Embankment lecture 1


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embankment dam types, earthen dam, rockfill dam, hydraulic fill dam, semi hydraulic fill dam, rolled dam, homogeneous dam, zoned dam, diaphram dam,

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Embankment lecture 1

  2. 2. INTRODUCTION • An embankment dam, as defined earlier, is one that is built of natural materials. In its simplest and oldest form, the embankment dam was constructed with low-permeability soils to a nominally homogeneous profile. • The section featured neither internal drainage nor a cutoff to restrict seepage flow through the foundation. Dams of this type proved vulnerable associated with uncontrolled seepage, but there was little progress in design prior to the nineteenth century. 1/8/2014 2
  3. 3. INTRODUCTION It was then increasingly recognized that, in principle, larger embankment dams required two component elements. 1. An impervious water-retaining element or core of very low permeability of soil, for example, soft clay or a heavily remoulded ‘puddle’ clay, and 2. Supporting shoulders of coarser earthfill(or of rockfill), to provide structural stability 1/8/2014 3
  4. 4. DESIGN AND ANALYSIS • Although the loads acting on the concrete gravity dam (discussed SOON) is the same acting on the embankment dam, the method of design and analysis of the two differ considerably. • This is mostly because the gravity dam acts as one monolithic structure, and it has to resist the destabilizing forces with its own self weight mainly. • Failure to do so may lead to its topping, sliding or crushing of some of the highly stressed regions. 1/8/2014 4
  5. 5. An embankment dam, on the other hand, cannot be considered monolithic. It is actually a conglomerate of particles and on the action of the various modes, which are much different from those of a gravity dam. Hence, the design of an embankment dam is done in a different way than that of a gravity dam. In fact, the design procedures are targeted towards resisting the failure of an embankment dam under different modes, which are explained in the next section. 1/8/2014 5
  6. 6. Embankment dam types • An embankment dam, whether made of earth completely or of rock in-filled with earth core, has a trapezoidal shape with the shoulder slopes decided from the point of stability against the various possible modes of failure, 1/8/2014 6
  7. 7. • The top crest is kept wide so as to accommodate roadway In order to check the seepage through the body of the dam, a number of variations are provided. For earthen embankment dams, these range from the following types: 1. Homogeneous dam with toe drain 2. Homogeneous dam with horizontal blanket 3. Homogeneous dam with chimney drain and horizontal blanket 4. Zoned dam with central vertical core & toe drain 5. Zoned dam with central vertical core, chimney filter and horizontal blanket 6. Zoned dam with inclined core, chimney filter and horizontal blanket 1/8/2014 7
  8. 8. Embankment dam types EARTHEN DAM ROCKFILL DAM Earth dams have been built since the early days of civilization. They are constructed mainly from earth (or soil). Fig.. Rockfill dams are constructed mainly from rockfill or pieces of rocks. Fig. 2.1(1 They require somewhat stronger foundations as compared to earth 1/8/2014 dams, 8
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  10. 10. RESERVOIR 1/8/2014 EARTHEN DAM 10
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  13. 13. For example, The Nurek dam across river Vakhsh in U.S.S.R, which is the second highest dam in the world is an earth dam and it is 300 m high 1/8/2014 but the foundation need not be as strong as those for gravity dams. 13
  14. 14. Composite earth and rockfill dams : These dmas are constructed from both earth and rockfill. Fig. The Rogun dam across river Vakhsh in U.S.S.R. is the highest dam in the work 1/8/2014 14
  15. 15. which is a composite earth and rockfill type embankment dam antf it is 335 m high. In India, Tehri dam across river Bhagirathi is the highest dam which is also a compositie earth and rockfill dam type embankment dam and it is 261 m high. 1/8/2014 15
  16. 16. Embankment Classified Earthen embankment dam Rockfill dam Composite dam 1/8/2014 16
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  19. 19. Method Of Construction • Rolled fill HOMOGE NEOUS DAM DIAPHR AM DAM ZONED EMBANKM ENT DAM • Hydraulic fill • Semi hydraulic fill dam 1/8/2014 19
  20. 20. Rolled fill • A rolled filled dam is the one which is constructed in successive, mechanically compacted layers. The material (sand, clay, gravel, etc.) is transported from the borrow Pits to the dam site by trucks or scrapers. • It is then spread within the dam section by Bulldozers to form layers of 15 to 45 cm thickness. Each layer is then thoroughly compacted bonded with the preceding layer by means of power operated rollers of proper design weight. 1/8/2014 20
  21. 21. Usually sheep foot rollers and heavy pneumatic-tyred rollers are used. For proper compaction the moisture content of the material should be near that for OMC (Optimum Moisture Content), for which required quantity of water is sprinkled on each layer of soil during compaction. 1/8/2014 21
  22. 22. Hydraulic fill • In the case of hydraulic fill dam the transportation of the material from borrow pits the dam site as well as its placement in final position in the dam is done by water. • In this case at the borrow pits the material is mixed with water to form slush which is transported through flumes or pipes and deposited near the faces of the dam. 1/8/2014 22
  23. 23. • The coarser material of the slush stay near the faces of the dam while the finer'ones moves towards centre and get deposited there. • This would provide a dam section with shoulders of coarse-free draining material and an imperious central core of fine grained material • as clay and silt. In this method no compaction is required. 1/8/2014 23
  24. 24. HYDRAULIC FILL DAM 1/8/2014 24
  25. 25. Semi-hydraulic fill dams : In semi-hydraulic fill dams, the coarse material is dumped from trucks as in case of rolled fill dam without use of water. Some of dumped material is then moved to its final position in the dam by action of water. For this jet of water are directed on dumped fill which causes finer material from fill near faces of dam to be washed away. 1/8/2014 25
