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

Posted by & filed under Costs, Design, Sustainability.

The pressure to deliver a cost effective drainage installation with minimum environmental impact has never been greater, but despite this, some Bedding Designpipelines are still fully surrounded in expensive imported granular bedding when a more considered approach to the design and selection of the pipe material could result in the use of a less costly, lower carbon installation.

 

There are various classes of bedding, providing varying degrees of support for the pipe. What is required will depend upon the material of the pipe, the surface load (traffic, weight of the ground and any overlaying fill) and the reaction from the ground below to support the pipe.

For structural design purposes pipe materials are classified – in BS 9295: Guide to the Structural Design of Buried Pipelines – as either rigid, semi-rigid or flexible. Concrete and clay pipes are defined as “rigid”; ductile iron and thick-walled steel are all classed as “semi-rigid” while thermoplastic, glass reinforced plastic and thin-walled steel pipes are all classed as “flexible”.

Bedding selection is particularly important in the case of flexible pipes because they have relatively little inherent strength and derive a significant proportion of the structural strength from the embedment placed at the sides of the pipeline. This means in practice that most standard plastic sewer pipes must be installed with Class S bedding, with the surrounding embedment taking the majority of the flexible pipe’s design loading.

Class S bedding requires a pipe to be completely surrounded with granular material – the effect is to distribute loads, significantly increasing the load-bearing capacity of the pipe. Bedding Classes N, F and B require considerably less granular material than Class S while still providing an enhancement to the pipe’s load carrying capabilities. The structural design of a pipeline using rigid pipes such as concrete may often result in the opportunity to use an alternative to Class S.

The material savings using bedding classes N, F and B can be considerable and have a significant impact on the installed cost of the pipeline project. The CPSA Material Cost Calculator is designed to allow the material costs of alternative designs to be compared quickly and easily – and when the results are examined, the value of doing this calculation is clear.

For example, using the online Material Cost Calculator with a 600mm diameter pipe, assuming the cost of imported bedding is £15/tonne and the disposal of excavated material is £6/tonne, with default values for bulk density, the cost (independent of pipe and fittings costs) will be £49.22 per linear metre for a plastic pipe with Class S Bedding compared with £28.50 per linear metre for a concrete pipe using Class B bedding (reducing further to £16.70 for Class F and £15.13 for Class N). This equates to £20,720 savings (Concrete Class B v Plastic Class S) per km of pipeline.

There is clearly a risk of undermining the performance of the pipeline if the bedding is incorrectly specified, which can lead to a default position where Class S Bedding is specified. However, this may involve a significant over specification with the consequent cost implications as outlined above.

The CPSA’s Structural Design Calculator identifies which bedding type is required for a pipeline installation while the Material Cost Calculator then provides a fast and reliable way to compare the cost of different solutions. The CPSA has invested in these tools because we are confident that concrete pipes, with their inherent strength, provide a compelling cost advantage in many situations and that this, coupled with concrete’s performance and longevity, will prove to be the most attractive option.

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