Reasons for lining

The question of whether to line a canal system, and which categories of canal to line, involves technical, economic, and financial considerations. Canal lining is a major cost item, but the record of performance ranges from excellent to very poor, underlining the need for full analysis of alternative courses and technical options in each particular case.

A principal reason for lining may be to reduce seepage losses. However, as an alternative to lining, seepage may be recoverable by groundwater extraction. Indeed recharge by canal seepage may be highly desirable if the quality of the groundwater and the nature of the aquifer favor well development in the particular area. However, if the groundwater is unsuited to irrigation or if the underlying formation is not favorable to well development, canal seepage may present a drainage problem. To illustrate, a canal was excavated in thinly-bedded sandstones dipping parallel with the ground surface, down slope from the canal. The bedding planes provided seepage paths for water, but insufficient yield for economic well development. The seepage produced waterlogging for a distance of about a kilometer down-slope from the canal and paralleling it for several kilometers. Lining was the only available solution, justified in this case by the loss of production from the water-logged area. Some credit could also be taken for the value of water lost from the canal, although seepage water is seldom entirely lost. It may reappear downstream as a contribution to stream-flow and may subsequently be developed for irrigation (although at some cost). However, it is lost to economic use if it joins a body of groundwater of quality unsuited to irrigation or if it is by unproductive evapotranspiration in areas of waterlogging caused by the seepage. A regional rise in watertable and threat of extensive waterlogging may in fact be the principal reason for lining a canal system. Even where the character of the formation is suited to groundwater development, cultivators in the areas may not generally be in a financial position to install wells, at least not on a sufficient scale to control the rise in watertable, preferring to use canal supply only. Further, once waterlogging has occurred the economic productivity of the area falls and cultivators are even less likely to be willing to undertake well installation. Large scale canal lining may than be the solution.

Another factor which may influence a decision to line is erosion, particularly in the vicinity of structures or bends in canal alignment. In some clay-soils (previously discussed) the impossibility of preserving the section of an unlined channel due to sloughing or lateral erosion of the canal banks may be a reason for lining.

The tertiary canal requires special consideration in the matter of lining. In view of the small flow in relation to the "wetted perimeter" of these small channels seepage in permeable soils can cause disproportionate loss. Lining of at least the main stem of the tertiary may be necessary in such soils, if reliable delivery is to be maintained at the outer perimeter of the tertiary command. In dune sand areas, lining of the whole length of the tertiary is virtually mandatory. Another reason for considering lining of tertiaries is heavy infestation with phreatophyte plants (particularly bull-rushes or "typha") in areas of perennially high watertable. Maintenance of a small unlined water-channel in such circumstances can be very difficult. This raises the question of responsibility for maintenance of tertiary channels, lined or unlined, also for meeting the cost of their construction. Payment by cultivators for the cost of tertiary lining has been successfully practiced to a limited extent, but is very much the exception. Government view, and that of some international financing agencies, is generally that tertiaries are the communal property of the cultivators within the tertiary command, not part of the canal system proper. The cost of construction and subsequent maintenance should therefore lie with the cultivators. Cultivators understandably take the opposite view. Their position is complicated by the fact that lining of the main stem of the tertiary, primarily for the benefit of cultivators on the outer (downstream) perimeter, does not significantly benefit upstream cultivators. So why should the upstream cultivators contribute to its cost? Acceptance of the idea of communal property does not apparently extend to acceptance of communality of costs. Efforts to provide credit to cultivators to meet the cost of lining, or to set up intermediary institutions which borrow for that purpose and endeavor to collect from cultivators, have not made payment of the cost of lining of tertiaries any more palatable to cultivators and have generally been unsuccessful. It has usually become a Government cost, and it can be a very substantial budgetary item. For this reason lining is often limited to the main stem, or a certain proportion of the length of the channel.

Causes of deterioration canal linings

Deficiencies encountered with linings are generally either leakage or physical deterioration, and frequently the two are associated. As linings are exposed to a wide range of temperature and to cyclic wetting and drying, some degree of expansion and contraction of any form of rigid lining is inevitable, whether the material is concrete, brick, or masonry. The movement is either at joints, as in formed-in-place concrete linings and pre-cast lining units, or if joints are not provided, it is distributed in capillary cracks as in brickwork or masonry. Leakage occurs at cracks, to a degree depending on their width and at joints unless flexible seals are provided.

