Though irrigation canals may be constructed in natural or compacted earth, these suffer from certain disadvantages, like the following
• Maximum velocity limited to prevent erosion
• Seepage of water into the ground
• Possibility of vegetation growth in banks, leading to increased friction
• Possibility of bank failure, either due to erosion or activities of burrowing animals
All these reasons lead to adoption of lining of canals, though the cost may be prohibitive. Hence, before suggesting a possible lining for a canal, it is necessary to evaluate the cost visa-versa the savings due to reduction in water loss through seepage.
Apart from avoiding all the disadvantages of an unlined canal, a lined canal also has the advantage of giving low resistance and thus reducing the frictional loss and maintaining the energy and water surface slopes for the canal as less as possible. This is advantageous as it means that the canal slope may also be smaller, to maintain the same discharge than for a canal with higher friction loss. A smaller canal slope means a larger command area.
Types of Lining
1. Concrete Lining:
Cement concrete lining made from selected aggregate gives very satisfactory service. Despite the fact that they are frequently high in their initial cost, their long life and minimum maintenance make them economical. Cement concrete lining are best suited for main canals which carry large quantities of water at high velocities. However, a firm foundation is essential for avoiding any possibility of cracking due to foundation settlement. Expansive clay soils should be avoided and proper moisture and density control of the sub grade soil should be maintained while lining. In areas where the ground water table is likely to rise above the invert level of the lining and cause undue uplift pressure, drains are laid below the lining to release the water and relieve the pressure, generally, a thickness of about 5 to 12 cm is generally adopted for larger canals and stable side slopes are considered to be between 1.5H: 1V to 1.25H: 1V. Reinforcement to the extent of 0.1 to 0.4 percent of the area in longitudinal direction and 0.1 to 0.2 percent of the area in the transverse direction reduces width of the shrinkage cracks, thereby reducing seepage. Further details regarding cement concrete linings may be had from Bureau of Indian Standards code IS: 3873-1987.
2. Shotcrete Lining:
Shotcrete, that is, cement mortar in the ratio of 1 cement to 4 sand proportions is through a pump-pipe-nozzle system on the surface of the channel. Wire mesh reinforcement is generally, though not necessarily, is clamped to the channel surface (as for a rocky excavation) before applying shot Crete. Equipment units used for shot Crete construction are relatively small and easily moved. They are convenient for lining small sections, for repair of old linings, and for placing linings around curves or structures. Shot Crete linings are generally laid in a thickness of about 3.5cm, but many standard code IS: 9012-1978.
3. Brick or Burnt clay tile lining precast concrete lining:
This type of lining is popular because of certain advantages like non-requirement of skilled mason or rigid quality control. Further, since it is more labour intensive, it generates employment potential. Brick tiles can be plastered to increase the carrying capacity of canal with same section and help in increasing the life span of the lining. Sometimes a layer of tiles is laid over a layer of brick masonry. The top layer is generally laid in 1:3 cement mortal over 15mm thick layer of plaster in 1:3 cement plaster. The size of tiles is generally restricted to 30mm x 150mm x 53m. This type of lining is stable even if there is settlement of foundation, since the mortar joint between bricks or tiles provides for numerous cracks so fine that seepage is insignificant.
4. Boulder Lining:
Also called dry stone lining or stone pitching, is used for lining the earthen canal cross section, by proper placement and packing of stones, either after laying a filter layer over the soil surface or without any such filter, depending upon the site requirement. To reduce the resistance to flow, a 20 to 25mm thick cement plaster is provided as a finishing surface. Stones are generally placed on levelled sub-grade, and hand packed. This type of lining is of course suitable where stones of required specification are available in abundance locally. For details of this type of lining, one may refer to the Bureau of Indian standard code IS: 4515-1976. One advantage of this type of lining is allowing free flow of water from the submerged or saturated sub-grade into the canal. Hence, this type of lining does not need any drainage arrangement or pressure relief values, etc. which may be required for concrete or brick lining.
5. Earth Lining
The different types of earth linings that are used in canals include the following:
Stabilized earth linings: Here, the sub-grade is stabilized using either clay for granular sub-grade or by adding chemicals that compact the soil.
Loose earth blankets: Fine grained soil is laid on the sub-grade and evenly spread. However, this type of lining is prone to erosion, and requires a flatter side slopes of canal.
Compacted earth linings: Here the graded soil containing about 15 percent clay is spread over the sub-grade and compacted.
Buried bentonite membranes: Bentonite is a special type of clay soil, found naturally, which swell considerably when wetted. Buried bentonite linings for canals are constructed by spreading soil-bentonite mixtures over the sub-grade and covering it with gravel or compacted earth.
Soil-cement lining: Here, cement and sandy soil are mixed and then compacted at optimum moisture content or cement and soil is machine mixed with water and then laid.