In this article we will discuss about:- 1. Introduction to Canal Lining 2. Types of Lining in Canals 3. Essential Properties 4. Design 5. Advantages 6. Disadvantages.
Contents:
- Introduction to Canal Lining
- Types of Lining in Canals
- Essential Properties of a Good Lining in Canals
- Design of Lined Canal
- Advantages of Canal Lining
- Disadvantages of Canal Lining
1. Introduction to Lining in Canals:
Canal lining is a treatment given to the canal bed and banks, so as to render the canal section impervious. Since imperviousness of the canal section is achieved mostly by making canal section pucca, either by cement concrete, or bricks, the lined canals are also sometimes known as pucca canals.
Lined canals are mostly referred as pucca canals. It has been estimated that seepage losses in irrigation canals may amount from 30% to 50% of the water admitted in the canals at diversion head works. This much loss of water in unlined channels cannot be afforded, as resources of water in India are limited in relation to irrigable area available. Hence in order to reduce or rather eliminate the seepage losses, lining of canals is the need of the hour.
2. Types of Lining in Canals:
The linings may be classified under the following four heads:
I. Hard Surface Lining:
Following types of linings come under this category:
1. Cement Concrete Lining:
This lining has excellent hydraulic properties but being costly its use in India has been limited. The success of this lining depends upon the stability of foundation layer to a large extent. Bed sleepers and precast sleeper on sides are provided at regular intervals. Bed and side sleepers act as gauges for the dressing and preparation of the sub-grade. The joints of concrete lining are always located at sleepers so as to reduce seepage to minimum.
The thickness of lining depends upon the desired imperviousness, and structural strength. In case of ordinary M150 concrete thickness of lining varies from 5 cm to 10 cm depending upon the discharge the canal is to carry. If lean concrete say M100 has been used the thickness may vary from 7.5 cm to 15 cm.
Concrete lining is liable to crack on account of shrinkage and temperature changes. The cracking of lining may be prevented either by providing reinforcement or by providing contraction joints. In case of continuous lining operations only transverse or transverse and longitudinal grooves at 3 m to 5 m interval should be formed.
When lining is cast in panels, slabs are laid in alternate compartments at an interval of at least one day. The grooves in the joints should be filled with suitable sealing compound. Various types of joints provided in concrete lining are shown in Fig. 20.1.
2. Shot-Crete Lining:
In this lining a cement sand (1 : 4) slurry is prepared and is shot on the prepared sub-grade with the help of pneumatic pressure. This lining is costlier than cement concrete lining of equal thickness. Its utility lies in plugging the joints of old concrete lining and fissures in rock formations. This lining is not common.
3. Precast Concrete Lining:
In this lining, the precast cement concrete slabs are prepared at factory site and then used for lining the canals. Concrete blocks most commonly used are 50 cm long, 25 cm wide and 5 cm thick. The blocks have interlocking arrangement with the help of which different blocks can be inter-locked and sealed by sealing compound. The slabs are laid on well compacted sub-grade. For precast slabs, the concrete should not be of leaner grade than M150.
4. Cement Mortar Lining.
It has never been used all alone as lining material. It is mostly used as a sandwich material between brick layers. Thickness of this layer may vary from 1 cm to as much as 4 cm. It is also not in very common use.
5. Brick of Tile Lining:
This lining has been extensively used for lining of canals in India. Rajasthan canal project is also adopting this lining. The lining maybe singled layered or double layered. The subgrade is prepared well and first layer of tiles or bricks is laid on 15 mm thick layer of 1: 6 cement mortar.
A 15 mm layer of 1 : 3 cement mortar is placed on the first layer of tiles and then second layer of tiles is laid. The tiles of second layer should be laid so as to break the joints of the bricks of first layer. The usual size of tiles used for lining is 30 x 15 x 5 cm.
Following are inherent advantages of brick lining:
(i) Ordinary mason can lay tiles.
(ii) Rounded work at corners can be easily done without any form work.
(iii) Strict quality control is not required.
(iv) Expansion joints are not required to be provided.
Single tile lining consists of a single layer of tiles, laid on 10 cm thick 1 : 5 cement mortar, on well compacted sub-grade. A 20 mm cement plaster 1 : 3 is laid over tiles and given a smooth finish.
If this lining fails at few spots it can be easily repaired. Sub-sided lining is excavated and new lining is placed in its place.
6. Stone Masonry Lining:
This lining consists of undressed stone blocks. The subgrade is prepared as usual and stone blocks are laid over it in cement mortar. Lining may be made from dressed blocks but it proves very uneconomical. This lining offers a very large resistance to flow as its rugosity coefficient is very high.
7. Asphaltic Concrete Lining:
A mixture of asphalt with graded aggregate is prepared and placed at a high temperature of about 200°C. The layer so laid is covered with earth layer of at least 30 cm thickness. Earth layer provides protection to the asphaltic concrete layer.
II. Buried and Protected Type Membrane Lining:
These linings are such lining in which water proof thin membrane is place on the prepared subgrade and thereafter it is covered by a protective layer of earth or gravel. Protective layer provides protection to the lining against damage due to outside effects and 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. 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.
