Misplaced Pages

#398601

34-791: The São Simão Dam is an embankment dam on the Paranaíba River near São Simão in Goiás / Minas Gerais , Brazil . It was constructed for hydroelectric power production and flood control . The dam was completed in 1978 and all generators were operational by 1979. In 1977, the first use of roller compacted concrete in Brazilian dam construction occurred on the São Simão. The dam is 3,600 metres (11,800 ft) long and 127 metres (417 ft) high embankment dam with concrete spillway and power plant sections. The dam's reservoir has

68-418: 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

102-505: A loss of stability, increases in pore pressure and clogging of the permeable layer. Experimental results show that close to the geometrical limit, the point at which the fine particles can just pass between the coarse particles(the filter criterion), erosion initiation and failure are much more likely. It is possible to interrupt the process of internal erosion with the use of filters . Filters trap eroded particles while still allowing seepage, and are normally coarser and more permeable than

136-414: A pipe, surface instability, and, lastly, initiation of a breach . Internal erosion is also classified in four types, dependent on failure path, how the erosion initiates and progresses, and its location: Concentrated leaks occur when cracks form in the soil. The cracks must be below reservoir level, and water pressure needs to be present to maintain the open pipe. It is possible for water flow to cause

170-434: A simple embankment of well-compacted earth. A homogeneous rolled-earth dam is entirely constructed of one type of material but may contain a drain layer to collect seep water. A zoned-earth dam has distinct parts or zones of dissimilar material, typically a shell of locally plentiful material with a watertight clay core. Modern zoned-earth embankments employ filter and drain zones to collect and remove seep water and preserve

204-436: A single one. The stability of the pipes is dependent on the head, and once this is larger than a critical value (0.3-0.5 of flow path length), the channel extends upstream. Beyond this, at any head greater than the critical value, erosion progresses until eventually, the pipes break through to the upstream reservoir, at which point a breach occurs. In order for backward erosion to occur, the dam or levee body must form and maintain

238-404: A small sustained overtopping 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 reservoir as the mass of water still impounded behind the dam presses against the lightened mass of the embankment, made lighter by surface erosion. As the mass of the dam erodes, the force exerted by

272-430: A surface area of 703 square kilometres (271 sq mi) and a capacity of 12.5 cubic kilometres (10,100,000 acre⋅ft) of which 5.54 cubic kilometres (4,490,000 acre⋅ft) is live. The dam's power station contains six 285 megawatts (382,000 hp) generators powered by Francis turbines for a total installed capacity of 1,710 megawatts (2,290,000 hp). There is additional room for four more generators in

306-686: A thick suspension of earth, rocks and water. Therefore, safety requirements for the spillway are high, and require it to be capable of containing a maximum flood stage. It is common for its specifications to be written such that it can contain at least a one-hundred-year flood. A number of embankment dam overtopping protection systems were developed in the early 21st century. These techniques include concrete overtopping protection systems, timber cribs , sheet-piles , riprap and gabions , Reinforced Earth , minimum energy loss weirs , embankment overflow stepped spillways , and precast concrete block protection systems. All dams are prone to seepage underneath

340-515: A ‘roof’ for the pipe. Suffusion occurs when water flows through widely-graded or gap-graded , cohesionless soils. The finer particles are transported by seepage, and the coarse particles carry most of the effective stress. Suffusion can only occur provided the fine soil particles are small enough to pass between the coarse particles and do not fill the voids in the coarser soil. Water flow velocity must also be sufficient to transport those fine particles. Suffusion leads to increased permeability in

374-630: Is a large dam on the Indus River in Pakistan , about 50 km (31 mi) northwest of Islamabad . Its height of 485 ft (148 m) above the river bed and 95 sq mi (250 km ) reservoir make it the largest earth-filled dam in the world. The principal element of the project is an embankment 9,000 feet (2,700 m) long with a maximum height of 465 feet (142 m). The dam used approximately 200 million cubic yards (152.8 million cu. meters) of fill, which makes it one of

