Minimum self-cleansing velocities are based on empirical research and experience. The values used for design purposes are based on typical sediment loads expected in normal conditions. However, caution should be taken to ensure that the anticipated sediment load and the composition the material carried in the flow is within normal expectations for foul and surface water wastewater systems. If excessive sediment loading is anticipated it is best practice (even in "normal" situations) to incorporate an effective means of sediment control at the head of the flow, e.g. with a gully, silt trap or hydrodynamic separator.
Remember, the larger the pipe and the flatter the gradient, the greater the flow rate required to achieve self-cleansing velocity. It is unwise to select too large a pipe "to allow for possible development" as this may lead to settling out of solids, long retention periods, blockages and build-up of septicity. For in-line surface water attenuation systems, this can sometimes be overcome with "low flow" channels built into the invert of the larger pipe or box culvert.
It should also be remembered that self-cleansing velocities may only be achieved periodically and accumulation of detritus can occur within the system at lower flows until the higher flow "design" events take place. In these situations, the accumulated material may not be flushed through the system when self-cleansing velocities occur and over time the sewer may become blocked. Regular inspection and maintenance of sewers is therefore critical to ensure their optimum performance. The minimum design self-cleansing velocities recommended in Sewers for Adoption are:
Foul sewers: 0.75m/sec at one third design flow. Surface water sewers: 1.0m/sec at full pipe flow.
Limits are not normally placed on maximum velocity but certain criteria such as the potential for erosion should be considered for steep gradients. It is wise to eliminate conditions such as supercritical flow and cavitation in the design of sewers.
Whilst differing opinions exist regarding an upper limit for the velocity of flow within sewers, values of between 8 - 14 metres/second have been used on some projects. For particularly high velocities and flow rates, the use of thrust blocks at bends may be necessary.
There have been various calculation methods used in the UK for the hydraulic design of sewers. The most commonly used methods include Manning (frequently used for open channel flows) which uses a dimensionless "n" value for hydraulic roughness and the Colebrook-White equation for transitional flow which has been generally accepted as the industry norm.
The linear measure of effective hydraulic roughness using the Colebrook-White equation is expressed as a "Ks" value. There has been extensive research on the comparative hydraulic roughness of pipes of different materials. The findings of HR Wallingford and incorporated within 'Design & Construction Guidance' suggest that regardless of pipe material the effective hydraulic roughness values are:
Foul sewers: Ks = 1.5mm Surface water sewers: Ks = 0.6mm
The intended use of concrete pipes is to the conveyance of sewage, rain water and surface water under gravity or occasionally at a low head of pressure in pipelines that are generally buried. The British Standard sets the performance requirement, under type test and routine tests of 0.5bar (i.e. 5m static head of water). This does not confer a pressure rating on the pipeline and is not to be considered as a value for a permanent working head of water a pipeline can withstand.
The minimum design self-cleansing velocities recommended in 'Design & Construction Guidance' (DCG) published by Water UK are:
Foul sewers: 0.75m/sec at one third design flow.
Cover depths less than the minimum values published in industry specifications and Standards should only be used with the appropriate authority's permission. a) lt is common practice that pipes laid under roads should have cover over the pipe of not less than 1.2m to avoid conflict with other services. This cover should be maintained for main roads, light roads (which may on occasion carry main road traffic) and for pipes laid under grass verges adjacent to a road. Where pipes have to be laid with less than 1.2m cover special consideration is needed to reduce the risk of damage. For concrete pipes, according to TRL tables, the cover depth under highways can be reduced down to a minimum depth of 0.6m when installed in conjunction with a full granular bed and surround (Bedding Class S).
b) For pipes laid in fields a minimum cover of 0.6m should be provided. At shallower depths there is a risk of damage from agricultural operations. Where pipes are required to be laid at cover depths less than 0.6m, the pipes should be protected as per the recommendations of BS 9295 Annex A, A13. Where a shallow depth of cover cannot be avoided (i.e. less than 0.6m for concrete pipes) , the preferred method of protection is the use of a reinforced concrete slab installed over the pipeline. It is important that the slab extends sufficient distance beyond the trench and would depend on soil conditions (minimum bearing of 300mm each side advised). A layer of compressible material placed directly over the pipeline aids in the prevention of the slab loading directly onto the pipeline should settlement occur. Another method of protection at shallow cover depth is with the use of a concrete surround (Bedding Class A). It is important in such installations to install compressible material at least every other pipe joint to ensure that the pipeline retains its flexibility at the joints.
