Lining in expansive soils is avoided. Where unavoidable to traverse a reach of expansive clay, if in thin layer or in small pockets may be overexcavated and replaced with a suitable non-expansive soil and compacted. Swelling of clay can be controlled by loading the surface with 60 cm thick non- expansive compacted soil or gravel and filled to the grade of the underside of lining with good draining material. The excavated surface of expansive clay be given a’coat of asphalt before loading.
In other than predominantly sandy reaches where dry bulk density of natural soil is not less
than 1.8 g/cm3, initial excavation is done to about 30 cm above the final section and cutting to
final shape is done immediately before the lining.
Anti-salt Treatment: In reaches where salt contents are over 1.0 per cent or sodium sulphate is over 0.36 per cent, the subgrade is first covered with about 2 mm thick layer of bitumen obtained by evenly spraying bitumen at a rate of about 2.35 11m2. Crude oil at a rate of 0.5 l/m2 is sprayed over it in advance of spraying bitumen to get a good bond between bitumen and soil.
The essential requirements of a satisfactory type of lining are (i) low cost, (ii) impermeability, (iii) hydraulic efficiency (i.e., reduction in rugosity coefficient, (iv) durability, (v) resistance to erosion, (vi) repairability, and (vii) structural stability. The selection of particular type of lining is governed by the following considerations, the important factors being imperviousness and smoothness, position of water table, climatic conditions, size of canal, etc.
Imperviousness. To save seepage losses and as an important antiwater logging measure, it should ensure maximum degree of water-tightness. Seepage is proportional to the number and size of cracks. The type of lining selected should, therefore, ensure minimum cracks to limit seepage losses to the minimum. Cement concrete lining is more impervious than tile lining.
Hydraulic Efficiency. The carrying capacity of a channel varies inversely with the value of coefficient of rugosity of the lined surface. The coefficient of rugosity increases with the deterioration of lined surface with the passage of time. A reasonable smooth surface, i.e., low coefficient of rugosity is desirable to ensure maximum velocity with corresponding economy in section. Concrete and tile linings are hydraulically most efficient.
The lining of an irrigation canal has the advantages (i) Reduction in seepage losses from canals reaching water table and raising it resulting in waterlogging and reduction in yield, (ii) Reduced seepage losses by as much as 75 per cent means saving of water which otherwise would have required construction of bigger reservoir and dam for the same amount of actual
water delivered to the field which implies more capital expenditure without much gain, (iii) Reduction in losses and thereby making available more water for extension of irrigation to new areas and improvement of irrigation facilities in the areas afready under irrigation, (iv) Flatter slope in lined canal system results in low height of dam and consequent saving in cost of dam construction, (v) Stable channel section, (vi) Brings more area under command due to very flat slope possible, (vii) Steeper side slopes and bed slope possible as the lined section is immune from erosion, (viii) Higher velocity permissible, resulting in proportionate saving in cross sectional area, land width, quantum of earthwork excavation and construction of bridges and cross drainage works which in certain cases may offset completely the extra cost of lining, (ix) Possibility of breaches are remote owing to sound structural stability of lined section, hence improvement in operational efficiency, (x) More hydel power generation possible with saving in water from losses and conservation of head losses due to flatter bed slope possible, (xi) Low coefficient of rugosity resulting in high velocity with the same slope and hence reduced cross sectional area compared with unlined section, (xii) Lined canal water does not pick up harmful salts from the soil through which it passes because canal water does not come in contact with the subsoil, (xiii) Permits more winding alignment resulting in saving in embankment and cutting costs, (xiv) Prevents weed growth thereby resulting in saving of expenditure incurred on weed removal in the case of earthen channels, (xv) Stable section which in the case of distributaries and minors reduces remodelling and alteration of outlets, (xvi) Theft of water by cultivators is stopped, (xvii) High velocity provided in lined section shall carry forward the blown in sand in sandy tracts, (xviii) Recurring charges on silt clearance inherent in unlined canals are avoided in the case of a silting canal, with the same slope. Existing silting channel when lined at the same slope generates higher velocity to carry forward the sediment, (xix) Considerable economy in the acquisition of cultivable land due to relatively narrow section of the canal, (xx) Improvement in cropping pattern, (xxi) Environmental bettennent, (xxii) Greatly reduced maintenance and operational charges of the canal, (xxiii) Reduction in evaporation and transmission losses due to reduced exposed area, (xxiv) Reduction in erosion which occurs in unlined channels constructed in steep lands.
Disadvantages of Lining
The widely publicized lining of channels is not free from disadvantages, some of which are (i) Higher initial investment; a lined canal is 3 to 4 times costlier than an unlined one of the same capacity, (ii) Costly repairs, (iii) Shifting of outlets is very costly because it involves dismantling and relaying of lining, (iv) Longer construction period, and (v) More sophisticated construction equipment and skilled labour are required.
Lining of irrigation channels is necessitated to achieve all or some of the objectives, keeping in view economy as well. These are (i) Reduction of seepage losses through unlined canal section thereby making available additional supplies for extension of irrigation to new areas, (ii) Prevention of waterlogging in the area adjacent to the canal by reducing seepage of canal water to water table, (iii) Improvement of discharging capacity of existing channel, (iv) improvement of operational efficiency, (v) Elimination of silting problems inherent in unlined canals due to higher velocities possible in lined channels, (vi) Reduction in maintenance cost, (vil) Reduction in cost of excavation due to reduced cross section (and thereby saving in land) with increased velocity permissible in lined section, (viii) Increase in available head for power generation as also to command more area as a flatter gradient can be provided in lined channels, and (ix) Prevention of weed growth. The necessity of lining has to be established by quantifying the contribution of each of the above factors and assessing the overall economy to be achieved by lining a canal.
The benefits that accrue from lining of the channels generally justify the initial capital cost and as a consequence there is now better appreciation of the need for lining of channels.
Conservation of water supplies is important as the demand continues to increase and new sources of supply are becoming increasingly scarce. The principle of conservation requires that full use be made of available water supplies, the most promising being reduction in the amount of water lost through seepage during conveyance in the canals. Avoidable seepage losses from unlined canal system not only entail sizeable wastage of investment made on the construction of dam to impound water, and canal system to distribute it, but also calls for further investment to control the menace of waterlogging created in the canal irrigated areas. Lining offers a solution because it helps conserve the costly impounded water otherwise lost during conveyance due to high seepage losses in unlined section. Thus, the principal objects of lining irrigation channels are to (i) Minimize losses due to seepage, (ii) Protect the area prone to waterlogging due to rise in watertable, and (iii) Pass increased discharge through the canal due to the adoption of higher velocity, (iv) Rigid lining also results in improvement of command and larger working head for power generation. Lining of water courses in general and in the areas irrigated by tubewells assumes significance as pumped water is costly.