An interim report from the Association of State Dam Safety Officials reports that a clear factor in the failures of two dams built in the 1920s was static liquefaction (flow) instability of saturated, loose sands in the downstream section of their embankments. The Edenville and Sanford Dams along the Tittabawassee River in north central Michigan failed on May 19, 2020, after two days of heavy rain, resulting in flooding and property damage.
Static liquefaction has rarely been observed in dam failures and is more commonly associated with earthquakes, according to the forensic team. It occurs when the mobilized shear strength in saturated, loose sand decreases rapidly to values significantly less than the applied static shear stresses, resulting in a force imbalance. The sand quickly liquefied, became mud and then accelerated the velocity of the already fast-moving water with its mass.