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Types of Canal Lining
The following are the different types of canal lining which are generally recommended according to the various site conditions.
1. Cement concrete lining
2. Pre-cast concrete lining
3. Cement mortar lining
4. Lime concrete lining
5. Brick lining
6. Boulder lining
7. Shot Crete lining
8. Asphalt lining
9. Bentonite and clay lining
10. Soil-cement lining
Cement concrete lining
This lining is recommended for the canal in full banking. The cement concrete lining ( cast in-situ) is widely accepted is as the best impervious lining. It can resist the effect of scouring and erosion very efficiently. The velocity of flow may be kept above 2.5 m/sec. It can eliminate completely growth of weeds. The lining is done by the following steps –
(a) Preparation of sub-grade
The sub-grade is prepared by ramming the surface properly with a layer of sand (about 15 cm). Then, a slurry of cement and sand (1: 3)is spread uniformly over the prepared bed.
(b) Laying of concrete
The cement concrete of grade M15 is spread uniformly according to the desired thickness (generally, the thickness varies from 100 mm to 150 mm). After laying, the concrete is tapped gently until the slurry comes on the top. The curing is done for 2 weeks. As the concrete is liable to get damaged by the change of temperature, the expansion joints are provided at appropriate places. Normally no reinforcement is required for this cement concrete. But in special cases, a network of 6 mm diameter rods may be provided with spacing 10 cm centre to centre.
Pre-cast concrete lining
This lining is recommended for the canal in full banking. It consists of pre-cast concrete slabs of size 60 cm x 60 cm x 5 cm which is set along the canal bank and bed with cement mortar(1:6). A network of 6 mm diameter rod is provided in the slab with spacing 10 cm centre of the centre. The proportion of the concrete is recommended as 1:2:4. Rebates are provided on all the four sides of the slab so that proper joints may be obtained when they are placed side by side. The joints are finished with cement mortar (1:3). Expansion joints are provided at a suitable interval. The slabs are set in the following sequence –
(a) The sub-grade is prepared by properly ramming the soil with a layer of sand. The bed is levelled so that the slabs can be placed easily.
(b) The slabs are stacked as per estimate along the course of the canal. The slabs are placed with cement mortar(1:6) by setting the rebates properly. The joints are finished with cement mortar (1:3).
(c) The curing is done for a week
Cement mortar lining
This type of lining is recommended for the canal fully in cutting where hard soil or clayey soil is available. The thickness of the cement mortar (1:4) is generally 2.5 cm. The sub-grade is prepared by ramming the soil after cutting. Then, over the compacted sub-grade, the cement mortar is laid uniformly and the surface is finished with neat cement polish. This lining is impervious but is not durable. The curing should be done properly.
Lime concrete lining
When hydraulic lime, Surkhi and brick ballast are available in plenty along the course of the canal or in the vicinity of the irrigation project, then the lining of the canal may be made by the lime concrete of proportion 1:1:6. The procedure of laying this concrete is the same as that of the cement concrete lining. Here, the thickness of concrete varies from 150 mm to 225 mm and the curing should be done for a longer period. This lining is less durable than the cement concrete lining. However, it is recommended because of the availability of the materials and also because of economics.
This lining is prepared by the double-layer brick flat soling laid with cement mortar (1:6) over the compacted sub-grade. The first class bricks should be recommended for the work. The surface of the lining is finished with cement plaster (1:3). The curing should be done perfectly.
This lining is always preferred for the following reasons –
(a) This lining is economical.
(b)Work can be done very quickly.
(c) Expansion joints are not required.
(d)Repair works can be done easily.
(e) Bricks can be manufactured from the excavated earth near the site.
However, this lining has certain disadvantages –
(a) It is not completely impervious.
(b) It has low resistance against erosion.
(c) it is not so much durable.
In hilly areas where the boulders are available in plenty, this type of lining is generally recommended. The boulders are laid in a single or double-layer maintaining the slope of the banks and the bed Label of the canal. The joints of the boulders are grouted with cement mortar(1:6). The surface is finished with cement mortar (1:3). Curing is necessary for this lining too. This lining is very durable and impervious. But the transporting cost of the material is very high. So, it cannot be recommended for all cases.
Shot Crete lining
In this system, the cement mortar (1:4) is directly applied to the subgrade by the equipment known as cement gun. The mortar is termed as shot create and the lining is known as shotcrete lining. The process is also known as the guniting, as a gun is used for laying the mortar. sometimes, this lining is known as gunited lining. The lining is done in two ways –
(a) By dry mix
In this method, a mixture of cement and moist sand is prepared and loaded in the cement gun. Then it is forced through the nozzle of the gun with the help of compressed air. The mortar spreads over the sub-grade to a thickness which varies from 2.5 cm to 5 cm.
(b) By wet mix
In this process, the mixture of cement, sand and water is prepared according to the approved consistency. The mixture is loaded in the gun and forced on the sub-grade.
This type of lining is very costly and it is not durable. It is suitable for resurfacing the old cement concrete lining.
This lining is prepared by spraying asphalt (i.e. bitumen) at a very high temperature ( about 150-degree Celsius) on the subgrade to a thickness varies from 3 mm to 6 mm. The hot asphalt when becomes cold forms a waterproof membrane over the sub-grade. This membrane is covered with a layer of earth and gravel. The lining is very cheap and can control the seepage of water very effectively but it cannot control the growth of weeds.
Bentonite and clay lining
In this lining, a mixture of bentonite and clay are mixed thoroughly to form a sticky mass. This mass is spread over the sub-grade to form an impervious membrane which is effective in controlling the seepage of water. but it cannot control the growth of weeds. This lining is generally recommended for small channels.
This lining is prepared with a mixture of soil and cement. The usual quantity of cement is 10% of the weight of dry soil. The soil and cement are thoroughly mixed to get a uniform texture. The mixture is laid on the sub-grade and it is made thoroughly compact. The lining is efficient to control the seepage of water, but it cannot control the growth of weeds, So, this is recommended for small channels only.
Book – Irrigation Engineering, Writer – N N Basak