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The revised standard for structural design of buried pipelines will soon bring methods into line with contemporary design practice. Marshalls CPM’s director of technical & engineering, Mark Flavell, who represented BPDA on the drafting committee, explains the importance of this update.

At the end of November 2019 a revised version of British Standard BS 9295:2010 Guide to the structural design of buried pipelines will be released. The new BSI document will include all relevant design information for all types of buried pipelines, which was previously split across two different standards.

Standards for buried pipelines make it possible to demonstrate that a pipeline is structurally sound, especially when it passes under a highway or motorway. Adopting authorities require uniform documentation, that is understood by all parties, to confirm the infrastructure is fit for purpose and can take the traffic load.

Currently BS EN 1295-1:1997 details the UK nationally established method of design for rigid concrete pipelines, while BS 9295:2010 gives further information on the structural design of buried pipelines under various conditions of loading using the established UK method.

When BS EN 1295-1 came up for its five-year review, BSI’s Management Committee for Wastewater Standards gave a mandate to revise and it was decided to withdraw the method of design from this standard and revise BS 9295 to include all the detailed design method together with the guidance on its use.

BS 9295:2019 will be the key standard for anyone in the construction industry that designs or builds drains, sewage systems and underground pipes. Revision of the standard has provided a suitable opportunity to review various shortcomings in current UK design methods which have arisen due to changes in the nature of pipelines over the period since those methods were originally published. In the case of concrete pipelines, this is over 50 years.

The revised standard consolidates and updates the various documents that together describe the UK method and brings practice into line with contemporary design methods. Alignment with the European Eurocode standards for structural design was also considered wherever practical.

The timing of the publication of the revised standard is of particular importance because the HA 40/01 Determination of pipe and bedding combinations for drainage works from the Design Manual for Roads and Bridges (DMRB) is also being revised and will be redrafted around BS 9295:2019.

One of the principal changes to the documentation will be to ensure that traffic loading used is consistent with British and European standard BS EN 1991-2 Traffic Loads on Bridges. This will align pipeline design with design requirements for all other buried structures. It is also consistent with the vertical test loads for manhole cover slabs as specified in BS 5911-3:2014.

The revised standard places more emphasis on consideration of the wide-trench formula for pipe design. Using wide-trench principles increases potential load on pipelines, however higher bedding factors have been introduced for designs using wide-trench design. Narrow-trench design is still permissible where the designer has sufficient knowledge of the installation conditions to make a well-informed decision on trench width.

Narrow trench design has been used by the Industry for many years without any concerns being raised regarding the structural integrity of concrete pipelines. This demonstrates the robust structure of concrete pipes and the conservative nature of design methods. However, the standard steers designers towards wide-trench as ideally a maximum permissible trench-width should be stated.

Two new tables are included in the revised standard – one for updated bedding factors and one detailing permitted installation cover depths for both narrow and wide trench applications.

A limit state design (LSD) method is introduced for concrete pipelines which means they can be designed to withstand all actions likely to occur during their design life and remain fit-for- use, with an appropriate level of reliability for each limit state.

A limit state is a condition beyond which a structure no longer fulfils the relevant design criteria. The condition may refer to loading or other actions on the structure, while the criteria refer to structural integrity, durability or other design requirements.

The procedure requires both an ‘ultimate’ and ‘serviceability’ bedding factor to be calculated and appropriate bedding factor assigned. All Eurocode European standards are based on the LSD concept in conjunction with a partial safety factor method.

Finally, an annex to the section gives typical examples of pipeline designs covering various design situations. This includes a comparison of wide-trench and narrow-trench design for the same diameter pipe and reinforced and unreinforced scenarios.

In summary, the new standard brings methods into line with current design practice while aligning with Eurocodes, introducing LSD and making trench-width a primary consideration. With release of the revised DMRB in 2020, the UK will have a joined-up contemporary approach to buried pipeline design.

Structural Design Calculator update planned

BPDA plans to bring its own Structural Design Calculator app for pipe design into line with the new standard in early 2020. The Calculator, which is available from any app store, simplifies concrete pipeline design calculations.

It offers all the basic values including external design loads and bedding factors and takes into account the pipe crushing strength. It then offers advice on what type of bedding to use. The calculated load, which is the total load a concrete pipe in a trench is required to sustain, is used in the design formula.

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