1. Earthen Dam-Embankment Dam
Dr. Kiran Yarrakula
Associate Professor,
Civil Engineering Department,
GKCIET - A CFTI Under Ministry of Education, Government of India
Malda, West Bengal
Mobile No:
Id:

2. Content Dam Definition Types of Dams Problems in Dam Construction
Selection of Type and their Classification
Factors affecting Selection of particular type of Dam
Nagarjuna Sagar Dam
Types of Earthen Dams
Methods of Construction
Causes of Failure of Earthen Dams
Design Criteria for Earth Dams
Seepage Control in Earth Dams
Slope Protection

3. Definition of Dam and Uses
A dam may be defined as Obstruction or barrier built across a stream or a river.
At the back of this barrier water gets collected forming a pool of water.
This side on which water gets collected is called upstream side and
The other side of the barrier is called downstream
The lake of water which is formed upstream is often called a reservoir, or a dam reservoir, or a river reservoir, or a storage reservoir.

4. Uses Irrigation Drinking purpose Recreation uses Industrial Needs
At the time of floods, dams are used for protection of towns and cities
Navigation
Electricity generation
Flood control
Planning for war strategy on enemies
Dams are frequently opened in times of war

5. Planning for War Strategy on Enemies
The Dutch breached their dikes during II world war to bedevil the invading Germans
Chinese used to destroy their dikes to flood out enemy
Russian army partly destroyed the famous Dneprostroi dam in the Ukrain to keep its power plant from falling into the hands o Hitler’s men

6. Types of Dams Earth Dams Rock fill dams Solid masonary gravity dams
Earth dams are made of soil that is pounded down solidly
They built in areas where the foundation is not strong enough to bear the weight of a concrete and
Where Earth materials more easily available as a building material compared to concrete or stone or rock.

7. Important Earth Dams of the World
Green Mountain Dam on Colorado River in U.S.A
Swift Dam in Washington in U.S.A
Side flanks of Nagarjuna Sagar Dam in India
Trinity Dam in California in U.S.A
Maithan Dam in India (it is partly Earthen and partly Rockfill)

8. Rockfill Dams
Rockfill are formed of loose rocks and boulders piled in the river bed
A slab reinforced concrete is often laid across the upstream face of a rockfill dam to make it water tight
Important Rockfill Dams of the World
The Salt Springs Dam in California (345‘ height) in USA
The San Gabriel No.1 Dam (331‘ height) in USA
Cougar Dam on Mc-Knezie River on Oregon (445‘ height) in USA

9. Solid Masonry Dam
These big dams are expensive to built but more durable and solid than earth and rock dams.
They can be constructed on any dam site, where there is natural foundation strong enough to bear large weight of the dam.
These above three dams are well found in ancient days.
In recent days, four other types of dams have come into practice. These are
Hollow masonry gravity dams
Timber dams
Steel dams
Arch dams

10. Classification of the Type of Dam and Their Classifications
Classification according to material used for dam construction
Classification according to use
Storage dams
Diversion dams are also called weir or barrage
The detention dams –to trap sediment- also called debris dams
Classification according to Hydraulic Designs
Overflow dams - Spillways
Non-Overflow dams – Many times Overflow dams and
Non-Overflow dams combined together to form a composite single structure.
Rigid dams – Rigid materials like masonry, concrete, steel, timber
Non-rigid dams – Earth and rock fill materials

11. Factors affecting Selection of a Particular Dam
Topography
Geology and Foundation Conditions 
Availability of Materials
Spillway size and location
Earthquake zone
Height of Dam
Other consideration

12. Selection of Type of Dam
Topography
A narrow U-shaped valley flowing between high rocky walls would suggest a concrete dam.
A low-rolling plane country would suggest an earth fill dam.
A narrow V-shaped valley indicates choice of an arch dam.
Availability of Materials
In order to achieve economy in the dam, the materials required for its construction must be available locally at short distance from the construction site.

