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.
- Design of Components of Earthfill Dam
Design of Components of Earthfill Dam
1. Top Width of Earthfill DamThe 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.
2. Central Core of Earthfill DamCentral 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|
3. Outer Shell of Earthfill DamOuter 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 DamBerms 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.
5. Freeboard of Earthfill DamFreeboard 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|
6. Cutoff Trench of Earthfill DamThe 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.
7. Slope Protection of Earthfill DamBoth 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.
8. Drainage System in Earthfill DamTwo type of drainage systems are required for the earth dams and they are
- Internal Drainage System
- Surface drainage System
Internal Drainage SystemInternal 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