Materials for lining watercourses and field channel, Mechanics of Tillage and Traction | Mechanics of Tillage and Traction Notes- Agricultural Engg - Agricultural Engineering PDF Download

Introduction

A watercourse, as defined in the Northern India canal and drainage act, means any channel which is supplied with water from a canal but which is not maintained at the cost of the state government and all subsidiary works belonging to any such channels. The watercourses pass through the common land and are maintained by the farmers. Field channels carry water to the individual fields from the watercourse. In irrigated rice areas of the south, however, field channels are normally absent and the conventional practice is to irrigate from field to field .the overflow from the upper field irrigates the adjoining lower field and so on successively.

20.1 OBJECTIVES OF LINING

1) To control seepage

2) To prevent water logging

3) To increase the capacity of canal

4) To increase the command area

5) To protect the canal from the damage by flood

6) To control the growth of weeds.

20.2 ADVANTAGES OF LINING

1) It reduces the loss of water due to seepage and hence the duty is enhanced.

2) It controls the water logging and hence the bad effects of water logging are eliminated.

3) It provides smooth surface and hence the velocity of flow can be increased.

4) Due to the increased velocity the discharge capacity of a canal is also increased.

5) Due to the increased velocity, the evaporation loss also be reduced.

6) It eliminates the effect of scouring in the canal bed.

7) The increased velocity eliminates the possibility of silting in the canal bed

8) It controls the growth of weeds along the canal sides and bed.

9) It provides the stable section of the canal.

10) It reduces the requirement of land width for the canal, because smaller section of the canal can produce greater discharge.

11) It prevents the sub-soil salt to come in contact with the canal water.

12) It reduces the maintenance cost for the canals.

20.3 DISADVANTAGES

1) The initial cost of the canal lining is very high. So, it makes the project very expensive with respect to the output.

2) It involves much difficulties for repairing the damaged section of lining.

3) It takes too much time to complete the project work.

4) It becomes difficult, if the outlets are required to be shifted or new outlets are required to be provided, because the dismantling of the lined section is difficult.

20.4 TYPES OF LINING

The following are the different types of linings, 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

20.4.1 CEMENT CONCRETE LINING

This lining is recommended for the canal in full banking. The cement concrete lining in widely accepted as the best impervious lining. It can resist the effect of scouring and erosion very efficiently. The velocity of flow may be kept above 2.5m/sec. It can eliminate completely growth of weeds. The lining is done by the following steps,

20.4.1.1 Preparation of sub-grade

The sub-grade is prepared by ramming the surface properly with a layer of sand (about 15cm). Then slurry of cement and sand (1:3) is spread uniformly over the prepared bed.

20.4.1.2 Laying of concrete

The cement concrete of grade M  is spread uniformly according to the desired thickness ((generally, the thickness varies from 100mm to 150mm).  After laying, the concrete is tapped gently until the slurry comes o the top. The curing is done for two weeks. As the concrete is liable to get damaged by the change of temperature, the expansion joints are provided at appropriate places. Normally no re-inforcement is required for this cement concrete. But in special cases a network of 6 mm diameter rods may be provided with spacing 10 cm center to center.

20.4.2 PRE CAST CONCRETE LINING

This lining is recommended for the canal in full banking. It consists of pre-cast concrete slabs of size 60cm´60cm´5cm, which are set along the canal bank and bed with cement mortar (1:6). A network of 6mm diameter rod is provided in the slab with spacing 10cm center of center. The proportion of the concrete is recommended as 1:2:4. Rebates are provided on all the four sides of he 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

1. The sub- grade is prepared by properly ramming the soil with a layer of sand. The bed is leveled so that the slabs can be placed easily.

2. 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).

3. The curing is done for a week.

20.4.3 CEMENT MORTAR LINING

This type lining is recommended for the canal fully in cutting where hard soil or clayey is available. The thickness of a cement mortar (1:4) is generally 2.5cm. 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. The lining is impervious, but is not durable. The curing should be done properly.

20.4.4 LIME CONCRETE LINING

When hydraulic lime, surki and brick ballast are available in plenty along the course of the canal or in the vicinity of the irrigation project, then lining of the canal may be made by the lime concrete of proportion 1:1:6.  The procedure of laying this concrete is same as that of the cement concrete lining. Here the thickness of concrete varies from 150mm to 225mm and the curing should be done for 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 the economics.

20.4.5 BRICK LINING

This lining is prepared by the double layer brick 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,

1)    This lining is economical.

2)    Work can be done quickly.

3)    Expansion joints are not required.

4)    Repair work can be done easily.

5)    Bricks can be manufactured from the excavated earth near the site.

However this lining has certain disadvantages,

1)    It is not completely impervious.

2)    It has low resistance against erosion.

3)    It is not so much durable.

20.4.6 BOULDER LINING

In hilly areas where the boulders are available in plenty, this type of lining is generally recommended. The boulders are laid in single or double layer maintaining the slope of the banks and the bed level of the canal. The joints of the boulders are grouted with cement mortar (1:3). Curing is necessary in this lining too. This lining is very durable and impervious. But the transporting cost the material is very high. So, it cannot be recommended for all cases.

20.4.7 SHOT CRETE LINING

In this system cement mortar is (1:4) is directly applied on the sub-grade by an equipment known as cement gun. The mortar is termed as shot Crete and the lining is known as shot Crete lining. The process is also known as gunning, as the gun is used for laying the mortar. Sometimes this lining is known as gunned lining. The lining is done in two ways,

20.4.7.1 By dry mix

In this method a mixture of cement and moist sand 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 2.5cm to 5cm.

20.4.7.2 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 lining is very costly and it is not durable. It is suitable for the old cement concrete lining.

20.4.8 ASPHALT LINING

This lining prepared by spraying asphalt (i.e. bitumen) at a very high temperature (about 150 c) on the sub-grade to a thickness varies from 3mm to 6mm. The hot asphalt when becomes cold forms a water proof membrane over the sub-grade. This membrane is covered with layer of earth gravel. The lining is very cheap and can control the seepage of water very effectively but it cannot control the growth of weeds.

20.4.9 BENTONITE AND CLAY LINING

In this lining a mixture of Bentonite and clay 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.

20.4.10 SOIL CEMENT LINING

This lining is prepared with a mixture of soil and cement. The usual quantity cement is 10 percent of the weight of dry soil. The soil and the cement is thoroughly mixed to get a uniform mixture. The mixture is laid on the sub-grade and it s 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.

 Manning’s N value for different types of lining:

The Rugosity co-efficient to be used in the design of dimensions of lines channel are tabulated below.

Table 20.1 Recommended values of N for field channels.