After reading this article you will learn about the lining of irrigation canals which is the most effective method of preventing waterlogging in a canal. Also learn about: Types of Lining and Its Failure.
Preventing Waterlogging in Irrigation Canals:
The most effective method of preventing waterlogging in a canal irrigated area, however, is to eliminate or reduce the seepage of canal water into the ground. This can be achieved by the lining of irrigation canals (including watercourses, if feasible).
Most of the irrigation canals in India today are earth channels. The major advantage of an earth channel is its low initial cost.
The disadvantages of an earth channel are:
(i) The low velocity of flow maintained to prevent erosion necessitates larger cross-section of channels,
(ii) Excessive seepage loss which may result in water-logging and related problems, such as salinity of soils, expensive road maintenance, drainage activities, safety of foundation structures etc.,
(iii) Favourable conditions for weed growth which further retards the velocity, and
(iv) Breaching of banks due to erosion and burrowing of animals. These problems of earth channels can be got rid of by lining the channel.
A lined canal decreases the seepage loss and, thus, reduces the chances of waterlogging. It also saves water which can be utilised for additional irrigation. A lined canal provides safety against breaches and prevents weed growth thereby reducing the annual maintenance cost of the canal. Because of relatively smooth surface of lining, a lined canal requires a flatter slope.
This results in the increase of command area of the canal. The increase in the useful head is advantageous in case of power channels also. Lining of watercourses in the area irrigated by tube-wells assumes special significance as the pumped water supply is more costly.
The cost of lining a canal is, however, the only factor against lining. A detailed cost analysis is essential for determining the economic feasibility of lining a canal. The true cost of lining is its annual cost rather than the initial cost.
The cost of lining is compared with the direct and indirect benefits of lining to determine the economic feasibility of lining of a canal. Besides economic factors, there might be intangible factors, such as high population density, aesthetics etc. which may influence the final decision in lining of a canal.
Types of Lining:
Types of lining are generally classified according to the materials used for the construction of lining. Concrete, rock masonry, brick masonry, bentonite- earth mixtures, natural clays of low permeability and different rubble, plastic and asphaltic compounds are the commonly used materials for canal lining.
The suitability of the lining material is decided by:
(i) Economy,
(ii) Structural stability,
(iii) Durability,
(iv) Repair-ability,
(v) Impermeability,
(vi) Hydraulic efficiency, and
(vii) Resistance to erosion.
The principal types of lining are as follows:
1. Concrete lining
2. Shot Crete lining
3. Precast concrete lining
4. Lime concrete lining
5. Stone masonry lining
6. Brick lining
7. Boulder lining
8. Asphaltic lining
9. Earth lining
(i) Concrete Lining:
Concrete lining is probably the best type of lining. It fulfills practically all the requirements of lining. It is durable, impervious and requires least maintenance. The smooth surface of the lining increases the conveyance of the channel.
Properly constructed concrete lining can easily last about 40 years. Concrete linings are suitable for all sizes of irrigation canals, and for both high and low velocities. The lining cost is, however, high but can be reduced by using mechanised methods.
Thickness of concrete lining depends on canal size, bank stability, amount of reinforcement and climatic conditions. Small channels in warm climates require lesser thickness of lining.
Channel banks are kept at self-supporting slope (1.5H: IV to 1.25 H: IV) so that the lining is not required to bear earth pressures and its thickness does not increase. Concrete lining is laid without form work and, hence, the workability of the concrete should be good. Besides, experienced workmen are required for laying the concrete lining.
Reinforcement in concrete linings usually varies from 0.1 to 0.4 % of the area in the longitudinal direction and 0.1 to 0.2 % of the area in the transverse direction. The reinforcement in concrete linings prevents serious cracking of the concrete to reduce leakage, and ties adjacent sections of the lining together to provide increased strength against settlement damage due to unstable subgrade soils or other factors.
The reinforcement in concrete linings does not prevent the development of small shrinkage cracks which tend to close when irrigation canals are operated and linings are water-soaked.
The damage due to shrinkage and temperature changes is avoided/reduced by the use of special construction joints. Reinforced concrete linings may result in increased water tightness of the lining. However, well-constructed unreinforced concrete linings may be almost equally watertight.
