ABSTRACT
The concrete that has been referred to in previous chapters was made onsite, which means that it was mixed, poured, extended and compacted next to or over the actual canal. It is well known that concrete can be used to produce high quality pre-
fabricated pieces, which can also be employed to line canals. The method used is that of employing thin slabs, which is intended to
help reduce both transport and installation costs. The actual dimensions are very variable, from 30cm×50cm to 2m×1m. The material’s strength depends on its thickness and this has to permit
the slabs to be transported, raised and installed over the trimmed canal excavation. Of course, if there are any protruding points in the excavation (though everything must be done to avoid this), the slabs should be strong enough to withstand the water pressure without breaking. It must also be taken into account that the maximum aggregate size in
the concrete, in the case of lining and piping, should not exceed one-third of the total lining thickness. This will greatly limit the maximum gravel size and could lead to high cost increases that might negate the original savings due to the lower volume of concrete required. Perhaps the greatest advantage associated with prefabrication is the excel-
lent product quality that is obtained, which is much better than concrete that is mixed onsite, and it is also less difficult to control and involves fewer weather-associated problems for the people working with it, all of which lead to better overall quality results. The concrete production plant is able to provide much better concrete control with laboratory and inspection techniques than when it is mixed and poured onsite, since these will be separated by large distances. The concrete must possess very high strength and so great care must be taken, with respect to not only the aggregate size, but also the water-cement ratio, which should be as low as that permitted by the available means of compaction. In order to achieve lower water-cement ratios during slab production,
employed, some of which include suction
systems to remove any excess water. Formwork that incorporates closewoven fabric on the upper slab surfaces is used and this enables the elimination of excess water, but without allowing the cement to pass and results in water-cement ratios that are impossible to achieve under normal production conditions. The strength of the concrete produced in this way is extremely high. This, then, is the main reason why, under normal circumstances, thin slabs
can be achieved without any requirement for reinforcement or prestressing. Another significant advantage for certain climate regions is that it now
becomes possible to take most advantage of the times of the year when, due to adverse weather conditions, production would be otherwise very low. This would be the case, for example, in regions where there are very welldefined wet and dry seasons. During the rainy season, it would be possible to prefabricate a large number of slabs and store them under some form of suitable cover that would protect the personnel involved from the adverse weather conditions. Then, on arrival of the dry season, large surface areas of lining could be installed employing simple machinery basically consisting of lorries and small cranes for raising the slabs. This would obviously involve taking precautions to prepare accesses and to carry out the excavation profiling or trimming. In order to obtain maximum benefit from this method, the associated
canal must be large enough to justify and amortize the installation of a prefabricated slab production plant, which would normally be sited in a location with all-round easy access and electric power. To facilitate this amortization in situations where not just a canal is to
be constructed, but a complete irrigation network for a region, including the execution of smaller canals, it is recommended that the entire network be prefabricated. In this case, it is recommended that the contents of Chapter 18 concerning such small canals and irrigation channels be taken into consideration. One basic characteristic of prefabricated slab linings is that they have a
joint length per square metre that is much greater than in the case of onsite concrete. This is still true even with very large slabs, and even more so in the case of smaller ones (Figure 6.1). The result is that the probability of filtration is much greater in a slab-
lined canal. This means that any decision should only be made after very careful analysis, with special attention being paid to whether the canal is basically being constructed with small infiltration. If, in spite of everything, it is decided that a prefabricated slab canal is the most recommended, extreme caution must be taken with the joints between slabs in order to obtain maximum impermeability guarantees, otherwise it might turn out to be difficult to achieve desired losses of less than 25-50 l/m2, unless, of course, the underlying ground itself possesses sufficient level of
A very different case is that of a canal that is lined to prevent ground erosion, facilitate its cleaning, and improve the rugosity factor and hence the canal capacity, etc. The profusion of joints in this case should not be a problem. The quality achieved in the excavation trimming or profiling has tremen-
dous significance in the use of prefabricated slabs in canal linings. Not only must there not be any irregularities in the ground that couldpossibly break the slab, but the excavated ground plane must match the desired surface. However, themost important point is that theremust not be any excessive excavation at any point. If this were to occur, then the hollow should be filled with low-cement concrete since it is completely unacceptable to use soil for this purpose,whichwould not be suitably compacted, as can be seen in Figure 6.2. Figure 6.3 shows a clear example of how an excavation should not be
slabs are to be installed.