SECTION 10

#1732779591399

408-670: Is a related phenomenon and is defined as the progressive development of internal erosion by seepage, appearing downstream as a hole discharging water. Piping is induced by regressive erosion of particles from downstream and along the upstream line towards an outside environment until a continuous pipe is formed. According to the International Commission on Large Dams (ICOLD), there are four general failure modes for internal erosion of embankment dams and their foundations: The process of internal erosion occurs across four phases: initiation of erosion, progression to form

442-449: Is initiated, a cavity is formed, leading to a reduction of stress. The roof of the cavity then collapses; the collapsed material is transported away resulting in a larger cavity. The process continues until the formation of a sinkhole. It is possible for a cavity to not collapse; this will lead to backward erosion occurring. Soil contact erosion can occur between any granular layer and a finer soil such as in silt-gravel, and often results in

476-426: The U.S. Bureau of Reclamation Internal erosion Internal erosion is the formation of voids within a soil caused by the removal of material by seepage . It is the second most common cause of failure in levees and one of the leading causes of failures in earth dams , responsible for about half of embankment dam failures. Internal erosion occurs when the hydraulic forces exerted by water seeping through

510-637: The asphalt make such dams especially suited to earthquake regions. For the Moglicë Hydro Power Plant in Albania the Norwegian power company Statkraft built an asphalt-core rock-fill dam. Upon completion in 2018 the 320 m long, 150 m high and 460 m wide dam is anticipated to be the world's highest of its kind. A concrete-face rock-fill dam (CFRD) is a rock-fill dam with concrete slabs on its upstream face. This design provides

544-549: The concrete slab as an impervious wall to prevent leakage and also a structure without concern for uplift pressure. In addition, the CFRD design is flexible for topography, faster to construct and less costly than earth-fill dams. The CFRD concept originated during the California Gold Rush in the 1860s when miners constructed rock-fill timber-face dams for sluice operations . The timber was later replaced by concrete as

578-406: The core is separated using a filter. Filters are specifically graded soil designed to prevent the migration of fine grain soil particles. When suitable building material is at hand, transport is minimized, leading to cost savings during construction. Rock-fill dams are resistant to damage from earthquakes . However, inadequate quality control during construction can lead to poor compaction and sand in

612-406: 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 beyond its spillway capacity will cause its eventual failure . The erosion of the dam's material by overtopping runoff will remove masses of material whose weight holds the dam in place and against the hydraulic forces acting to move the dam. Even

646-490: The dam, but embankment dams are prone to seepage through the dam as well; for example, the Usoi landslide dam leaks 35-80 cubic meters per second. Sufficiently fast seepage can dislodge a dam's component particles, which results in faster seepage, which turns into a runaway feedback loop that can destroy the dam in a piping-type failure. Seepage monitoring is therefore an essential safety consideration. gn and Construction in

680-450: The dam. The critical hydraulic shear stress τ c required for the initiation of concentrated leak erosion can be estimated using laboratory testing, such as the hole erosion test (HET) or the jet erosion test (JET) . Backward erosion often occurs in non-plastic soils such as fine sands . It can occur in sandy foundations, within the dam or levee, or in cofferdams under high flood pressures during construction, causing unraveling at

714-491: The dam. The core can be of clay, concrete, or asphalt concrete . This type of dam is a good choice for sites with wide valleys. 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 grades to get the right range of size for use in an embankment dam. Earth-fill dams, also called earthen dams, rolled-earth dams or earth dams, are constructed as

SECTION 20

#1732779591399

748-456: The design was applied to irrigation and power schemes. As CFRD designs grew in height during the 1960s, the fill was compacted and the slab's horizontal and vertical joints were replaced with improved vertical joints. In the last few decades, design has become popular. The tallest CFRD in the world is the 233 m-tall (764 ft) Shuibuya Dam in China , completed in 2008. The building of

