This is mainly addressed to Roger, but some others maybe interested.  As a civil engineer, it certainly is of interest to me.

The American Society of Civil Engineers recently release their report on the dam failures at Edenville and Sanford.  A couple of stories on their findings.

https://www.asce.org/publications-and-news/civil-engineering-source/civil-engineering-magazine/article/2021/09/static-liquefaction-likely-caused-edenville-dam-failure-report-says

https://www.asce.org/publications-and-news/civil-engineering-source/article/2021/09/22/failures-at-edenville-and-sanford-raise-concerns-about-other-us-dams?utm_medium=email&utm_source=rasa_io&PostID=38199516&MessageRunDetailID=6486574554

Seems like the plausible root cause was uncompacted sand used in the dam while the specifications called for the sand to be compacted.

Non-compacted sand allowed it to liquify when the water pressure on the soil got too high.  A video shows the initial failure.  I had not seen this video until I read this report.  Notice the dam fails from inside the dam and then the water breaks through.  The dam was not overtopped as originally thought.

https://www.youtube.com/watch?v=Hc3u_CHVHJ8

Some of these earthen dams are over 100 years old and were built before engineers understood the engineering properties of soils.  We would never build a dam like this today.

Rob