13. Geology and Foundation Conditions
The foundation strata should be strong enough to carry weight of dam. Hence the dam site must be surveyed by geologists so as to detect the thickness of foundation strata, presence of faults, fissured material etc. The various types of foundations generally encountered are :-
Solid-rock foundation:
Similar like granite, gneiss etc. Almost every type of dam can be built as it is strong enough to withstand the self weight of the dam and external forces acting on the dam.
Gravel foundation:
This type of foundation is suitable for earth and rock fill dams. For this foundation, low concrete gravity dams up to height of 15 meter  may also be suggested.
Silt and fine sand foundation:
This type of foundation suggests the adoption of earth or very low gravity dam, up to height of 8 meter.
Clay foundation:
This foundation may be accepted for earthen dam after special treatment.

14. Spillway size and location
The cost of constructing a separate spillway may be enormous or sometimes suitable site for construction of spillway may not be available. In such cases combining the spillway and dam into one structure may be desirable.
Earthquake zone
If dam is located in earthquake zone, its design must include the earthquake forces.
Height of Dam
Earthen dams are usually not provided for height more than 30 meter. Hence for greater height, gravity dams are generally preferred.
Other consideration
It includes life of dam, the width of roadway to be provided over the dam, overall cost of the construction and maintenance.

15. Site Selection Criteria for Dam
Suitable foundation must be available.
For economy, the length of the dam should be as small as possible and for given height it should store maximum amount of water.
A suitable site for the spillway should be available in or nearby vicinity.
The bed level at the dam site should preferably be higher than that of the river basin. This will reduce the height of the dam and will facilitate the drainage problem.
The reservoir basin should be reasonably water-tight.
Material required for the construction of the dam should be easily available locally.
The value of the land and property submerged by the proposed dam should be as low as possible.
The dam site should be easily accessible, such that it can be economically connected to important towns.
Site for establishing labour colonies and healthy environment should be available near by vicinity.

16. Earthen Dams
Earthen dams and earthen levees are the most ancient type of embankments.
They can built with natural materials and process with minimum primitive equipment.
The cost of gravity dams gas gone up because of increase in cost of concrete, masonry etc..
Earthen dams are still cheap as they can use locally available materials and require less skilled labour.

17. General view of an Earthfill dam

18. Conditions favouring an earth dam
When funding is low, the construction of an earth dam is appropriate.
When the foundation strut is not sufficiently strong.
In this situation, when sufficient spillway space is not available.
When there is the availability of local material and sufficient quarry material near the dam site, this dam is most suitable and economical in such a situation.
If the river is perennial (i.e. flows continuously throughout the year), then in such a situation, the earth is more favourable for dam construction.
When the construction of a large dam is not possible.

19. Types of Earthen Dams Homogeneous Embankment type Zone embankment type
Diaphragm type
Homogeneous Embankment Type. The simplest type of an earthen embankment consists of a single material and is homogeneous throughout.
Sometimes, a blanket of relatively impervious material may be placed on the upstream face.
A purely homogeneous section is used, when only one type of material is economically or locally available.
Such a section is used to moderately high dams for levees. Large dams are elected designed as homogenous embankments.

21. Zoned Earth Dams
A zoned earth dam make up of more than one type of material. This is the most common type of a role fill dam in which a central impervious core is flank shells of previous material on the upstream and downstream of the dam.
A transition filter generally provides between the core and the shell to prevent piping.
The central core checks the seepage through the dam, it consists of clay, silty clay or clayed silt.
The upstream and downstream pervious zone gives stability to the dam and it consists of sand, cobbles or rock or a mixture of these materials.
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 transition filters prevent the migration of the core material into the pores of the previous zone material. The downstream transition filter is useful during the steady seepage condition.
However, The transition filters are omitted if the difference in the particle size of the shell material and core material is no much or when the seepage gradient line trough dam is flat.
In place of central vertical cops them, the sloping core also provides in a zoned type earthen dam.
The main advantage is that the downstream portion to the dam can construct first and the core can be constructed later.

