A buttress dam or hollow dam is a dam with a solid, water-tight upstream side that is supported at intervals on the downstream side by a series of buttresses or supports. The dam wall may be straight or curved. Most buttress dams are made of reinforced concrete and are heavy, pushing the dam into the ground. Water pushes against the dam, but the buttresses are inflexible and prevent the dam from falling over. Buttress dams do however, run into certain problems that are not as prevalent in other dams. Even though they tend to be more economical, they run the risk of not reaching the proper height required to become effective. An example would be as follows: with a slope of 50 degrees the crown of each arch will have to be 0.84 feet farther forward at the crest than at the base for each foot of height. This defect will demand that the dam is either much taller than calculated or a lot wider to accommodate for the problem. Another issue is that the dam has to have a proper foundation of bedrock or other suitable material. If the buttresses for the dam are not properly seated the dam runs the chances of a possible collapse. Some problems that might occur during the construction of a buttress dam is cracks along the convex side near the cresting of the dam due to thermal stresses and the dam not being 'loaded'. Once the dam has been reinforced if cracks continue to occur the cracks will close once the force of the water is against it.
John S. Eastwood an engineer, was a designer and proponent of buttress dams. Buttress or hollow gravity dams were originally built to retain water for irrigation or mining, the benefits of multiple-arch dams can be most readily seen when looking into small agricultural towns. These people lack the necessary capital to build massive and expensive gravity dams but they could finance lighter, cheaper, multiple-arch designs. An example of the benefits that come from buttress dams was Littlerock dam, completed in 1924 it helped gather water from the Littlerock and Palmdale district in California. Throughout their history prominent engineers have denounced multiple-arch dams due to having financial and professional stakes in gravity dams. However, a buttress dam remains a good choice in wide valleys and areas were solid rock is not present.
As designs have become more sophisticated, the virtues and weaknesses of the buttress type dams have become apparent. The Romans were the first to use buttresses to increase the stability of a dam wall. Buttress dams of slab concrete construction became popular in the United States in the early 20th century with the patented process of Norwegian-American civil engineer Nils F. Ambursen.
Types of buttress damEdit
Multiple dome buttress damEdit
Within the buttress dam family there exists a dam called a multiple dome buttress dam. Most of its characteristics are the same as a multiple arch dam, however instead of having arches it has domes instead. Applying multiple domes help reduce the number of buttresses required to make sure the dam is stable.
Multiple dome buttress dams are very similar to multiple arch buttress dams, but domes are built in place of arches. The main advantage of a multiple dome buttress dam is that domes can be spaced farther apart than arches can be placed. This helps save material, and reduces cost when designing the dam itself. Similarly to multiple arch buttress dams, multiple dome buttress dams are heavily reliant on every dome in the dam. If one of the domes fails, it is likely that the whole dam is compromised. Multiple dome or multiple arch buttress dams are most likely to fail at either end of the dam in the first dome or arch. This is because those two arches or domes take most of the force being exerted on the dam itself. When compared to multiple arch buttress dams, multiple dome buttress dams do not hold up as well in high stress environments. This is due to the fact that there is less reinforcement in the domes than in the arches. The multiple dome buttress dam is best used in places with little to no risk for earthquakes, and in waterways where flooding is minimal.
Deck slab buttress damEdit
The deck slab buttress dam tends to be smaller ranging from 20 to 50 metres (66 to 164 ft). The slab is inclined to between 40 and 55 degrees, to help provide for necessary stability when under load. The deck slab buttress dam can also be categorized into three types: fixed deck slab, free deck slab, or cantilever slab each providing its own benefits and drawbacks. A benefit of the deck slab dam is that the deck slabs work together like a multiple arch dam for proper support. However, unlike the multiple arch dam, if one of the slabs gets damaged the damage will not affect the other slabs. If a dam is to be constructed in an area with loose soil the engineers may elect to build a free deck slab dam. Unlike the fixed deck slab dam it will provide necessary support downstream of the face of the dam.
Buttress dams can be referred to as gravity dams or arch dams. It is a dam in which the concrete and material of the dam is designed to hold all the horizontal force and pressure of the water. The water pressure is distributed to the slab of the buttresses. Distributing this force will reduce the load on the wall, allowing the dam to last longer.
