My wife is reading about the dam busters bombers, which set me to reading about dam failures. I found an incredible* presentation about the St Francis Dam failure. It is entitled a “Reassessment of the St Francis Dam failure” This is a dam which burst in 1928 after standing for two years, and took 420 people along, when it did. The (re)assessment is from an engineering and geological point of view, and the writer suggests that there were many flaws in the design and investigation.
The interesting thing is how difficult it is to do a forensic examination of a single known event (a broken dam) with a large amount of relevant information about the initial conditions (photographs, engineering drawings, surveys), and this, relatively soon after the event. This is a real example of geological forensics — in a flood context nogal. The degree of remaining uncertainty after all is said and done is quite remarkable.
An interesting statement which the paper makes, in assessing how all manner of misunderstanding (including geological) contributed to the disaster. It gives an idea of how geologists tend to think:
“We will not identify those geologic features or structures for which we are not specifically looking. We have to have in mind what we are seeking, realizing that we will seldom be able to recognize those features with which we’ve had little prior experience”
I wonder …
- Considering how carefully one has to work with well documented events to reach tentative conclusions, how much confidence should we have in geology that works without any written historical testimony, yet purports to tell us exactly how events unfolded over bzillions of years?
- One of the problems noted in the design of the St Francis Dam is “Hydraulic uplift not relieved on sloping abutments ( a common problem until 1960s)”. As a layman, that means that the water presses down onto the surface below, and that pushes the walls up. I wonder about the dams around South Africa which were built before the 1960’s — Kariba Dam in Zimbabwe was built in the 1950’s. Bert worked on the Kariba dam, and one of the things he was involved with was flushing out crumbling wet material, and replacing it with concrete, because the dam wall began vibrating as the dam filled up (that’s apparently not cool). He said this was because the geological survey up the side of the slope was done to the required depth, but relative to the surface, rather than relative to the lowest point. The presence of porous rock way beneath the dam wall was not known as a result. I wonder if they ran into this problem because of a lack of more recent engineering insights, and I also wonder if the fix for the problem solved other problems at the same time.
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* and when I say “incredible”, I don’t mean that it is not credible.
