Buried and Protected type Membrane Lining
- These linings are such lining in which a waterproof thin membrane is place on the prepare subgrade.
- Thereafter it is covered by a protective layer of earth or gravel.
- The protective layer provides protection to the lining against damage due to outside effects.
- Membrane itself provides imperviousness.
- The commonly used buried membranes may be:
1. Sprayed asphaltic lining.
- In this hot asphalt is sprayed on the subgrade which acts as water proofing barrier.
- 2. Prefabricated asphaltic membrane lining.
- In this, asphalt lined papers, clothes, mats etc. are used to put a barrier against seepage.
- All these fabrications are available in marked in form of rolls.
- The membrane is laid on smooth well prepared subgrade and covered with a fine soil or earth.
3. Plastic or rubber membrane lining.
- In this case, plastic or synthetic rubber membrane is used as water proofing membrane.
- Out of numerous such membranes polyethylene film has shown encouraging results.
- This lining is liable to be easily ruptured by sharp stones or weed growth and as such.
- The subgrade should be prepared smooth and treated with herbicide so as to prevent weed growth. .
4. Bentonite and clay membrane lining.
- It has not been used on irrigation canals as yet.
- The main characteristic of Bentonite is swelling due to absorption of water.
- On swelling the membrane becomes perfectly water-tight and controls seepage from canals.
- The bentonite layer is formed by laying 2.5 to 3 cm thick layer of bentonite over a prepared subgrade.
- The layer is lastly covered with 15.30 cm protective blanket of suitable earth or gravel.
Under this category of lining following linings come.
(i) Soil cement lining.
- In this lining, 2 to 5% cement is mixed dry with fine soil of which 35% passes through 200 sieve.
- This mixture is prepared in dry state of the soil.
- The water is sprinkled on this mixture and compacted on the prepared subgrade.
- The lining is kept wet for about a week before water is allowed to flow in the canal.
(ii) Clay puddle lining.
- The clay puddle is prepared by first excavating the clay and then exposing it to weathering.
- After a week or 10 days of exposure, water is added and clay is pugged throughly.
- The pugged clay is known as clay puddle.
- The pugged clay is put along the perimeter of the canal to act as lining.
- The lining of clay puddle may be about 30 cm.
- It is generally protected by earth layer so that it is not exposed and cracked when canal is closed for few days.
(iii) Sodium carbonate lining.
- A mixture is prepared with local soil by mixing about 10% clay and 6% sodium carbonate.
- This mixture is added with water and laid in a layer of 10 cm thickness.
- It may be used on water courses or other small channels.
- It is not suitable for larger canals as it is not durable.
Porous Type Lining
- In head reaches, the groundwater table is generally much higher than the bed level of the main canal.
- The porous lining is advisable in such circumstances.
- The porous lining allows water pressure to be released and thus occurrence of back pressure is eliminated.
- 15 cm inverted filter is spread evenly on the prepared sub-grade and stone pitching is done by hand packing.
- If stones are not available near the site, brick pitching may be done.
- This lining does not provide any imperviousness but is used for the drainage of the banks.