1. Slope Stability Failure
THE VAJONT DAM
Slope Stability Failure

2. INTRODUCTION
The Vajont Dam is located 100 km north of Venice under Mount Toc in the Italian Alps.
Completed in 1961, it was the biggest dam in the world measuring metres high, 27 metres thick at the base and 3.4 at the tip.
It was built as a part of the on-going, post-war development of Italy in order to provide HEP for the rapidly-expanding northern cities of Milan, Turin and Modena.

3. INTRODUCTION
Whilst a proposal to site a dam at this location was made in the 1920's, excavation of the site began in 1956 and the dam was completed in 1960.
The completed doubly curved arch dam was, at metres above the valley floor, the worlds highest thin arch dam. The chord of the dam was 160m, and the volume of impounded water was 115 million m³.

4. LOCATION AND BACKGROUND

5. BACKGROUND
The dam was built across the Vajont Valley, a deep, narrow gorge.
Testing of the Mount Toc, which took place after the project had already been approved, found that the mountain had a tendency to free huge amounts of rock, causing unexpected landslides.
The project went ahead…

6. Schematic Cross-Section through the Vajont Valley

7. PRIOR TO COMPLETION
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.

8. FILLING OF RESERVOIR Filling was initiated in February 1960.
The artificial lake was continuously filled and flared in order to test the stability of Mount Toc, the drainage basin and the subsoil. This caused the detachment of rock which caused minor landslides.
It was realized by the designers of the dam that the large mass of the left bank was inherently unstable.

9. CATASTROPHIC FAILURE
At 22:38 GMT on October a 250 million metres cubed rock mass slid approximately 500m northwards at up to 30 m/sec.
The mass completely blocked the gorge to a depth of up to 400m, and it traveled up to 140m up the opposite bank.
Movement of the landslide mass ceased after a maximum of 45 sec.
At the time the reservoir contained 115 million m³ of water. A wave of water was pushed up the opposite bank and destroyed the village of Casso, 260 m above lake level before over-topping the dam by up to 245m.

10. CATASTROPHIC FAILURE
The water, estimated to have had a volume of about 30 million m³, then fell more than 500m onto the villages of Longarone, Pirago, Villanova, Rivalta and Fae, totally decimating them.
A total 2500 lives were lost.
However the dam was not destroyed and is still standing today. The by-pass tunnel is used for the generation of HEP.

11. CAUSES OF FAILURE
Since the catastrophic failure, a huge range of work has been undertaken on the causes of the failure.
Initially the was a large amount of speculation about the location of the sliding surface, recent studies have confirmed that it was located in thin ( cm) clay layers in the limestone.
It is claimed by some that as such it represents a reactivation of an old landslide whilst others claim that it was a first-time movement.
It is likely that increasing the level of the reservoir drove up pore pressures in the clay layers, reducing the effective normal strength and hence the shear resistance

12. FAILURE OF DAM
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.
Remarkably the dam remained unbroken by the flood.

13. Credits Chief Whip - Paul Rogers Dr. of Love - Alan Mathews
Music - Laura Hartmann
Editor - Paul Conlon
In Association with Group 14 Productions