Civil Engineering Reference
In-Depth Information
(1963)
A 276 m (900 ft) high double-arched dam across the Vajont River valley in Northern
Italy was constructed between 1957 and 1960. The dam created a reservoir with an
estimated capacity of about 169 million cu m (220 million cubic yards). In 1959
concerns about potential slope stability problems in the reservoir were raised, which
resulted in further analyses being undertaken. These studies confirmed that there was
a slide problem; however, there was disagreement as to the volume of material which
would be involved in a slide. Predictions ranged from relatively small volumes
associated with local surficial movements 10 to 20 m (33 to 66 ft) deep up to volumes
of the magnitude of the actual slide as a result of deep-seated movements.
Recognition of potential slope stability problems resulted in the installation of
a monitoring program in 1960 and a staged reservoir filling schedule. It was
considered that the rate of movements could be controlled by raising and lowering the
reservoir water level. Results from the monitoring program over the next 3 years,
which relied primarily on devices to detect surface movements, did confirm a
relationship between the reservoir level and the slide mass movement; however, it
failed to give a warning of the rate at which the failure ultimately occurred.
On October 9, 1963, the southern rock slope of the reservoir failed over an
approximately 2 km (1.2 miles) length. Movement rates of the slide mass of
approximately 275 million cu m (360 million cubic yards) during the failure were
estimated to be of the order of 25 m per sec (80 ft per sec) as opposed to the typical
rates recorded during the previous three years of monitoring of less than 1 cm per day
(0.4 in. per day) to at most 20 cm per day (8 in. per day) on the day of the failure. The
massive slide mass came to rest approximately 360 m (1,180 ft) laterally, and 140 m
(429 ft) upward, on the opposite bank of the reservoir (Figure 2-1). The top of the
slide mass was 160 m (525 ft) above the crest of the arch dam. At the time of the
failure, the reservoir was about 66% full and contained an estimated 115 million cu m
(150 million cubic yards.) of water. The water level had been lowered in a controlled
fashion by approximately 10 m (33 ft) in the preceding two weeks. As the slide mass
plunged into the reservoir, the water was displaced over the dam crest in a stream
estimated to be up to 245 m (800 ft) above crest level. Five villages and 2,040 lives
were lost.
Lessons Learned
The massive slide into the Vajont reservoir yielded important lessons in regard to the
analysis and monitoring of slope movements. The difficulty of predicting when a
slide mass will accelerate or fail became evident and the difficulty of estimating
changes in states of stress and strength during sliding was reinforced.
