dam engineer utilizes the full civil engineering degree. Every dam project is a new and exciting puzzle to solve. While the design of new dams is not as common as it once was, extending the life of America’s critical infrastructure and improving dam safety is a crucial need and provides a rewarding career path.

About the Author

Aimee Corn, PE, M.ASCE, is Sr. Project Structural Engineer at Gannett Fleming specializing in the analysis of existing concrete dams. Corn served on the SEI Board of Governors from 2018 to 2022 and was the 2023 recipient of the ASCE Edmund Friedman Young Engineer Award for Professional Achievement.