  26. 26. The finer material moves towards the central position of the dam and is deposited there thus forming an impervious central core while the coarser material stays near the face of dam. However absence of proper jetting action the dumped fill faces of dam may be more dense and impervious than the material immediately below it which may result in failure. In this case no compaction is done. 1/8/2014 26
  27. 27. SEMI HYDRAULIC FILL DAM 1/8/2014 27
  29. 29. HOMOGENEOUS DAM 1/8/2014 29
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  33. 33. HOMOGENEOUS TYPE DAM A purely homogeneous type earth dam is composed of a single kind of material, dams are usually built of impervious or semi-impervious soils, but several successful have been built of relatively pervious sands and sand gravel mixture. These dams are used only for low to moderate heights 1/8/2014 33
  34. 34. A purely homogeneous section has been replaced by a modified homogeneous section in which internal drainage system in the form of a horizontal drainage blanket or rock toe or a combination of both is provided. By providing drainage system, dams with much steeper slopes may be designed and also the phreatic line (or top seepage line) is kept well within the body of the dam. 1/8/2014 34
  35. 35. Zoned type dam : • A zoned embankment dam is composed of more than one kind of material. This is most common type of a rolled fill dam in which a central impervious core is flanked by zones of materials considerably more pervious. • The pervious zones enclose, support and protect the impervious core. The upstream pervious zone provides free drainage during sudden drawdown and the downstream pervious zone acts as a drain to control the phreatic line • The impervious core checks the seepage. 1/8/2014 35
  36. 36. PERVIOUS SHELL Transition Filter 1/8/2014 36
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  39. 39. The pervious zones may consists of sand, gravel, cobbles or rock or mixtures of these materials while the core consists of impervious soil such as clay, silt, etc. In general, dam is considered as zoned embankment type dam if the horizontal width of impervious core at any elevation equals or exceeds the height of the dam above in the dam and is not less than 3 m. 1/8/2014 39
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  41. 41. Diaphragm embankment type : In a diaphragm type earth dam the bulk of the dam is constructed of pervious materials, (i.e. sand, gravel or rock) and a thin core usually known as diaphragm of impervious material is provided to check seepage. As such these dams are also sometimes known as thin core dams 1/8/2014 41
  42. 42. The position of the diaphragm may vary from a central vertical core to a blanket directly on the u/s face of the dam and in between these extreme positions there may be several intermediate positions of the diaphragm in which an inclined diaphragm sloping upstream may be provided. The inclined diaphragm is also sometimes known as burried blanket. 1/8/2014 42
  43. 43. Diaphragm type dam : - 1/8/2014 43
  44. 44. • Scan p.n.modi page no 448 1/8/2014 44
  45. 45. Shear strength of Soils : Coulomb's law : The resistance offered by a soil mass against the shearing forces, is known as shear strength of the soil. In order to ensure that the foundation soil and the soil of the body of the dam are stable, we must ensure that the shearing strength of this soil mass is sufficiently greater than the shear stresses developed within the soil mass by external forces. 1/8/2014 45
  46. 46. According to Coulomb's law, the shear strength of soil mass is provided by two factors: (1) Cohesion between the soil grains. (2) Internal friction between the soil particles. 1/8/2014 46
  47. 47. The Coulomb's law for shear strength is given by : 1/8/2014 47
  48. 48. Void ratio (e) : Porosity (n) : Bulk unit weight (y to yb) : Dry unit weight (YD) : Saturated unit weight (Ysat) : 1/8/2014 48
  49. 49. Submerged density (ysub or y') : Degree of saturation (Sr) : When soil is fully saturated, voids are completely filled with water. There is no air, Sr = 1 When soil is fully dry, voids are completely filled with air. There is no water, Sr = 0 1/8/2014 49
  50. 50. Pore water pressure and its significance in the desing of Earth Dams : 1/8/2014 50
  51. 51. Saturated sediments are more likely to undergo mass wasting because pore pressure decreases the frictional force. in a saturated sediment the weight of all the water above produces a pore pressure that tends to push the grains apart 1/8/2014 51
  52. 52. the effective normal stress is the lithostatic stress (from the weight of the sediments in grain on grain contact) minus the pore pressure 1/8/2014 52
  53. 53. total effective pressure: y1 y 1/8/2014 53
  54. 54. • Soil mass contain some voids or pores which are partly or fully filled with water, let us consider a soil mass below the water table BB as shown in fig. • The soil mass below water table BB is fully saturated and the voids are fully filled up with water . Thus soil below water table is subjected to hydrostatic uplift. • Hence as explained above 1/8/2014 54
  55. 55. Now as we know total normal pressure = density x height = p =wh At level CC 1/8/2014 55
  56. 56. • It, therefore, become evident that the effective normal stress is much less than the total normal stress, as a part of the total stress gets consumed by water as pore pressure. • The pore water pressure acts equally in all directions. It does not press the soil grains in its frictional resistance, that is why, it is called a neutral stress. • When the pore water pressure is considered, the Coulomb's law will become 1/8/2014 56
  57. 57. This clearly indicates that the shear strength of soil gets reduced due to the presence of pore pressure. The pore water pressure gets developed in the body of the earthen dam when seepage takes place through the body of the dam, thus reducing the shear st strength of the soil 1/8/2014 57
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