The Design And Construction Of Dams

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Introduction

This technical report will explain how dams are built through the processes of design and construction. It will go into detail about who is involved in these areas and what roles they play. It also explains what methods are used to physically construct an arch dam and an embankment dam. Finally a case study of what was, for a long time, the largest dam in the United States, and now one of the most famous dams in the world will be used as an example: The Hoover dam.

A short history of the uses of dams

The first known large dam was called Sadd el Kafara. Built around 4000 years ago near Cairo, it is thought to have been built to control major floods that came quite infrequently but with disastrous effects. Sadly it was one of these floods that destroyed it before it had been finished and because there was no attempt to rebuild it for 800 years, it is thought to have caused far more harm than good. However it does highlight one of the many reasons why dams are built to this day; to control the flow of water in a river. Every year thousands of people's homes are destroyed by rivers flooding and dams can help control this by monitoring the flow of water through them. There are also many other uses including irrigation, energy, a water source and navigation. Naturally there are many different shapes and sizes of rivers and valleys that would be more appropriate than others for providing these services and different types of dams need to be available to accommodate the varying specifications. A number of different dams have been designed since its creation to tackle new world needs and the problems that arose from them. This may have included the use of larger valleys or different shaped valleys. New materials meant dams could be larger and stronger.

Types of dam

Consequently there are now four main types of dam. Each of their designs makes some better than others at coping with varying situations. The largest and often the most dramatic is the arch dam. Given its name due to its shape and design where the dam itself has a convex shape towards the water, displacing the force of the reservoir into the sides of the valley or gorge. Buttress dams tend to be used in wider valleys. This is so called because of the buttresses that support the dam from behind. Although often smaller buttress dams can still look very impressive such as the Daniel Johnson dam in Canada which is 1.3km long and over 200m tall. Embankment dams are much smaller due to the method of construction and materials. They are however the most common in the UK claiming around 3000. Finally, gravity dams are medium sized concrete dams with a very wide base and triangular shape. This pushes the weight of the water behind it into the ground, again giving it its name. How these various dams are physically constructed will be delved into in more detail later in this report but firstly, it needs to be explained who is involved in the design side of construction.

Designing dams

As I stated earlier the designs and subsequent capabilities of dams changed over time to accommodate new needs and situations. Now a particular design of dam is chosen depending on where it is to be built and its purpose. For a steep sided gorge aimed at holding a large amount of water, an arch dam will almost certainly be used whereas in a wide valley, it is far more likely that a buttress or embankment dam would be used, and this decision may in turn be decided by the availability of material for the construction of an embankment from local sources. There are a number of people whose specialist knowledge is used in helping to decide what type of dam is needed and then what the dam must be capable of achieving.

The role of surveyors in design

Surveyors map the surrounding land. Their job is to specify the best site for the dam in a designated area. This will be in terms of the contours and the physical lay of the land. They will produce an accurate map of the whole area in question, that can then be used to make a final decision. Ultimately the best place would be somewhere where there is the best ratio between the size of the dam and the amount of water it would hold back. Of course this will not be decided until further more detailed information about the ground and water movement have been researched.

the role of geologists

Geologists help to find out some of this information. Their main job is to find out what the ground is made of. This will include soil types and depth and the bedrock. They will also look for ruptures in the ground and any fault lines. They may also look into potential natural hazards such as earthquakes. In the case of embankment dams the geologists will also look for near-by sources of material that can be used in the construction.

the role of geotechnical engineers

Geotechnical engineers will work closely with the geologists. They use all the information the geologists have found and look at it more mathematically. It is their job to work out whether the ground will physically be strong enough to support the weight of the dam and water. They will also make sure the type of ground is impermeable. A decision will then be made as to the choice of site or whether the site could be treated. Geotechnical engineers are also in charge of investigating the chance of landslides and the effect it would have on the dam. A geotechnical engineer must investigate the properties of all materials in question and use the information found to give their input into the design process.

the role of hydrologists

Hydrologists, as the name suggests investigate all the potential flows of water around the intended site. They will look at average yearly precipitation rates to find extremes as well as flows of water underground and through flow. They need to produce accurate figures of the amount of water that will be moving through the dam in each season, as well as the potential for unaccountably large precipitation in a short amount of time. For example if the dam was built in the mountains below a glacial region, in spring when the ice was melting a much larger volume of water would flow towards the dam and this must be accounted for. The civil engineer in charge of the project will then look at all of the information and, after doing many tests and calculations to model the dam in every scenario, design an appropriate dam. This whole process can often take years but once the designs are intricately drawn up by the civil engineers the dam is ready to be built.

