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GENERAL
The ability of a rigid pipe to support the total load transmitted
to it is established by reference to actual crushing tests to cause
fracture. Flexible pipes such as those made from unplasticized PVC
do not fracture under load but they are liable to deformation.The
extent of this deformation depends largely upon the compaction of
the immediate surrounding fill. For this reason, flexible pipes should
always be surrounded with non-cohesive material.
This surround should extend to the trench width in normal trench situations.
The external loads (backfill and surcharge) imposed on a pipe of rigid
material, (such as vitrified clay, concrete, asbestos cement or cast
iron) are supported mainly (sometimes wholly) by the resistance of
the pipe to circumferential bending. On the other hand unplasticized
PVC pipes, being relatively flexible, offer less resistance to circumferential
deformation and rely partly on external support to resist deformation.
Therefore, it is of primary importance for unplasticized PVC pipes
that fill material, particularly the bedding and sidefill, should
be properly compacted in order to prevent excessive deformation.
It is desirable that vertical deformation should be limited to 5%
on completion of the backfilling, which can only be achieved by proper
compaction of the backfill (Please refer to Codes of Practice BS5955
and BS8301).
It is essential to avoid high stress concentrations and sharp objects
such as large stones or flints which should not be allowed to come
into contact with the surface of the pipe.
The flexible nature of unplasticized PVC pipes helps them to accommodate
deformations resulting from ground movement or from other differential
settlement under normal circumstances.
Except in special circumstances, e.g. at very shallow cover depths
or where it is necessary to safeguard the foundations of existing
structures, the use of concrete for bedding or surrounding the pipes
is unnecessary. Figure 4 (in "Special Protection -Ground Loads"
section) illustrates the use of concrete in special local circumstances.
Normally drainage pipework is laid in straight lines. However, in
special circumstances and subject to approval it may sometimes be
acceptable to "spring" the jointed pipes to a slight curve
to avoid an obstacle, or to follow the curvature of a street. If this
is done, and the joints are of the push-in type, care has to be taken
not to spring the pipework to, too sharp a curve or the joints may
be overstrained and fail later.
The manufacturer should be consulted as to the minimum radius that
can be accommodated in this way. Straining of the joints can be minimised
by firmly backfilling a short length of pipe. The pipe should be anchored
in this position by further backfilling before the next joint is made,
and the process repeated as necessary. The trench may need to be widened
on the curve to accommodate the pipe in its straight position. It
is essential that the jointing is always carried out in the straight
position.
FLOW PROPERTIES
The following is based upon information given in the code of practice
BS5955: Part 6: 1980.
For the purposes of calculating flow rates through PVCu pipes, reference
should be made to the Colebrook-White equation. Figures have been
derived using values for roughness (k s ) given in the "Hydraulic
Research Station Charts", 4th edition (metric), 1978 and "Tables
for the hydraulic design of pipes" (metric edition), 1977 for
the sizes of PVCu pipes dealt with in this code. It is recommended
that the information given in Figure 3(b) is used for velocities less
than 1m/s.
These values of roughness are for guidance only and may need future
modification in the light of continuing research work. |
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