CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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Box 4.5 The formers method for lining canals
The method developed by Practical Action in Peru and
described here is generally known as the ‘formers method’
for constructing canals. The formers method permits savings
in time and material in constructing concrete channels by
reducing the need for pouring concrete into conventional
formwork. Precise placement of formers and lines tied
between them enable the concrete to be plastered to the
insides of the trench and finished with a trowel (see Figure
4.4).
The method involves placing a
layer of concrete on the bottom
and sides of the canal to form a
uniform thickness and a smooth
finish. Levelling and finishing the
surface is done according to the
former.
of oil or petrol is placed on the formers before carrying
out the lining. This also assists with the preservation
of the formers. Care should be taken to avoid damage
to the edges of the lining when the formers are taken
out.
Curing the concrete. To reach the required strength
and durability, fresh concrete should be cured. This is
achieved by filling the surrounds with water so that
Procedure
Setting out for the formers.
Locate pegs every 10 metres Figure 4.4 Formers method of lining canals.
in straight sections and every
5 metres in curved sections, taking into account the
the linings remain soaked for a period of a minimum
slope of the design. It is preferable to use a builder’s
of 10 days. This is easy to carry out by locating turns or
level to achieve the required precision.
earth banks at each end, which retain the water. During
Fixing the formers. Locate formers on each peg at right
rainy periods a spillway can be formed to allow excess
angles to the centreline of the canal, vertical and exactly
water to escape, which will also offer a check on the
in line. They are fixed to the pegs using No. 16 gauge
slope. The curing of concrete is very important and
wire and nails, after which intermediate formers are
should not be overlooked.
located every 2.5 metres in straight sections, the
Expansion joints. Expansion joints are required in the
required slope being checked with a pipe level to give
spaces that are left when the formers are taken out
5 mm drop every 2.50 metres (a slope of 2 in 1000).
every 2.5 metres in straight sections and variable in
Each former is checked for line, level, that it is
curved sections. These permit the concrete to expand
perpendicular to the canal centreline and fixed firmly.
and contract without cracking the linings. To fill the
Lining the canal. Prepare a 1:1.5:3 concrete mix
joints the following work is required:
After making the dry mix, turning the mix a
a) Clean the joints of debris and unwanted materials
minimum of three times to mix thoroughly, add water,
with an angular palette whose dimensions are
which should have a quantity no greater than one-
suitable for the width of the joint.
half of the total weight of the cement (i.e. for mix
b) Prime the inside surface of the joint with a solution
with 1 kg of cement put ½ litre of water). Next the
of tar with kerosene in proportions 1:3 so that it
sides of the canal are plastered and compacted. The
has the viscosity of paint. This solution should be
pegs are taken out after the finish is completed. Then
applied with a brush.
fine sand is sprinkled with cement to give a mix of
c) Place a hot mix of tar with fine sand, in proportions
1:3 and a plastering board is used to give a smooth,
of 1 can of tar to 4 cans of sand. First heat the tar
impermeable finish to the lining. When the sides of
and then gradually add the sand while mixing
the canal are completed the same procedure is
until it has the consistency of black sugar. This
followed for the bottom. To finish the edges, care is
mixture is placed first at the sloping sides of the
required to ensure that the formers remain in line.
channel and then at the bottom. It is placed in
They should be checked using a cord or rule.
layers and compacted with the angular palette.
Extracting the formers. Formers are taken out after 24
The finished level of the joint should not exceed
hours in cold climates. To make extraction easy, a layer
the level of the canal lining.