Notice: This blog piece was created prior to the formation of the British Precast Drainage Association.

Posted by & filed under Costs, Design.

CPSA_FB_POSTBedding is an essential component in every “open cut” pipeline installation. Its primary function is to transfer loads between the pipe and the surrounding soil.  However, the design of the bedding system can have a significant impact on the cost of an installation and also on the environmental impact of the project as a whole.    We believe that overspecification of bedding systems contributes unnecessary cost to pipeline installations.

There are numerous pipe bedding Classes such as Class S, B, F and N with different construction details using varying amounts of imported granular material and different load distribution characteristics.  Bedding Classes B, F and N require significantly less granular material than Class S which requires a pipe to be completely surrounded.

The actual bedding Class required can differ depending on the type of drainage pipe used.  British Standard 9295: Guide to the Structural Design of Buried Pipelines provides examples of common pipe materials, their strength and their classification.   Pipes will generally be considered either “rigid”, “semi-rigid” or “flexible”.

Flexible pipes, such as thermoplastics, derive up to 95% of their structural strength from the embedment either side of the pipeline.  Generally this means that flexible pipes have to be fully surrounded by the correct granular material, sufficiently compacted, as defined under Class S.

This design of embedment will require considerable quantities of aggregate to be extracted and transported to site while the material excavated from the trench will often need to be removed and disposed of in landfill. This process has both a financial cost and an impact on carbon emissions.

By contrast in rigid pipes, such as precast concrete, up to 80+% of the design strength is inherent in the pipe itself, which in many cases allows users to choose from a range of bedding solutions requiring less granular embedment.

An acceptable design for a concrete pipeline in many instances is bedding Class B, which entails only surrounding the lower half of the pipe with bedding material – often termed 180o granular bedding.

Given the pressure on contractors to minimise cost, installation time and environmental impact, choosing the most appropriate bedding design is clearly in the best interests of the project. However, in many instances there is a lack of understanding of pipe embedment design.  This can lead to a conservative approach being adopted and the unnecessary overspecification of a bedding solution.

The materials savings achieved from using bedding Classes B, F and N as opposed to Class S can be considerable.  To help designers and installers assess the scale of savings, the CPSA has developed a web-based Structural Design Calculator to help in the selection of appropriate bedding Classes for buried pipelines.  www.concretepipes.co.uk/page/structural-design

When bedding Classes have been selected, the costs of alternative designs can be compared using the CPSA’s online Material Cost Calculator www.concretepipes.co.uk/calculators/material-cost

 

 

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