It was 92 years ago today.
This week we get an extra story of engineering failure courtesy of EBL. The St. Francis Dam Disaster. This is the worst US civil engineering failure of the 20th Century.
The St. Francis Dam, was a curved, concrete gravity dam about 40 miles from Los Angeles. It was constructed between 1924 and 1926 and it failed March 12-13, 1928. 400 people died and property damage was estimated to be $7 million, in 1928 dollars.
Click thru the link above for a short video (about 6 and a half minutes) on the dam, the need for water in Los Angeles and William Mulholland, the man who oversaw construction of the dam, some links to info on the dam, and a musical-interlude-video, “St. Francis Dam Disaster” by Frank Black and the Catholics. (Yes, that Frank Black, AKA Black Francis.)
For more details, we can consult Damfailures.org – Case Study: St. Francis Dam (California, 1928).
While the dam’s upstream face exhibited a nearly vertical profile, the downstream side was equipped with a stair step design that resulted in base and crest thicknesses of 175 and 16 feet, respectively. The main structure reached a height of 205 feet and spanned 700 feet along its curvilinear crest.
It was supposed to impound a 1-year-supply of water for the City of Los Angeles.
I suppose I should not be shocked that one part of the design was changed, while the impacts of that change were not accounted for. It is not the first time I’ve run across that, and the opinion of the folks in charge is usually (always?) the same; it is never feasible to start over, even though (as in this case) you should start over. Or at least make some major changes.
Multiple instances of poor judgment by Mulholland and several of his subordinates significantly contributed to the cause of the failure of St. Francis Dam. Plans for the dam were based upon those previously prepared by Mulholland for the Mulholland Dam with little regard for site-specific investigations. When these plans were finalized and after construction began, the height of the dam was raised by ten feet on two separate occasions in order to provide additional reservoir storage needed to sustain the growing community surrounding the dam. Although these modifications increased the dam’s height by twenty feet, no changes were made to its base width. As a result, the intended safety margin for structural stability decreased significantly. Mulholland’s team recognized this effect, however the engineering analysis, acquiring of additional materials, and extended construction time to properly mitigate the height increase were considered to be too costly to the project and to those stakeholders who were financially invested in the completion and operation of the dam.
Those damn engineers and their numbers. Don’t they know how much it costs to build a dam? And it’s only a little higher. What could go wrong?
Of course I’m sure that none of the those financial stakeholders lost lives or property when the dam succumbed to uplift failure.
It isn’t clear that accounting for the changes in height would have saved this dam, because underlying aspects of the geology had been ignored. It was considered another detail, not worth worrying about, and besides we built dams like this before and they are fine. Or something.
Although opinions vary, more recent and more thorough investigations assign the ultimate failure mode to weakening of the left abutment foundation rock due to the saturated condition created by the reservoir which essentially re-activated a large landslide that combined with a destabilizing uplift force on the main dam caused failure to initiate at the dam’s left end. In quick succession as catastrophic failure was occurring at the left end, the maximum height section tilted and rotated which destabilized the right end of the main dam causing catastrophic failure at the right end as well.
Mulholland was entrusted with the dam’s design and construction, not because he was a brilliant engineer, but because he was well known for having completed other water projects for Los Angeles. This is also not the first time that “reputation” was used to substitute for “engineering qualifications.” And so the dam looked like a previous dam he had built, that was in a geologically different area. Details.
Small changes in the details can destroy a project. This is not a lesson we learned in 1928 – because it has come back again and again. I doubt that the powers-that-be, or the media talking heads know that even now. I mean, they can’t divide 500 by 327.