The Nature and Construction of DamsLower-Sixth General Studies ProjectJon GoddardThis paper is copyright - don't plagerise other peoples work but by all means use my work as a reference. IntroductionI have always been interested in how man uses nature's resources so engineering as a career has always seemed a possibility. Until recently, when I started to do some research, I did not know which branch of engineering would suit me best. As engineering is such a large field the choice was not going to be easy. At first I narrowed the choice down to mechanical and structural engineering which appealed to me most. Further investigation led me to choose structural engineering. Mechanical engineering involved the study of the properties of metals and alloys and how they react to stress and strain, expansion and contraction, and friction and the ways that they can be changed by heat treatment. This all seemed very interesting but I felt that structural engineering would suit me better. This makes use of mechanical engineering skills but also uses surveying and building principles, involving understanding things like concrete construction, structural steel work and traditional brickwork. To further my investigation of this field I chose to do more research in the form of this project on : The Construction of Dams. What is a Dam ?A dam is an obstacle built across a river or a lake to hold back water. The reservoirs which form behind them are used to store water which is used for four major uses:
It is usually the case that a dam performs more than one of these functions. Dams in HistoryThe first dams were built for the irrigation of crops. They were built by piling up banks of earth or by building barriers of wood and reeds; some were made with rocks and stone. The remains of a dam that the Egyptians built across the River Nile, at Sadd el-Kafara 32 km south of Cairo, about 2000 BC can be seen today. About 2000 years ago the Romans built two dams near Merida in Spain to provide water for the town. They are still used today to provide water for irrigation. In the 16th century dam builders in Spain discovered that a curved dam is much stronger that a straight dam and could be built much higher. The Tibi Dam on the Monegre River near Alicante was built in 1589 is still used today for irrigation and is over 44 metres high. During the 19th century engineers learned how to make dams strong enough to withstand the immense power of running water. Larger and higher dams have been built all over the world, often in remote places where construction has been very difficult. Siting a DamWhen it is decided that a dam is to be built for any of the different purposes mentioned above, an important job is the selection of a site. During this process the engineer must consider the purpose of the dam and how much water the reservoir needs to hold. He must find out the quantity of water which flows down the river during the rainy seasons and in times of drought, and whether more water can easily be brought from streams in the neighbourhood by tunnels. One of the prerequisites for a site is a natural basin which will hold the required amount of water without vast amounts of adaptation and which has a narrow enough neck across to be spanned by a dam. When the dam is built, the water level will rise causing flooding of the land directly behind the dam. This may flood farms, villages and transport systems, so the engineer has to consider whether and where these can be moved or new ones built. In 1987 the Indian government had to reconsider its plan to build dams in the Namada Valley when it realised that more than one million people, from 435 villages and one large town, would have to be relocated. Sometimes ancient monuments and wild life are endangered. The temples and statues at Abu Simbel were built by the Egyptians in about 1250 BC. Before the completion of the Aswan High Dam they were cut from the rock face and moved to a higher level above the rising waters of the reservoir - Lake Nasser. Thousands of wild animals had to be rescued and moved to safety when the Kariba Dam was built thirty years ago. The engineer must also consider the natural beauty of the countryside although sometimes new reservoirs may enhance it. Careful thought needs to be given. Will the benefits outweigh the cost, both in monetary terms and physical sacrifices. This demands competent and accurate surveying and a detailed knowledge of soil mechanics and geology besides engineering skill in planning a structure which will work. Planning a DamThe engineer then needs to decide what type of dam can be conveniently and economically built across the outlet, how high it must be to hold the required amount of water, how strong to hold the weight of water and whether the foundations are good. In considering the height, consideration must be given to the amount of land that will be flooded behind the dam. Modern day engineers use computers to calculate the forces that will exert pressure upon the dam. (In the past all the preparatory work was done on paper and in the engineers head. This took a fair degree of skill.) These forces include the weight of the water that will push against the dam and the forces with which the dam and the sides of the valley will push back against the water. All the forces acting together must allow the dam to remain in place with the water against it and perform the function it was built for. Types of DamModern dams fall into two categories - embankment (earth or rock fill) and masonry (concrete)
Generally these are built in valleys which are wide and shallow, and when the rock in the walls of the valley are not hard enough to support the weight of a concrete dam. An embankment dam forms úa massive barrier across a valley. The front of the dam slopes down to the bottom of the valley, and it is covered with grass or bare rock. Often there is a road or other transport system running along the top of the dam. It is made of layers of soil or sand around a central clay or rock core. Gravel, stones or large pieces of rock found in the vicinity can also be used. The weight of the water on the sloping side helps to keep the dam stable. A couple of examples of embankment dams are: the High Island Dam in Hong Kong, the Mica Dam in Canada and the Marchlyn Dam in North Wales made of rock , the 154m high Goschener Dam in Switzerland made of broken stone, and the Aswan High Dam in Egypt.
