WET News

August 2017 WET NEWS 11 Considering the choice of pipeline material and its associated Class of embedment can prove very beneficial. stuart Crisp, of the CPsA, reveals all. Pipeline bedding – in a class of its own INSIGHT Insight Pipes & drainage D esigners and installers are under increasing pressure to deliver a cost-effective drainage installation with minimum environmental impact. There are many factors to consider in deciding on the optimum pipeline solution; one of the most important of which is pipeline bedding. The forces acting on a buried pipe include: those from above, such as traffic and the weight of the ground; and those from below, including reaction from the ground supporting the pipe. There may also be hydrostatic forces from groundwater and, in the case of very large pipes, consideration may need to be given to the load imposed by the water flowing through the actual pipe. There are various bedding configurations, known as Classes, which provide varying degrees of support for a buried pipe. The most significant difference between the various bedding Classes is the extent of area around the pipe's circumference where bedding is placed. Class S bedding, for example, requires a pipe circumference to be fully surrounded with granular bedding material, whereas Classes B, F and N do not require the pipe to be fully surrounded and so the quantity of bedding required is significantly less. The material from which a pipe is manufactured will define how it will interact with the surrounding embedment. The standard, BS 9295: Guide to the Structural Design of Buried Pipelines includes simplified embedment tables for materials commonly used for buried pipelines based on their structural classification. Concrete and clay THE CONCEPT • Classes are various bedding configurations that provide varying degrees of support for a buried pipe • the structural Design Calculator helps specifiers and installers select the appropriate bedding Class • the Material Cost Calculator enables cost comparisons to be made for different pipe materials and bedding class options NEED TO KNOW 1 Bs 9295: guide to the structural Design of Buried Pipelines includes simplified embedment tables for materials commonly used for buried pipelines 2 the material from which a pipe is manufactured will define how it will interact with the surrounding embedment 3 surrounding a pipe with Class s compliant granular bedding will add significantly to the installation cost THE VERDICT "We had every intention of using plastic pipes on this project, but…the CPsA calculators gave us the opportunity to present a more cost-effective and efficient approach to the job to our client and builder Chartford Homes, which took on board our proposal" Jonathan Hotham, Athena Civil Engineering pipes have a structural classi- fication of rigid, ductile iron and thick-walled steel pipes are classed semi-rigid, while thermo- plastics, glass reinforced plastic and thin-walled steel are all termed flexible. Flexible sewer pipes, such as HDPE, have very little inherent strength and the pipeline consequently derives a sign- ificant proportion of its structural strength from the surrounding embedment to help prevent the pipe from flattening under load, which could otherwise result in compromised structural and hydraulic performance from a process known as ovalisation. This lack of inherent strength in the flexible pipe itself means that in reality most standard plastic sewer pipes are installed with Class S bedding fully surrounded by granular bedding to ensure that the pipe receives adequate structural support. Bedding costs money. Surrounding a pipe with Class S compliant granular bedding will add significantly to the cost of an installation. It will also add to the environmental impact of an installation because considerable quantities of aggregate will need to be extracted and transported to site while the additional spoil removed from the trench to make space for the bedding will need disposal, perhaps by trucking to landfill. Class S bedding is oŽen the default bedding specification for buried pipelines, regardless of pipe material, but this could be a costly over specification. There other pipe material / bedding Class combinations which could offer a more cost-effective solution. To make it easier to consider the various material and bedding options for buried pipelines the CPSA has developed two web-based tools – a Structural Design Calculator to help specifiers and installers with the selection of the appropriate bedding Class; and a Material Cost Calculator to enable clear cost comparisons to be made for different pipe materials and bedding class options. Leeds-based ground worker and civil engineer Athena Civil Engineering demonstrated the value of these tools when selecting the most cost-effective option to construct a stormwater attenuation tank for a new housing development in Round- hay, Leeds. Using the Structural Design and Material Cost Calculator, Athena was able to determine the optimum bedding design for a concrete solution and to compare that with the cost of a plastic- based system with Class S full granular bedding surround. The Structural Design Calculator showed that for this application a concrete pipe solution with Class B embedment would provide an effective solution, delivering significant cost savings and helping to minimise the scheme's environmental impact. The installed tank included four runs of 12 standard 1,800mm diameter, 2.5m long spigot and socket pipes; four 1,800mm diameter pipes with end caps; and four 1,800mm diameter pipes with end caps and cast-in side entry access, all manufactured by FP McCann. Jonathan Hotham, project manager for Athena Civil Engineering, says: "We had every intention of using plastic pipes on this project, but… the CPSA calculators gave us the opportunity to present a more cost-effective and efficient approach to the job to our client and builder Chartford Homes, which took on board our proposal. We shall certainly be using the CPSA Material Cost Calculator on future schemes and a closer consideration of pipe choice in conjunction with bedding options will be something we'll continue to develop." There was a similar outcome with the materials used for the underground drainage scheme at St Wilfreds Academy, Blackburn, where an option of using plastic pipe with a full granular surround (Class S) was under consideration. However, switching to a precast concrete pipe option, with Class N bedding, delivered both cost and environmental savings. The final installation used 450mm and 525mm diameter concrete pipes, manufactured by CPSA member CPM Group, which led to reduced environmental impact and cost savings. The scheme also used CPM Group's precast concrete Perfect Manhole system. Ray Harding, of Peter Harding Construction, says: "The overall solution not only accelerates the installation of the drainage requirements, it also allows for the installation of the drainage in poor ground conditions, faster, easier and more economically. n Stuart Crisp is business development director of the CPSA. The CPSA Structural Design Calculator and Material Cost calculators can be accessed via www.concretepipes. co.uk. The calculators are also available via a web App for Apple, Android and Amazon platforms at www. concretepipes.co.ukcalculator-app • Athena used the calculator tools to select the most cost-effective option for a stormwater attenuation tank scheme in Leeds the Class bedding configurations provide varying degrees of support for a buried pipe

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