Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | DECEMBER 2014 | 25 In the know Getting to grips with Pipe Bedding Bedding Down Pipe bedding is a vital component of any pipeline project, yet many contractors fail to choose the most appropriate type You wouldn't throw money down a drain, so why use large amounts of imported granular bedding when you don't have to? It sounds crazy, yet that is precisely what many contractors are doing by installing pipelines with Class S bedding when frequently a less expensive bedding class could have sufficed. And they are doing so at a time when the water industry is under pressure to be increasingly cost and carbon efficient. What is pipe bedding? When a pipeline is buried in the ground it is surrounded by bedding material. The embedment surrounding the pipe is designed to ensure that the pipeline attains its full structural potential by transferring loads between the surrounding soil and the pipe. There are three main forces on a Stuart CriSp BusinEss DEvEloPMEnT DiRECToR, ConCRETE PiPElinEs sysTEMs AssoCiATion typical buried pipeline: the weight of overlying fill, pressures transmitted to the pipe from surface loads such as traffic, and the supporting reaction of the ground beneath the pipe. In addition, the load imposed by the water in the pipe may also need to be considered for larger diameter pipes. What are the options for pipe bedding? There are various different bedding designs, or classes. These are defined in British Standard 9295: Guide to the Structural Design of Buried Pipelines. The main difference between the classes is the type of bedding material and the areas around the pipe that the bedding is placed. Class S requires a pipe to be completely surrounded with granular material whereas other classes, such as B, F and N, use significantly less granu- lar material. When more granular material is used there is usually a corresponding increase in the volume of excavated material from the trench that will require disposal, oŒen through transportation off site to landfill, which increases the cost of the installation and the amount of carbon embodied in the installation. How is the bedding class selected? The class of bedding required for a particular installation will depend on the material from which the drainage pipe has been fabricated, the strength characteristics and structural classi- fication of the pipe itself, traffic load- ing and the depth at which the pipe is to be installed. BS9295 provides examples of common pipe materials, their strength and their structural classification. Concrete, clay and reinforced con- crete pipes all have a structural clas- sification of 'rigid'. Ductile iron and thick-walled steel pipes are termed 'semi-rigid'. Thermoplastics, glass reinforced plastic and thin walled steel are all classed 'flexible'. Why is bedding so critical for flexible pipes? Flexible pipes need to derive a sig- nificant proportion of their structural strength from the embedment either side of the pipeline. This is because they can deflect under load if not adequately supported, flatting on top and spreading laterally - a condition termed ovalisation. As a result, a flexible pipe system will depend heavily on a suitably designed, high quality installation where the surrounding embedment takes around 85-95% of the pipeline's designed loading with the remaining 5-15% taken by the (deformed) pipe. As a flexible pipe ovalises its load-carrying capacity can reduce, which can lead to an even greater dependence on the lateral support from the side fill embedment. Without 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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