68 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
(wet sacks) to cover the masonry helps to retain moisture
and cures the canal structure better. An uncured canal
will not gain full strength. During hot and dry weather
pouring water on the masonry canal will be required
frequently to ensure that the canal walls remain moist
and reach their full strength. Curing is more important for
plasters since they are thin surfaces and can easily crack
if they dry up quickly. Note that if the plaster is done at a
later stage (and not immediately after the masonry work),
it will require further curing for at least another 4 days.
Also, the masonry should be wetted before applying the
plaster.
For plain concrete lining, use 80 mm thickness. A minimum
curing period of 7 days is recommended.
For reinforced concrete see Section 8.5.2.
4.8 Checklist for headrace works
Check the headrace alignment for stability. Is the area above
and below the headrace alignment stable? Refer to Chapter 2
for signs of instability. Remember that earth canals are the
most economic option where the headrace alignment is on
stable ground and seepage is not likely to contribute to slope
instability.
To minimise costs, stone masonry in cement mortar canals
and headrace pipes should only be used along the difficult
stretches of the alignment.
While fixing the headrace alignment do not make the invert
slope steeper than necessary, since any loss in head here
leaves less head for power generation. Minimise the length
unless longer length is required to avoid costly crossings.
Try different options so the design is economical and the
construction is practical.
Has the headrace canal or the pipe size been calculated
based on the available site data? If an irrigation canal is
being refurbished into a headrace, note that there may not
much control over the invert slope. Decide on the type of
canal by trying different cross sections and calculating the
corresponding velocities such that they are within the limit
shown in Table 4.1.
For HDPE headrace pipes, be sure to follow Figure 4.8 for
pipe burial details. HDPE pipes should not be exposed.
For the construction work refer to Section 4.7.