This is a typical sequence of events for constructing a dam and creating a reservoir:
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Diverting the river
A dam is usually constructed across a river to create a reservoir in the valley behind by storing the water that flows into it naturally. Sometimes, they are built across dry valleys, or valleys with small streams, to create a storage area for water that is transported from elsewhere.
The diversion tunnel excavated through soft
rock for the stream at Baoshan 2 Reservior
in Tiwan
Streams and rivers have to be diverted to create a dry area to construct the dam. Small rivers and streams are usually diverted through a tunnel, or a channel that is constructed around the side of the dam. Soft soils and rocks are excavated to form the route, while harder rocks have to be blasted with explosives.
Sometimes, dams are built across wide rivers with large volumes of flowing water. It would be impracticable and too expensive to construct a separate channel to divert the water. Instead, a dry construction pit is formed on one side of the river, leaving the other side open for the water to flow through. The first portion of the dam is constructed in the dry pit. When it is finished, another dry area is formed on the other side of the river, and the remaining part of the dam is built. Meanwhile, the river flows through openings in the completed portion of the dam, and the reservoir can start to fill behind it.
Find out about the construction sequence for the Three Gorges Dam that is being built across the River Yangtze in China.
Preparing the foundation
The rock foundation for a gravity
dam being prepared
A dam is built on a soil or rock foundation, depending on the type of dam. The foundation must be strong enough to carry the weight of the dam, and the water pressures acting on the dam. The dam foundation is below the original ground level. It is covered by weaker soils or rock that have to be removed either by excavation or by blasting with explosives.
Sometimes, areas of weaker soils or rocks are found below the planned foundation level and these have to be removed and replaced with stronger materials
Some rock foundations have cracks and fissures in them. These have to be filled with grout to stop water leaking out from the reservoir through the cracks once the dam is completed. This is done by drilling holes down into the rock, and by pumping grout into them, which spreads outwards to fill the cracks.
Building a concrete dam
A concrete batching plant on the
construction site for a gravity dam
As so much concrete is required in a concrete dam, it is mixed together in a special building called a concrete batching plant, which is built on the construction site. For very large dams, there may be a number of batching plants. The concrete can be placed in the dam by two different methods.
Pouring Concrete
The traditional method is to pour a wet mixture of concrete into a mould made in the required shape of the dam. The mould, called formwork, is usually made from sheets of timber. The mould is not made to the full height of the dam, as the dam is built upwards in stages of about 1 to 2 metres at a time. The concrete is left to dry before the next section is formed on top.
The concrete can be transferred from the batching plant to the dam by a number of methods. Sometimes, a system of conveyor belts is used and sometimes the concrete is taken by trucks to the bottom of the dam where it is poured into skips which are lifted to the top of the dam by cranes.
Compacting Concrete with Rollers
Concrete being placed and compacted
on Stillwater gravity dam in the
United States
Roller Compacted Concrete (RCC) dams are formed by spreading a drier of mix of concrete and compacting it down using rollers. The dam is raised in steps of about 600 mm. The edges are formed first, by making low concrete walls on the upstream and downstream faces of the dam. Concrete is then transported to the area between the walls and spread in a thin layer using bulldozers. Rollers are then driven over the concrete to compact it down.
Building an embankment dam
Masjed-e-Soleiman embankment dam
under construction in Iran
Embankment dams are constructed upwards in a series of thin layers. For each layer, the fill material is transported to the dam by trucks and dumped there. Then bulldozers are used to spread the material in a thin layer. The thickness of the layer depends on the material being used. Earthfill layers are about 300 mm thick and rockfill layers can be up to 1 metre thick. The layers are compacted down using rollers.
When the dam has a core, that is also constructed in layers to keep it at the same height as the rest of the dam.
The reinforced concrete layer being placed on the upstream face of the Messochora rockfill dam in Greece.
Once the dam has been raised to its full height, a protective layer is formed on its upstream face. This layer protects the dam against damage by waves, and sometimes provides waterproofing too.
A dam is a structure commonly built across a river or stream to create a large reservoir behind it. We use dams for various human consumption purposes such as irrigation, hydroelectric power, reducing peak floodwater, and improving navigation.
Dams also have extra works such as spillways, valves, and moveable gates that control water movement downstream. They are sometimes connected to intake structures such as canals, waterways, and pipelines to convey water in distant places.
In construction engineering, dams fall into different categories depending on the structural type, use case, and materials used to construct. This article will discuss the different types of dams and their uses.
TABLE OF CONTENTS
The types of dams
We classify dams into different types depending on their uses, structural types, and materials used in construction. Choosing a particular type of dam depends on the foundation conditions, accessibility of the dam site to transport networks, availability of construction materials, and financiers.
Below are the most common types of dams in construction.
Arch dam
An arch dam is a concrete dam curved into the shape of an arch. The curved part points back to the water. When pressure from the water presses against the arch, the water pressure makes it straighten slightly, thus strengthening the structure as it pushes back its foundations and abutments.
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Arch dams are best suited for narrow canyons and gorges to support the structure&#;s stresses. Generally, arch dams are thinner than other dams; hence, they consume lesser construction materials than other dams. Due to the small base width of arch dams, they have fewer problems with uplift pressure as only a tiny part of the water load is transferred to the foundation.
Arch dams require skilled labor to construct, and the construction speed usually is prolonged. They also need strong abutments that can resist thrust.
We classify arch dams into constant radius dam, variable radius dam, and constant angle dam.
In the constant radius arch dam, the radius of the arch in either the upstream or downstream face is continuous; in the variable arch dam, the radius of the upstream face changes, and in the constant angle arch dam is a unique variation of the variable radius dam. It has different horizontal arch rings on the upstream face, but all these rings are from the same central angle and have the same magnitude in all elevations.
