What do you understand by the term ‘lining of irrigation channels’ what is the necessity for lining the irrigation channels? State the different materials used the lining the water course with their relative merits and demerits?by Madan Mohan
Purpose of Canal Lining:
Implementation of irrigation scheme which includes extensive distribution system is an expensive proposition. Hence it is very necessary to ensure that large losses do not occur in transit due to seepage. In practice, however, considerable loss of water takes place due to seepage in earthen canals. This loss is unavoidable unless the canal is lined. An irrigation canal is said to be lined when the bed and the sides of the canal cross- section are protected with impervious or fairly impervious material of sufficient strength. Thus main aim of lining the irrigation canals with impermeable material is to stop seepage, thereby saving valuable irrigation water. When canal is lined considerable additional area can be commanded with the help of the saving resulted from lining the canals. When a canal is to be lined the canal may be built to much smaller cross-section than an unlined one as there is practically no loss in the lined canals due to seepage. Condition of canal is also improved a lot and as a result it requires less maintenance. Experience has shown that lined canals check water-logging in the areas adjoining the canals. Existing canals may also be lined, but then the construction work may interrupt the irrigation supplies. The canal in head reach does not need lining. The obvious reason is the seepage water is not lost totally. It goes back to the river. However, sometimes it may become necessary to line the canal in the head reach also. Then the maintenance of the canal becomes an important problem as the waters table in the head reach is usually high and water behind the lining may create unstable conditions. The lining of existing or proposed irrigation canal is justified economically when the cost of the lining can be recovered from the increased monetary benefits in a reasonably short period.
Drawbacks of Canal Lining:
Though the benefits derived from the lined canal are so great there are some disadvantages also.
They are the following:
- Once the lining is damaged it is difficult to repair it.
- The lined canals are constructed without berms. Moving vehicles, pedestrians, animals are liable to fall in the canal directly in the absence of safety berms.
- The lined canal is a permanent structure hence it is difficult to shift the outlets often.
- Lining of a canal can be done only after incurring heavy expenditure.
Economics of Canal Lining:
Though lining has advantages to its credit a lined canal costs 2 to 2.5 times as much as an unlined canal. Hence, it is very essential to make sure before lining the canal that the cost incurred on lining is recoverable during the life time of the canal. While considering the economics of a canal lining it is necessary to evaluate the actual benefit and to compensate them with the cost of the lining. If the value of the annual benefits exceeds the annual cost of the lining then construction of the canal lining may be considered to be a good investment. The indirect benefits may be considered as an extra addition to saving which will provide sound economy to a lining project.
The annual cost of canal lining includes:
- Annual interest charge on the average investment during the life time of the lining.
- Annual depreciation changes.
P be the first cost of lining per km r be the rate of interest
n be the estimated useful life of canal in years (before renewal of canal is required)
d be the annual depreciation
i be the average annual interest charge.
Annual depreciation charge, d = P/n = First cost/Useful life
Average annual interest charge, i = P/2. r = average cost x rate of interest
The obvious reason for taking average cost is that the first cost is reduced every year due to the process of charging depreciation. The reduction in the cost each year will be equal to the annual depreciation charge.
Then average annual cost of lining per km is
d + i = P/n + P/2. r …………………………….. (1)
The annual benefits actually derived by lining the canal include:
- Saving in seepage loss
- Saving in maintenance and operation cost
- Other additional benefits.
A be the saving in seepage loss per km length of lined canal B be the saving in maintenance and operation cost per km C be the additional benefits.
Then total annual benefits per km = A + B + C ………………….. (2)
For economical justification annual benefits in (2) should be sufficiently more than annual cost in (1).
It may be mentioned here specially that if lining is planned before constructing the canal much greater saving would be possible. Thus because of greater overall savings, the need for lining the canal should be weighted carefully well in advance and if required and considered proper should be included in the original construction programme.
Types of Lining:
The canals can be made fairly watertight by lining the canal section with various materials. The materials which are commonly used for lining are cement, bricks, puddle clay, stone blocks, and sodium carbonate, asphalt and road oils. The various types of linings vary in first cost, construction procedure.
Cement Concrete Lining:
There are various types of lining in which cement is used for lining the canal. This term is reserved for either plain or reinforced plastic concrete made with cement, coarse and fine aggregate and water. Experience has shown that this type of lining is very durable. It is capable of reducing the losses due to seepage by about 90 to 95 per cent. No general rule can be stated for determining the thickness of the concrete linings.
However, proper thickness of the lining should be adopted to fulfill two basic requirements, namely:
- Water-tightness, and
- Sufficient structural strength to resist cracking.
It has been proved by experience that for small canals plain concrete lining of 4 to 6 cm thickness is satisfactory. While for big canals lining may be provided up to thickness of 15 centimeters. To relieve the canal of various unaccounted forces it is essential to limit the side slopes. The slopes adopted should be such that for the backfill the angle of internal fiction is not exceeded. Then the backfill will be stable and it will not exert any pressure on the lining. Thus the side slopes are limited to a range of 1.25: 1 to 1.5: 1. Sometimes side slopes as steep as 1: 1 may be adopted.
