FUNCTIONS AND DESIGN REQUIREMENTS OF COMPONENTS OF EARTH DAM

The main components of the earth dam are described below:

1. Cut off
2. Core
3. Casing
4. Internal drainage system and foundations
5. Slope protection
6. Surface drainage
7. Impervious blanket

1. Cut off

The cut off is required,

• To reduce loss of stored water through foundations and abutments
• To prevent sub-surface erosion by piping.
• The type of cut off should be decided on the basis of detailed geological investigations. It is desirable to provide positive cut off. Where this is not possible, partial cut off with or without upstream impervious blanket may be provided. In any case, adequate drainage arrangements may be provided on the downstream.
  The following guidelines may be adopted for design of cut off.
• The cut off shall be located such that its centre line should be within the base of impervious core and should be upstream of centre line of dam.
• The positive cut off should be keyed at least to a depth of 0.4 metre into continuous impervious sub stratum or non-erodible rock formation.
• A minimum bottom width of 4.0 metre is recommended.
• Side slopes of at least 1:1 or flatter may be provided in case of over burden while 1/2:1 and 1/4:1 may be provided in soft rock and hard rock respectively.
• The back fill material for cut off trench shall have same properties as those specified for impervious core.
• The cut off in the flanks on either side should normally extend upto the top of impervious core.
• If cut off trench is terminated in rock formation which is weathered or have cracks, joints and crevices; and if percolation test exhibit a lugeon value of more than 10, then rock foundation below the bed of cut off trench should be grouted.

2. Core

The core provides impermeable barrier within the body of the dam. Impervious soils are generally suitable for the core. However soils having high compressibility & liquid limit, and having organic contents may be avoided, as they are prone to swelling & formation of cracks.
Following guidelines are recommended for design of core.

• The core may be located either centrally or inclined upstream.
• The minimum top width should be kept 3 metre.
• The top level of the core should be fixed at 0.5 m above MWL.
• The side slopes may be kept 0.5:1 and 1:1.
• Thickness of core at any section shall not be lesser than 30% (preferably not less than 50 percent) of maximum head of water acting at that section.

3. Casing

The function of casing is to impart stability and protect the core. The relatively pervious materials, which are not subjected to cracking on direct exposure to atmosphere, are suitable for casing. Top width of dam should be provided as 4.5 m (minimum). The berms may be provided for the dam, which are more than 10 m in height. Minimum berm width may be kept as 3 m.

4. Internal Drainage System

To ensure safety of dam, it is very important to handle the seepage water in the dam so as to maintain the original particles of soils in their place. The measures commonly adopted for safe disposal of seepage water through embankment dams are;

• Inclined or vertical filter (chimney filter)
• Horizontal filter
• Rock toe
• Toe drain

As far as possible locally available sand, gravel etc should be used. Inclined or vertical filter is provided just on downstream slope of core. Its thickness is kept 1.0 metre (minimum). Horizontal filter collects the seepage from chimney filter & foundation, and carries to the rock toe & toe drain. Its thickness is kept minimum as 1.0 metre. The standard filter criterion between filter and adjoining soil (casing or foundation) should be satisfied .In case of dam portions, where the head of water is 3 m or less it is not required to provide chimney filter or horizontal filter. Adequate toe protection shall however be provided. The height of rock toe is generally provided as 0.2 H, where H is the height of embankment. However minimum height of rock toe be kept as 1.0 metre. Rock toe is not necessary where height of embankment is 3 m or less.
The toe drain is provided at the downstream toe of the earth dam to collect seepage from horizontal filter, rock toe & through foundation and to discharge it away from the dam by suitable surface or sub surface drains. The section of toe drain should be adequate enough to carry seepage. The bed of toe drain should be given a suitable slope to lead the seepage to natural drains. Depth of toe drain is usually provided as 1.5 m with bottom width of 1 m minimum and side slopes of 1:1.
The filter material should satisfy the following criteria with the base material:

• D15 (f) / D15 (b) > 4 and < 20
• D15 (f) / D85 (b) < 5
• A filter that satisfies the above criteria may yet fail if it has an excess or lack of certain sizes or is not uniformly graded. The following criteria must be fulfilled.

D50 (f) / D50 (b) < 25

The gradation curve of the filter material should be nearly parallel to the gradation curve of the base material.
Note: The suffix ‘f‘ stands for the filter material and ‘b’ for the base material. 15, 50, 85 percent particles, by weight, respectively are finer than D15, D50 and D85 particle size.

5. Slope Protection

Upstream slope:

The upstream slope protection is ensured by providing riprap. A minimum of 300 mm thick riprap over 150 mm thick filter layer may be provided upto the top of dam.

Downstream slope:

The downstream slope protection is ensured by turfing or riprap. It is usual practice to protect the downstream slope from rain cuts by providing suitable turfing on the entire downstream slope from top to toe.

6. Surface Drainage

For surface drainage of downstream slope, a system of open paved drains (chutes) along the sloping surface terminating in the longitudinal collecting drains at the junction of berm and slope shall be provided at 50 m c/c to drain the rain water. The section of drain may be trapezoidal having depth of 30 cm. From longitudinal collecting drain, the rain water is carried through 15 cm diameter pipes placed at 50 m c/c into paved chutes on the d/s slope. Where no berm has been provided, the open paved drains (chutes) should terminate in the downstream rock toe or toe drain.

7. Impervious Blanket

The horizontal impervious blanket is provided to increase the path of seepage when full cut-off is not practicable impervious foundation. The impervious blanket shall be connected to the core of the dam. To avoid formation of crack, the material should not be highly plastic. A 300mm thick layer of random material over the blanket is recommended to prevent cracking due to exposure to atmosphere. As a general guideline, impervious blanket with a minimum thickness of 1.0 metre and a minimum length of 5 times the maximum water head measured from upstream toe of core may be provided.

REFERENCES:

1. IS 12169:1987 Criteria for design of small embankment dams
2. IS 8237:1985 Code of practice for protection of slope for reservoir embankment
3. IS 9429:1999 Code of practice for drainage system for earth and rock fill dams
4. IS 8414:1977 Guidelines for design of under seepage control measures for earth and rock fill dams
5. IS 6066:1994 Pressure grouting of rock foundations in river valley projects – Recommendations (Second revision)
6. IS 10635:1993 Guidelines for free board requirement in embankment dams
7. IS 1498:1970 Classification and identification of soils for general engineering purpose
8. IS 7894:1975 Code of practice for stability analysis of earth dams