What are the recommended self-cleansing velocities for foul and surface water concrete sewers?
Minimum self cleansing velocities are:
Foul sewers: 0.75m/sec at one third design flow.
Surface water sewers: 1.0m/s at full pipe flow.
What is the maximum permissible velocity in concrete pipes?
Limits are not normally placed on maximum velocities but certain criteria such as the potential for erosion should be considered for steep gradients.
Are there any precautions to take for concrete pipes laid to steep gradients?
It is recommended that anchor blocks be cast into the ground every 3 or 4 pipe joints behind the socket (also consider installing packers on the face of sockets to prevent concrete to concrete contact).
What are the appropriate ‘roughness values’ for concrete pipes?
Roughness values are:
Foul sewers: 1.5mm.
Surface water sewers: 0.6mm.
What is the maximum working pressure that concrete pipes can withstand?
Concrete pipes are designed to operate at low heads of water. The BS performance requirement is set at a minimum of 0.5 bar.
What are the recommended minimum depths of cover for concrete pipes?
Minimum depth of cover to top of pipes is 0.6m with bedding class S (360o granular bedding). For cover depths less than 0.6m, then the pipeline should be protected with either a full concrete surround, or a reinforced concrete slab should be placed over it (spanning sufficient distance onto trench walls).
What materials can be used as backfill for a concrete pipeline?
In many situations the ‘as-dug’ trench material is suitable for backfill (should be readily compactable and free from large boulders etc).
What sizes of concrete pipes are subject to testing when installed?
It is common practise to carry out air or water testing of pipes normally up to DN750 and a visual inspection above this size.
What is the most likely cause of air test failure for a concrete pipeline on site?
Common causes of air tests failures include:
- Displaced seals (usually due to use of wrong pipe lubrication during installation).
- Poor laying technique.
- Faulty testing equipment.
- Poor seal between testing equipment and face of concrete pipe.
How frequent should I be testing the pipeline during installation?
It is highly recommended to test the pipeline every 3 or 4 pipes laid as an absolute minimum (do not wait until whole line is installed before carrying out such test).
Do you recommend surrounding pre-cast concrete manholes with in-situ concrete?
Concrete surrounds to manholes are not always necessary in the majority of situations. However if the sewer is to be adopted, this may be a requirement.
What typical joint gaps should be achieved when jointing concrete pipelines?
Joint gaps between pipes should be measured on the internal surface and should ideally be between 5-25mm.
What strength of concrete pipes is available?
Circular concrete pipes are all manufactured to strength class 120 (minimum crushing load for a particular size of pipe is equal to 120 multiplied by DN of pipe divided by 1000).
What concrete pipes are generally reinforced?
Marshalls CPM concrete pipes are generally reinforced in sizes DN675 and above (it is possible to reinforce some of the smaller sizes and also to provide un-reinforced pipes in some of the larger sizes – usually made to order).
What is the standard position of the access opening in cover slabs?
The standard access position is 150mm in from the chamber wall below (measured from centre of nearest access edge to inside of chamber wall below). Other positions/access sizes/multiple access slabs can be made to order.
Do you supply certificates of conformity for your standard products?
No – they are not required as our standard products are certified under the BS ‘Kitemark’ system – certification can be viewed here.
What is the recommended maximum jetting pressure acceptable for concrete products?
5000 psi is the generally accepted maximum, although higher pressure can be accommodated by concrete drainage products.
Where can I get additional assistance on the use of concrete drainage products?
Additional assistance can be had via the Concrete Pipeline Systems Association, see www.concretepipes.co.uk