Vajont Dam failure – slope stability problem
Vajont Dam was completed in 1961. It is located 100 km north of Venice, Italy. When
completed, the doubly curved arch dam was, at 265.5 metres high above the valley
floor, the tallest thin arch dam in the world. It measured 27 metres (89 ft) thick at the
base and 3.4 metres (11 ft) at the top. The chord of the dam was 160 m, and the
volume of water retained by it was 115 million metres cubed.
The Vajont Valley is a deep, narrow gorge. It had been stated that the geological
detail of the valley was fully understood. The company building the dam, stated that
the geology had been studied, including analysis of ancient landslides, and that it was
believed to be sufficiently stable.
In general the valley’s geological structure is considered to be a syncline cut by the
valley (see figure below). The syncline is based in middle Jurassic limestone, overlain
with successive layers of upper Jurassic limestone with clay and Cretaceous
limestones.
The form of the strata and the possibility of old slides in this area led to a lot of
discussion about whether the valley walls were stable enough considering the above
structure.
The dam design team were of the opinion that a landslide was very unlikely to occur
due to two aspects: 1) the asymmetric form of the syncline was expected to act as a
natural break on possible slope movements, and 2) the quality of the in-situ rock, as
detailed in seismic surveys, which appeared to be very firm in-situ rock with a high
modulus of elasticity. Areas of weakness were not identified in the three test borings
that were conducted.
Smaller slides in the looser surface layers were thought to be likely. However the
designers thought that the volume and velocity of any land movement would be low.
Filling of the reservoir began in February 1960, before the final completion of the
dam (September 1960). By March 1960 the level of the reservoir had reached 130 m
above the level of the river. It was at this time that the first small movement occurred.
Filling of the reservoir continued and monitoring of the movements in the banks was
undertaken concurrently. By October 1960 the depth of the reservoir had reached 170
metres. At this point a rapid increase in the rate of displacement was observed
amounting to approximately 3.5 cm per day. In parallel to this, a huge joint of 2 km
length opened up. The movement encompassed an area about 1700 m long and 1000
m wide and this suggested that a very large landslide had occurred. On November 4th,
with the depth of the reservoir at 180 m, a large failure occurred when 700,000 m3 of
material slid into the lake in approximately ten minutes. This reduced the level of the
reservoir back down to 135 m. Subsequently movement was reduced to approx. 1 mm
per day.
At this point the designers realised that the large mass of the left bank was inherently
unstable. Therefore they reduced the water level behind the dam and refilled the
reservoir under a strict monitoring regime. Calculations showed that catastrophic
failure was unlikely and it was concluded that the left bank of the valley could be
stabilized over time in this manner. Hence the reservoir was filled and slowly emptied
three times.
On October 9, 1963 at approximately 10:35pm, the combination of the third drawingdown of the reservoir and heavy rains resulted in an enormous landslide of about 260
million cubic metres of the valley falling into the reservoir at up to 110 km per hour
(68 mph). This displaced the water behind the dam and approximately 50 million
cubic metres of water swept over the dam in a 250-metre high wave. However only
the top metre or so of masonry was washed away and the basic structure remained
intact.
See following figure for precipitation levels and changes to reservoir level with the
corresponding displacement of the landmass.
What caused the failure?
Initial thoughts were centred on speculation of the location of the sliding surface.
However more recent studies confirmed that it was located in thin (5 - 15 cm) clay
layers in the limestone. Some experts claim that it represents a reactivation of an old
landslide (Hendron and Patten, 1985; Pasuto and Soldati, 1991), whilst others claim
that this was a first-time movement (Skempton, 1966; Petley, 1996).
The increase in level of the reservoir is thought to have resulted in an increase in pore
pressures in the clay layers. This had the effect of reducing the effective normal
strength and hence the shear resistance.
Resistance to movement was created by the chair-like form of the shear surface.
Dropping the level of the reservoir induced hydraulic pressures that increased the
stresses as water in the jointed limestone tried to drain.
It has been estimated that the total thrust from this effect was 2 - 4 million tonnes
(Muller, 1964). It is postulated that failure occurred in a brittle manner and this
resulted in a disastrous loss of strength. The speed of movement is attributed to the
possible frictional heating of the pore water in the clay layers (Voight and Faust,
1982, 1992).