Page 254 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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natural ground has to be sufficient to mobilize adequate resistance from the soil
against pipe thrust. Where a bend points down, a weight block below the upper
joint is usually necessary, the joint being strapped down to it. Particular care
has to be taken when a bend down is required at the bank of a river or stream.
There may then be very little ground resistance to prevent the bend blowing
off if the joint is the usual push-in type, hence the upper joint of the bend may
require tying back to a suitable thrust block to resist the hydraulic force tending
to push the bend off.
The watertightness test for a pipeline should be prescribed in the specifica-
tion. Care must be taken to fill the pipeline slowly to ensure release of all air. The
usual practice is to fill from one end, having all washouts and hydrants open.
The latter are progressively closed along the line when they cease to emit air or
mixed air and water. This may take a considerable time, as air pockets may
get trapped and only slowly disperse. A satisfactory test cannot be achieved if
an air pocket is left in the pipeline. After filling, the pipe should stand under
pressure for 24h before testing. This permits pipe expansion and absorption
of water by asbestos cement or concrete pipes or by any mortar lining of steel or
iron pipes. Normally pipelines are tested between valves, but on a trunk main
these may be so far apart that temporary stop ends may be needed to test the
pipeline in reasonable lengths. Especial care is needed in testing partly com-
pleted pipelines to ensure they are properly restrained against the high test
pressures.
An experienced pipelaying contractor will know that care in making joints
on a pipeline is rewarded many times over when the pipeline test shows the
line is satisfactory. It is rarely possible to leave joints exposed before testing, so
if a test fails it may take much time and trouble to find the leak causing it. When
finally found it may be as simple as a twisted rubber ring in a push fit joint. It is
not easy to make clean perfect joints in a muddy trench. The contractor can
save himself money if he gives the jointer every facility to make a good joint –
easy access to see the underside of the pipe spigot, buckets of water and plen-
tiful clean rags to ensure that joint faces and joint ring are scrupulously clean
before the joint is assembled. No grease or jointing compounds should be
permitted other than that which the pipe manufacturer recommends. Welders
also need sufficient room and good lighting to make sure welds are adequate
and should not be expected to weld up badly aligned pipes.

18.9 Handling and jointing large pipes and fittings

All ductile iron or steel pipes and fittings must be handled with proper wide lift-
ing slings to prevent damage to their sheathing or coating. The use of chains
or wire ropes ‘blocked off’ pipes with pieces of wood should not be permitted
by the resident engineer. Apart from possible damage to the coating, the packing
pieces may slip out when the chain or wire rope slackens and the pipe may
fall. Handling of heavy pipes and fittings must be done with every precaution.