22. Diaphragm Earth dam
A diaphragm type earth consists of a thin impervious core known as diaphragm, which is previous shells.
It is sometimes know as a thin core dam. The impervious core is made-up of impervious soils, concrete, steel, timber, or any other material.
It acts as a water barrier to prevent seepage through the dam. The position of the diaphragm many very from a central vertical core or at the upstream faces a blanket, also sometimes known as a buried blanket.
It may be noted that the difference between a zoned earth dam and diaphragm type earthen dam is only in the thickness of the core.
If the thickness of the diaphragm core at any elevation is less than 10 m or less than the height of the embankment above the corresponding elevation.
The dam generally considers being the diaphragm type. On the other hand, if the thickness of the core equals or exceeds those limits. The dam considers being the zoned type earth dam.

23. Diaphragm Type Embankment

24. Failure of Earthen dam and embankment
Earth dams are very huge earth structures that store a large quantity of water in the upstream reservoir of the dam. The failure may cause vast damages and loss of human lives. Earth dams may fail, like other engineering structures, due to improper designs, inadequate care in the construction and maintenance, lack of maintenance, etc.. Various causes by the failure of earth dam may group Hydraulic failure-35% Seepage failure-38% Structural failure-7%

25. Hydraulic Failures of Earthfill Dams
4 out every 10 earthen dams have failed due to hydraulic failures. The Hydraulic failure of an earthen dam arise due to any of the following causes.
Erosion of Upstream face
Erosion of Downstream toe
Due to Over-topping
Due to Frost Action
Due to formation of Gullies

26. Erosion at Upstream Erosion of Upstream face
The upstream face of dam may subjected water waves which are formed due to winds and these waves notch-out the soil from upstream face which causes erosion.
Sometimes this may also cause slip of upstream slope.
To overcome this, stone pitching or rip rap should be provided on upstream face of dam.

27. Erosion of Downstream toe
The tail water present on the downstream side or cross-currents may come from spillway buckets may affects the downstream toe portion and erosion occurs. To prevent this problem, stone pitching or rip rap should be provided on the downstream face of dam up to a depth of normal tail water level. To prevent cross currents from spillway, sidewalls of spillway should be constructed with adequate height and length.

28. Due to Over-topping
Hydraulic failure also occurs when the water over-tops the dam crest. This may be due to insufficient capacity of spillway or design without proper estimation of flood level or problem in lifting of spillway gates during flood times.
To prevent over-topping of dam, adequate freeboard should be provided.

29. Structural Failure
The slope fail during construction
When an earth dam constructed of relatively impervious soil, the drainage is extremely slow.
The pore water pressure develops during and immediately after construction, especially if the construction is relatively fast.
The pore water pressure up too almost 100% of the vertical stress due to the total weight of the soil is development, therefore the slopes may fail.
Slope failure during steady seepage
This critical condition occurs when the reservoir is full and steady seepage is at its maximum rate.
The direction of seepage forces in the dam is to decrease the stability of the dam by increasing. The actuation forced and decreasing the resisting forces.

30. Failed during sudden drawdown condition
The most critical condition occurs when the reservoir suddenly gets empty without allowing appreciable drainage from the saturated soil mass. This stage is known as a sudden drawdown.
The rate at which the water is withdrawn from the reservoir would develop the sudden drawdown stage. Which mainly depends upon the type of soil on the upstream side of the dam. (Earthen Dam)

31. Design Criteria for Earth Dams
Safe against overtopping during design flood by providing adequate\spillway and outlet capacity,
Spillway be of sufficient capacity to pass peak flood by providing adequate spillway and outlet capacity to pass peak flood
Safe against overtopping by wave action providing adequate free board,
Side slopes stable during construction and under all conditions of reservoir operation. Upstream slope is safe against rapid drawdown conditions while the downstream slope is safe against sloughing,
Side slopes upstream and downstream are flat enough so that the shear stress-induced in the foundation is within the shear strength of the material comprising the foundation with a suitable factor of safety,
Upstream slope is protected against erosion by wave action while the crest and downstream slope is protected against erosion due to wind and rain. Horizontal berms at suitable intervals in upstream and downstream faces may be provided for this purpose,
Downstream slope is safe during steady seepage under full reservoir condition,
Portion of the dam downstream of the impervious core is properly drained by the provision of suitable drainage System,
Seepage flow through the dam and foundation is so controlled that there is no danger of fine particles getting washed out from the downstream by the efflux of seepage. Moreover, quantity of seepage loss is restricted to the minimum,

