Civil Engineering Project Management (4th Edition)
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234 <strong>Civil</strong> <strong>Engineering</strong> <strong>Project</strong> <strong>Management</strong><br />
natural ground has to be sufficient to mobilize adequate resistance from the soil<br />
against pipe thrust. Where a bend points down, a weight block below the upper<br />
joint is usually necessary, the joint being strapped down to it. Particular care<br />
has to be taken when a bend down is required at the bank of a river or stream.<br />
There may then be very little ground resistance to prevent the bend blowing<br />
off if the joint is the usual push-in type, hence the upper joint of the bend may<br />
require tying back to a suitable thrust block to resist the hydraulic force tending<br />
to push the bend off.<br />
The watertightness test for a pipeline should be prescribed in the specification.<br />
Care must be taken to fill the pipeline slowly to ensure release of all air. The<br />
usual practice is to fill from one end, having all washouts and hydrants open.<br />
The latter are progressively closed along the line when they cease to emit air or<br />
mixed air and water. This may take a considerable time, as air pockets may<br />
get trapped and only slowly disperse. A satisfactory test cannot be achieved if<br />
an air pocket is left in the pipeline. After filling, the pipe should stand under<br />
pressure for 24 h before testing. This permits pipe expansion and absorption<br />
of water by asbestos cement or concrete pipes or by any mortar lining of steel or<br />
iron pipes. Normally pipelines are tested between valves, but on a trunk main<br />
these may be so far apart that temporary stop ends may be needed to test the<br />
pipeline in reasonable lengths. Especial care is needed in testing partly completed<br />
pipelines to ensure they are properly restrained against the high test<br />
pressures.<br />
An experienced pipelaying contractor will know that care in making joints<br />
on a pipeline is rewarded many times over when the pipeline test shows the<br />
line is satisfactory. It is rarely possible to leave joints exposed before testing, so<br />
if a test fails it may take much time and trouble to find the leak causing it. When<br />
finally found it may be as simple as a twisted rubber ring in a push fit joint. It is<br />
not easy to make clean perfect joints in a muddy trench. The contractor can<br />
save himself money if he gives the jointer every facility to make a good joint –<br />
easy access to see the underside of the pipe spigot, buckets of water and plentiful<br />
clean rags to ensure that joint faces and joint ring are scrupulously clean<br />
before the joint is assembled. No grease or jointing compounds should be<br />
permitted other than that which the pipe manufacturer recommends. Welders<br />
also need sufficient room and good lighting to make sure welds are adequate<br />
and should not be expected to weld up badly aligned pipes.<br />
18.9 Handling and jointing large pipes and fittings<br />
All ductile iron or steel pipes and fittings must be handled with proper wide lifting<br />
slings to prevent damage to their sheathing or coating. The use of chains<br />
or wire ropes ‘blocked off’ pipes with pieces of wood should not be permitted<br />
by the resident engineer. Apart from possible damage to the coating, the packing<br />
pieces may slip out when the chain or wire rope slackens and the pipe may<br />
fall. Handling of heavy pipes and fittings must be done with every precaution.