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