Water & Wastewater Treatment Magazine
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The Knowledge: plastic pipes www.wwtonline.co.uk | WWT | NOVEMBER 2017 | 29 Below ground plastic pipes: Strength through flexibility? Below-ground plastic pipes are known to be flexible under loads, but this can be an advantage rather than a drawback when a sewer or drainage system is considered in its entirety. Here we look at some practical advice for designing pipelines for longevity and resilience structural analysis: stiffer elements will attract greater proportions of shared load than those that are more flexible. In other words, the more flexible pipe will attract less crown load than a rigid pipe of the same outer geometry. This is because the rigid pipe does not transmit the loads into the surrounding material but the loads are transferred through the pipe wall into the bedding and, therefore, are subject to much greater load than the flexible one. How can this be a benefit? In general, flexible pipes rely upon their deformation from imposed loads to mobilise the support of material on both sides of the pipe. Their primary structural function is distributing the imposed vertical loads to the surrounding soil and bedding material. Only a small portion of the imposed loads are actually carried by the flexible pipe itself. W ith increasing pressure to mini- mise disruption in highways, the need for trouble-free drains and sewers is well understood. But how does the flexibility or rigidity of the chosen pipe affect the performance and reliability of the system? A flexible pipe is, by definition, a pipe which will deflect when subjected to external loads. It is a commonly held assumption that any deformation of a plastic pipe is inherently detrimental and indicative of failure of the pipe to perform properly. This is a fundamental miscon- ception. Flexibility in pipes is a desir- able attribute. Understanding how the flexible pipe relates to its neighbouring soils – thereby establishing a functional pipe/soil composite structure – is key to successful design. How does a thermoplastic pipe per- form under load? A buried pipe and its adjacent soil will attract earth loads and live loads in accordance with a basic principle of By Caroline Ayres, Director, BPF Pipes Group