Sixty years ago : the Malpasset Dam disaster.
It was a marvel of modern civil engineering. Sixty-six meters high, two hundred and twenty two meters wide. A slender (6,5 m at the base, only 1,5 m at the top), innovative, elegant arch shaped concrete work of art, designed by France’s -and the world’s - undisputed expert in dam construction, André Coyne.
The Malpasset Dam had been built in a rather dry area in southern France, on the Reyran river, Var Department. Its purpose was to collect a water reservoir for drinking water and irrigation. The dam had been completed in 1954, and since, the reservoir was filling up.
The filling up lasted until November 1959. Continuous rainfall made it reach its maximum design level for the first time. On December 2nd 1959, at 9:13 p.m., the Malpasset Dam, collapsed. Within minutes, a 40 m high wall of water made its way through the valley. It swept through the city of Fréjus, killing about 423 people.
Was there a design failure? Had André Coyne overestimated his own expertise and taken too much risks? The Malpasset Dam was reputed as ‘the thinnest dam in Europe’, as ‘hardly an egg shell’ in the river valley.
Investigations showed however no errors in Coyne’s design, properly. The advantage of arch dams was, that they can be built so thin and still be technically safe. The problem had been in the soil.
The name of the dam site should have been a warning. ‘Malpasset’ indicates a place difficult to cross (‘mal à passer’). Because of landslides, of friable grounds, of dangerous rocks. Actually, geologists deemed the whole valley as a difficult area for dam construction. But politics wanted the dam.
The subsoil of the dam consisted of gneiss, a kind of rock comparable to granite. Gneiss, like granite, normally offers a solid foundation, impermeable for water. But under the Malpasset Dam, the gneiss was very brittle, particularly under the left bank of the valley. The brittle structure allowed water from the reservoir to penetrate into the rock under the reservoir. Under the dam itself, however, the gneiss had been compressed by the weight of the concrete structure, down to a fault which had not been detected before the disaster. The compressed rock and the fault, created an impervious shield for the infiltrating water. The compressed gneiss acted as a sort of subsurface continuation of the dam, against which the infiltrating water exerted an upward pressure. This underground dam, compressed, but still of brittle gneiss, had much less resistance against such pressure, than the dam itself.
When, on December 2nd 1959, the water in the reservoir rose to its maximum height, the upward pressure of the infiltrating water on the compressed gneiss became so high, that the brittle rock could no longer hold. A block of rock moved and the left bank side of the dam got detached from its rock foundation, and subsequently broke under the pressure of the reservoir.
The ruins of the Malpasset Dam still stand in the Reyran river valley. Particularly the side at the right bank. At the left bank, most of the structure has completely collapsed.
The disaster is since often cited as a classic case of miscommunication between engineering and geological expertise. A brief geological investigation had determined the gneiss nature of the soil, which offered for the engineers, by experience, sufficient guarantee for a stable foundation. No detailed geotechnical investigations had been carried out at the building location of the dam properly. Some geologists had expressed doubts about the suitability of the local bedrock to serve as a dam foundation. But geologists only make a diagnosis, engineers make calculations. In that professional culture, hard numbers prevail over qualitative estimations. So, the geologist’s advice had been overruled. It must also be pointed out that neither rock sampling techniques, nor knowledge of rock mechanics were at today’s levels, but the disaster was an incentive to improve these techniques, and to instigate a dialogue between the geology and geotechnics specialists, and the building experts.
André Coyne, devastated by the disaster, died a few months later.
(Incidentally, these days, the Var Department, is once more struck by heavy rainfall, and suffers from inundations).