Building the dam

Spillways

The first thing to do when building a dam is to decide where the continually flowing water will go. Often rivers are large and clearly a dam can't just be built with the water constantly crashing against it, so a diversion is created. There are three more common ways this can be achieved including tunnels, channels or dry construction pits. What method of diversion is used, will of course depend on what the lay of the land is. For example in steep sided gorges tunnels diverting the river through the valley sides is far easier that cutting a channel through the cliffs. Channel diversion is therefore much more likely in smaller dams or shallower valleys. The third diversion mentioned was to create a dry construction pit. This for example was used in the construction of the three gorges dam. The river at the intended site of the dam is halved by the construction of, again a temporary dam. This will enclose the area in the other side of the river and creates a dry pit where the construction of that side of the dam can commence. Once this is complete and the water gates are functional the other half of the river can be cut off and the water flow through the gates of the half built dam. This will now make it possible for the construction of the other side to begin. So to jump back a step, the dam had been designed in intricate detail and the decision of how to divert the river has been made. The foundations of the dam are the first part of construction.

Foundations of arch dams

An arch dam must almost always be built on solid rock. This often leads to large amounts of preparation. The first part of which is more commonly known as stripping. This is where layers of top soil or weak rock must be removed so work can progress on the harder bed rock. Similarly dental work is where the weaker rock must be removed in specific areas. This can sometimes be very extensive and if there is physically too much, unviable, and another solution is needed. Work called scaling must also be done on the valley sides, where the dam will eventually press against. Remember the shape of the arch dam is designed to deflect the force of the weight of water into the sides of the valley rather than the base. Another common problem is if there are fissures or cracks in the bed rock and these must be filled with concrete in a process called grouting. Physical foundations will then be dug for the base of dam and work on the actual dam can commence.

Foundations of embankment dams

Embankment dams are different. Because they are made out of materials excavated from the ground such as soil and loose rock they do not need the same foundation work nor do they need to be built on bed rock. There will however be an impermeable core, often made of concrete, which needs to have a deeper foundation and a good bond between the original ground and the new layers of the dam is needed. Like the foundations for concrete and arch dams, there are methods for embankment dams to strengthen and improve the soil on which the dam is to be built. These include dynamic compaction, pre-consolidation and densification. Respectively, these involve compressing the soil by dropping weights, often 10 to 20 tons, building up the foundation in compressible soils foreseeing that it will sink with the weight of the dam, and using vibration to compact the soil. The dam itself can now start to be built.

The construction of concrete dams

Formwork

In arch dams Formwork is the most commonly used method of construction. Made of steel or wooden frames, the blocks are filled with concrete. These blocks are normally 1.5m high and 15m long but can be anything up to 3m high in some larger dams. This would make construction quicker with fewer surfaces to clean before further concrete is poured and fewer forms are needed, however the concrete would be more prone to cracking, take longer to dry and heavier equipment is needed to lift the forms into place.

Built in features

Unlike embankment dams concrete dams and especially arch and buttress dams will often have machine rooms and other built in items such as turbines, sluice gates or even observatories within the actual dam. These have to be built at the same time as the concrete is being laid and can often lead to delays. Prefabricated units are often used be it pre-cast concrete or steel shells. These areas must be meticulously designed as they can act like large cracks if they are not built to the same specification and strength of the rest of the dam. These prefabricated units must be able to work with the rest of the dam for the final product to be homogenous and flawless.

Curing the concrete

One very important factor of laying concrete in this way is that is has to be cured so cracks cannot form and the dam be weakened. One problem that may induce the cracking is the temperature. The temperature must be regulated whether it is too high or too low or fluctuating too much. One way to combat this is to lay pipes 0.25 and 0.75 of the way up each lift. Water can then be flowed through these to either keep the concrete warm or cool as it sets.

Embankment dams

How are they built?

