A 40-meter wave devastated Fréjus in 1959 when the Malpasset dam collapsed, revealing a hidden geological fault, not a concrete failure, as the real cause.
Malpasset dam collapse in 1959
On December 2, 1959, the Malpasset arch dam collapsed during its first filling, releasing 50 million cubic meters of water into the Reyran valley. The disaster resulted in 423 deaths and missing persons, with around 7,000 people affected.Cause of the collapse
The dam did not fail due to a weakness in its concrete structure. Instead, a hidden geological fault beneath the left bank caused the collapse. A gneiss rock block, lifted by water pressure in fractures, slid and undermined the dam’s foundation.Design and construction of the Malpasset dam
Designed by engineer André Coyne, the dam was 66 meters high and 222 meters long, constructed at a cost of 588.45 million francs. It was intended to provide irrigation and water supply for the Var region, relying on the surrounding rock to support its structure.Geological factors behind the disaster
The left bank of the dam was built on gneiss, a metamorphic rock with natural fractures. These fractures created a wedge-shaped block that slid under the pressure of infiltrating water, a phenomenon engineers call underpressure. This was a geological trap that early rock mechanics failed to detect.Concrete arch wrongly accused
For years, the failure of the dam was wrongly attributed to a structural flaw in its concrete. However, expert analysis showed the concrete was intact. The dam collapsed because the ground beneath it gave way, not because of a design or construction error.Legal and engineering consequences
The Court of Cassation ruled in 1967 that no fault had been committed, ending the legal case. This decision highlighted the limitations of rock mechanics in 1959. The disaster led to a permanent change in dam engineering, emphasizing the importance of studying the geology of support areas.