Earlier practice of reinforced concrete lining is now being replaced with well-constructed unreinforced concrete lining. However, reinforcement must be provided in (a) large canals which are to be operated throughout the year, (b) sections where the unreinforced lining may not be safe, and (c) canals in which the flow velocities are likely to be very high.
Proper preparation of subgrade is essential for the success of concrete lining which may, otherwise, develop cracks due to settlement. Natural earth is generally satisfactory for this purpose and, hence, subgrade preparation is the least for channels in excavation. Thorough compaction of subgrade for channels in filling is essential for avoiding cracks in lining due to settlement.
Some cracks usually develop in concrete lining. These can be sealed with asphaltic compounds. The lining may be damaged when flow in the canal is suddenly stopped and the surrounding water table is higher than the canal bed. This damage occurs in the excavated channels and can be prevented by providing weep holes in the lining or installing drains with outlets in the canal section.
Minimum thickness of concrete lining based on irrigation canal capacity have been specified, as given in Table 6.1.
Concrete lining has been used in Nangal Hydel canal, Amaravathi, Krishnagiri Reservoir project and several other projects. The use of concrete lining in India is, however, limited because of relatively small cost of water and higher cost of lining. The Bureau of Indian Standards does not specify use of reinforcement for cement concrete lining.
(ii) Shotcrete Lining:
Shotcrete lining is constructed by applying cement mortar pneumatically to the irrigation canal surface. Cement mortar does not contain coarse aggregates and, therefore, proportion of cement is higher in shotcrete mix than in concrete lining.
The shotcrete mix is forced under pressure through a nozzle of small diameter and, hence, size of sand particles in the mix should not exceed 0.5 cm. Equipment needed for laying shotcrete lining is light, portable and of smaller size, compared to the equipment for concrete lining. Thickness of shotcrete lining may vary from 2.5 to 7.5 cm. The preferred thickness is from 4 to 5 cm.
Shotcrete lining is suitable for:
(a) Lining small sections,
(b) Placing linings on irregular surfaces without any need to prepare the subgrade,
(c) Placing linings around curves or structures, and
(d) Repairing badly cracked and leaky old concrete lining.
Shotcrete linings are subject to cracking and may be reinforced or unrein- forced. Earlier shotcrete lining was usually reinforced. Larger thickness of shotcrete lining was preferred for convenient placing of reinforcement. The reinforcement was in the form of a wire mesh. In order to reduce the cost of lining, shotcrete linings are not reinforced these days particularly on relatively small jobs.
(iii) Precast Concrete Lining:
Precast concrete slabs, laid properly on carefully prepared subgrades and the joints effectively sealed, constitute a serviceable type of lining. The precast slabs are about 5 to 8 cm thick with suitable width and length to suit channel dimensions, and to result in weights which can be conveniently handled.
Such slabs may or may not be reinforced. This type of lining is best suited for repair works as these can be placed rapidly without long interruptions in canal operation. Side slopes of Tungbhadra project canals have been lined with precast concrete slabs.
(iv) Lime Concrete Lining:
The use of this type of lining is limited to small and medium size irrigation channels with capacities up to 200 m3/s and in which the velocity of water does not exceed 2 m/s. The materials required for this type of lining are lime, sand, coarse aggregate and water. Lime concrete mix should be such that it gives minimum compressive strength of about 5.00 kN/m2 after 28 days of moist curing.
Usually, lime concrete is prepared with 1:1.5:3 of kankar lime: kankar grit or sand: kankar (or stone or brick ballast) aggregate. The thickness of lining may vary from 10 to 15 cm for discharge up to 200 m3/s. Lime concrete lining has been used in the Bikaner canal taking off from the left bank of Sutlej.
(v) Stone Masonry Lining:
Stone masonry linings are laid on the canal surface with cement mortar or lime mortar. The thickness of stone masonry is about 30 cm. The surface of the stone masonry may be smooth plastered to increase the hydraulic efficiency of the irrigation canal.
Stone masonry linings are stable, durable, erosion resistant, and very effective in reducing seepage losses. Such lining is very suitable where only unskilled labour is available and suitable quarried rock is available at low price. This lining has been used in Tungbhadra project.
