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Dams.ppt
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- 2. Dams and reservoirs- literature • Bell F.G., Engineering geology and geotechnics
- 3. Dams Dam is asolid barrier constructed at a suitable location across a river valley to store flowing water. • Storage of water is utilized for following objectives: • Hydropower • Irrigation • Water for domestic consumption • Drought and flood control • For navigational facilities • Other additional utilization is to develop fisheries
- 4. Structure of Dam Heel Gallery Toe Spillway (insidedam) Crest NWL Normal water level MWL Max. level Free board Sluice way Upstream Down stream
- 5. Parts of adam • Heel: contact with the ground on the upstream side • Toe: contact on the downstream side • Abutment: Sides of the valley on which the structure of the dam rest • Galleries: small rooms like structure left within the dam for checking operations. • Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry. • Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side • Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.
- 6. Dams: forces applied •Vertical static forces • Lateral force applied by water body • Dynamic forces – wave action – overflow of water (controlled by spillway channels) – earthquakes and tremors – ice/freezing
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- 8. ACCORDING to theSIZE of the DAM 1. Large (Big) dam 2. Small dam • International Commision on Large Dams, (ICOLD) assumes a dam as big when its height is bigger than 15m. • If the height of the dam is between 10m and 15m and matches the following criteria, then ICOLD accepts the dam as big: • If the crest length is bigger than 500m • If the reservoir capacity is larger than 1 million m3 • If the flood discharge is more than 2000 m3/s • If there are some difficulties in the construction of foundation
- 9. ACCORDING to theSTATICAL DESIGN of DAM BODY • Gravity Dams • Arch Dams • Butress Dams • Embankment Dams • Composite Dams
- 10. Dams: types • Gravitydam: rigid monolithic structure – Trapezoidal cross section – Minimal differential movement tolerated – Dispersed moderate stress on valley floor and walls • Arch dam: high strength concrete wall – Convex faces upstream – Thin walled structure – Relatively flexible – Huge stresses imposed on valley walls and floor • Earth dams: bank or earth or rock with impermeable core – Core of clay or concrete, extended below ground – Sand or gravel drains built to cut fluid pressure – Low stress applied to valley floor and walls
- 11. TYPES OF DAMS •Gravity Dams: • These dams are heavy and massive wall-like structures of concrete in which the whole weight acts vertically downwards Reservoir Force As the entire load is transmitted on the small area of foundation, such dams are constructed where rocks are competent and stable.
- 12. • Bhakra Damis the highest Concrete Gravity dam in Asia and Second Highest in the world. • Bhakra Dam is across river Sutlej in Himachal Pradesh • The construction of this project was started in the year 1948 and was completed in 1963 . • It is 740 ft. high above the deepest foundation as straight concrete dam being more than three times the height of Qutab Minar. • Length at top 518.16 m (1700 feet); Width at base 190.5 m (625 feet), and at the top is 9.14 m (30 feet) • Bhakra Dam is the highest Concrete Gravity dam in Asia and Second Highest in the world.
- 13. Buttress Dam: • ButtressDam – Is a gravity dam reinforced by structural supports • Buttress - a support that transmits a force from a roof or wall to another supporting structure This type of structure can be considered even if the foundation rocks are little weaker Buttress Dams use multiple reinforced columns to support a dam that has a relatively thin structure. Because of this, these dams often use half as much concrete as gravity dams
- 14. • These typeof dams are concrete or masonry dams which are curved or convex upstream in plan • This shape helps to transmit the major part of the water load to the abutments • Arch dams are built across narrow, deep river gorges, but now in recent years they have been considered even for little wider valleys. Arch Dams: Arch Dams utilize the strength of an arch to displace the load of water behind it onto the rock walls that it is built into.
