WET News

18 WET NEWS JUNE 2015 Selecting the right said than done, to be considered. The challenge: Putting pipes INSIGHT CSOS & draiNagE The most significant differences between bedding Classes is the extent of the area around the pipe where the granular bedding material is placed W hen a drainage pipeline is buried in the ground, loads are transferred between the pipe and the surrounding earth by the bedding surrounding the pipe. There are three main types of loads that apply: the weight of the overlying trench-fill, pressures transmitted to the pipe from surface loads such as traffic, and the supporting reaction from the ground beneath the pipe. In addition, for larger diameter pipes the load imposed by water in the pipe may also need to be considered. There are various bedding solutions or Classes described in British Standard 9295: Guide to the Structural Design of Buried Pipelines. The most significant differences between bedding Classes is the extent of the area around the pipe where the granular bedding material is placed. Class S bedding, for example, requires a pipe to be completely surrounded by granular material. Other Classes, such as B, F and N require significantly less granular material. The actual bedding Class specified for a particular 1) The CPSA has a web-based Structural Design Calculator tool to help select appropriate bedding Classes for buried pipelines. www.concretepipes. co.uk/page/structural-design 2) Materials costs of alternative designs can be compared using the CPSA's online Material Cost Calculator. www.concretepipes.co.uk/calculators/ material-cost A solution: Go rigid Using a rigid pipe material provides scope to use economical bedding, says Stuart Crisp. R igid pipes do not deform. Most of their design strength is inherent in the pipe itself. As a consequence, rigid pipes made out of materials such as precast concrete may have less reliance on precise placement and compaction of the correct type of embedment, except as a means of distributing loads and providing a supporting reaction under the pipe. Because of their inherent strength, it is not always necessary to fully surround a rigid pipe with granular embedment, saving costs and resources. Instead, the inherent strength of a rigid pipe gives installers, specifiers and asset owners a wider choice of bedding options (see Table 1). In contrast to flexible pipes, the need to fully surround a pipe with Class S bedding can o'en be an over-specification where precast concrete pipes are used. A common bedding solution for concrete pipeline installations is Class B. This Class uses much less bedding because it requires only the lower half of the pipe to be surrounded with granular material, which is why it is o'en termed 180 degree granular bedding. According to BS9295 standard concrete pipes, installed with cover depths of between 2m and 4m, can be used with Class B bedding under a main road. In some instances, Class B can even be used for pipelines installed at depths of up to 8m - a depth many flexible pipeline systems may not be able to meet even with Class S bedding. By opting for rigid pipe drainage contractors will have scope to select the most economical bedding solution based on material cost, installation time and environmental impact by including the options of using bedding Classes B, F and N in addition to a Class S solution. n Stuart Crisp is business development director at the Concrete Pipelines System Association Pipe diameter (mm) Flexible pipe Class S Rigid pipe Class B Rigid pipe Class F Rigid pipe Class N 300 8.72 4.41 2.80 2.57 600 20.64 10.40 5.32 4.57 1200 52.51 26.07 12.43 10.16 The carbon impact (kgCO2e/linear meter) of importing additional granular for bedding Table 1: The carbon emissions associated with importing granular material to site and removing pipe trench spoil from site for bedding class options B,F or N compared to bedding Class S.

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