Construction of Concrete Lining:
The first phase in construction of concrete lining is preparation of sub-grade. Ordinally sub-grade should be sufficiently porous to permit drainage. When sub-grade is made up of the soil like clay in which great volume changes occur due to moisture it becomes essential to control moisture content of the soil. Secondly subgrade should be sufficiently compact. When the lining is to be placed against a trimmed subgrade in undisturbed soil no compaction is needed. But when the canal is in filling, the banks need compaction. The compaction can be done successfully at optimum moisture content with a sheep-foot roller. It is seen that when concrete is placed on the subgrade directly the subgrade absorbs the water from the concrete. As a result the concrete is rendered weak due to moisture deficiency. Then curing is not satisfactory and concrete becomes porous. Obviously it is very essential to see that when the subgrade is made of very dry soil it should be sufficiently wetted before placing the concrete. Even this is not sufficient in many cases. Then other methods also are used as a supplement.
(a) Laying a base coat of 1: 4 cement-sand slurry:
The slurry of 1: 4 cement-sand may be applied to the subgrade to give a 3 mm thick base layer. This slurry is generally placed few hours before placing the concrete.
(b) Spreading oil paper on the subgrade:
Before laying the concrete oil paper is spread on the subgrade. It has been experienced that it gives good results.
(c) Spraying of crude oil on the subgrade:
Before placing the concrete crude oil may be sprinkled on the subgrade to give a good coating.
(d) Applying 1: 6 cement plaster on the subgrade:
Initially subgrade is plastered with 1: 6 cement plaster before placing the concrete. It is not necessary to make the surface of the plaster smooth. It has been experienced that the method of applying cement-sand slurry is cheapest. The second phase in construction of lining is actual placing of concrete. Mixing of concrete may be done either at site or at centrally located batching and mixing plant. Proportion of Z concrete may be 1: 3: 6 or 1: 5: 6 or 1: 4: 8 depending upon the requirements.
The concrete may be placed by hand or by machinery. When hand methods are used temporary form work is constructed and placed transversely across the canal. The size of the form work should be equal to the size of the blocks to be provided. Blocks are generally laid alternately. The time interval between two adjacent placements should be about 7 days. For by canals concrete may be laid making use of machinery. The machines travel on the rails. The rails are laid on the banks of canal. The trimining operation for sub-grade may also be done with the help of a machine travelling on the same rails. The third phase is curing of concrete and is of great importance. Proper curing may be accomplished in two ways. Firstly by covering the fresh lining with moist earth. Secondly, the concrete may be kept wet continuously for about 5 days. In large canals, on the side slopes ladders are provided at suitbale intervals along the length of the lined canals. The ladder serves dual purpose. Firstly, it helps during inspection of the dry canal. Secondly, it provides easy means to come out of the running canal safely to fallen human beings. The ladders are provided on both banks say about 250 to 300 m apart. On two banks they are arranged in a staggered manner. When the canal enters any underground structure, the ladders are invariably provided about 30 metres upstream of the entry point of the canal. The ladder is made of smooth round mild steel bars, and the ladder rungs are constructed in the canal lining. The steel rungs are galvanised or coated with coal tar after installation to avoid its rusting.
Other Types of Lining Using Cement:
- Shotcrete lining:
It is a type of lining in which slurry of Portland cement, processed sand and pure water is applied pneumatically through the nozzles on the surface of the subgrade. Ordinarily a mixture of cement and sand (1: 4) is used. The jet of cement mortar slurry is shot at the subgrade and hence this type of lining is called shotcrete. The thickness of this type of lining generally varies from 2.5 cm to 6 5 cm. This type of lining gives good working rate irrespective of the nature or condition of the subgrade. It works equally successfully on smooth, uneven or cracked sub-grade surfaces. It is of course true that for irregular and cracked subgrade surfaces the amount required is more. This type is rich in cement and hence its cost is high. Satisfactory curing of shotcrete is very important requirement for successful lining. This type of lining has been found to be most successful.
- Precast concrete lining:
This type of lining is constructed with precast concrete slabs. The slabs are manufactured at a suitably located central place. The slabs may then be taken to the site at the time of construction. The size of concrete slab should be such that it can be handled by one or two men. The size may be 50 cm x 30 cm. The thickness of the slab may range from 5 to 6.5 cm. The blocks are manufactured with some interlocking arrangement at ends. The provision of a suitable joint ensures continuity of the lining. The slabs are then laid on a well compacted subgrade. The joints are sealed afterwards with asphalt or cement grout to prevent leakage.
- Cement mortar lining:
In this type of lining it is very essential to have well graded sand. The sand should range from fine to coarse to meet the requirements of durability and appearance. The amount of cement required is more and hence the cost is also more. The thickness of this type of lining may vary from 9 mm to 38 mm. The method of construction is similar to that of concrete lining. This type is not durable hence it can be used only in conjunction with some other protective material. To give an example it may be sandwiched between two layers of bricks when later is used as a lining material. It has been proved that 25 mm thick cement mortar layer reduces seepage by about 75 per cent.
