Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | NOVEMBER 2017 | 31 The design of a gravity pipe uses this stabilisation effect of the soil to minimise deflection in the pipe, by constraining it in the horizontal direction, and simul- taneously transferring the surface loads being inflicted by the ground above the pipe, traffic, structures etc. to the soil. During construction (the installation phase) the weight of the soil above the pipe will cause it to deflect and this will continue as the soil is compacted (the settlement phase). With good installation these deflections are very small – less than 2%. Therea er, during the lifetime of the pipe, there is no significant change in the shape of the pipe. In very well pre- pared ground, the deflection during the settlement phase can be almost zero. The pipe can happily sit there for many years operating as planned whilst withstanding the external loads placed on it. How does installation affect perfor- mance? Extensive field and laboratory testing was carried out and reported by TEPPFA (the European Plastic Pipes and Fittings Association) in their Design of Buried Plastic Pipes study (1999). More informa- tion on the study can be found on www. discover-plasticpipes.com/uk. During this work, trials were carried out in both clay and sand, and in well The Knowledge: plastic pipes When a leading UK housebuilder wanted to ensure a proposed stormwater solution would withstand the worst case predicted storm events, it turned to large diameter HDPE plastic pipes and gained consider- able additional benefits. In the Bedfordshire town of Houghton Regis, the Regent's Place large-scale residential development of 180 houses required a below ground engineered stormwater drainage solution, able to store up to 2,052m3 of surface water run-off to cater for both '1 in 30' year and '1 in 100 plus 30% climate change' year storm events. Nearby public spaces meant a lim- ited footprint available for drainage design, and it was also necessary to work with the sloping gradient found on site. Taking on the challenge, BPF Pipes Group member Polypipe Civils worked closely with the housebuilder and the consultant engineers to design a stormwa- ter solution that met site requirements and adopted elements laid out under a Section 104 agreement for the approving water company, Anglian Water. The attenuation tank supplied for the Regent's Place project boasts a 3m diam- eter offset vortex flow control chamber which controls discharge water from the tank at an agreed rate of 10 l/s, so the local watercourse is not overwhelmed. To control additional flow when required, the chamber also had penstock flow control. Over 500m of pipes forming 14 pipe runs in 2100mm diameter were supplied. The technical team engineered the pipe to ring stiffness SN2 to meet ground condi- tions, burial depths, native soil pressures and loading on-site. The system was tested to pass required deformation and buckling checks in accordance with BS EN 1295-1. To accompany the pipe runs, the system incorporated 34 pre-fabricated modularised fittings which included 90° bends and 'F', 'T' and double 'T' fittings to fit the limited on-site dimensions perfectly. The socket and spigot nature of these engineered fittings kept jointing to a mini- mum, allowing easy pipe alignment and a speedy install using electro-fusion jointing. In order to make maintenance work less strenuous and safer overall for installers, the pipe supplier also provided 10 access manholes in 1200mm and 1800mm diam- eters for the use of engineers. Keith Millard, Senior Engineer at Taylor Wimpey said: "This stormwater attenua- tion system was designed to not only fit the available area, but it also saved us time and costs on the installation by having the system and fittings delivered as one-piece modular units ready to install." CASE STUDY: LARGE DIAMETER HDPE PIPES compacted through to non-compacted ground. It was, unsurprisingly, found that pipe deflection in the settlement phase depended on the compaction of the ground around the pipe. However, even when reality kicks in and compac- tion is impacted by weather conditions, change in soils along the pipeline, type of fill available, the measured deflection in a plastic pipe is still well within the practical limits set for the UK (6% adopt- able sewers, 5% highway applications) and represents a very large factor of safety compared to the capability of the pipe material. Clearly, good preparation is always to be encouraged, but the engineering properties of plastic pipes mean that less than perfect preparation should not pose a problem to the final installation.