Embankment dam
Encyclopedia
Dam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
. It is typically created by the emplacement and compaction
Soil compaction
In Geotechnical engineering, soil compaction is the process in which a stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water being displaced from between the soil grains then...
of a complex semi-plastic
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...
mound of various compositions of soil, sand, clay and/or rock. It has a semi-permanent waterproof natural covering for its surface, and a dense, waterproof core. This makes such a dam impervious to surface or seepage erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
. The force of the impoundment creates a downward thrust upon the mass of the dam, greatly increasing the weight of the dam on its foundation. This added force effectively seals and makes waterproof the underlying foundation of the dam, at the interface between the dam and its stream bed
Stream bed
A stream bed is the channel bottom of a stream, river or creek; the physical confine of the normal water flow. The lateral confines or channel margins, during all but flood stage, are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto...
. Such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than the use of a cementing substance.
Types
Embankment dams come in two types: the earth-filled dam (also called an earthen dam or terrain dam) made of compacted earth, and the rock-filled dam.A cross-section of an embankment dam shows a shape like a bank, or hill. Most have a central section or core composed of an impermeable material to stop water from seeping through the dam. The core can be of clay, concrete or asphalt concrete
Asphalt concrete
Asphalt concrete is a composite material commonly used in construction projects such as road surfaces, airports and parking lots. It consists of asphalt and mineral aggregate mixed together, then laid down in layers and compacted...
. This dam type is a good choice for sites with wide valleys. Since they exert little pressure on their foundations, they can be built on hard rock or softer soils. For a rock-fill dam, rock-fill is blasted using explosives to break the rock. Additionally, the rock pieces may need to be crushed into smaller chunks to get the right range of size for use in an embankment dam.
Safety
The building of a dam and the filling of the reservoir behind it places a new weight on the floor and sides of a valley. The stress of the water increases linearly with its depth. Water also pushes against the upstream face of the dam, a nonrigid structure that under stress behaves semiplastically, and causes greater need for adjustment (flexibility) near the base of the dam than at shallower water levels. Thus the stress level of the dam must be calculated in advance of building to ensure that its break level threshold is not exceeded.Overtopping or overflow of an embankment dam outside of its spillway
Spillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...
s will cause disastrous flooding through the eventual failure of the dam. In the failure process the sustained hydraulic force and pressure caused by an overtopping surface runoff; immediately erodes the dam's material structure as it flows over the top of the dam. Even a small sustained overtopping surface flow can remove thousands of tons of overburden soil from the mass of the dam within hours. The removal of this mass unbalances the forces that stabilize the dam against its impoundment. The mass of water still impounded behind the dam presses against the lighter mass of the embankment, (made lighter by surface erosion). As the mass of the dam gets lighter, the impoundment begins to move the whole structure. The embankment, having almost no elastic strength, begins to break into separate pieces, naturally allowing the impounded water to flow between them eroding and removing more material as it goes. In the final stages of failure the remaining pieces of the embankment offer almost no resistance to the flow of the water; as they continue to fracture into smaller and smaller sections of earth and/or rock. The overtopped earth embankment dam disintegrates into a thick mud soup of earth, rocks and water.
Therefore safety requirements for the spillway are high, requiring the spillway to be capable of containing a maximum flood stage. Specifying a spillway able to contain a five hundred year flood is common. Recently a number of embankment dam overtopping protection systems were developed. These techniques include the concrete overtopping protection systems, timber cribs, sheet-piles, riprap and gabions, reinforced earth, minimum energy loss weirs, embankment overflow stepped spillway
Stepped spillway
Recent advances have permitted the construction of large dams and reservoirs and channels. These have required the development of new spillway designs. A key feature of a spillway system is the safe dissipation of a major component of the kinetic energy of the waters to avoid damage and failure...
s and the precast concrete block protection systems developed in Russia.