One thing often misunderstood in drainage pipeline bedding design, is the wide range of bedding options available for specifiers. Without proper guidance and assessment of available options, the likelihood of missing out on the most cost-effective ones is always high.
Structural design of pipeline installations involves dealing with three main types of load: The weight of the trench-fill, loads transmitting to the pipe from the surface such as traffic, and the supporting reaction from the ground under the pipe. Loads imposed by wastewater/ stormwater in the pipe may also be considered for larger diameter pipes. Pipe beddings play a major part in supporting the pipeline in dealing with these loads and are an integral part of a pipeline’s structure. A number of pipe bedding solutions are available. These are described in detail in BS 9295, and other industry standards, and are usually identified as “bedding classes”.
Class S bedding is the most extreme option, structurally, as the pipe is surrounded fully by the granular material. In such option, a significant proportion of the structural strength is derived from the embedment either side of the pipeline, protecting the pipe from the superimposed loads. Other pipeline Bedding Classes, such as Classes B, F and N require significantly less material and would rely more on the pipe’s inherent strength.
Figure 1. Bedding classes used in pipeline structural design.
Due to their lack of inherent strength, flexible pipe installations depend heavily on a high-quality installation where the surrounding embedment takes most of the loading. The options for bedding are usually limited and the installation will more likely need a Class S full surround bedding with a high degree of compaction and sometimes some level of site supervision to ensure that the installation is carried out precisely to the engineer’s specification.
The use of Class S full granular bedding will lead to more material cost and more vehicle movements to deliver the granular to site. More soil will need to be removed from site and sent to landfill or off-site use. A full surround will also need more time laying and compacting the granular around and on top of the pipeline. This can add to the overall cost significantly. For many pipe bedding installations where stronger pipe material is used, less bedding support may be needed, and the pipeline bedding may need significantly less granular material.
Specifying Class S as a default, without knowing, understanding or considering the other options, is a very common problem in pipeline design. In many cases we have seen in the past, Class S was specified for concrete pipeline installations that would have worked perfectly well with a Bedding Class B or even Classes N & F. To help make these options more transparent and easier to detect, the British Precast Drainage Association (BPDA) developed a web-based Structural Design Calculator tool to help designers and specifiers identify the range of Bedding Classes available for their projects. BPDA also developed another tool to help specifiers understand the costs associated with bedding choices. The Material Cost calculator offers alternative costs for both a concrete and flexible pipe option based on pipe type and diameter, imported bedding costs and the cost of dug soil disposal to landfill.
The BPDA Structural Design Calculator can be accessed here: https://www.precastdrainage.co.uk/page/structural-design
The Material Cost Calculator is available at this link: https://www.precastdrainage.co.uk/calculators/material-cost
Both calculators are available via a mobile phone app through Apple, Android and Amazon platforms: https://www.precastdrainage.co.uk/calculator-app