Water & Wastewater Treatment

26 | DECEMBER 2014 | WWT | www.wwtonline.co.uk In the know Getting to grips with Pipe Bedding adequate lateral support a exible pipeline's integrity may progressively decline at an accelerated rate with a greater risk of failure. To help limit the extent of ovalisa- tion an appropriate width of pipe trench should be speci ed and the exible pipe should, in most cases, be fully surrounded by properly com- pacted granular material as de ned under Class S. The time required to install exible pipes may also be increased by the e• ort needed to adequately compact the granular material, particularly Bedding classes N, F, B and S: the amount of granular material varies from a horizontal layer in Class N, to Class S which requires the pipe to be completely en- closed by the material "The use of Class S bedding for rigid pipes is o en likely to be an over- specifi cation in terms of a pipeline's structural design." if well-rounded, single size material is not locally available, to ensure it provides the support needed to pre- vent ovalisation. The option of using an alternative to Class S bedding is, generally, unlikely for most standard plastic sewer pipes. What about rigid pipes? 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. As a result, they have less reliance on the bed- ding, except as a means of distribut- ing loads and providing a supporting reaction under the pipe. This means the use of Class S bed- ding for rigid pipes is oƒ en likely to be an over-speci cation in terms of a pipeline's structural design. Instead, the inherent strength of a rigid pipe gives installers, speci ers and asset owners a wider choice of bedding options based on cost, environmental impact and ease of installation – see Table 1 (previous page). Class B, for example, is a common bedding solution for concrete pipeline installations. This class requires only the lower half of the pipe to be surrounded with granular material – termed 180 degree granular bedding. BS9295 tables con rm that standard concrete pipes installed with cover depths of between 2 metres and 4 me- tres, can be used with Class B bedding under a main road. In some instances Class B can even be used for pipelines at depths of up to 8 meters, a depth that many exible pipeline systems may not be able to meet, even with Class S. A rigid pipe solution will give contractors the exibility to select the most economical bedding solution based on material cost, installation time and environmental impact by providing the choice of bedding Classes B, F and N in addition to a Class S solution. How can I fi nd out more? The Concrete Pipeline Systems As- sociation (CPSA) has developed a web-based Structural Design Calcu- lator tool to help in the selection of appropriate bedding classes for buried pipelines (www.concretepipes.co.uk/ page/structural-design). The CPSA's website also includes an online Mate- rial Cost Calculator; more information on the in uence of bedding class on pipe material choice is available at www.concretepipes.co.uk/downloads. For more info go to wwtonline.co.uk

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