Special consideration should be given where construction plant has to cross pipelines with shallow cover depth. Where possible, traffic should be routed over dedicated crossing points. Crossing points may consist of heavy steel plates to transfer vehicle loads or temporary additional cover placed over the pipeline.
In deep trenches it can be critical for the trench width to be kept minimal (narrow trench design method) for a distance above the crown of the pipe. Any slight increase over the designed trench width can greatly increase the pipeline's loading. Under embankments, loadings can differ - not simply due to height of embankment, but also on the method of emplacement of the fill and the stage that the pipeline is installed i.e. the pipe may be installed in a shallow trench and an embankment constructed over it, or the embankment may be constructed with the pipeline subsequently laid with a newly excavated trench cut into it.
The 'narrow trench' condition should be used where possible. Recommended trench widths are given in the Total Design Loads Tables within the Technical Design Guide. Trench widths for concrete pipes are automatically calculated within the on-line Structural Design Calculator and Material Cost Calculator. An advanced user function is provided within the Material Cost Calculator that enables users to optimise costs based on minimum trench widths. Additional knowledge relating to the site conditions and engineering experience and judgement is required in order to use this function effectively.
It is good practice to state the maximum design trench width for each diameter in the pipeline on the construction drawings.
Concrete pipes manufactured in accordance with Clause 4.3.8 of BS EN 1916 are suitable for slightly aggressive chemical conditions. In the UK, some soils are more aggressive in nature. As a safeguard, the provision of a concrete suitable for ACEC AC-4 conditions as described in Building Research Establishment Special Digest 1 2005 is specified.
The preferred method to achieve AC-4 for a 100 year intended working life is the use of a DC-4 concrete with surface carbonation (i.e. precast concrete) and low permeability concrete to Clause 4.2.6.2 of BS EN 1916 without the need for additional protective measures.
The provisions in the relevant product Standards are adequate for 100 years working life for the vast majority of discharges in normal conditions of use. However, further consideration should be given to suitable additional protective measures in the following cases: - where a sewer, drain or other component within the system is liable to carry untreated or corrosive trade effluents - a rising main discharge - septic sewage - in situations without adequate ventilation.
Circular concrete pipes are manufactured to Strength Class 120. This defines the minimum crushing load under test conditions and is equal to 120 multiplied by the nominal diameter (DN) of a pipe in metres. For example, a Class 120 concrete pipe of size DN600 has a minimum crushing strength of 120 X 600/1000 = 72kN/m. The test load is applied at the crown of the pipe along its length and the pipe is supported on longitudinal bearers. Test conditions are generally more onerous that pipes installed in the ground, where different embedment material placed at specific locations around the pipe provides varying degrees of additional support.
The different embedment details are known as “bedding classes” and each bedding class has a “bedding factor” which relates to the additional support provided to the installed pipe compared to the test condition. Ovoid pipes are available from some MPA Precast Drainage members and are manufactured to Strength Class 150 - this would use the maximum bore width of the pipe (WN) rather than the DN in the strength calculation.
Do not confuse the term "backfill" with "embedment". Embedment is the material that surrounds the pipe and is engineered to provide adequate structural support. There are a number of different designs of embedment for concrete pipes providing varying degrees of structural support.
It is important to choose an appropriate design of embedment for the type of pipe being used, the loading that it will be subjected to and the characteristics of the native soil. Backfill is the material that is placed in the trench above the pipe and embedment. In many situations the 'as-dug' trench material may be used for backfill provided for example, it is readily compactable, free from tree roots, vegetation, building rubbish and frozen soil.
Adopting Authorities may have standard backfill details in particular for roadways and reference should be made to such details.
For further information on backfill and pipe bedding, refer to the Technical Design Guide.
It is recommended to cast anchor blocks into the ground at every 3 or 4 pipe joints behind the socket. Also consider compressible packers (e.g. MDF) on the face of sockets to prevent concrete to concrete contact. Also, on steep gradients or where dewatering has taken place, it is important to restrict ground water movement within the completed trench.
Selection of appropriate bedding or clay dams across the full width of the trench will assist in this. The inlets/outlets from manholes should also be considered, particularly when the use of precast manhole bases is intended. Precast manhole bases have a predetermined fall and angle of entry/exit. This needs to be considered when using steeper gradients to avoid any joint gaps/steps. This would also be the case when in-situ constructed manhole bases are used.
There may be differential settlement between a structure and the pipeline resulting in angular deflection of the joint. This creates no problem for the joint itself but when this movement is ?excessive? there is a shear force that can cause structural failure on the pipe, either shear behind the collar or from beam fracture of the pipe barrel. To prevent this, the first pipe in the line can be restricted in length.
