The bedding Class specified for buried pipes can vary depending on a number of factors including the material and type of pipe installed. Stuart Crisp, Business Development Director of the Concrete Pipelines System Association outlines the savings that can be made from using rigid pipeline systems.
The pressure to deliver a cost effective drainage installation with minimal environmental impact has never been greater. Despite this, some pipeline installers are still fully surrounding drainage pipelines 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.
Bedding, or embedment as it is sometimes termed, is used to transfer loads between the buried pipe and the surrounding earth. The forces acting on a pipe include surface loads, for example traffic; the weight of the ground and any overlaying fill above; and the reaction from the ground below to support the pipe. Sometimes hydrostatic forces will also need to be taken into account in areas liable to high groundwater or flooding, In very large pipes the additional load imposed by the water in the pipe itself may also need to be considered.
There are various Classes of bedding, which provide varying degrees of support for the pipe and distribute the imposed loads. The main differences between bedding Classes is the type of material used and the areas around the circumference of the pipe where the bedding material is placed. Class S bedding is commonly used; it requires a pipe to be completely surrounded with granular material to effectively distribute any loads, which can significantly increase its load-bearing capability. Bedding Classes N, F and B all require a pipe to be surrounded by considerably less granular material than Class S but, nevertheless, these Classes still provide an enhancement to the pipe’s load carrying capabilities achieved under test conditions defined in BS EN1916.
British Standard 9295: Guide to the Structural Design of Buried Pipelines includes simplified embedment tables for pipe materials commonly used in the construction of buried pipelines. The pipe material will define how the pipe will interact with its surrounding embedment. For structural design purposes pipe materials are defined as either rigid, semi-rigid and flexible. Concrete and clay pipes are defined as “rigid” in the Standard, while ductile iron and thick-walled steel are all classed as “semi-rigid”. 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. If this lateral support is inadequate flexible pipes can flatten under load – a process termed ovalisation. Lateral deflection can be limited in a flexible pipe through the use of a trench of appropriate width and by fully surrounding the pipe with properly compacted granular material. In reality this means that most standard plastic sewer pipes are installed with Class S bedding, with the surrounding embedment taking the majority of the flexible pipe’s design loading.
Consideration should also be given to the additional time that it takes to install flexible pipes in order to adequately place and compact the granular bedding material, particularly if a well-rounded, single sized material is not available locally.
Rigid pipes do not deform. Typically more than 60% of the design strength of pipelines constructed with rigid pipes, such as precast concrete, is inherent in the pipe itself. Such pipes rely on bedding simply as a means of distributing loads and to provide a supporting reaction under the pipe. As a consequence, using standard production rigid pipes means that installers, specifiers and asset owners have a wider choice of bedding options from which to choose to satisfy cost, environmental criteria and installation requirements. The inherent strength of rigid pipes means the use of Class S bedding is often likely to be an expensive over-specification when cheaper, less carbon intensive options can be utilised.
Class B is a common, cost effective and less carbon intensive bedding option for rigid pipes. With this bedding Class only the lower half of the pipe needs to be surrounded by granular bedding material, often termed 180-degree granular bedding. In the UK, Class B bedding is generally suitable for concrete drainage pipeline installations under a main road, with cover depths of between 2 metres to 4 metres according to BS 9295: 2010. In some instances Class B can even be used for pipe cover depths of up to 8 metres, a depth that many flexible pipeline systems may not be able to meet, even with Class S bedding.
The material savings using bedding Classes B, F and N can be considerable. The CPSA website features a recently updated structural design calculator to assist in the design of buried pipelines and selection of appropriate bedding classes. Go to: http://www.concretepipes.co.uk/calculators/structural-design
When a suitable bedding Class has been selected, the material costs of alternative designs can then be compared using the CPSA’s recently updated online Material Cost Calculator http://www.concretepipes.co.uk/calculators/material-cost
The variances in material costs based on different bedding Classes can be considerable. 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: Plastic Class S) per km of pipeline.
From an environmental perspective, the additional quantity of granular bedding material necessitated by the use of Class S for flexible pipe installations increases the embodied carbon in the installed pipeline as a consequence of the use of increased quantities of aggregate that have to be extracted and transported to the site. Similarly the amount of excavated material that will be displaced by the bedding will also increase; often this material will have to be trucked from site and disposed of in landfill.
The use of rigid pipes, such as precast concrete, will often give pipeline installers the option of using alternatives to Class S bedding for a buried pipeline installation. This change in Class could result in significant financial, environmental and time savings.
For more information on concrete pipes, or to access the Material Cost Calculator or the Structural Design calculator go to www.concretepipes.co.uk
The CPSA has also launched a new Concrete Pipe Calculator web app optimised for Apple and Android portable devices which replicates the online Material Cost and Structural Design calculators.