III. Earth Lining:
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 thoroughly. 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.
IV. Porous Type Lining:
In head reaches, the ground water table is generally much higher than the bed level of the main canal. Porous lining is advisable in such circumstances. 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.
3. Essential Properties of a Good Lining in Canals:
Following are the essential properties of a good lining:
1. The lining should be completely water tight.
2. The rugosity coefficient of the lining material should be low, so as to make the section more efficient, hydraulically.
3. The lining should be strong and durable.
4. Initial cost of lining and its subsequent maintenance cost should be low.
5. The lining should not get damaged by tramping of catties.
6. It should resist growth of weeds and attack of burrowing animals.
7. Lining should not get damaged when flow in the canal is stopped.
4. Design of Lined Canal:
Design of lined canals is always done by Kennedy’s Theory.
Following equations given by Kennedy are used in the design:
The values of N for different types of linings are given in Table 20.1. These values of N are as per IS : 4745 — 1968.
The value of N for protected type of linings is taken same as for natural soil. The value of N of natural soil varies from 0.02 to 0.025.
In order to obtain the most economical section, it is necessary to adopt the best discharging section. The flow will be greater when the friction is least. This happens when wetted perimeter is least for any particular given area of the channel. In other words, the discharge will be maximum when Hydraulic mean depth (H.M.D.) is maximum.
A semi-circular section of channel is considered as the best theoretical section. But Trapezoidal channel section is mostly adopted on practical grounds. In order to increase the H.M.D., the angle subtended at the corners at bottom should be same as side slopes of channel make with the horizontal. See Fig. 20.3.
Design Steps:
Following data should be known before design of the canal can be carried out:
(i) Discharge of the channel (Q).
(ii) Rugosity Coefficient (N)
(iii) Longitudinal slope (S)
(iv) Slope of banks.
(v) Permissible velocity of flow (V).
Use following equations:
Steps:
(i) Knowing the limiting values of V, N, and S find out the H.M.D. (R) from equation-
(ii) From an expression in terms of Bed width (B) and depth (D) by pudding values in R = A/P.
(iii) From another equation in terms of B and D from equation Q = AV.
(iv) By solving equations formed in step (ii) and (iii), find out the unknown values of B and D.
For different values of side slopes, the sectional area, wetted perimeter, in terms of B and D are given as follows. The radius of arc for monitoring bottom corners is taken equal to depth of water.
Note- If channel alignment is not straight loss of head by resistance would increase.
Use of Design Chart:
Design of lined canals can be done by using Plate 20.1. This chart is made by adopting 1 ½ : 1 side slope, NI = 0.018, and slope of 1 in 1000. This chart can be made use of for other values of N and S.
Plate 20.1:
The method of use of this chart for other values of N and S is explained as follows:
1. For other values of N chart is used as follows:
2. For other values of slope (S), the chart can be made useful as follows:
5. Advantages of Canal Lining:
Following are the benefits of lined canals:
1. Seepage losses are practically eliminated or reduced to minimum.
2. Maintenance cost of the canals is reduced.
3. Canals can be run with increased velocity.
4. Section of the canal is considerably reduced as due to increased velocity, same discharge can be passed through smaller sectional area.
5. Cost of earth work is reduced, as smaller section will have to be constructed.
6. Silting does not take place, as canals are run at considerably larger velocities than silting velocity.
7. No scouring occurs anywhere in canal section.
8. Less width of land is required for the canal.
9. Weed growth in the-canal is reduced.
10. Harmful salts from adjoining soils do not get dissolved in the canal water.
11. More head for power generation becomes available as lined canals can be laid at flatter gradient. This measure also helps in bringing more areas under command.
12. Possibilities of canal breaches are eliminated.
13. Additional areas may be brought under irrigation from the water saved by lining.
14. Possibilities of water logging of adjoining areas are reduced.
15. Losses due to evaporation are reduced, as due to increased velocity, water takes less time to reach the outlet.
6. Disadvantages of Canal Lining:
Following are the disadvantages of the canal lining:
1. It requires a very heavy initial expenditure.
2. When lining gets old it generally develops cracks. Leakage through cracks is very difficult to check.
3. Joins in lining always create problems.
4. Position of outlets cannot be shifted easily.
5. Lined canals do not have berms. If some person or animal happens to fall in the canal, it is very difficult to pull him cut. To overcome this difficulty there should be stepped sections at regular intervals along the length of the canal.
6. Lined canals are generally seep. Only that man can enjoy bath in the canal who knows swimming.
7. As seepage of water is almost completely stopped, rows of trees cannot be grown along the canal as in case of Kucha canals.
8. In order to run with greater velocity, lined canals may have to be given increased longitudinal slopes. This may result in loss of command in the area.
9. Lining materials like clay, bitumen, etc. get easily destroyed under the feet of animals crossing the canal.
Financial Viability:
In order to justify the lining of canal it is essential to analyse on one hand the extra capital cost of providing lining and on the other hand an evaluation of the benefits of lining. For lining to be economically feasible, the capitalized value of benefits should be equal to or greater than extra cost involved in providing lining. In other words the capitalized value of benefits divided by extra capital cost of providing lining should always be more than one.