This is known as a 'rocker pipe'. The likelihood of differential settlement should be assessed and rocker pipes used as appropriate. In certain conditions where excessive differential movement is possible, for pipes ≥ DN750, it may be advisable to use multiple rocker pipes to avoid unacceptable angular deflection or shear force at the joint. Check with Precast Drainage Members for size and availability of rocker pipes. Guidance on rocker pipes may be found in ‘Civil Engineering Specification for the Water Industry’ and the 'Design & Construction Guidance’ published by Water UK.
See the detailed guidance given in the Technical Design Guide. A proprietary Concrete Pipe Lifter can be fitted to your excavator for mechanical offloading and laying of concrete pipes. It is a safer, quicker and lower cost way of handling and installing concrete pipes. Further information and videos can be found at Concrete pipelifters (mpaprecast.org).
Under no circumstances should blocks, bricks or any other incompressible material be placed beneath concrete pipes for levelling as this can result in beam loading, for which the pipe is not designed. Any pegs used for setting out or levelling must be removed.
It is recommended the pipeline is air tested at least every 3 pipes laid. Do not wait until whole line is installed before carrying out an air test. The 'Design & Construction Guidance’ published by Water UK prescribes that air/water tests are normally applied to pipes up to DN750, with visual inspection used for pipelines with diameters above this size.
CESWI 7th edition provides guidance for testing pipe diameters greater than DN750, with an increased duration For further information refer to BS EN 1610 Construction and testing of drains and sewers. See also MPA Precast Drainage guide to concrete pipeline air testing video at https://www.youtube.com/watch?v=UdohjdbKP0o
Common causes of air test failure include:
Joint gaps between pipes should be measured internally. Due to the manufacturing tolerances applied, joint gaps of between 5mm and up to 25mm should be of no concern. Note the minimum gap is important to allow flexibility of the pipeline.
Where there is difficulty obtaining joint gaps of 25mm or less, seek advice from the manufacturer. When required, it is advised that an approved lubricant that has been specifically designed for the jointing of concrete pipes is used at all times. Please note that lubricant should not be applied when using "rolling ring" and pre-lubricated seals.
5000 psi (340 bar) is the generally accepted maximum, although a higher jetting pressure can usually be accommodated by concrete drainage products.
Our concrete pipe sector EPD in 2017 suggested a carbon footprint of around 145 kg CO2e/tonne of unreinforced concrete. We believe that that footprint has since dropped as the carbon intensity of OPC cement (CEM I) has since dropped by well over 8%., the grid electricity carbon intensity has also went down significantly since 2017, and some of our rebar suppliers are already reporting embodied carbon intensities as low as 330 kg CO2e/tonne.
There is much confusion within the construction industry over the accurate estimation and reporting of the carbon footprint of materials and construction methods. The only method which should be used to assess the carbon footprint of pipeline systems should be via 3rd party EPDs in accordance with EN 15804 using appropriate Declared Units and Functional Units. MPA Precast Drainage Memebrs has undertaken extensive research and published numerous independent studies. This work uses recognised industry calculation methodologies and transparent reporting of data to demonstrate that the embodied carbon of concrete pipes compares favourably against plastic pipes. For further information visit Sustainable Drainage Systems.
It is generally not practical to increase the wall thickness of concrete pipes. Due to the way pipes are commonly manufactured changing the wall thickness would require significant investment in moulds and development, and so the end cost to the client would be exceptionally high.
It is recommended that the distance between pipelines should be no less than the recommended minimum bedding depth i.e. 1/6th Bc (Bc = outside of pipe barrel dimension) or 100mm.
Some pipes are available with dry weather flows (limited availability, so please contact individual members for availability of products). A range of alternative ‘ovoid’ and ‘elliptical’ shaped products are also available.
Perforated pipes can be supplied to meet with the requirements of the Department of Transport Specification for Highways Works. Perforated pipes are made to the same product standards as standard pipes, with the exclusion of the ‘watertightness’ requirement. Porous pipes used to be made, but are no longer available.
Under the scope of BE EN 1917 and BS 5911-1 product standards, the products are designed to cope with occasional low pressure water surges as they would be subjected to during storm events. The typical installed test requirement is not greater than 6m head at the lowest point of the section, as specified in ‘the 'Design & Construction Guidance’ published by Water UK. This does not confer a pressure rating on the pipeline and is not to be considered as a value for a permanent working head of water a pipeline can withstand.
The products should have an asset life of 100+ years. MCDHW (Volume 2, Series NG1700) suggests that the intended working life should be 120 years. See also MPA Precast Drainage factsheet which also covers service life.