The main components of Prithvi Dam are described below:
- cut off
- Internal drainage system and foundation
- slope protection
- surface drainage
- impermeable blanket
1. cut off
Cut off is required
- To reduce the loss of water stored 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 investigation. It is desirable to provide a positive cut off. Where this is not possible, a partial cut off may be provided with or without an upstream impermeable blanket. In any case, an adequate drainage system can be provided at the downstream.
The following guidelines can be adopted for the design of cut off.
- The cut-off should be located such that its center line is within the base of the impermeable core and above the center line of the dam.
- Positive cut-off should be done in continuous impervious sub-stratum or non-erodible rock formation to a depth of at least 0.4 m.
- A minimum bottom width of 4.0 m is recommended.
- In case of heavy loads, a side slope of at least 1:1 or flatter may be provided, while 1/2:1 and 1/4:1 may be provided in soft rock and hard rock respectively.
- The back fill material for the cut off trench shall have the same properties as are specified for the impermeable core.
- The cut off in the flanks on both sides should normally be extended to the top of the impenetrable core.
- If the cut off trench ends up in a rock formation that is weathered or has cracks, joints and cracks; And if the percolation test shows a Lujian value of more than 10, the rock foundation under the bed of the cut off trench should be grouted.
The core provides an impenetrable barrier within the body of the dam. Impermeable soil is usually suitable for the core. However, soils with high compressibility and liquid limit, and organic materials can be avoided, as they are prone to swelling and cracking.
The following guidelines are recommended for the design of cores.
- The core can be either centrally located or inclined upwards.
- The minimum upper width should be kept 3 meters.
- The top level of the core should be fixed at 0.5 m above the MWL.
- Lateral slopes can be kept 0.5:1 and 1:1.
- The thickness of the core in any section shall not be less than 30% (preferably not less than 50%) of the maximum head of working water in that section.
The function of the cover is to provide stability and protect the core. Relatively deformable materials, which are not subject to cracking in direct contact with the atmosphere, are suitable for cladding. The upper width of the dam should be provided 4.5 meters (minimum). Beams can be provided for dams whose height is more than 10 metres. The minimum berm width that can be maintained is 3 metres.
4. internal drainage system
To ensure the safety of the dam, it is very important to handle the seepage water so that the original soil particles are retained in their place. Embankments are the commonly adopted measures for the safe disposal of seepage water through dams;
- Inclined or vertical filter (chimney filter)
- horizontal filter
- rock toe
- toe groove
As far as possible locally available sand, gravel etc. should be used. An incline or vertical filter is provided on the downstream slope of the core. Its thickness has been kept at 1.0 meter (minimum). The horizontal filter collects the leak from the chimney filter and foundation, and carries it to the rock tow and tow drain. Its thickness has been kept at least 1.0 m. The standard filter criteria must be satisfied between the filter and the surrounding soil (cover or foundation). In the case of part of the dam, where the top of the water is 3 meters or less, there is no need to provide chimney filters or horizontal filters. However adequate toe protection should be provided. The height of the rock tow is usually provided as 0.2 h, where h is the height of the embankment. However, the minimum height of rock tow should be kept at 1.0 m. Where the height of the embankment is 3 meters or less, rock tow is not necessary.
A toe groove is provided at the downstream toe of the earth dam to collect the seepage through horizontal filters, rock tows and foundations and remove it from the dam by suitable surface or sub surface drains. The section of the toe groove should be large enough for seepage. The toe drain bed should be given a suitable slope to allow the seepage to the natural drains. Toe-groove depth is typically provided as 1.5 m, with a bottom width of 1 m minimum and a side slope of 1:1.
The filter material should meet the following criteria along with the base material:
- d15(f) / d15(b) > 4 and < 20
- D15(F) / D85(B) < 5
- A filter that meets the above criteria may still fail if it has an excess or lack of certain sizes or is not classified equally. The following criteria must be met.
D50(F) / D50(B) < 25
The gradation curve of the filter material should be approximately parallel to the gradation curve of the base material.
pay attention: suffix ‘F‘ stands for filter for material and ‘B’ base material, 15, 50, 85 percent of the particles, by weight, are finer than the D15, D50 and D85 particle sizes, respectively.
5. slope protection
Upstream slope protection is ensured by providing riprap. A 150 mm thick filter layer up to the top of the dam can provide a riprap of at least 300 mm thick.
Downstream slope protection is ensured by turfing or riprap. It is common practice to protect the downstream slope from rain cut by providing suitable turfing on the entire downstream slope from top to foot.
6. surface drainage
For surface drainage of downstream slope, a system of open pucca drains (chute) along the sloping surface terminating in longitudinal collecting drains at the junction of berm and slope shall be provided to drain rainwater at 50 mC/c. The section of the groove may be trapezoidal with a depth of 30 cm. From the longitudinal collection drain, rainwater is carried to paved slopes on d/s slope through 15 cm diameter pipes placed at 50 mc/c. Where no berms are provided, open paved drains should terminate at the downstream rock tow or tow drain.
7. impermeable blanket
When full cut-off is not practical impervious foundation, horizontal impermeable blanket is provided to increase the leakage path. The impermeable blanket would be attached to the core of the dam. To avoid the formation of cracks, the material should not be excessively plastic. A 300 mm thick layer of random material is recommended over the blanket to prevent cracking due to exposure to the atmosphere. As a general guideline, impermeable blankets with a minimum thickness of 1.0 m and a minimum length of 5 times the maximum water head measured from the upstream tow of the core can be provided.
- IS 12169:1987 Criteria for the design of small embankment dams
- IS 8237:1985 Code of practice for slope protection for reservoir embankments
- IS 9429:1999 Code of practice for earth and drainage systems for rock fill dams
- IS 8414: 1977 Guidelines for the Design of Under Seepage Control Measures for Earth and Rock Fill Dams
- IS 6066: 1994 Pressure Grouting of Rock Foundations in River Valley Projects – Recommendations (2nd Amendment)
- IS 10635:1993 Guidelines for requirement of free board in embankment dams
- IS 1498:1970 Classification and identification of soils for general engineering purposes
- IS 7894: 1975 Code of Practice for Sustainability Analysis of Earth Dams