Malpasset Dam Failure

Odds are, you don't know as much as you think you do. Malpasset Dam (France, 1959) Pronounced "Mal-pa-say."

The image is from Wikimedia Commons. Click for a larger view and more information.

Construction started in 1952. Filling of the reservoir started in 1954. Failure occurred December 2, 1959 just before the reservoir was completely full. 421 people were killed by the flood.

At the time of completion in 1954, it was reported as the thinnest arch dam of its height (218 feet) with a maximum thickness of 22.2 feet. The dam was equipped with one un-gated, notched spillway at the center of its 736-foot-long crest.

The dam was situated on a type of gneiss (metamorphic rock) which has proved to be impervious enough to water to make reservoirs possible. But this particular gneiss had series of very thin layers (foliation - from the Latin for leaves) which made the foundation unstable.

Geological investigations that took place after the dam’s failure revealed that it had been built on a gneiss formation with a foliation structure exhibiting a slope of thirty to fifty degrees in the downstream direction of the dam. In addition, a fault oriented perpendicular to the river was discovered just downstream of the dam.

The foliation pattern of the foundation in combination with the presence of the fault and the forces associated with the water accumulating behind the dam caused the gneiss along the left abutment to enter a compressive state in which the permeability of the formation decreased with the increasing pressure behind the dam. Uplift pressure at the abutment caused by this phenomenon increased with the filling of the reservoir

The height of the dam was higher than the terrain on the left side, and so a thrust block was added to make up the difference. Uplift caused the thrust block to move, and the dam to fail.

A more complete report on the failure is available at the following link. The traps behind the failure of Malpasset arch dam, France, in 1959 I think this was written by someone whose first language was French. Or maybe it is just a very dry technical treatise on the geology of dam foundations. I could not get through all of it.

One of the things that paper makes clear, is that the assumptions of the day lulled the engineers and builders into feeling confident that the dam would last a long time. In other words, the moral of the story is: You don't find problems that you don't know to look for.

This Movietone News footage from the UK, shows images of the aftermath. (The footage of a dam bursting is, I think, lifted from a Hollywood film.)