- Soil cement lining:
Sometimes cement may be mixed with the water and locally available soil. The soil before using should be thoroughly analysed in laboratories by conducting various tests. The water soil and cement is mixed to get a workable mixture. After spreading this mixture on the subgrade it is compacted to attain maximum density. This type of lining may be constructed with a travelling mixer with a slip form. It has been experienced that this type of lining can be constructed rapidly and has a low cost.
- Brick Lining:
The canal is said to be lined with bricks when the sides and bed are protected with brick surfacing laid in cement mortar.
It has been experienced that the brick lining generally fails due to:
(a) Spilling of water:
It may be checked by providing sufficient freeboard.
(b) Settlement of weak subgrade:
It may be prevented by giving adequate compaction to the back fill.
(c) Pressure of water behind lining:
The failure of lining may be prevented by providing properly designed drainage system in the banks.
To make the lining successful the lining may be constructed of two layers of bricks laid flat in mortar. The first layer is laid on a 12 mm layer of 1: 6 cement mortar. This 12 mm thick layer of cement mortar is spread on the properly compacted and wetted subgrade. Then on the top of first brick layer, a 12 mm layer of 1: 3 cement mortar is given. It is allowed to cure for two days. Finally second layer of bricks in mortar is laid on the top. Thus a 12 mm layer of rich cement mortar (1: 3) is sandwiched between two brick layers and this layer IS actually responsible to make the lining practically watertight. The size of the bricks generally used is 30 cm x 15 cm x 6.5 cm. However, this size of bricks IS not standard. To give additional stability to the lining, masonry may be reinforced by using mild steel bars. This reinforcing steel is generally laid in the cement mortar layer sandwiched between the two brick layers. The bricks used for lining should be manufactured with good earth. Good earth is one which contains about 10 to 20 per cent clay and in which calcium carbonate is less than 2 percent and salt content is not more than 0.3 per cent. The bricks should be thoroughly wetted with water before use. The brick lining has been successfully done on various canals in Punjab, for example, Haveli canal and Bhakra canal. Sarda canal in Uttar Pradesh is also lined with bricks. The lining of Haveli canal damaged at many places within a year after its construction. The main reason for failure was that the lining was not designed and constructed to fulfill the safety requirements strictly. (Improper compaction, defective material and insufficient freeboard were the main reasons.)
Miscellaneous Types of Canal Lining:
(i) Clay puddle lining:
The canal may also be lined with a clay puddle. The puddle clay is fairly impervious when properly pugged and saturated with water. It can be laid on the subgrade to form a fairly watertight coating. The thick layer of puddle clay is then protected with 30 cm thick layer of silt soil. It has been seen that it prevents about 80% seepage loss.
(ii) Lining with stone masonry:
This type of lining is constructed with dressed stone blocks laid in mortar. Properly dressed stones are not available in nature. Irregular stone blocks are dressed and chipped off as per requirement. It makes the type costly. When roughly dressed stones are used for lining, the surface is rendered rough which may put lot of resistance to low. Technically the coefficient of rugosity will be higher. Thus the stone lining is limited to the situation where loss of head is not an important consideration and where stones are available at moderate cost.
(iii) Plastic lining:
As a modern technique use of plastics in canal lining holds good promise.
There are three types of plastic membranes which are used in canal lining, namely:
- Low density polyethylene (LDPE)
- High molecular high density polythene (HM); and
- Polyvinyl chloride (PVC)
The plastic as a material for canal lining offers certain characteristic advantages like negligible weight, easy for handling, spreading and transport, immune to chemical action and speedy construction. The plastic film is spread on the prepared subgrade of the canal. To anchor the membrane on the banks ‘V trenches are provided. The film is then covered with protective soil cover. The plastic films are available like cloth in 3 m width. The thickness of plastic membrane varies according to its quality, for example, LDPE, HM and PVC qualities are used with 250, 100 and 15 micron thickness respectively. The plastic sheets can be welded together at site to increase the width. Considering factors like initial capital cost, installation cost and effectiveness plastic lining will go a long way in reducing loss of water in the water conveyance system.
Selection of Type of Lining:
The selection of a particular type of lining should be based on the following considerations:
1. Functional Success:
The canals are lined to prevent seepage loss. Obviously the type which gives maximum required water-tightness to the canal section should be chosen.
2. Economic Consideration:
It is very essential to make sure that the type selected is such that the benefits derived from it are sufficient to balance the annual cost of the lining with safe margin.
3. Structural Stability:
The type of lining selected should be sufficiently strong to resist the damaging forces. Lining should be sufficiently flexible to allow moderate settlement of subgrade without cracking.
4. Hydraulic Efficiency:
The lining selected should give smooth finish to the surface. If the surface is rough the coefficient of rugosity will be high. It reduces the velocity of flow and consequently discharge capacity of the canal is reduced.
The type of lining selected should have sufficient working or useful life. The lining should be resistant to wearing, weathering and chemical attack.
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