Embankment dams are built in layers around a central impermeable core. These layers are normally between 0.3m and 1m depending on whether it is soil or rock fill respectively. The material, having been unloaded on the dam will be spread evenly by bull dozers then compressed and flattened using steam rollers. As the height of material rises so the core will be built up as well. This is necessary because as well as it being potentially very dangerous having a very tall slab of concrete resting on the ground with little in the way of foundations, the core itself can be inclined or slightly sloped called rolled rockfill.

Materials

Embankment dams have been defined as “any dam constructed of excavated materials placed without addition of binding materials other than those inherent in the natural material. The materials are usually obtained at or near the dam site” (ICOLD). However there are often extra integral parts, unmentioned in this statement. The core mentioned earlier can be made from a range of materials either brought from nearby or elsewhere. Some common core materials such as clay and gravels can potentially be found close to the dam site however cement or asphaltic concrete are more likely to have been brought from mixing plants often further afield. There are regulations for the permeability of these core materials which must not exceed 10-5 cm/s. They may also have a layer on the outside of the excavated material such as rip rap or again, concrete. Rip rap is when rocks are lain on a slope, often in cages, to break up the force of the water and holds together the softer material underneath.

Seepage

Seepage can be a major problem in embankment dams. More commonly known as piping it occurs when water from the reservoir finds a route of least permeability and flows slowly through the dam. This does however cause a weakness in the dam and over time can seriously damage its strength. There are some ways to prevent it, the most common of which is the impermeable core. If the water is seeping underneath the foundations of the dam impermeable blankets are needed that extend upstream underneath the reservoir or alternatively cut off walls can be used. These are simply impermeable walls extending underneath the foundations of the central core.

Releasing the water

Unlike how many might expect, where the water is suddenly released behind the dam crashing against it, this is actually a very slow process. Water is released from the spillway into the reservoir in measured quantities, while measurements of force and structural integrity are meticulously calculated on a regular basis. If applicable other utilities such as turbines, sluice gates and power stations will be tested for the first time with water. This monitoring will continue until the reservoir reaches its limit, however checks will still be made at designated intervals for the rest of the dam's life.

How does the Hoover dam show characteristics of the methods of design and construction mentioned in this report.

In the case of the Hoover dam, the sides of the gorge were very steep and around 300m high, diversion by channels was not an option. The river was also so large that in the end four tunnels, 17 meters in diameter and four kilometres long were built through the solid rock of the valley sides as a diversion for the water. Of course in 1932, no mechanical tunnelling machines were available so 2 storey trucks loaded with workers and pneumatic drills were used; and incredibly dangerous solution. The sides of the gorge between the entrance and exit of these tunnels could then be blown in to form a temporary (coffer) dam and create a dry zone where the actual dam was to be built. Scaling of the sides of the valley caused many more fatalities when workers had to drill away loose rocks while hanging from ropes. Once the building started the first ever cable way was used to move the concrete into position to pour into the formwork. This was a rapid process and competition between groups of workers in separate areas was encouraged by the chief engineers who were granted larger bonuses, the earlier they finished. The Hoover dam began operation in 1936, 5 years after construction started.

Summary

This report has been a basic report touching on the broadest areas of dam design and construction. It has been written for people who know very little about dams and want a rough knowledge of what or who is involved in the design and construction of them. It covered the uses and types of dam currently in operation followed by a paragraph as to why they are designed the way they are and who is involved in that process. How the dams were physically built and some common problems and solutions were described and finally a short case study of the one of the most famous dams in the world was made describing how several areas of this were report were used in the actual construction of a dam.

References

  1. The British dam society [Online] [January 4, 2010]. http://www.britishdams.org/about_bds/
  2. GRAHAM, A.T., DAVID, 1997-last update, dam design [Homepage of Andrew Graham], [Online] [January 4, 2010]. http://www.dur.ac.uk/~des0www4/cal/dams/fron/contents.htm
  3. ROBIN FELL, PATRIC MACGREGOR, DAVID STAPLEDON, GRAEME BELL, 2005. Geotechnical engineering of dams. Leiden, Netherlands: A.A. Balkema Publishers.
  4. THOMAS, H.H., 1979. The Engineering of Large Dams. Colchester, London: William Clowes and Sons Ltd.

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