CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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that debris cannot get in and block the pipe.
As far as possible, the pipe alignment should be such
that it is always sloping downhill. This ensures that
there is always a positive head over the pipe and the
chance of it being blocked is also reduced.
If there is a need for inverted siphons (or the pipe
needs to go uphill for some length due to the ground
profile), air release valves should be provided at high
points along the alignment. Similarly, flush valves
should also be provided at low points to flush sediment
from the pipes and hence prevent them from being
clogged.
Note that the setting out and preparation of the bench
for headrace pipe is similar to the headrace canal
discussed in Section 4.4. As mentioned earlier, HDPE
pipes should always be buried. A minimum buried depth
of 1 m with sieved soil 150 mm to 300 mm around the
pipe is recommended as shown in Figure 4.8. The use
of sieved soil ensures that the pipe is not punctured by
pointed rocks during compaction, distributes the loads
evenly and prevents future differential settlements
above the pipe. The 1 m depth minimises the
overburden loads over the pipe such as when people or
cattle walk over it. Also, in areas where freezing is
expected during mid-winter, 1 m is usually sufficient
to be below the frost line.
At inlet and outlet sections of a headrace pipe, it is
recommended to provide inlet and outlet structures of
stone masonry or concrete.
Photo 4.16 Break-pressure tank, with ice around the walls (Jhong)
Photo 4.17 Flush-outs should be provided at low points in pipelines so that
heavy debris can be periodically removed (Siklis)