Vajont

The Vajont reservoir disaster is a classic example of the consequences of the failure of engineers and geologists to understand the nature of the problem that they were trying to deal with. During the filling of the reservoir a block of approximately 270 million m3 detached from one wall and slid into the lake at velocities of up to 30 m sec-1 (approx. As a result a wave over topped the dam by 250 m and swept onto the valley below, with the loss of about 2500 lives. Remarkably the dam remained unbroken by the flood.

Vajont is located in the south-eastern part of the Dolomite Region of the Italian Alps, about 100 km north of Venice. The chord of the dam was 160 m, and the volume of impounded water was 115 million m3.

Figure 1: Schematic map of the location of the Vajont dam and reservoir. The dam was built across the Vajont Valley, a deep, narrow gorge.

ORDER OF EVENT

1. Prior to the Completion of the Dam it appears that during the construction of the dam the chief engineer was concerned about the stability of the left bank of the dam, and a number of reports were compiled on this during 1958 and 1959, which identified a possible prehistoric slide on the right bank.

2. During the First Filling of the Reservoir Filling was initiated in February 1960, before final completion of the dam (which occurred in September 1960). By March 1960 it was realised by the designers of the dam that the large mass of the left bank was inherently unstable.

Thus it was decided that an attempt could be made to gain control of the sliding mass by varying the level of water in the reservoir whilst controlling the joint water thrust within the rock mass by means of drainage tunnels.

The experiences gained from the second phase of filling and the subsequent draw-down confirmed to the engineers that control of the landslide was possible by altering the level of the reservoir.

Between April and May 1963 the reservoir level was rapidly increased to 231 m and then the engineers changed their opinions and begun to empty the lake and a night of october the mountain fell down!