(vi) Brick Lining:
Bricks are laid in layers of two with about 1.25 cm of 1:3 cement mortar sandwiched in between. Good quality bricks should be used and these should be well soaked in water before laying on the moistened canal surface.
Brick lining is suitable when concrete is expensive and skilled labour is not available. Brick lining is favoured where conditions of low wages, absence of mechanizations, shortage of cement and inadequate means of transportation exist. Brick linings have been extensively used in north India.
The Sarda power channel has been lined with bricks. The thickness of the brick lining remains fixed even if the subgrade is uneven. Brick lining can be easily laid in rounded sections without form work. Rigid control in brick masonry is not necessary. Sometimes reinforced brick linings are also used.
(vii) Boulder Lining:
Boulder lining of irrigation canals, if economically feasible, is useful for preventing erosion and where the ground water level is above the bed of the canal and there is possibility of occurrence of damaging back pressures.
The stones used for boulder lining should be sound, hard and durable which can sustain weathering and water action. Rounded or sub-angular river cobbles or blasted rock pieces with sufficient base area are recommended types of stone for boulder lining.
Dimensions of stones and thickness of lining are as given in Table 6.2:
Wherever required, a 15 cm thick layer of filter material is to be provided. For laying of boulders, the subgrade (both bed and side slope) of the canal is divided into compartments by stone masonry or concrete ribs. These compartments will not have dimensions more than 15 m along and across the centre line of the irrigation canal.
(vii) Asphaltic Lining:
Material for asphaltic lining is asphalt-based cement and sand mixture which is mixed in hot condition.
The most commonly used asphaltic linings are:
(a) Asphaltic concrete, and
(b) Buried asphaltic membrane.
Asphaltic linings are relatively cheaper, flexible and can be rapidly laid during any part of the year. Because of their flexibility, minor movements of the subgrade are not of serious concern. However, asphaltic linings have short life and are unable to permit high velocity of flow. They have low resistance to weed growth and, hence, it is advisable to sterilize the subgrade to prevent the weed growth.
Asphaltic concrete is a mixture of asphalt cement, sand and gravel mixed at a temperature of about 110°C, and is placed either by manual labour or with laying equipment. Experienced and trained workmen are required for this purpose. Lining is compacted with heavy iron plates while hot.
A properly constructed asphaltic concrete lining is the best asphaltic lining. Asphaltic concrete lining is smooth, flexible and is erosion resistant. Since asphaltic concrete lining becomes distorted at higher temperatures, it is unsuitable for warmer climatic regions. It is preferred in comparison to concrete lining in situations where aggregate is likely to react with the alkali constituents of Portland cement.
Buried asphaltic membrane can be of two types:
(a) Hot-sprayed asphaltic membrane, and
(b) Pre-fabricated asphaltic membrane
Hot-sprayed asphaltic membrane is constructed by spraying hot asphalt on the subgrade to result in a layer of about 6 mm thickness. This layer, after cooling, is covered with about 30 cm thick layer of earth material.
Asphalt temperature is around 200°C and the spraying pressure is about 3 x 105 N/m2. For this type of lining, the channel section has to be over excavated. The lining is flexible and easily adopts to the subgrade surface. Skilled workmen are required for the construction of this type of lining.
Pre-fabricated asphaltic membrane is prepared by coating rolls of heavy paper with a 5 mm layer of asphalt or 3 mm of glass fibre-reinforced asphalt. These rolls of pre-fabricated asphaltic membrane are laid on the subgrade, and then covered with earth material. These linings can be constructed by commonly available labour.
Materials used for covering the asphaltic membrane determine the permissible velocities which are generally lower than the velocities in unlined canals. Maintenance cost of such linings is high. Cleaning operations should be carried out carefully so as not to damage the membrane.
(ix) Earth Linings:
Different types of earth linings have been used in irrigation canals. They are inexpensive, but require high maintenance expenditure.
The main types of earth linings are:
(a) Stabilized earth linings,
(b) Loose earth blankets;
(c) Compacted earth linings,
(d) Buried bentonite membranes, and
(e) Soil-cement linings.
