DHS introduction to dams and hydraulic structures.pptx

DHS presentation on unit 1

1. Unit – IV a) Earth Dam What is Earthen Dam Earth fill dam also called Earth Dam or Embankment Dam Dam built up by compacting successive layers of earth,using the most impervious materials to form a core and placing more permeable substances on the upstream and downstream sides A dam built of soil material ( sand,clay,loam and so on) with a trapazoidal or nearly trapezoidal cross section. 1

2. Earth Dam An earth dam is composed of suitable soils obtained from borrow areas or required excavation and compacted in layers by mechanical means. Following preparation of a foundation, earth from borrow areas and from required excavations is transported to the site, dumped and spread in layers of required depth. The soil layers are then compacted by tamping rollers,sheeps foot rollers,heavy pneumatic tired rollers,vibratory rollers,tractores or earth hauling equipment. One advantages of an earth dam is that it can be adopted to a weak foundation 2

3. Images :- Earthen Dam 3

4. Factors Affecting Selection of Type of Dam Whenever it is decided to construct a dam, the first question that one face is which type of dam will be most suitable and most economical ? * Following are the factors affecting selection of dam site by dam type. Topography Geology and Foundation Conditions Availability of materials Spillway size and location Earthquake zone Height of the Dam Other factors such as cost of construction and maintenance , life of dam, aesthetics etc. 4

5. Conditions favoring selection of earth and rock fill dams are as follows : (i) Significant thickness of soil deposits overlying bedrock. (ii) Weak or soft bedrock which would not be able to resist high stresses from a concrete dam. (iii) Abutments of either deep soil deposits or weak rock. (iv) Availability of a suitable location for a spillway. (v) Availability of sufficient and suitable soils from required excavation or nearby borrow areas. 5

6. ADVANTAGES OF EARTH DAMS 1. Availability of earth materials Earth is readily available in most parts of the world close to possible dam sites. . 2. Easy handling of earth Earth can be excavated by hand, transported in baskets and compacted with cattles walking over it. In suitable situation, earth can be handled and moved with latest machinery. 3. Foundation condition Earth dams are suited to the sites where a masonry dam cannot be used for structural reasons. These dams can be constructed even on compressible foundations. The intensity of foundation stress due to earth is less than that due to solid masonry. 4. Cost of construction The most important advantage of an earth dam compared to masonry dam, is its lesser cost. It has been observed that the total cost of an earth dam, is roughly one-half of a concrete dam . 6

7. DISADVANTAGES OF EARTH DAMS Non-availability of the materials at or near the dam site . Greater maintenance cost as compared to a good concrete dam . Earthen dams are unsuitable for spillways . If it is not adequately compacted during construction, the dam will have weak structure hence prone to seepage . Earth dams require continual maintenance to prevent erosion, tree growth, subsidence, animal and insect damage and seepage. 7

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9. Earth dams are further divided into the following types: The main types of earthen dam Homogeneous earth dam Diaphragam type earthen dam Zoned earth dam 2. Based on Method of construction Rock fill earth dam Hydraulic fill earth dam Semi-hydraulic earth dam 9

10. Homogeneous earth dam A modified homogeneous section in which internal drainage system ( Hz.filter drain or rock toe) is provided ) Advantages Control the action of seepage,so as to permit much steeper slopes. 10

11. (ii ) Diaphragam type earthen dam A thin diaphragm of impervious material is provided to check the seepage. Materials:- Impevious soil,cement concrete,bituminous concrete. Types :- Central vertical core :- A blanket at the upstream face 11

12. iii) Zoned embankment type Zoned embankment type earth dam is the one in which the dam is made up of more than one material Consists of :- Central impervious core Shells: flanking the core and more pervious Transition zone (Optional) Drainage system 12

13. Rock –fill earth dam Rockfill dams are mainly made from dumped and compacted rock fill. Rockfill dams are permeable. They have an impermeable core or an impermeable layer on the upstream face of the dam to prevent seepage through the porous core. 13

14. ii) Hydraulic –fill earth dam . A hydraulic fill dam is one in which the material is transported in suspension in water to the embankment where it gets placed by sedimentation. Hydraulic fill is also a term used in hard rock mining and describes the placement of finely ground mining wastes into underground  stopes in a slurry by boreholes and pipes to stabilize the voids. 14

15. ii) Semi-hydraulic earth dam. 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. 15

16. M ain components parts of the earth dam  : Cut off. Core. Casing. Hearting Rock toe Pitching Berms Internal drainage system and foundations. Slope protection. Surface drainage. Impervious blanket. Drains- L- drain Cross drains Toe Drain Transition filter 16

17. C omponents   of earthen dam   17

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20. Failure of earthen dam  : Hydraulic Failures Erosion of Upstream face Erosion of Downstream toe Due to Over-topping Due to Frost Action Due to formation of Gullies Toe erosion 2.Seepage Failures Piping through Foundation Piping through Dam body Sloughing of Downstream toe 3.Structural Failures Sliding of Foundation Sliding of Embankment Failure due to earth quake Failure due to leaching of water soluble salts. 20

21. Various Causes of Failures of Earth dams Erosion of Downstream Due to Spillway Overflow Erosion of Upstream Hydraulic Failures:- 21

22. Over-topping of Dam Formation of Gullies Due to Rainwater 22

23. 2.Seepage Failures iii) Sloughing of Downstream  ii) Piping Through Dam Body near Conduit Region i) Piping through foundation 23

24. 3.Structural Failures Sliding of Foundation Upstream slope failure 24

25. Control of seepage through Embankment   : Provision of Impervious core, Provision of rock toe H orizontal drainage blanket Relief wall Chimney Drain Control of seepage through the foundation   : Impervious cutoff Upstream impervious blanket Drainage trenches Relief wall Downstream seepage berms 25