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The Vaiont Dam Disaster

The Vaiont dam, one of the highest in the world, was completed in 1961 and is used to generate hydro-electric power for the rapidly expanding northern cities of Milan, Turin and Modena. After heavy rains in 1963, landslides into the Vaiont reservoir caused the stored water to spill over the dam, sweeping away the village of Longarone and flooding nearby hamlets. The Vaiont 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 cubic meters detached from one wall and slid into the lake at velocities of up to 30 meters per second. As a result a wave over topped the dam by 245 m and swept onto the valley below, with the loss of about 2500 lives. Remarkably the dam remained unbroken.

The slip surface of the slide was along weak bedding planes in the dipping limestone valley. Inadequate geological surveying of the mountain surrounding the projected lake contributed greatly to the disaster. The measures taken by the authorities,(lowering the water level in the lake after prolonged rainfall), while intended to stop movement of the hillside, actually made the situation worse.

As the valley filled with water after completion of the dam in 1960, an ancient landslide on its upper southern side, adjacent to the dam, began to move again, in episodes of slow creeping movement. It was later established that this was due to groundwater, unable to escape into the floor of the now - flooded valley, saturating a layer of clay within the rocks beneath it. This accumulation of water high in the slope was most marked during periods of heavy rain, although the association of heavy rainfall and creep went unnoticed. However, the presence of the impermeable clay layer in the bedrock was also not recognized at the time and it was assumed that the movement was due to local saturation of the rocks below the level of water in the reservoir, at the toe of the creeping landslide, rather than accumulation of water pressure in the entire mountainside. It was therefore proposed to regulate the movement of the landslide, and thus allow it to settle to a new equilibrium, by lowering the level of water in the lake when an episode of creep was in progress, until the toe of the landslide was no longer saturated and the creeping stopped The reservoir was then allowed to refill and the drainage cycle repeated whenever creeping movement occurred again.

The engineers concluded that if there were any shearing, it would take place such that it would cause a chair like deformation. This would then result in a braking reaction. After analyzing the seismic information gathered, it was determined that the walls had a very high characteristic of elasticity and that even though small slides were likely to occur, the outcome of these slides could be managed. Around February 1960, the dam was filled and only one month later a tiny detachment slide occurred. Significant amounts of mass material continued to slide and eventually on November 4th, 70 000 cubic meters of material slid down into the lake. The team of engineers decided to solve the problem by varying the water level in the reservoir by constructing drainage and bypass tunnels. From October 1961, to September 1963, the engineers who increased and decreased height of the water controlled the reservoir levels. This was done with the intent to control the landslides. On October 9th 1963, a mass of earth and rock slid down the hill and blocked the gorge completely. The material eventually travelled 140 meters up the opposite bank and finally terminated its movement after 45 seconds. At the time the reservoir contained 115 million cubic meters of water. A wave of water was pushed up the opposite bank and destroyed the village of Casso, 260 meters above lake level, before over-topping the dam by up to 245 meters. The water, estimated to have had a volume of about 30 million cubic meters, then fell more than 500 meters onto the villages of Longarone, Pirago, Villanova, Rivalta and Fae, totally decimating them. However the dam was not destroyed and is still standing today. The by-pass tunnel is used for the generation of hydroelectric power. The main cause of failure of the dam was due to the sliding surface in the clay layers of the limestone of the surrounding area.

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. 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 tons. Failure occurred in a brittle manner, inducing catastrophic loss of strength. The speed of movement is probably the result of frictional heating of the pore water in the clay layers.

The Vaiont slide was a reactivation of an old slide. The age of the old slide is unknown, but it probably occurred in postglacial times but before the period of recorded history of the Vaiont Valley. The evidence for an old slide is strong and diverse. It includes many aspects of the surface morphology, the talus infilling of a reoccurring crack at the head scarp which formed breccias of differing characteristics, the basal rupture plane, and remnants of a previous slide mass or masses on the north side of the valley. The elements of the surface morphology, which are indicative of an old slide, include deranged drainage, enclosed depressions, bulging slopes, and other related alignments and patterns evident in aerial photos.

The slide mass moved upon one or more clay layers which were continuous over large areas of the surface of sliding. Multiple clay interbeds occur near the base of the Lower Cretaceous stratigraphic units and were observed at many locations within the slide. Clays occur on the slide surface, below the slide surface, and also form the matrix of the lower portions of the slide mass. Thick clay fragments and layers are abundant in the debris. Clay interbeds were found outside the slide area in stratigraphic positions corresponding to the surface of sliding of the 1963 slide. The field evidence for the presence of clay along the surface of sliding is compelling because of the number of locations where clays were noted on the failure surface and because of the details of the geology at these locations. It is apparent that clay, which is predominantly calcium montmorillonite or a closely related clay mineral, occurs on the failure surface in many locations.

