Abstract
This report analyses the disastrous failure of Vaiont Dam on 9th October 1963. The chronology before the final landslide at each stage is presented in this report. The reservoir level with the velocity of the movements in each stage are also outlined in the report. The factors that caused the giant landslide were in strong debate in the past years in a lot of literatures. However, the actual reasons are still not convinced. In this report, the final landslide will be analysed and personal ideas on the causes of the event will be given based on previous researches.
Introduction
The Vaiont dam is a double-arched dam located on the valley of the Vaiont River in northern Italy, which was the highest dam in the world that time with a height of 276 meters. The construction took place from 1957 to 1960 and the filling started in February 1960 (Muller-Salzburg, 1987). The programme was sponsored by the government, hoping to make a large development in hydroelectricity as the country was exhausted after the Second World War. On 9th October 1963, Italy witnessed a catastrophe which is still a memorable lesson for civil engineers and geologists after over a half century. When around 500 million tonnes of mountainside fell into the reservoir, a massive wave was generated. The wave overtopped the dam and swept away the town that was crowded with people. The tragedy happened in less than 5 minutes and led to the death of around 2500 people (The Guardian, 2000).
Chronology
In March 1960, the first small movements in the Vaiont slope occurred after the first filling and the level of the reservoir that time was 590 m above sea level, which was at the same level as the feet of the old failure surface. However, it was not paid serious attention on because it was just considered as the behaviour in accordance with the change in the weight of water in the reservoir (Genevois and Ghirotti. 2005).
Three months later, when there was a raise of over 10 m in the reservoir level, new small movements were observed near the lake (Semenza and Ghirotti, 2000).(Genevois and Ghirotti. 2005). when the existence of the old failure surface were firstly under consideration (Semenza and Ghirotti, 2000). For the purpose of ensuring the assumption, as well as trying to find the location of it, three boreholes were drilled. However, as they were not as deep as it was expected, the action was kind of useless (Genevois and Ghirotti. 2005).
In late October 1960, a crack of 2.5 km long and a short mass movement with a rate of over 30 mm/day appeared (The Guardian, 2000), which was in correspondence with the result of the second geological survey carried out in summer. This crack, on one hand, verified that the moving slope did not belong to the original mountain but the relic of an ancient landslide (The Guardian, 2000). On the other hand, it also confirmed that the filling of the reservoir was the reason of the reactivation of the ancient landslide (Semenza and Ghirotti, 2000).
On 4th November 1960, a mass of about 700,000 m3 fell down from the western part into the reservoir and created waves of the height of 30 m (Genevois and Ghirotti. 2005). In order to mitigate the high velocity, Muller suggested that the reservoir level should be lowered carefully. In January 1961, the reservoir level was reduced to 600 m a.s.l. and the velocity of movements decreased accordingly until stopped (Semenza and Ghirotti, 2000). To make sure that the reservoir level was not higher than that of the village, a bypass tunnel was built afterwards on the right side of the Vaiont Valley for safety reasons (Semenza and Ghirotti, 2000).
The construction of the bypass tunnel was completed in October 1961. Since then, the level of the reservoir had been increased until it reached 700m a.s.l. in December 1962 and the velocity of the movements raised again. To stop the movements, the lake level was lowered to 650 m a.s.l. in March 1963 (Semenza and Ghirotti, 2000).
In April 1963, the lake level was raised again. When it reached 700m, movements started again. However, as the velocity was not high that time, the water level was raised once more. In early September, a sudden raise in the slope velocity came under observation, which reached 20 cm/day when the reservoir level was lowered to 700m (Genevois and Ghirotti. 2005).
In the evening of 9th October 1963, the tragedy happened. The rate of movements was at a low level, however, the reservoir level reached 710m, which was only 22.5m smaller than the maximum value that can be obtained. The sudden collapse surprised everyone and killed 2500 people.
Causes
The possible causes of the Vaiont landslide various and had been discussed in literatures a lot. Unfortunately, there is still no speculation that was generally agreed with by all experts. In my personal view, based on the previous literatures, the main causes are the existence of an ancient slide, the heavy rain before the final slide, the high velocity of the movements and the negligence of researchers.
The first, as well as the main cause of the landslide in my view is the existence of an ancient slide surface. Hendron and Patton researched the landslide and concluded that the Vaiont Slide was reactivated by an ancient slide, which was not in the recorded history of the Vaiont Valley, based on strong evidence in terms of many different aspects (Hendron and Patton, 1987). For example, Many experts agreed with their viewpoint according to an experienced knowledge that “slides along prehistorically developed surfaces are activated much easier than those in a comparable rock mass without predefined failure structure†(Muller-Salzburg, 1987). Semenza and Ghirotti even suggested that it was the main factor (Semenza and Ghirotti, 2000).
The heavy rain two weeks prior to the final landslide made great contribution to the landslide. On one hand, the rainwater cannot escape because the normal routes were blocked by the water in the reservoir. The adjacent cracks in the old landslide were separated by the trapped water so that the entire mass slid into the reservoir (The Guardian, 2000). On the other hand, it can also be explained in terms of the existence of aquifers. It was put forward by Hendron and Patton as one factor of the landslide. The hydrogeology of the whole area was then re-examined because of this reason (Semenza and Ghirotti, 2000) and it was found that the lower confined aquifer was basically filled by the water in the basin of Monte Toc, in this case, the rainfall conditions. As a result, the water level of the lower aquifer reached higher values than that of the upper aquifer. Therefore, the production of neutral pressures would have led to the decrease in the shear resistance along the failure surface which cause the mass become unstable (Semenza and Ghirotti, 2000).
According to Muller, the final slide was in his expectation based on the monitoring of previous three years, however, the unexpected high rate of the slide moving caused the massive failure (Muller-Salzburg, 1987). According to Figure , it can be easily found that generally the higher the reservoir level is , the higher speed the movements have. However, the situation of the final landslide in 1963 did not suit the theory as the speed unexpectedly greatly raised when the lake level was lowered.
To some degree, it is also agreeable that the deficient research on the slope instability led to the disastrous landslide. Nonveiller believed that if the calculation of the stability were put into action before the construction of the Vaiont Dam, the landslide in 1963 may not happen (Muller-Salzburg, 1987). In the period of the construction of the Vaiont dam, there was no compulsory request for the study on the stability of the slope of the reservoir as it was often not included in the panning of the project (Semenza and Ghirotti, 2000). Therefore, the ancient landslide on the southern slope of Mountain Toc not founded in time, which caused the tragedy in 1963 to some extent (Genevois and Ghirotti. 2005).
Conclusion
In conclusion, although a large amount of investigations and researches have been carried out since 1963, the exact reasons for the tragedy are still uncertain. This report states the factors being the ancient slide, the heavy rain, the high velocity and the disregard of researchers. The failure of the Vaiont Dam caused a lot of loss both in economics and in human lives and is a lesson that must be memorized by all civil and geotechnical engineers.