The deck slab buttress dam is similar to a gravity dam. This is a type of buttress dam that is supported by the corbels of the buttresses. The flat slab is supported by the buttress heads. Similar to a normal buttress dams this dam is reinforced by the buttresses. The buttress spacing depends on the height of the dam. The concrete slab is tilted horizontally about 40 to 55 degrees. This allows the deck of two buttresses to become one single part. There are multiple types of deck slab buttress dams including the fixed deck slab, a free deck slab and a cantilever. The fixed deck slab is a dam in which the deck and the buttresses are cast all in one single pour of the concrete. This means there is only one part of concrete rather than having a disconnect between the slab and buttresses. The free deck slab dam is also referred to as a simple deck slab buttress dam. The cantilever type buttress dam is one in which the deck slab is supported by a cantilever which is a long-extended beam. The upstream face is built like a brace and is on both ends of the dam.
Multiple arch buttress damEdit
Gravity dams and buttress dams must be designed in a way where every section of the dam is balanced and durable. Buttress dams are also referred to as gravity dams or arch dams. The buttress dams are types of gravity dams. The multiple arch type of buttress dam has arch slabs that are on the upstream face of the buttress dams. Each of the arches on the dam are supported by the buttresses. Multiple arch buttress dams are more enduring and flexible then other buttress dams, such as a deck slab buttress dam. The dam can either be constructed as a singular stiffened wall or a double hollow one. The centers between the arches are removed. The biggest disadvantage is that the buttresses depends on each other meaning that if one buttress develops problems the whole dam will lose its efficiency. If one arch develops problems it will trickle down the sequence or arches. The depth of these arches often varies. This dam is best for larger heights preferably above 50 meters. The center angle of the arch must be 150 to 180 degrees to work. The area between the buttresses must be between 15 and 21 meters.
Massive head buttress damEdit
Massive head buttress dams are a type of gravity dam. They are constructed with large amounts of concrete, and little reinforcement; this makes their construction relatively easy compared to other types of buttress dams. The sheer weight of the concrete makes massive head buttress dams very heavy, and very resistant to sliding. This type of buttress dam will not have a slab or arch at the upstream face like other buttress dams. Instead of having a slab at the upstream face the massive head buttress dam has buttress heads that are expanded and connected with other buttress heads. The larger buttress heads can be made into different shapes including round and diamond. These extensions of the buttresses is strengthened by using copper strips in the construction. Overall construction of massive head buttress dam is easier compared to other types of dams. The massive head buttress dam is resistant to sliding because of is weight. The massive head buttress dam is a heavy because of the large amounts of concrete used in construction. Massive head buttress dams are mainly designed to withstand static loads, and they can endure additional forces from natural occurrences such as silt build up, or a high water year. In general, massive head buttress dams do not hold up well against large floods or earthquakes due to concrete's fragile properties. This fact, coupled with the lack of reinforcement in massive head buttress dams, means that they are not the best dams to have in earthquake prone areas.
Columnar buttress damEdit
With the columnar buttress dam the columns support the deck slab of the dam. The columns are inclined to better support the flat deck of the dam. The flat deck slab are made to replace the buttresses. It is an altered deck slab buttress dam. It needs a very durable base. It requires skilled personnel to create the buttresses. This is why it is not used as much at the other types of buttress dams.
Existing dams can also be modified to possess qualities of the buttress dams. An example of this modification would be the Butt Valley dam in Plumas county, California. The dam is located within 0.5 km (0.31 mi) of the Butt Valley Fault zone and within 7 km (4.3 mi) of the Lake Almanor Fault. The location of the dam makes it highly susceptible to seismic failure leading engineers to reinforce it with rock fill buttresses. Other alternatives were considered; however, due to multiple factors such as reliability, environmental impact, materials available, cost, expedience, and construction practicality they were rejected. The evaluation led to the selection of the upstream and downstream rock fill buttresses. The application of buttresses would protect intake towers and assure upstream and downstream stability under difficult conditions. The Butt Valley dam is able to return to normal operations due to the fact that applying buttresses takes the least amount of time when compared to other practical options. Through multiple experiments the conclusion was reached that the reinforcements would be sufficient enough to pass MCE regulations allowing for the appropriate amount of deformation.
Another example of applying buttresses would be East River from Astoria to the Bronx, where it was necessary to construct a concrete blanket to resist the upward thrust of water under considerable pressure. There was insufficient room for a blanket that would provide the required resistance due to its weight, so the concrete was laid in the form of an inverted arch.
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