(a) Stabilized earth linings:
Stabilized earth linings are constructed by stabilising the subgrade. This can be done either physically or chemically. Physically stabilized linings are constructed by adding corrective materials (such as clay for granular subgrade) to the subgrade, mixing and then compacting.
If corrective materials are not required, subgrade can be stabilized by scarifying, adding moisture and then compacting. Chemically stabilized linings use chemicals which may tighten the soil. Such use of chemicals, however, has not developed much.
(b) Loose earth blankets:
This type of lining is constructed by dumping fine-grained soils, such as clay, on the subgrade and spreading it so as to form a 15 to 30 cm thick layer. Such lining reduced seepage only temporarily and are soon removed by erosion unless covered with gravel.
Better results can be obtained by saturating the clay and then pugging it before dumping on the subgrade. The layer of pugged clay is protected by a cover of about 30 cm silt. This type of lining requires flatter side slopes.
(c) Compacted earth linings:
These linings are constructed by placing graded soils on the subgrade and then compacting it. The graded soil should contain about 15% of clay. The compacted earth linings may be either thin-compacted or thick compacted. In thin-compacted lining, the layer thickness of about 15 to 30 cm along the entire perimeter is used.
Thick-compacted linings have about 60 cm thick layer on the channel bed, and 90 cm thick layer on the sides. If properly constructed, both types are reasonably satisfactory. However, thick type of lining has been generally preferred.
Compacted earth linings are feasible when excavated materials are suitable or when suitable materials are available nearby. Compaction operations along the side slopes are more difficult (particularly in thin-compacted lining) than along the channel bed. The lining material should be tested in the laboratory for densities, permeability and optimum moisture contents. The material must be compacted in the field so as to obtain the desired characteristics.
(d) Buried bentonite membranes:
Pure bentonite is a hydrous silicate of alumina. Natural deposits of bentonite are special types of clay soil which swell considerably when wetted. The impurities of these soils affect the swelling and, hence, the suitability of these as canal lining material.
Buried bentonite linings are constructed by spreading soil-bentonite mixture over the subgrade and covering it with about 15 to 30 cm of gravel or compacted earth. Sandy soil mixed with about 5 to 25 per cent of fine-grained bentonite and compacted to a thickness of 5 to 7.5 cm results in a membrane which is reasonably tough and suitable for lining.
(e) Soil-cement linings:
These linings are constructed using Portland cement (15 to 20 % by volume) and sandy soil (not containing more than about 35 % of silt and clay particles). Cement and sandy soil can be mixed in place and compacted at optimum moisture content. This method of construction is known as dry-mixed soil-cement method.
Alternatively, soil-cement lining can be constructed by machine mixing the cement and soil with water and placing it on the subgrade in a suitable manner. This method is called plastic soil-cement method and is preferable. In both these methods, the lining should be kept moist for about seven days to permit adequate curing.
The construction cost of soil-cement lining is relatively high. But these resist weed growth and erosion, and also permit velocities slightly higher than in unlined earth channels. The use of soil-cement lining for irrigation canals is restricted to small irrigation canals with capacities up to 10 m3/s and in which the velocity of water does not exceed 1 m/s.
Failure of Lining:
The main cause of failure of lining is the water pressure that builds up behind the lining material due to high water table, saturation of embankments by canal water, sudden lowering of water levels in the channel and saturation of embankment sustained by continuous rainfall. Embankment of relatively pervious soil does not need drainage measures behind lining.
In all situations requiring drainage measures to relieve pore pressures behind lining, a series of longitudinal and transverse drains satisfying filter criteria are provided. A typical arrangement of longitudinal filter drain is as shown in Fig. 6.1.
Growth of weeds on canal banks and other aquatic plants in channels may not result in failure of lining but affect the conveyance of channels which may be lined or unlined. Weeds and aquatic plants consume water for their growth and thus consumptive use of irrigation water increases. The weed growth increases the channel roughness and, hence, reduces the flow velocity thereby increasing evaporation losses.
Cleaning of channels having excessive weed growth is, therefore, a vital maintenance problem. Cleaning operations can be carried out manually or by mechanical devices such as dragline excavation and tractor-drawn cranes. Commonly used methods for cleaning of channels are pasturing, moving, burning and applying chemical weed killers.