782-490: The downstream face. It also occurs in landslide and flood-prone regions where slopes have been disturbed. Backward erosion is most often exhibited by the presence of sand boils at the downstream side of dams. Experiments from Sellmeijer and co-workers have shown that backwards erosion initiates in a slot through the strata that overlays the eroding soil (e.g. through excavations or drainage ditches) and then progress in many, smaller pipes (less than 2mm in height) rather than

816-405: The embankment core, greater seepage velocities and possibly hydraulic fractures. It can also lead to settlement if it occurs in the dam foundation. Soils subject to suffusion also tend to be affected by segregation . The Kenney-Lau approach is a renowned method for the analysis of suffusion, which uses the particle size distribution to assess the internal stability of a soil, which directly affects

850-578: The embankment which can lead to liquefaction of the rock-fill during an earthquake. Liquefaction potential can be reduced by keeping susceptible material from being saturated, and by providing adequate compaction during construction. An example of a rock-fill dam is New Melones Dam in California or the Fierza Dam in Albania . A core that is growing in popularity is asphalt concrete . The majority of such dams are built with rock and/or gravel as

884-453: The integrity of the downstream shell zone. An outdated method of zoned earth dam construction used a hydraulic fill to produce a watertight core. Rolled-earth dams may also employ a watertight facing or core in the manner of a rock-fill dam. The frozen-core dam is a temporary earth dam occasionally used in high latitudes by circulating a coolant through pipes inside the dam to maintain a watertight region of permafrost within it. Tarbela Dam

918-449: The largest man-made structures in the world. Because earthen dams can be constructed from local materials, they can be cost-effective in regions where the cost of producing or bringing in concrete would be prohibitive. Rock -fill dams are embankments of compacted free-draining granular earth with an impervious zone. The earth used often contains a high percentage of large particles, hence the term "rock-fill". The impervious zone may be on

952-416: The likelihood of suffusion occurring. Soil contact erosion occurs when sheet flow (water flow parallel to an interface) erodes fine soil in contact with coarse soil. Contact erosion is largely dependent on the flow velocity, which must be sufficient to detach and transport the finer particles, as well as the finer soil particles being able to pass through the pores in the coarse layer. When contact erosion

986-460: The pores and cracks of the material in the dam and/or foundation are sufficient to detach particles and transport them out of the dam structure. Internal erosion is especially dangerous because there may be no external evidence, or only subtle evidence, that it is taking place. Usually a sand boil can be found, but the boil might be hidden under water. A dam may breach within a few hours after evidence of internal erosion becomes obvious. Piping

1020-444: The power stations but there are no plans to have them installed currently. Embankment dam 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

1054-424: The primary fill. Almost 100 dams of this design have now been built worldwide since the first such dam was completed in 1962. All asphalt-concrete core dams built so far have an excellent performance record. The type of asphalt used is a viscoelastic - plastic material that can adjust to the movements and deformations imposed on the embankment as a whole, and to settlement of the foundation. The flexible properties of

São Simão Dam - Misplaced Pages Continue

1088-501: The reservoir begins to move the entire structure. The embankment, having almost no elastic strength, would begin to break into separate pieces, allowing the impounded reservoir water to flow between them, eroding and removing even more material as it passes through. In the final stages of failure, the remaining pieces of the embankment would offer almost no resistance to the flow of the water and continue to fracture into smaller and smaller sections of earth or rock until they disintegrate into

1122-465: The sides of the pipe to swell, closing it and thus limiting erosion. Additionally, if the soil lacks sufficient cohesion to maintain a crack, the crack will collapse and concentrated leak erosion will not progress to a breach. Cracks that allow concentrated leaks can arise due to many factors, including: Longitudinal cracks arise from the spreading of the embankment, while transverse openings, which are much more common, are due to vertical settlement of

1156-404: The upstream face and made of masonry , concrete , plastic membrane, steel sheet piles, timber or other material. The impervious zone may also be inside the embankment, in which case it is referred to as a "core". In the instances where clay is used as the impervious material, the dam is referred to as a "composite" dam. To prevent internal erosion of clay into the rock fill due to seepage forces,

#398601