The failure surfaces of the 1963 slide and the prehistoric slides appear to correspond closely to one or more faults of possible tectonic origin formed much earlier in geologic times. A well-cemented breccia with a grooved and striated surface described here as a “tectonic breccias” is believed to have been formed by this faulting. This breccia can be observed at many locations throughout the exposed portions of the failure surface. The eastern boundary of the slide appears to have been formed by one or more lateral faults associated with or postdating the decollement-fault already mentioned.

The great majority of the slide both east and west of the central Massalezza Ditch moved as a unit. The evidence for this is the surface morphology of the slide and the geological features of the area mapped before and after the slide by Rossi and Semenza. A secondary slide movement formed an area called the Eastern Lobe. This movement was presumably triggered by the loss of toe support caused by the movement of the main slide. The resulting unstable mass overran a large area on the uphill side of the eastern part of the slide. As the main slide came to rest, differential movements developed within it as a result of differences in the geometry of the valley in the toe areas and differences in the momentum of various sections of the slide mass.

A significant area of pronounced karstic and/or combined karstic and glaciated terrain exists above the slide near the top of Mt. Toc. Evidence of minor and incipient karstic terrain is found just above the slide and on its western boundary. The bedding in these areas also dips towards the slide at angles of 13 degrees to 45 degrees or more. Solution features were observed at three areas immediately below the main surface of sliding. Undoubtedly more solution features existed. This evidence strongly suggests that the conditions were present to enable the transmission of high water pressures developed due to infiltration from precipitation or snowmelt on the mountain above. These high water pressures could therefore develop along the surface of sliding.

High groundwater pressures with respect to the reservoir levels were measured in piezometer P-2 in the vicinity of (probably just above) the failure surface. These measurements were taken prior to the slide and apparently before sufficient slide movement occurred to damage the piezometer. This water pressure fluctuated both with changes in the reservoir level and with rainfall. Initially the piezometer level in P-2 was 90 meters above reservoir level. This represents a water pressure difference which was probably lower than the real difference because the piezometer tip was not well sealed. Also, the 90 meter difference was observed in a period of low to moderate rainfall and could have been higher in periods of higher rainfall.

The lower permeability of the clay layers and the higher permeability of the intervening limestones and cherts must have combined to significantly increase the hydraulic conductivity along the bedding relative to that across the bedding. This effect results in a near classic case of an inclined multiple-layer artesian aquifer system at and below the surface of sliding. Such a system would be expected to produce the high piezometric levels observed at P-2.

Landslides are rock, earth, or debris flows on slopes due to gravity. They may take place on any topography depending on the conditions of soil, humidity, and the slant of the ground. Important to the normal development of the earth's shell geology, landslides provide a way to restructure soil and deposits by sudden give ways or in slow slides. Landslides are also know as mud flows, debris flows, earth failures, slope failures, etc., they can happen in natural ways such as earthquakes, heavy rain, and flooding. They can also be caused by man-made action such as overdevelopment of land, grading, and ground cutting and filling also through many other ways. Because landslides can be triggered by man-made activity and by natural, geological activity, landslides can occur in urbanized places, non-urbanized places or any where the land was changed for road ways, home construction, utilities, other construction buildings, and even from landscaping or altering your own backyard. They occur everywhere in the United States, although some smaller than others and less damaging, some places are marked as hazardous natural disaster areas.

Landslides can be categorized by 3 main factors that may occur on the Earth’s surface that control the type and the rate of slope movement. The first factor is Slope gradient, which means the steeper the incline of the terrain; the more chance there is of mass wasting to happen. Second is slope consolidation, this refers to deposits and cracked or badly cemented rocks and the deposits are fragile, and this means there is a better chance of mass wasting. Finally water, if the terrain substances are soaked in water, they may lose structure and slide easier.

When we can see that rocks have fallen and/or mountain side materials move as a consistent chunk, we can determine that a side has occurred. There is more than one type of slide however one of the most known types of slide is called a slump. This slump happens when a piece of hillside slides or falls down because of gravity. A slump has a distinguishing form, with a cliff at the top of the slump, and a lump of matter that can be called the toe of the slump at the bottom of the slump. A slump is the type of mass wasting that happened in the Vaiont Valley.

After the Vaiont Dam Disaster three engineers were found guilty with a sentence of six years for multiple counts of manslaughter. There can be many preventative measures taken to avoid, or decrease the likely hood of a disaster of this magnitude from happening again. Before any structure that is dependent on geology, is designed, see if the location is suitable. Investigate the terrain, the likelihood of plate movement, and the types of rock substructure present. Evaluate the strength of the materials, administer proper fill tests in which the environment of the valley could be recreated providing more accurate and relevant statistics. Anticipate the trends that occur in the area, that is since landslides were known to happen, the dam should have been designed to anticipate a large-scale landslide, even though they weren't a normal occurrence. This area had experienced landslide activity before, so before completion of such a large potentially environmentally altering structure, the safety of the residents should have been considered a priority.

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