| Design and operation of smallholder irrigation in South Asia |
|Chapter 10 - Durability of 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).