gravity dam
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What is a gravity dam?
A concrete gravity dam is a massive structure that runs straight across a wide valley and uses its weight to resist the water's horizontal pressure. The main forces on it are the water's thrust, the dam's weight, and the foundation's pressure. Other factors to consider include silt and ice thrust, seismic inertial forces, and buoyant uplift from water seepage.
Uplift from seepage can destabilize gravity dams, so design and construction require great care. Cutoffs and underdrainage are often incorporated into dams. Post-tensioned steel can further strengthen them. Gravity dams can withstand minor overtopping better than earthfill dams but frequently include spillways to handle floodwaters because heavy spillage could erode the foundation.
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How does a gravity dam work?
Gravity dams are typically straight structures that span across broad valleys, using their weight to resist the horizontal force of the water they hold back. The main forces on a gravity dam are the water's thrust, the dam's weight, and the foundation's pressure. They are often constructed from unreinforced concrete but can be strengthened with post-tensioned steel.
Careful design and construction are needed to prevent destabilization from seepage, which can create uplift. This is countered using methods like grouting and underdrainage systems. Gravity dams are more resistant to overtopping than earthfill dams and are frequently equipped with spillways to handle floodwaters.
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What materials are usually used to build gravity dams?
Gravity dams are typically built as straight structures across wide valleys, using their weight to resist the water's horizontal force. The most common material used in the construction of gravity dams is unreinforced concrete, which can be strengthened with post-tensioned steel.
For gravity dams founded on solid rock, a downward projection of concrete into the rock is generally enough to cut off seepage and eliminate uplift pressures. However, since rock foundations are often permeable, an extensive system of grouting the fissured rock and drainage is required to relieve uplift pressures. Gravity dams are not usually built on soil foundations.
Compared to concrete, soils and rock fragments lack strength and possess less resistance to deterioration and disturbance by flowing water.
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Why is the shape of a gravity dam important for its strength?
The shape of a gravity dam is important for its strength because the dam resists the horizontal thrust of water by its own weight. The shape and large footprint of gravity dams are designed to counteract forces like the thrust of stored water, the dam's weight, and foundation pressure.
The shape helps manage uplift from seepage, which can destabilize the dam. Design and construction carefully address this, using methods like grouting and drainage systems to relieve pressure. Innovative techniques, such as incorporating post-tensioned steel, can further strengthen the dam, allowing for a more slender design while maintaining resistance against overturning.
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What are some advantages and disadvantages of gravity dams compared to other types of dams?
Gravity dams offer unique advantages and disadvantages compared to other dam types:
Advantages:
* They can be further strengthened with posttensioned steel, and have greater resistance to minor overtopping than earthfill dams.
* Concrete dams usually do not experience great accelerations when shaken by earthquakes because their foundations are typically keyed into bedrock.
* Buttress dams have smaller uplift forces than gravity dams because buttresses can be founded individually.Disadvantages:
- Gravity dams have large footprints and can be destabilized by uplift from seepage, which requires care in design and construction.
- They are typically used to block streams running through relatively narrow gorges, as in mountainous terrain; although the structures may be very high, the total amount of material required for such sites is limited.
- Heavy spillage can erode the foundation at the downstream toe, so gravity dams are often fitted with spillways to accommodate floodwaters.
- They are not typically built on soil foundations.
- Compared to concrete, soils and rock fragments lack strength, are much more permeable, and possess less resistance to deterioration and disturbance by flowing water.
- The characteristics of strong ground motions at a given site cannot be predicted, and all types of dams possess some degree of freedom, imperfect elasticity, and imprecise damping.
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How do engineers ensure the safety and stability of gravity dams?
Engineers ensure the safety and stability of gravity dams through careful design and construction, addressing key forces such as the thrust of water, the dam's weight, and foundation pressure. They consider factors like silt and ice thrust, seismic activity, and buoyant uplift from water seepage.
To counter destabilization from seepage, engineers employ strategies such as grouting fissured rock, underdrainage systems, and concrete cutoffs into solid rock foundations. They may also use post-tensioned steel to strengthen the dam, resisting overturning. Regular monitoring through instruments embedded in the dam helps detect movements, strains, and other factors, providing early warnings of potential issues and confirming design assumptions.
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gravity dam, a concrete or masonry dam so proportioned that it will resist overturning and sliding forces by its own weight.
Form and mechanics
Concrete gravity dams usually run in a straight line across a broad valley and resist the horizontal thrust of the retained water entirely by their own weight. The three main forces acting on a gravity dam are the thrust of the water stored in the reservoir, the weight of the dam, and the pressure exerted by the foundation. It is also essential to consider the thrust exerted on the upstream face by silt deposited in the reservoir or by ice on the water surface, the inertial forces that can be caused by seismic action, and, in particular, the buoyant uplift force of water seeping under the dam or into the horizontal joints.
Given their large footprints, gravity dams can be destabilized by uplift from seepage, a problem that calls for the greatest of care in design and construction. Where a dam is founded on solid rock, a simple downward projection of concrete into the rock will generally suffice to cut off seepage and eliminate uplift pressures. Usually, however, the rock foundation is permeable, sometimes to considerable depths, so construction of an absolutely reliable cutoff is either difficult or impossible. Reliance must then be placed on an extensive system of grouting the fissured rock and on relieving uplift pressures by means of drainage. Many gravity dams possess both cutoffs and underdrainage and are not usually built on soil foundations.
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Gravity dams are commonly constructed with unreinforced concrete and can be further strengthened by the use of posttensioned steel. A series of vertical steel rods near the upstream water face, stressed by jacks and securely anchored into the rock foundation, resist the overturning tendency of this more slender section. This system has also been used to raise existing gravity dams to a higher crest level, economically increasing the storage capacity of a reservoir. Gravity dams have a greater resistance to minor overtopping than earthfill dams. However, given that heavy spillage can erode the foundation at the downstream toe, gravity dams are often fitted with spillways to accommodate floodwaters.
Examples of gravity dams
Hoover Dam, built on the Colorado River at the Arizona-Nevada border between 1930 and 1936, is an outstanding example of a curved gravity dam built in a narrow gorge across a major river and employing advanced design principles. It has a height of 221 metres (726 feet) from its foundations, a crest length of 379 metres (1,244 feet), and a reservoir capacity of 37 billion cubic metres (48 billion cubic yards).
Of special interest are three concrete gravity dams that feature a straight sloping downstream face. Bratsk Dam, built across the Angara River at Irkutsk in Russia, was completed in 1964. It stands 125 metres (410 feet) above foundation level and, excluding the earthen side dams, is nearly 1,525 metres (5,000 feet) in length; it contains 4,500,000 cubic metres (5,900,000 cubic yards) of concrete. Grand Coulee Dam, completed in 1941, was built across the Columbia River in Washington state, U.S. Its main structure is 168 metres (550 feet) high and 1,592 metres (5,223 feet) long and contains almost 9,000,000 cubic metres (12,000,000 cubic yards) of concrete. Grande Dixence Dam in Switzerland, completed in 1962 across the narrower valley of the Dixence, has a crest length of 700 metres (2,296 feet) and contains approximately 5,960,000 cubic metres (7,790,000 cubic yards) of concrete. At 285 metres (935 feet) it was the highest dam in the world until the Nurek Dam on the Vakhsh River in Tajikistan was completed in 1980.