These are most often built when the valley is deep and narrow. A concrete dam can be tall and thin-walled yet very strong. However the rock in the floor and walls of the valley must be hard, so that it will help to support the base and sides of the dam. There are four main types of masonry dams:-
Building a DamOnce the detailed plans have been drawn up and a cost benefit analysis has been approved, the dam can be built. Consideration must then be given to the financial implications of building the dam. Most dams are built in isolated places so a special camp has to be built to house the workers with all the facilities they will require - a mini town in fact. Complicated transport arrangements must be made for the delivery of materials to the site and for the removal of any excavated material. Cranes, light railways or temporary roads are usually built to solve these problems. They can cheaply and easily carry away any excavated earth and rock and to bring new materials to the point where they are needed. All new materials have to be thoroughly tested before use on the site. Strength and resistance to penetration by water are the most important factors, especially when building a dam. Heavy machinery is now used in the building of dams but in the past all the excavation of foundations and rock supports had to be done by hand! As the dam has to be built across a river bed, the water has to be drained away before work can begin. Sometimes it is possible to divert the river into a temporary channel but, if this isn't possible, a U-shaped cofferdam is built out from one bank into the middle of the river. The water is pumped out from inside the cofferdam and half of the dam is constructed in the dry. When that is finished the cofferdam is flooded and removed and a new one built out from the other bank, inside which the rest of the dam is constructed. Whilst this is happening the river passes through temporary openings or through the sluices of the completed part. The aim of a cofferdam is simply to cut down the inflow of water to the working area to manageable proportions. Water that does get in is led to sumps, away from the working area, and pumped out continuously. It would be far too costly to make cofferdams completely watertight. In order to make sure the section of river below the dam does not run dry, some water must be allowed to flow downstream and not be captured by the dam. In times of flood, the water-level in the reservoir must be controlled to avoid so additional flooding does not occur above it. A spillway over the top of the dam is probably the easiest way of achieving this. Other methods of discharging surplus water are culverts, or channels, built into the dam; by spilling around the side; or by a tunnel cut through the flanking rock. If the water is carrying a lot of silt this has to be carried down with the water to prevent the reservoir choking up. In the Nile Barrages in Egypt sluices have been placed at bed level in the dam so that the silt laden water can pass through. ProblemsDams play a vital part in the lives of many of us. They provide water and power and control floods. Yet some of the changes they bring about are very unwelcome. If a dam takes water from a river, the river below the dam will be affected. For example, water that has passed through the dam has lost some of its silt which may have enriched the soil in the valley. This has been a particular problem in Egypt where the Nile River used to flood the plains every year but since it has been dammed the soil is not so fertile. The movement of fish up and down a river can be blocked although this can be alleviated by building a fish-ladder. Case Study : The Aswan High Dam.Egypt's High Dam across the Nile at Aswan is a stupendous piece of civil engineering. It was completed in 1970 after a decade of construction and an expenditure of over 1 billion dollars. It was undertaken as a joint venture between Egypt and Russia using plans drawn up by a West German firm. The plan was to capture the Nile where it was a third of a mile across and 100 feet deep. The dam had to withstand the pressure of a reservoir 300 miles long, containing 1.25 million acre-feet of water. The hydroelectric plant was to supply 2.1 million kilowatts to suffice most of Egypt's needs. The dam itself is a rockfill barrier encasing a central structure of sand, clay and concrete which is pegged to the bedrock by a huge curtain of epoxy-like grout. For added strength the design incorporated an arch which faced upstream and distributed water pressure such that the dam became locked in to the banks. Construction began in 1960 with the excavation of a diversion
canal to take the water away from the construction site, later it
became the principal spillway and a source of power. Two rockfill
cofferdams were also built; one to divert the water and one to
prevent erosion below the dam and to contain any fill materials
that could escape during the early stages of construction. Although the dam is built on sand, it is firmly founded due to the Soviet method of compacting sand to a stability approaching that of rockfill. This method involved mechanically vibrating 49 feet of dune sand at 1,750 cycles a second. The vibration increased the density and actually reduced the layer to 41 feet. The method employed to root the dam to the bedrock was no less ingenious. A series of holes four inches in diameter were drilled as deep as 700 feet. Then these were filled with grout which eventually linked up to other grout coming from holes about eight feet away. This formed a curtain 128 feet thick and 40 feet wide at its top. Above this was a core of impermeable clay which provided a spine like mass which was elastic enough to adsorb any of the inevitable settling movements of the foundation. A corrugated blanket of clay with an average thickness of 20 feet extended 600 feet upstream to prevent seepage. Finally on the surface of the dam 29.6 million cubic yards of rock fill were piled on, to add tremendous mass to the structure and to dissipate potentially damaging erosion by the wind-driven waves of the enormous reservoir. The great Aswan reservoir holds 136 million acre-feet of water; 26 million of this is kept as dead storage and so is set aside for the accumulation of silt (recent estimates have concluded the reservoir has a useful life of 300 years. The next 73 million acre-feet of water is kept for controlled irrigation throughout the year. Another 37 million acre-feet is available for water in the case of exceptionally large floods. Irrigation has been given priority over hydroelectric requirements so now 100 percent of the nation's arable land is irrigated and the target power output of 10 billion kilowatt capacity is short by 2.5 billion kilowatts each year. The dam has caused problems, as well as almost fulfilling its major purposes. Great Britain's Economist magazine has blamed the High Dam for causing the erosion of the Egyptian coastline, killing the sardine industry, depriving the delta of the yearly flood silt that gave Egypt the world's most fertile soil, and spawning a plague of the dangerous bilharzia parasite. A recent survey has established that the maintenance cost of the dam is $844 million whereas the benefits are estimated to be $1.1 billion. It is in this monetary sense that the dam has been declared a success. Recent surveys have, however, suggested that the dam has only a useful life of three hundred years, before it fills up with silt. This unexpected consequence has been occurred because the dam stops the annual flood, which used to deposit a lot of fertile silt on the flood plain. Nowadays farmers are being forced to buy chemicals to fertilise their land. Scientists are unsure of the long term consequences. In my opinion the dam's planners have made some fatal mistakes; such as allowing silt to build up behind the dam and not fertilise the land beyond it. As a result of the dam the land beyond is now irrigated but soon it could become polluted by the uncontrolled use of chemical fertilisers. Perhaps someone should have thought about this environmental aspect even before the design was even thought of. It seems to me that the structure was thought to be more important than the environment. This is clearly wrong. Bibliography
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