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.