Question
What are the recommended selfcleansing
velocities for foul or surface
water concrete sewers?
Answer
Minimum self-cleansing velocities are:
Foul sewers: 0. 75m/sec at one third
design flow
Surface water sewers: I0m/sec at
full pipe fl ow (This is consistent with
the water industry guide `Sewers for
adoption’)
Question
What is the maximum permissible
velocity in concrete pipes?
Answer
Limits are not normally placed on
maximum velocities but certain
criteria such as the potential for
erosion should be considered for steep
gradients.
Question
What are the appropriate “roughness
values” for concrete pipes?
Answer
Roughness values are:
Foul sewers: ks = I .5mm
Surface water sewers: ks = 0.6mm
(This is consistent with the water
industry guide `Sewers for adoption)
Question
What is the maximum working
pressure that concrete pipes can
withstand?
Answer
Pipes are designed to operate at
atmospheric or at occasional low
heads of water due to surcharge.
The BS performance requirement for
pipelines is a minimum pressure of 0.7
bar
Question
What are the recommended minimum
depths of cover for concrete pipes?
Answer
As a guide the minimum depth of
cover to top of pipes is. In roads and
adjacent verges: I.2m In fi elds: 0.9m |
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Question
Is the width of the trench important
when laying concrete pipes using
open-cut?
Answer
Yes the `narrow trench’ condition
should be used where possible, as the
`wide trench’ load the pipe more.
Question
What is the guidance on the selection
of the pipe strength class and
bedding class/factor for a given design
situation?
Answer
A combination of pipe strength and
bedding class should be chosen to
give an economical design. It is usually
better to select a higher pipe strength
than a higher bedding class.
Question
How can the strength class and sulfate
resistance be identifi ed on concrete
pipes and manhole units?
Answer
Precast concrete units are surface
marked in indelible paint or impressed
characters in accordance with the
requirements in the relevant part of
as 5911.
Question
Are there any precautions to take
for concrete pipelines laid with steep
slopes?
Answer
It is recommended that anchor blocks
be cast into the ground at every 3 or
4 pipe joints behind the socket Also
consider MDF packers on the face of
sockets to prevent construction of
a reinforced concrete raft over the
pipeline.
Question
If concrete pipes at shallow depths
require protection, what are the
recommended materials?
Answer
Pipes at shallow depths can be
protected with concrete surrounds or
by the construction of a reinforced
concrete raft over the pipeline. |
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Question
What is the view on using rocker pipes
at manholes for larger concrete pipe
sizes?
Answer
FP McCann advises that an assessment
should be made regarding possible
differential settlement and rocker
pipes used as appropriate.
Question
What are the recommended handling
methods for concrete pipes?
Answer
Visit our downloads section to download our safety data.
Question
Is it acceptable to place blocks under
each end of concrete pipes in order to
adjust levels?
Answer
Placing blocks under pipes is not
recommended as this can cause beam
loading when backfiled, for which
they are not designed.
Question
What sizes of concrete pipes are
subject to air/water tests when
installed?
Answer
Air/water tests are normaliy applied
to pipes up to DN7SO and visual
inspection is used above this size.
Question
What is the most likely cause of an air
test failure for a concrete pipeline on
site?
Answer
Common causes of an air test failure
include displaced joint seals or a fault
in the test equipment such as not
achieving a seal between the stopper
and the pipe wall
Question
Do you recommend surrounding
precast concrete manholes with in-situ
concrete?
Answer
Concrete surrounds to manholes
are not necessary in the majority
of situation. However if the sewer
is to be adopted, this may be a
requirement |
