concrete drainage

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Minimum cover depths for concrete pipes vary depending on the vehicular traffic loading likely to be sustained by the pipeline. There are various industry specifications and Standards that state values for minimum cover depths based on these loading conditions.

Most loading conditions used for design either relate to patterns of wheel loading generated from main road traffic or to pipelines installed within fields where only occasional trafficking, for example from agricultural equipment may be expected. A third loading scenario is also sometimes used which is based on light road trafficking, for example traffic within a residential area, although heavier vehicles may have access to these locations and the main road loading condition is often preferred for design.

Cover depths of less than the minimum values published in these documents should only be used with the appropriate authority’s permission.

Sewers for Adoption, for example, requires the minimum depth of cover to the crown of gravity pipes without protection to be as follows:

  • In domestic gardens and pathways where there is no possibility of vehicular access, 0.35 m;
  • 0.5 m for domestic driveways, parking areas and yards with height restrictions to prevent entry by vehicles with a gross vehicle weight in excess of 7.5 tonnes;
  • Domestic driveways, parking areas and narrow streets without footways (e.g., mews developments) with limited access for vehicles with a gross vehicle weight in excess of 7.5 tonnes, 0.9 m;
  • Agricultural land and public open space, 0.9 m;
  • Other highways and parking areas with unrestricted access to vehicles with a gross vehicle weight in excess of 7.5 tonnes, 1.2 m.

It is common practice that pipes laid under main roads to have at least 1.2m of cover to avoid conflict with other services. This is also true for the grass verges at the side of the road and for light roads, which may on occasion need to carry main road traffic.

However, minimum cover depths could be reduced (with the appropriate authority’s permission) if appropriately bedded concrete pipes are used according to transport research organisation TRL simplified tables of external loads on buried pipelines. It says the inherent strength of standard Strength Class 120 concrete pipes enables the cover depth to be reduced to a minimum depth of 0.6m beneath a highway when installed in conjunction with a full granular bedding surround, Bedding Class S.

For concrete pipes laid in fields the BPDA recommends a minimum cover of 0.6m should be provided to prevent damage from agricultural operations.

  • Where concrete pipes are required to be laid in fields at cover depths of less than 0.6m BS9295 Annex A, A16 gives recommendations for protection.
  • The preferred solution is to use a reinforced concrete slab installed over the pipeline which extends to provide at least 300mm bearing each side of the trench. 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 to use a concrete surround to the pipeline, Bedding Class A.

For pipelines under construction, where plant has to cross a pipeline, consideration should be given to providing dedicated crossing points which may consist of heavy steel plates bridging the trench to transfer vehicle loads away from the pipeline or additional cover material placed over the pipeline.

BPDA has an extensive library of useful information and guidance publications for designers, installers and asset owners. They are available to download free of charge from the BPDA’s website. The site also holds a comprehensive and searchable FAQ section and an online enquiry facility; go to www.precastdrainage.co.uk

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