32. There is no possibility of free flow of water from upstream through either the dam or the foundation,
Adequate impervious core to act as water barrier. Top of impervious core is maintained higher than the maximum reservoir level,
Seepage line, i.e., phreatic line is well within the downstream face so that no sloughing of the slope takes place,
Seepage flow through the embankment, foundation and abutments is controlled so that no internal erosion takes place. The amount of ater lost through seepage must be controlled so that it does not interfere with planned project functions,
Transition filters are incorporated to satisfy the two essential conditions of filters, viz., to prevent choking of the filter by soil and have minimum of head loss in the filter,
Dam as a whole is earthquake resistant. The seismic conditions of the region investigated with reference to geological map of vicinity. India has been divided into five seismic zones.

33. Seepage control in Earth Dams
The safety of earth dam depends almost entirely on seepage control through dam and its foundation.
Hence measures of seepage control are very important for the success of an earthen dam.
Seepage control through the dam.
Seepage control through the foundation.

34. Seepage Control Through the Dam
Rock Toe
Rock toe keeps seepage line well within the dam section.
It also helps a great deal for the drainage purposes.
The height of the rock toe is kept about one fourth of the height of the dam.
Rock toe should be designed like filter.

35. Horizontal Drainage Filter
It is provided at the base of the dam, starting from down stream end of the dam and extending backwards into the dam.
Backward extension of the filter depends upon so many factors.
But this extension may at the most be up to centre line of the dam.
This filter controls seepage line and does not allow it to get exposed on D/S face of the dam.
It also accelerates the process of consolidation.
It also causes drainage of foundation.
If seepage pressure at the D/S end of the dam is still excessive, the horizontal filter drain may be continued even beyond the D/S toe of the dam.
Sometimes rock toe and horizontal filter drains are dispensed with and entire D/S portion of the dam may be made from coarse-grained soil.

36. Chimney Drains
Under conditions of large stratification, the permeability in the horizontal direction is more than in the vertical direction.
This causes a greater speed of horizontal seepage than vertical seepage.
Chimney drain or filter, if correctly built, intercepts all the seepage from the dam regardless of the stratification in the dam.
Chimney drains also render earth dam earthquake resistant.

37. Seepage Control Through the Foundation
Impervious Cut-off
The cut-off is a wall of relatively impervious material.
It is used to prevent seepage through the foundation.
It is always led down into the foundation from the ground surface.
If possible, cut-off should extend upto the impervious strata lying below the ground level.
Partial cut-offs do not prove much effective in preventing seepage.
A 90%depth of cut-off reduces about 25% seepage.

38. D/S Seepage Berms
Additional berms may be built on the D/S side of the dam in continuation of the D/S end.
Such a bean is useful in controlling seepage where D/S top strata is relatively thin and uniform or even top strata is absent.
Drainage Trenches
This measure is adopted when top stratum is pervious and thin.
Porous drains remain enclosed in gravel filters.

40. Relief Well
It is such a well which if not constructed would cause formation of sand boils and possibly sub-surface piping.
They reduce the sub surface uplift pressure D/S of the dam.
They intercept the seepage through the foundation and control the outlet for seepage.
Relief wells become necessary when impervious layer overlies a pervious layer and the thickness of overlying impervious layer is less than the depth of water impounded.
Relief wells consist of 10 cm to 15 cm diameter holes filled with filter material.

41. Upstream Impervious Blanket
Such a blanket when constructed over a pervious foundation reduces the quantity of seepage on D/S side.
It also causes reduction in uplift pressure throughout the D/S side.
The provision of U/S blanket is found economical and more effective when the depth of previous overburden is large and the provision of the cut-off wall is uneconomical.
Blankets are particularly effective when there are cracks and fissures in the foundation beneath the dam structure. In such cases they seal such openings and reduce the seepage considerably.
The blanket should be composed of such material which is at least 100 times less pervious than the foundation material.