- 16. Earth Dams: • Theyare trapezoidal in shape • Earth dams are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth. • Earthen dams are relatively smaller in height and broad at the base • They are mainly built with clay, sand and gravel, hence they are also known as Earth fill dam or Rock fill dam
- 17. EMBANKMENT DAMS (Rock Fillor Earth Fill Dams) • They are mostly composed of natural materials such as, clay, sand, gravel etc... • Impervious core is placed in the middle of the embankment body • Generally riprap is used to control erosion
- 18. COMPOSITE DAMS • Compositedams are combinations of one or more dam types. Most often a large section of a dam will be either an embankment or gravity dam, with the section responsible for power generation being a buttress or arch. Gravity & Rock Fill
- 19. Dams and reservoirs Reservoirs •Site selection • Leakage from reservoirs • Sedimentation • Stability: effect of raised WT Dams • Types • Forces on a dam • Geology and dam sites • Rock types and dams • Dams on soils • Ground improvement
- 20. ENGINEERING GEOLOGICAL STUDIESfor DAM CONSTRUCTION
- 21. RECONNAISSANCE STUDY 1. Evaluationof the data having at archives 2. Field investigation for limited time (Reconnaissance Study) 3. Some maps in small scale, for example 1/25.000 or 1/50.000 4. Some hydraulic data about a. Basin b. Precipitation area c. Runoff, maximum discharge 5. Some approach to the reservoir area, dam site and type of dam and height of dam...etc 6. Photogeological studies 7. A preliminary report
- 22. PRELIMINARY STUDIES atthe RESERVOIR AREA and DAM SITE 1. Dam site investigations 1. Location of dam axis 2. Location of diversion tunnel 3. Location of spillway 4. Location of powerhouse...etc 2. Geological studies 3. Geophysical surveying 4. Underground investigations 1. Boreholes 2. Investigation galleries 3. Pitholes 5. Surveying for materials 1. Field surveying 2. Laboratory tests
- 23. 6. Slope stabilityinvestigations 7. Earthquake hazard & risk analysis 8. Environmental studies 9. Leakage possibilities from reservoir area 10. Leakage possibilities from dam site 11. Erosion, sedimentation & siltation
- 24. DETAILED INVESTIGATION atDAM SITE 1. Topographic surveyings 2. Geological mappings 3. Underground explorations Boreholes, adits....etc 4. Hydrogeological studies 5. Slope stability analysis
- 25. FACTORS AFFECTING tothe PLACE of the DAM AXIS • Topography • Geology • Materials • Spillway location availability • Derivation • Sediments in the flowing water • Water quality • Expropriation costs • Earthquake possibility • Downstream water rights
- 26. Site Selection –Geological Consideration • Narrow valley • Bed rock at shallow depth • Competent rock to offer stable foundation • Proper Geol structure
- 27. structure • Horizontal strata-- desirable – load of dam acts perpen to strata. • Alternate competent (qtzite) and incomp (shale) not desirable • Beds with gentle upstream dip = ideal, resultant force acts perpen / oblique to bedding plane, percolation of water directed upstream towards res
- 28. structure • Beds withgentle downstream dip = undesirable • WHY?
- 29. Reservoirs: purpose • Waterstorage • Flood prevention • Power
- 30. Reservoirs: site selection •Hydrological considerations • Fundamental controls – topography – climate – geology Water added Net amount of water available for storage Water subtracted Rainfall in river basin Infiltration Evaporation Transpiration Runoff
- 31. Reservoirs: leakage Water added Leakage from reservoir Water subtracted - Rainfallin river basin Infiltration Evaporation Transpiration Net amount of water available for storage Runoff - 1. Dam bypass 2. Water table effects
- 32. river reservoir before after water table divide Leakageto next valley Bedrock with a water table and finite permeability new water table Reservoirs: water table leakage-1
- 33. river before Bedrock with low permeability:aquiclude High permeability layer Water table in aquifer reservoir after High permeability layer Modified water table in aquifer Leakage to next valley Reservoirs: water table leakage-2
- 34. Most Important • Riveris Effluent or Influent • Influent = river feeds the GW • Effluent = river is fed by GW • Influent river = percolation will occur
- 35. Silting problem • Measures: 1. Terracing of slope and construction of retaining wall 2. Check Dam settling basins 3. Diversion of sed-loaded water