A relatively small incidence of cracking or joint leakage can cause a seepage rate not significantly different from that in unlined section or not sufficiently different to warrant the cost of lining, if reduction of seepage losses is its purpose. The seepage can also be the cause of progressive deterioration of the lining, which in turn increases the rate of seepage. The deterioration may result from slow erosion of fine material from behind the lining at the leaks due to movement of water in and out with fluctuating level in the canal. Collapse may eventually result. More commonly seepage attracts the root systems of canal-side plants, behind the lining, and pressure from the expanding roots displaces portions of the lining, again increasing seepage. Plant growth inside the canal at cracks or joints, particularly just below water-level, is also frequently disruptive.

Deterioration at initially small seepage sites can be aggravated by particular circumstances, notably the presence of gypsiferous soils behind the lining or the activity of crabs. Gypsum is highly soluble, and a slow leak can, in time, form a large cavity behind a lining, resulting in its eventual collapse at that point. The design of linings in gypsiferous soils is a special subject and calls for virtually zero seepage. Lining is resorted to in some areas because of severe leakage in unlined canals caused by the tunnelling activity of crabs. However, the same activity can cause the collapse of linings in some situations. This typically occurs where a canal, in embankment, runs at two different levels, seasonally. While the canal is at low level crabs may form tunnels, at a location of significant seepage, extending from water-level down to a seepage pool at the toe of the canal embankment. When the canal level is later raised seepage into these tunnels rapidly increases, causing erosion and cavity formation behind the lining, with eventual collapse. In such circumstances a very low level of initial seepage and crab control measures (drainage of the seepage pool) may be necessary.

Seepage is often initiated by structural cracking of a lining due either to differential settlement of the fill supporting the lining or to soil movement due to changes in moisture content if the canal is excavated in expansive clay soils. The latter can be a major problem in extensive areas of such soils, aggravated by the fact that an unlined section may not be a viable option in these soils due to the incidence of sloughing. The solution to the problem is generally excavation and replacement of the expansive clay in the vicinity of the channel with non-expansive material, an activity of substantial cost if haulage of the latter material is involved. A second alternative, applicable to smaller secondary canals and to tertiary channels, is to construct the canal in the form of a flume of reinforced concrete, with the base of the Qume being at ground surface. The freestanding sides of the flume are then not exposed to expansive soil pressures.

Structural failure of a lining may also be caused by hydrostatic back-pressure on the lining when a canal is drawn down or emptied. This may occur when a canal is in cut, and the watertable in the vicinity of the canal is high. On reducing the counter-balancing internal hydrostatic pressure on the lining, as a result of lowering the level in the canal, the lining is forced inward either collapsing or cracking sufficiently to relieve the external pressure. The solution to this problem is to provide drainage behind the lining, exiting into the canal via a one-way valve. However, the design of such drainage and particularly of the valve is the subject of continuing debate. As discussed earlier in connection with alternative operational systems, the problem of backpressure on linings in some circumstances can be an argument against rotational operation of secondary canals.

Efforts to reduce the first cost of canal linings may set the scene for early deterioration, and this is commonly the case with some types of masonry lining. A substantial masonry lining can have almost indefinite life. However, when the desire to reduce first cost, or inadequate quality control result in a lining consisting of random stone only nominally set in mortar, finished on the inside surface with a thin mortar plaster, deterioration can be very rapid. Crazing of the plaster on exposure to the sun and the frequent wetting and drying leads to peeling, which in turn exposes the very pervious, poorly cemented masonry to full hydrostatic head from the canal. Rapid seepage results, with leaching of the mortar, and failure.

Quality control is in fact a perennial issue in construction of canal linings. Cement is an expensive material, which provides a strong incentive for the contractor to "economize" in its use, with mutual distribution of the resulting "savings" Providing water for curing concrete, plaster, or mortar in brickwork, or for moisture control in embankment compaction, can also be a costly item for the contractor, who may have an incentive to reduce or eliminate its use, often with the collaboration of the inspector who may be under considerable pressure to cooperate. More extensive use of the non-destructive methods now available for testing the quality of completed work, provide independent back-up to routine inspection during construction and could help minimize this problem.

Finally, an important element in the deterioration of linings is often the cultivator himself. Stone slabs used in lining of some secondary or tertiary canals and pre-cast concrete slabs or tiles used for the same purpose are obviously of considerable value for paving or other home improvements, particularly in a muddy wet-tropic environment. Theft of such items from canal linings is consequently widespread and can have a bearing on the selection of these types of lining, versus other options, as well as on the method of their placement in the lining (to render removal more difficult).

