Every year or two, there seems to be a storm event that leads to a dam failure. The dam often fails due to inadequate spillway capacity and the dam is overtopped. This was the case at Oroville, which we have written about before. The emergency spillway overtopped and scoured out the downstream rock. When a dam is overtopped, the earthen portion of the dam quickly erodes and drains the reservoir. There are many reasons dams fail, but they rarely fail because they aren’t structurally “strong” enough to hold back water.
Edenville Dam failure investigation
On May 19th, 2020, the Edenville Dam in Michigan failed after two days of heavy rain. An independent team of national dam experts reviewed the dam failure. They concluded the dam did not fail by overtopping, but rather because the earth embankment became overloaded. They determined the actual failure mechanism was static liquefaction of the embankment soils. Put simply, this was a geotechnical issue, not a structural issue with the dam.
Liquefaction explained
Liquefaction is generally tied to earthquake events. During an earthquake, the ground vibrations put saturated soil particles in motion, and they lose particle-to-particle contact. Without particle contact, soil loses strength and cannot support loads from building foundations or water forces on a dam. In the case of static liquefaction, the saturated soils are loaded to their peak strength while the soil consolidates (or strains). As the soil strains, it cannot sustain the strength under loading and collapses internally. This is usually happens with round fine sand particles.
While liquefaction is typically a result of earthquake events, this was not the case with the Edenville Dam. The investigative team reviewed a video that showed the reservoir rose to within about two feet of the dam’s crest, and then the downstream slope quickly failed, releasing water downstream. Post failure testing of the embankment soils confirmed that the sands were very susceptible to static liquefaction.
Looking ahead
The Edenville dam failure will likely raise concerns with other similarly constructed dams. The focus will be on dams constructed with fine sands and those with marginal spillway capacity that can’t pass large rainfall events, which can result in a dam becoming fully loaded. This event will change the way we evaluate these types of conditions in the future.
Dams are critical infrastructure that communities rely on to keep people safe. It is of utmost importance that they be reliable and functioning correctly. We as engineers can take these events as vital learning opportunities. By striving to understand failures and apply lessons learned, we can better provide safe, reliable infrastructure for the communities we serve.
