BRANCH :civil-2

TOPIC :EARTHEN DAM

Rajesh Goswami

OUTLINES

• Introduction • What is earthen dam?• Needs for dams construction• Types of dam• Construction of dam• Requirements of earthen dam• Environmental impacts of dams• Uses of earthen dam

INTRODUCTION• The first known dam was built in 2900 B.C. across

the Nile River to protect the city of Memphis from flooding. Dam build was continued into the time of the Roman empire, after which dam construction was literally lost until the 1800s.

• Dams are a structure also seen in nature - beavers build dams to keep the water deep enough to cover the openings to their homes, protecting them from predators.

• A barrier constructed to hold back water and raise its level, forming a reservoir used to generate electricity or as a water supply.

• A dam is a barrier that impounds water or underground streams.

• The reservoirs created by dams not only suppress floods but provide water for various needs to include irrigation, human consumption, industrial use, aquaculture and navigability.

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 materials (sand, loam, clay, and so on), with a trapezoidal or nearly trapezoidal cross section.

Needs for Dam Construction1. Drinking and domestic water supply2. Flood control3. Irrigation4. Industrial water supply5. Hydroelectric energy production6. Retention and control of sediments and Inland navigation, Improvement of water quality, Fish

Farming, Recreation facilities

TYPES OF EARTHEN DAM• Earthen dams are usually of the fixed type (without

the flow of water over the crown); they are commonly used in many countries because of their simplicity of construction and maintenance.

• Six main types of earthen dams are distinguished, depending on the materials used for the body of the dam and the methods of providing water impermeability.

• 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.

ACCORDING to HEIGHT of DAM

• High Dam or Large Dam• If the height of the dam is bigger than 100m

• Medium Dam• If the height of the dam is between 50m and 100m

• Low Dam or Small Dam• If the height of the dam is lower than 50m

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, tractors, or earth-hauling equipment.

• One advantage of an earth dam is that it can be adapted to a weak foundation, provided proper consideration is given to thorough foundation exploration, testing, and design.

• Earthen dams are distinguished according to their construction method as fill dams, which are built up by dry pouring of soil, with artificial consolidation and without consolidation (with pouring of the soil into the water or by means of a directed explosion), and hydraulic fill dams, the construction of which is accomplished by the hydro mechanization method.

• The drainage of an earthen dam is usually made in the form of a drainage shell or a drainage mattress buried in the body of the dam.

• Rock fill and paving stone facing covered with concrete or reinforced-concrete slabs are used to protect the upstream (pressure) slopes of earthen dams against wave action.

• The downstream slopes are protected by planted grass, turf, poured gravel, and crushed stone.

CONSTRUCTION OF EARTHEN DAM

• Earthen dams may be built on virtually any type of ground base (except strongly liquescent muddy soil).

• The watertight part of the dam (the baffle and core) is usually joined to the rocky base by a cutoff or a concrete joint tongue, under which a grouted cutoff is set in fissured rock.

• On a non rock base, if the water-resistant material (clay or rock) is located at an acceptable depth, the impermeable part of the dam is joined to the water-resistant material by a positive ground cutoff, a sheet pile bulkhead, or a curtain (partial cutoff).

• A blanket or partial cutoffs and a sheet pile bulkhead, are installed for a deep-lying water support.

• 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.

• 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

Small earth dam design: A typical design of a small earth-fill dam is shown in Figure 8. For stability, the upstream slope must be a minimum of 3:1. Erosion protection is required to protect the dam from wave action. This protection can be achieved with a combination of smaller and larger rocks (or other suitable material).The downstream slope requires a minimum 2:1 slope, seeded with native grasses to prevent surface erosion. The top or crest of the dam should be a minimum of 2m feet wide for a stable embankment structure. The crest elevation should be a minimum of 0.5m above the Full Supply Level (FSL) of the reservoir. The dam should be fenced to prevent livestock traffic, as this traffic can be a major cause of slope and crest degradation. A textbook perfect dam embankment structure would look like the one in Figure 9. The crest width and the slope of embankments can be designed according the guidelines in Table 2 and 3.

Figure 8: Typical earth dam layout. (Ignore riparian pipe for our purpose)

Table 2: Crest width based on different dam embankment height

Requirements of earthen dam

• b. Basic requirements of an embankment dam. Dams are a critical and essential part of the Nation’s

• infrastructure for the storage and management of water in watersheds.

• To meet the dam safety requirements, the design, construction, operation, and modification of an embankment dam must comply with the following technical and administrative requirements:

• (1) Technical requirements.• The dam, foundation, and abutments must be

stable under all static and dynamic loading conditions.

• Seepage through the foundation, abutments, and embankment must be controlled and collected to ensure safe operation.

• The intent is to prevent excessive uplift pressures, piping of materials, sloughing removal of material by solution, or erosion of this material into cracks, joints, and cavities.

• In addition, the project purpose may impose a limitation on allowable quantity of seepage.

• The design should include seepage control measures such as foundation cutoffs, adequate and non brittle impervious zones, transition zones, drainage material and blankets, upstream impervious blankets, adequate core contact area, and relief wells.

• The freeboard must be sufficient to prevent overtopping by waves and include an allowance for settlement of the foundation and embankment.• The spillway and outlet capacity must be sufficient to prevent over-topping of the embankment by the reservoir.

(2) Administrative requirements.• Environmental responsibility.• Operation and maintenance manual

ENVIRONMENTAL IMPACTS OF DAMS

ENVIRONMENTAL IMPACTS of CONSTRUCTION PHASE of DAMS

• River pollution • Erosion• Loss of aesthetic view • Air pollution• Noise pollution• Dust

ENVIRONMENTAL IMPACTS of RESERVOIRS

• Loss of land• Habitat Destruction :

• The area that is covered by the reservoir is destroyed, killing whatever habitat existed there beforehand.

• Loss of archeological and histrorical places• Loss of mineral deposits• Loss of special geological formations• Aesthetic view reduction• Sedimentation• Change in river flow regime and flood effects• Reservoir induced seismicity• Change in climate and plant species

EFFECTS of DAMS to WATER QUALITY

• Change in temperature• Turbidity• Dissolved gases in the water

• Water discharged from the spillway contains 110-120% saturated nitrogen. This amount may be destructive for fish life.

• Eutrophication• It means increase in vegetation. If moss and other plants exist in

water, quality of that water gets worse.

Uses of earthen dam

• Generation of hydroelectricity .• Irrigation.• These are often diversion dams, which stop a

river’s natural course so that water can be sent off to a different place.

• Control flooding. • These are called detention dams, which are

constructed to either stop or slow the amount of water in a river.

• 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.

• Hydropower is often used in conjunction with dams to generate electricity.

• A dam can also be used to collect water or for storage of water which can be evenly distributed between locations.

• Modern mechanical excavation methods make possible the construction of earthen dams to heights of 150 m or more.

• Dams generally serve the primary purpose of retaining water, while other structures such as floodgates are used to manage or prevent water flow into specific land regions.

REFRENCES

• GOOGLE.COM• WIKIPEDIA.COM• YOUTUBE• www.cwc.nic.in• www.walkthroughindia.com