Building a ‘stronger’ dam

By Tim Wiles

The Lake Delhi dam failure will surely generate much speculation in upcoming months over the contributing factors. I read an article recently in which someone said that the new dam needs to be “stronger.” To a geotechnical engineer who studies soils as building materials, this is quite an understatement.

Stronger in what way? Under what conditions? How much stronger?

Answering these questions for earth dams in general requires an understanding of the factors that are critical to dam design and construction, not just failure. These factors include:

l Foundation stability. Earth dams are heavy. Even small dams are heavier and have a deeper, broader influence on their foundation soils than many buildings. If the foundations soils are compressible, the dam could settle, and a “shorter” dam is more easily overtopped by the flow. Settlement can also cause cracking within the dam. Water flowing through cracks can erode the dam from within, causing a collapse or slope failure. To address these important issues, engineers use soil borings, sampling and testing to evaluate foundation conditions.

l Water pressure. When a dam is working, there will always be more water on one side causing a difference in water pressure. At times, this pressure can exceed the strength of the soil, and then a failure occurs. The tedious work for engineers is considering these pressures throughout the normal life of the dam and during floods.

l Materials. Earth dams are most economical if they can be constructed from nearby soils, but they must have the right properties. Typically, clay soils are used because of their low permeability. Sometimes, composite dams are designed with more-permeable sand materials in certain zones to control seepage pressures described above. Organic material (topsoil, roots) cannot be used since these will decay over time. The economy of using unwanted construction debris (concrete, pavement, etc.) is tempting, but large pieces of debris will create voids or loose zones since it is impractical to compact between them. Destabilizing flow through these zones can cause dam failures.

l Construction. Once suitable material has been selected, it is very important to specify how the material is put in place. Soils should be placed in relatively thin layers (8 to 12 inches) and rolled over with heavy compaction equipment. The denser the material, the more resistant it is to failure. In addition to the density, the moisture content of the soils should be monitored. Soils should be wet enough so that they are more easily kneaded and compacted, but not too wet or they become soft and weak.

l Erosion control. The dam surfaces should be protected from the action of changing water levels, wave action and rainwater flow. Establishing shallow-root plant growth often works for the dry side slopes, and large rocks (riprap) can be placed on the reservoir side near the anticipated reservoir level to protect the dam.

l Maintenance. It's not over after design and construction. To maintain the dam, we still need to monitor settlement, observe signs of seepage, and verify that the riprap is protecting the embankment from waves, just to name a few maintenance issues.

In addition to these factors, overall economics plays a big role. Geotechnical engineers would love to design a super dam that would withstand the 2008 flood and more. But will anyone pay for it? Is it reasonable to design a dam that will withstand a flood event that will never happen?

Flooding in the past three years, or even dating back to 1993, has challenged the definitions of a 100- or 500-year flood. With all these variables, it is easy to see why “building a stronger dam” is not simple. I think the answer is to invest in exploration, design, construction and maintenance to determine an acceptable level of risk at an acceptable cost. Then, perhaps, our dams will perform to our expectations.

Tim Wiles is Principal Engineer at Braun Intertec in Cedar Rapids. Comments: twiles@braun

intertec.com