Buttress dam
A buttress dam is also referred to as a hollow dam. Buttress dams borrowed the concept of construction from gravity dams, except that buttress dams use way fewer construction materials. The buttress dam wall can be curved or straight.
Buttresses on the downstream side support the stressed areas of the dam. The buttresses are evenly spaced to resist the force of water trying to push the dam over.
There are five types of buttress dams. These are the deck slab, multiple arch, columnar, massive head, and multiple dome buttress dam.
We can construct buttress dams in relatively weak foundations, and they have no problem with uplift and foundation drainage. The uplift pressure acting on the buttress dam is also considerably smaller than gravity dams, making them more economical. Powerhouses and switchyards can also be placed between the buttresses, thus saving construction costs.
Cofferdam
A cofferdam is a temporary structure for allowing the process of diverting water, dewatering, or damming in an enclosed area. Building a watertight enclosure allows for pumping out water so that work can proceed in a safe and dry environment.
We use cofferdams to repair and construct bridges, piers, and oil platforms built in water.
Cofferdams can withstand very high pressure and are made by driving steel sheet piles to form a watertight bed. The cofferdam walls need to be sturdy enough to resist horizontal forces from the surrounding water. The design and shape of cofferdams depend on the soil type, working area, depth of cofferdam required, and water level fluctuations.
Cofferdams are typically dismantled after the completion of construction works.
Detention dam
The primary purpose of a detention dam is to regulate the flow rate and minimize flood impact in a water channel. Sometimes detention dams are also constructed to recharge groundwater systems or trap sediment.
Detention dams store water for extended periods for irrigation, livestock, hydroelectricity, municipal water supply, and recreation. In flood-prone areas, detention dams are built in areas higher than the flood area. The water collects in the basin above and is slowly released at a rate the flood zones and channels can accommodate.
The most significant danger of detention dams is overtopping, whereby the water in the dam exceeds the dam&#;s crest height. Designs need to account for overtopping as it poses a great danger to the dam structure when it occurs.
Diversion dam
A diversion dam is for diverting water from its natural course. The diverted water is mainly for supplying irrigation systems and reservoirs. Generally, detention dams do not impound water like other dams, but the waters are diverted through dikes, canals, or drain pipes.
Some diversion dams are built to catch surface runoff and trap sediments to make it easier to divert a watercourse downstream.
Embankment dam
An embankment dam is constructed from excavated construction materials or industrial wastes. The materials are then compacted to form a wall with varying soil compositions. The dam is semi-impervious, and this prevents seepage erosion. The interaction and friction of materials bind the particles together, making a stable mass.
Embankment dams are classified into an earth-filled dam and a rock-filled dam. The core of embankment dams is filled with an impermeable material such as clay or concrete to prevent water from seeping through. Embankment dams are a good choice, especially for sites with broad valleys.
These dams have a high resistance to settlement and movement of the ground, utilize locally available materials, and are relatively easy to construct.
Gravity dam
A gravity dam is a massive dam made from concrete or masonry designed to resist water weight from its self-weight. Each gravity dam section is stable and independent of other dam sections.
These dams need stiff foundations with high bearing strength to limit the resultant force from the water. It is best to test the bearing capacity of the soil on which the foundation rests to ensure it can support the weight of the dam and the water.
However, due to the stiffness of gravity dams, they are prone to cracking when there is a differential settlement. Gravity dams also have a significant footprint that makes them susceptible to uplift pressures that destabilize the dam.
Unlike embankment dams, gravity dams can tolerate minor overtopping flows since the concrete is scour-resistant.
Storage dam
Storage dams are constructed to capture and store water, especially during rainy seasons, for use by livestock during the dry season. We also use storage dams for municipal water supply, irrigation, hydroelectricity, and irrigation. These are the most common types of dams.
There are also unique storage dams for trapping sand and debris. Sand storage dams are progressively built in stages across a stream. They must be strong enough as they allow water to wash over their crests. With time, sand piles in layers behind the dam, which helps store water and prevent evaporation. We can then extract the water through a drainpipe, well, or the dam body.
How to build a dam
Dams are engineering marvels storing vast amounts of water for flood control, generating hydroelectricity, or recreation. However, this begs the question, how are dams built?
Dam construction is a complex process that needs a lot of workforces, raw materials, and resources. Below are the steps necessary in dam construction.
- Diverting the water: The first step is dewatering the area of the river where the dam will be built. Engineers do this by diverting the water using tunnels. The tunnels need to be deep enough to carry water without surface runoff. You do not need to empty the river but need to make it shallow enough to work in.
- Preparing the dam&#;s foundation: If you have not sufficiently diverted water from the river, you need to make a cofferdam to help you divert water from the river to the tunnel. When building the foundation, stack the heaviest rocks first, then increasingly smaller rocks to form a firm foundation bed.
- Assemble the main structure: Ensure there&#;s no loose rock on the riverbed, and construct a plinth (concrete foundation) to prevent water from leaking from the dam edges. The next step is actual dam construction, where you build your main structure on both sides of the foundation. Most dam construction projects use reinforced concrete steel to make the dam resilient against water flow.
- Filling the reservoir: After building the dam to the desired height, the next step is filling the reservoir. You then need to test whether the valves and floodgates work and monitor the behavior of the freshly built dam.
Summary
You now understand the types of dams available and how to construct a dam. We can use dams for domestic, industrial, and irrigation purposes. Dams are also used for navigation and hydroelectricity, and we can view dams as a sign of human ingenuity.
Looking for more construction resources? Check out our Resource Center full of plenty of guides, checklists, videos, and more!
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