ESSENTIAL PROPERTIES OF A GOOD LINING
Following are the essential properties of a good lining:
- The lining should be completely water tight.
- The rugosity coefficient of the lining material should be low, so as to make the section more efficient, hydraulically.
- The lining should be strong and durable.
- Initial cost of lining and its subsequent maintenance cost should be low.
- The lining should not get damaged by tramping of cattles.
- It should resist growth of weeds and attack of burrowing animals.
- Lining should not get damaged when flow in the canal is stopped.
TYPES OF LINING
The linings may be classified under the following four heads:
- Hard surface lining
- Buried and protected type membrane lining
- Earth lining
- Porous type lining, Each class of lining is discussed in details one by one.
Hard Surface Lining
The following types of linings come under this category:
- Cement concrete lining
- Shot-crete lining
- Precast concrete lining
- Cement mortar lining
- Brick or tile lining
- Stone masonry lining
- Asphaltic concrete lining.
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 then 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.
- 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 × 15 × 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.
- The 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.