Edenville and Sanford Dam colapse Followup 1

[FacEngrPE]

thread815-469386
Grady at Practical Engineering presents the information contained in the FERC Inteport.
What Really Happened At Edenville and Sanford Dams? Practical Engineering

The report can be found here,

https://www.damsafety.org/MI-Interim-Report

and is attached. Geotechs will find the proposed failure mechanism interesting.

 

[GeoEnvGuy]

I think the report focuses on liquefaction where the real culprit is the reason why the crest first settled and cracks developed. This is discussed in section 4.1.2 of the report which it was not able to definitely identify. As per my previous post I believe the independent report supports my initial comments.

It looks like a shallow failure occurred likely due to the high phreatic levels in the dam causing an elevated seepage emergence at the toe of the dam. This then allowed for an internal erosion mechanism to erode the core of the dam and finally cause a collapse of the crest to allow for continued erosion of the dam and large release of the upstream reservoir.

4.1.2
...Another possibility is that the subsidence was the result of an initial instability of the embankment prior to the overall instability failure. The subsidence could have contributed to the development of cracks, allowing water into the embankment, and may also have resulted in some shock stresses within the embankment, which contributed to the triggering of liquefaction.

4.1.3 Reasons for Location of the Failure
...The lack of drains in the failure location could have resulted in a higher phreatic surface at that location than elsewhere along the embankment.

 

[cvg]

no seepage was observed emerging from the toe and no shallow failure was observed

Internal erosion was judged to not be plausible as the primary mechanism of failure. The observed physical characteristics of the failure are not consistent with an internal erosion failure mode:

1. No seepage exiting the ground surface was detected; in fact, no water was detected on the downstream ground surface until just before failure.
2. No turbid water discharge was detected.
3. No evidence of a developing open pipe, sinkhole, or progressive sloughing that might indicate global backward erosion piping was observed.
4. The kinetics of the failure, in particular the global acceleration and velocity of the failure mass, are not consistent with historical observations of internal erosion failures.