Page 253 - Civil Engineering Project Management, Fourth Edition

Earthworks and pipelines
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minimum rise or fall. The preferred minimum gradients are 1:500 on a rising
grade in the direction of flow; and 1:300 on a falling grade. The former would
be to an air release valve, the latter from the air valve to a washout or hydrant.
Thus the levels of ground ahead of the pipelaying must be prospected to locate
suitable high and low points, and intermediate points where an increase or
decrease of grade is necessary. The pipeline between such pre-determined points
should follow an even grade. In flat ground it may not be possible to comply
with the foregoing grades, but it is still advisable to give uniform rises to air
valves and falls to washout positions. In built-up areas pipelines can generally
follow the requisite cover below ground surface because branches and connec-
tions will release air, and hydrants will be used as washouts.
Backfill to pipes should always be of selected soft or fine granular material
to 150mm above the crown of the pipe. Few contractors in UK would fail to
do this, but on some contracts overseas the resident engineer may need to stop
the contractor from dozing the excavated hard material straight back into the
trench irrespective of the rocks it contains which would at the least damage the
sheathing to pipes.

18.8 Thrust blocks and testing pipelines

The resident engineer may have to ensure that thrust blocks to pipes are
adequate. A block acts primarily to transfer pipe thrust to the natural ground
against which the block abuts. Hence, the nature of the ground is important.
The force to be resisted on a bend with push in joints is shown in Fig. 18.1. The
internal water pressure taken should be the maximum static pressure occur-
ring, plus an allowance for surge pressure. The block bearing area against the

Fig. 18.1. Thrust on a pipe bend. The maximum pressure taken should be maximum
static plus an allowance for surge