It was impossible to foresee what began as heavy rainfall on 18 May, 2020, would end with the brutal failure of both the Edenville and Sanford dams in Michigan, USA.
But the catastrophe could have been avoided, according to conclusions drawn by an Independent Forensic Team (IFT) in their report released in September 2021, by the Federal Energy Regulatory Commission.
“The main culprit here was the Edenville dam,” says Vincent Castonguay, Research Engineer, Seequent. “Had it not failed, the Sanford dam would most probably be still standing today.”
On 19 May at 5:35pm, overwhelmed by the deluge, the Edenville dam breached. It quickly overtopped the Sanford dam downstream, sending a wall of water along the Tittabawassee river.
Floodwaters forced the evacuation of 10,000+ people across the Midlands, Sanford, and Saganar communities, inundated over 2500 buildings, and caused more than $200 million in damage. Amazingly, no lives were lost.
Incredibly, eyewitnesses captured video footage at the exact moment the first dam failed.
“For engineers this is powerful, as we rarely get to see events like this unfold in real time,”
says Castonguay. The forensic team were then able to directly compare their GeoStudio software dam failure simulations with the actual recorded footage.
Their analysis results showed a perfect match and helped determine the cause as a relatively rare dam failure phenomenon – static liquefaction.
Steep embankments and loose soils
Edenville and Sanford were two of four dams owned by Boyce Hydro Power along the Tittabawassee river, built in the 1920s to generate hydroelectricity.
“If the Edenville dam was constructed today, it would not be prone to static liquefaction,” says Castonguay. “But it was built 100 years ago before most of our current understanding of geotechnical engineering and modern dam safety standards existed.”
Today, embankments are constructed slowly from the bottom up in densely compacted, individual layers or lifts.
“We use gentle slopes to increase long-term stability since most soils are naturally unstable on steep slopes,”
says Castonguay. “And during the design phase we have strict control over the types of soil used with constant testing to ensure the properties match or exceed our assumptions.”
For the 190-metre Edenville dam, a big part of the construction issue was the use of a technique called hydraulic filling. “Basically, piling up a heap of loose, saturated, uncompacted material,” says Castonguay.
“The embankment was also quite steep at 1.8: 1 (horizontal to vertical) – a slope ratio that would raise any engineers’ eyebrows, unless it was well-compacted, which it was not.”
Sanford dam was a similar, yet smaller structure to Edenville, with a gated spillway, an emergency spillway, and a powerhouse.
“While arguably not the most stable, both dams were still standing strong with a factor of safety that was sufficient to ensure stability in normal operating conditions,” says Castonguay.
Yet, the IFT’s geotechnical analysis, performed in GeoStudio’s SLOPE/W, SEEP/W, and SIGMA/W, showed that the unrelenting rains caused the reservoir level to steadily rise, worsening the dam’s stability.
“We’re talking loose, steep, saturated sands – the perfect conditions for static liquefaction leading to dam failure,” he says.
Piecing together the static liquefaction puzzle
The extreme weather event raised Wixom Lake, the manmade reservoir impounded by Edenville dam, to levels dangerously higher than ever before.
Waterfront residents whose homes flanked the entire periphery of the lake nervously watched as water reached just 30 centimetres shy of the embankment top.
According to the IFT, the reservoir level increased by approximately 1.8 metres in the two days leading to the failure (0.9 metres beyond the previous elevation record set in 1929).
GeoStudio’s SEEP/W software was used to simulate the rise of reservoir level during the rainfall and its impact on the overall stability of the dam.
“The way the IFT decided to use our software, to my mind, clearly demonstrates the extensive experience and wealth of knowledge of this team,”
says Castonguay. SEEP/W analysis simulates water seepage and transfer that occurs in a porous media such as an earthen dam.
“In this case the IFT simulated the response of Edenville dam to the large rainfall and how that affected the piezometric level inside the structure,” says Castonguay.
SLOPE/W stability analysis used results from SEEP/W to correctly establish where the piezometric level was positioned and then calculate how stable or unstable the dam was at various moments in the event timeline.
“SLOPE/W was extremely useful to understand when stability was more marginal or when it was more stable,” says Castonguay.
Saturated soils pushed beyond their limit
But these two types of analysis alone do not explain the failure. The stability was not marginal enough for failure to occur.
What’s interesting is that SEEP/W and SLOPE/W results were integrated into SIGMA/W to simulate the stress-strain behaviour.
“Specifically, the stress conditions within the dam which, once calculated, helped identify the zone where static liquefaction was likely to happen,” says Castonguay.
The IFT concluded that as the level of Wixom Lake rose, it increased the hydraulic load on the embankment and put more stress on the earthen structure than it had ever withheld before.
The higher level may have introduced water from the reservoir to permeable layers of the upper embankment. This likely caused the slump that formed prior to the failure – alerting eyewitnesses to the exact spot of the impending breach that they were able to catch on film.
“Increasing seepage and increasing pore water pressure had saturated the uncompacted, sandy soils within the structure. The toe of the dam unloading was all that was needed to trigger static liquefaction and the resulting collapse,” says Castonguay.
Innovation to make sense of extreme events
The job of any forensic team is to reconstruct the story by piecing together the evidence and making sense of events.
“This is where GeoStudio as a tool to easily and simply navigate complex challenges really shines,”
says Castonguay. “It’s a scalable solution that can accommodate any type of geotechnical engineering problem, on demand.”
Static liquefaction is a complicated issue, but in this case the IFT were able to easily solve how the large rainfall event affected slope stability and calculate the stress state within the dam.
The water conditions changed, the poor pore water pressure rose because of the rising reservoir, translating to lower effective stresses and lower resistance.
“The beauty of the GeoStudio products, SLOPE/W, SEEP/W and SIGMA/W, sometimes used in sequence and sometimes together, meant the forensic team could easily create a narrative to make sense of the liquefaction phenomena and explain the failure,” says Castonguay.
“Full credit to their imaginative solution – it was a great way to navigate a really complex challenge.”