Construction materials for primary and secondary canal linings

In view of the adverse effects of cracking on seepage and deterioration of rigid linings the use of flexible plastic sheet has received much attention in recent years. It may be used by itself, as the single lining material, or in association with other materials. However, while plastic sheet is now widely used in Western countries as a lining for storage ponds, its use as a single canal lining under South Asian conditions faces certain problems unique to that area, notably access of water buffaloes. The hooves of buffaloes easily penetrate a plastic lining, unless the lining is buried under soil cover. However, the soil in that situation is fully saturated and very soft, offering little protection from hoof penetration unless of substantial depth. Such a lining system would, in fact, be impractical for canals from which access of buffaloes could not be excluded. For major canals, in which such exclusion may be practical, soil cover would still be necessary to stabilize the lining against the forces of stream-flow. Vegetative growth rooted in the soil cover may then become a problem, either due to roots penetrating the plastic sheet or due to damage to the sheet during cleaning of vegetation. Damage during de-silting operations may be a further hazard. The use of plastic sheet as a single lining material is in fact restricted to special situations, generally relatively large canals, in which the necessary care in construction and maintenance can be assured.

The use of plastic sheet in conjunction with rigid linings is much more common and has considerable merit. The sheet may be regarded as the primary water barrier, the rigid lining providing mechanical protection, or as back-up to a rigid lining designed to be the primary barrier. Such composite linings may be applied to all categories of canal. The rigid linings for primary and secondary canals may be of cast-in-place concrete, pre-cast concrete panels slabs or tiles, brickwork or brick tiles, and stone slab or masonry. Continuously formed (slip-formed) concrete linings widely used in the western countries for canals of all categories are not generally employed in South Asia, probably because of difficulty in quality control. Concrete linings for major canals are either cast in place in panels of about 5 m width or are of pre-cast elements. The vulnerable point in either case is the joint.

It is not unusual to see heavy vegetative growth in joints between panels, and equally in the joints between pre-cast slabs, signalling leakage and the onset of deterioration. While a plastic sheet behind such linings would nominally contain the leakage, it would not stop vegetative growth within the joint and would eventually suffer from root penetration of the sheet unless unusually heavy-gauge sheet was used. For cast-in-place panels, the most satisfactory solution is probably the extruded rubber or plastic joint sealing strip embedded in adjacent panels, traditionally used elsewhere. Externally-applied joint sealants, while continually being improved, do not yet provide this degree of security. For the smaller pre-cast slabs, accurately formed shaped edges providing inter-lock when mortared into place with back-up plastic sheet can be a satisfactory compromise. It will not stop capillary cracking at joints, but will prevent displacement of slabs and will inhibit establishment of vegetation in the joints. Vigilance is necessary in intercepting vegetative growth within the canal in the joints and behind the lining in the embankment. The problem currently is that slabs generally have very poor edge detail, partly due to poor gradation of aggregate and partly to the methods used in their production. This difficulty is not insuperable. However, the problem is often aggravated by the mistaken view that if a plastic sheet backing is used little attention need be paid to joints.

Brick work linings constructed before the turn of the century and still in good condition testify to the virtues of the material, if well-constructed. On the other hand many much more recently constructed brick linings have deteriorated badly. The primary problems are poor quality mortar and inadequate compaction of the fill on which the channel is built, resulting in differential settlement and cracking. The lime mortar used earlier was plastic in consistency, facilitating full imbedding of bricks, and had low shrinkage. The straight Portland cement/sand mortars currently in use, unless with very well graded sand, are likely to be harsh, non-plastic, and permeable. This again is a situation not without remedy. The use of plastic sheet behind brick lining can contain the inevitable seepage through capillary cracks, but it should not be taken as an excuse for poor quality brickwork.

Construction materials and production methods of tertiary canal linings

Tertiary canal linings may be made of the materials discussed above for primary and secondary canals, but being smaller also offer the possible use of integral (single-piece) pre-cast or preformed units. Such units one to two meters in length and placed end-to-end, comprise the whole "lining". It is more correctly a flume, as the units are structurally independent of support from the adjacent fill. Semi-circular or "half-round" spun cast units, lightly reinforced, are a typical example. Trapezoidal sections, produced in conventional molds, are also in use.

A new material recently installed on a pilot scale is G.R.C (glass reinforced mortar). This is similar in some respects to asbestos cement but the reinforcement, instead of asbestos, is alkaliresistant (zirconia-based) glass fibre. The material is sprayed on to a shaped mould, or alternatively on to a flat plastic sheet which is then draped over a mould. G.R.C has a number of desirable features including thinner section and much lighter weight than the equivalent concrete unit.

The critical item with all such integral linings is again the joint between units where movement occurs as a result of changes in temperature and moisture. A rigid jointing material such as the conventionally used cement mortar does not prevent such movement, and capillary cracking and leakage occur. A back-up plastic sheet can contain such leakage. However, elastomeric bitumens have been successfully used as flexible joint sealants, in a lap-joint configuration.