Design of Earthfill Dam Components

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Earthfill dam contains various components such as top width, core, freeboard, casing, drainage system etc. which are designed using empirical rules or on the basis of existing dams of similar characteristics. Each component of earthfill dam should be designed for critical loading conditions.

Contents:

Design of Components of Earthfill Dam

Design of Components of Earthfill Dam

1. Top Width of Earthfill Dam

The top width or crest width of dam should be sufficient to withstand seismic forces and wave action. The seepage line should be within the dam width when the reservoir is full. The top width of a dam is decided based on the height of dam. Following are the formula for top width for different heights of dams.

For very low dams

W = H/5 + 3 ------- (1)

For dams lower than 30 m

W = 0.55(H)^1/2 + H/5 ---------- (2)

For dams higher than 30 m,

W = 1.65(H + 1.5)^1/3---------- (3)

W = 1.67(H)^1/2---------- (4)

Balustrades are provided at the end of the roads to prevent car falling off the slopes.

Fig 1: Top Width of Dam Using as Pavement

2. Central Core of Earthfill Dam

Central core of dam is made of impervious materials and it acts as a barrier against seepage flow of water through dam. The shear strength of core materials should be less than that of embankment materials. Sometimes there is chance of seepage occurrence due to capillary action of water to prevent this, the height of central core should be minimum 1 m more than the maximum reservoir level and its thickness should not less than 50% of maximum head of water. Different types of soils classified based on gradation and their suitability for core construction is tabulated below. Table: Different Types of soils and their suitability for core construction

Type of soil	Suitability
Very well graded mixtures of sand, gravel and fines of which 15% of material (D₈₅)are coarser than 50 mm and 50% of material (D₅₀) are coarser than 6 mm.	Very Good
Well graded mixture of sand, gravel and clayey fine, D₈₅ coarser than 25 mm. Fines consisting of inorganic clay (clay with plastic index > 12) or high plastic tough clay (clay with plastic index > 20).	Good
Fairly well graded, gravelly, medium to coarse sand with cohesion less fines, D₈₅ coarser than 19 mm, D₅₀ between 0.5 to 3 mm. Clay of medium plasticity (clay with plastic index > 12)	Fair
Clay of low plasticity with little coarse fraction. Plastic index between 5 to 8, liquid limit > 25 Silts of medium to high plasticity with little coarse fraction having plastic index greater than 10.	Poor

Fig 2: Core and Outer shell of Earthfill Dam

3. Outer Shell of Earthfill Dam

Outer shell also called as casing and its function is to protect the central core of dam. It covers the upstream and downstream slopes and is made of pervious soil. The slope maintained on both sides of dam is decided based on the type of soil used. Table: Side slopes of earth dams for different type of soils

Type of soil	Upstream Slope ( H : V)	Downstream Slope (H : V)
Homogeneous well graded	2.5 : 1	2 : 1
Homogeneous course silt	3 : 1	2.5 : 1
Homogeneous silty clay ( H < 15 m)	2.5 : 1	2 : 1
Homogeneous silty clay ( H > 15 m)	3 : 1	2.5 : 1
Sand with clay core	3 : 1	2.5 : 1
Sand with RC diaphragm	2.5 : 1	2 : 1

4. Berms of Earthfill Dam

Berms are provided in earth fill dams to perform following functions.

To observe the conditions of slope protection.
For maintenance and repair works purposes.
To increase seepage flow line length.

Berms are provided on downstream side with a width of 1 - 2 m and at an height interval of 10 to 15 m.

Fig 3: Berms on downstream side of Earthfill dam

5. Freeboard of Earthfill Dam

Freeboard is the vertical distance between the crest of dam to the water level surface. If the vertical distance is measured from normal pool level to the top of dam then it is called as Normal freeboard. If it is taken from maximum reservoir level to the top of dam then it is known as minimum freeboard. The distance between normal and minimum freeboards is called surcharge head. For any type of dam, adequate freeboard must be provided to prevent over-topping.

Table 1: Minimum freeboard values for earth dam recommended by U.S.B.R

Type of Spillway	Height of Dam	Minimum Freeboard
Uncontrolled spillway	For all heights	2 – 3 m
Controlled Spillway	Up to 60 m	2.5 m
Controlled Spillway	More than 60 m	3 m

Additional freeboard of 1.5 m is provided if dam is situated in very low temperature regions.

Fig 4: Freeboard of Earthfill dam

6. Cutoff Trench of Earthfill Dam

The seepage loss of water through foundation and abutment of earth dam can be prevented by cutoff trench. Subsurface erosion affected by the piping action can also be prevented by providing cutoff trench. Following are guidelines to be considered while designing cutoff trench for earth fill dam.

The center line of cutoff should be within the base of impervious central core.
Bottom width of cutoff trench should be at least 4 m.
Back fill material used for cutoff trench should same as the material used for impervious core.
The positive cutoff should be provided at least to a depth of 0.4 m into continuous impervious sub-stratum.
If there is a weathered rock stratum or rock formation with cracks and joints is found then cutoff trench should be terminated.

Fig 5: Cutoff Trench

7. Slope Protection of Earthfill Dam

Both upstream and downstream side of earth dam are subjected to different type of problems and to prevent this slope protection is required. Upstream slope of dam is always subjected to wave action by water and deterioration of dam may occur. To prevent this, 300 mm rock rip-rap layer is provided using stone boulders or hand packed stones. It is constructed over 150 mm filter layer. Downstream slope of dam may damage due to severe rains or during floods. The cheap and best method to protect the slope on downstream side is by planting grass on it. Then the water will easily slipped away from the surface.

Fig 6: Slope protection on both sides

8. Drainage System in Earthfill Dam

Two type of drainage systems are required for the earth dams and they are

Internal Drainage System
Surface drainage System

Internal Drainage System

Internal drainage system is provided for the safe disposal of seepage water. The filter material generally used in this system is sand, gravel etc. The seepage water is disposed through four stages which are as follows:

Chimney filter
Horizontal filter
Rock toe
Toe drain

Chimney filter is provided either vertically or with inclination on the downstream side of core. The minimum thickness of chimney filter should be 1 m. Seepage water collected by the chimney filter is received by the horizontal filter and transfers to the rock toe. Horizontal filter thickness should also be minimum 1 m. chimney and horizontal filters are not necessary if the head of water is 3 m or less. Rock toe collects the water from horizontal filter and Carries to the toe filter. The height of rock toe should be minimum 1 m or 20% of height of dam whichever is more. It is not necessary if the height of dam is less than 3 m. Toe drain is the last of internal drainage system which is arranged at the downstream toe of dam. It collects the seepage water coming from rock toe and carries it into the surface drains.

Fig 7: Internal Drainage System

Surface drainage System

Surface drainage system is provided to drain the rainwater from the surface of dam. In this system, 30 cm deep trapezoidal shaped open drains are provided on the sloped surface of earth dam at the junction of berms. 50 m center to center distance is maintained between these drains. The collected rain water is carried to the toe drain provided at the downstream toe through 15 cm diameter pipes. Also Read: Types of Earthfill Dams - Applications and Advantages