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How are dams built?

How are dams built

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 engineering, dams fall into different categories depending on the structural type, use case, and materials used for construction. This article will discuss the different types of dams and their uses.

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.

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

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 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.

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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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.