CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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7. Anscuhpoprobrltopcikesrsand
7.1 Overview
An anchor block is an encasement of a penstock designed to
restrain the pipe movement in all directions. Anchor blocks
should be placed at all sharp horizontal and vertical bends,
since there are forces at such bends which will tend to move
the pipe out of alignment. Anchor blocks are also required to
resist axial forces in long straight sections of penstock. Support
piers are short columns that are placed between anchor blocks
along straight sections of exposed penstock pipe. These
structures prevent the pipe from sagging and becoming
overstressed. However, the support piers need to allow pipe
movement parallel to the penstock alignment which occurs
due to thermal expansion and contraction.
7.2 Anchor blocks
7.2.1 GENERAL
Locations at which anchor blocks are required and their
construction are described in this section.
7.2.2 LOCATION OF ANCHOR BLOCKS
Anchor blocks are required at the following locations:
At vertical or horizontal bends of the penstock as shown in
Photograph 7.2. A filled penstock exerts forces at such bends
and the pipe needs to be properly ‘anchored’.
Immediately upstream of the powerhouse. This minimises
forces on the turbine housing.
At sections of the penstock where the straight pipe length
exceeds 30 m. This is to limit the thermal expansion of the
pipe since an expansion joint will be placed downstream of
the anchor block.
7.2.3 CONSTRUCTION OF ANCHOR BLOCKS
Anchor blocks should normally be constructed of 1:3:6
concrete (1 part cement, 3 parts sand, 6 parts aggregate) with
40% plums and nominal reinforcement. Plums are boulders
that are distributed evenly around the block such that they
occupy about 40% of the block volume. The boulders add
weight to the block and therefore increase stability while
decreasing the cement volume required. Hoop reinforcement
is required around the pipe to resist cracking of the concrete
due to tensile forces from the pipe. Three 10 mm bars are
generally sufficient, as shown in Figure 7.1
Photo 7.1 A straight penstock with four supports and an anchor
block before the power house
Photo 7.2 Jhankre mini-hydro anchor block for an
upward vertical bend