IGN 4-11-02 Issue 1 Revised bedding factors for vitrified clay drains and sewers

IGN 4-11-02 Issue 1 February 1988 has been archived.

The reasoning for this step is set out below. The Clay Pipe Development Association has confirmed its acceptance of this step.

IGN 4-11-02: Issue 1 Revised bedding factors for clay drains and sewers was published in February 1988. It has not been reviewed for 34 years. The research and advice contained within the document remains fundamentally current, but the revised bedding factors have now been incorporated in BS 9295:2020 – The Structural design of buried pipes and the bedding factors now apply to all rigid pipes, rather than just vitrified clay pipes.

The IGN was written following the completion of an important research programme, which showed that the bedding factors used at the time were too conservative. The UK Water sector agreed that the new bedding factors could be used for vitrified clay pipes and IGN 4-11-02 was published.

In 1997, BS EN 1295-1:1997 – Structural design of buried pipelines under various conditions of loading was published and provided guidance for the structural design of all commonly used pipes, including vitrified clay. The option of applying revised bedding factors for vitrified clay pipes was included in the UK Annex of the standard in Table NA.6 and paragraph B.1.12 (point 9). This cross-referenced IGN 4-11-02.

As noted in the second paragraph of IGN 4-11-02:-

• “It is the intention to pursue the principle of common bedding factors for all rigid pipes and it is envisaged that work in progress will enable advice to be given for other rigid pipe materials in pursuance of this aim. This interim Guidance Note therefore contains advice which will be incorporated into guidance relating to bedding factors for all rigid pipes at an appropriate time.”

This intention was not implemented until 2020, when BS 9295:2020 [Ibid] was published. Before 2020, the structural design of buried pipes was covered by two documents:-

• BS EN 1295-1:1997- Structural design of buried pipelines under various conditions of loading
• BS 9295:2010 – Guide to the structural design of buried pipes

BS EN 1295-1 was updated in 2019 and the UK Annex was removed. BS 9295:2020 [Ibid] now includes an updated version of the material that used to be included within the UK annex of BS EN 1295-1: 1997 as well as all the guidance that was included in BS 9295:2010. This overhaul of BS 9295 also saw the introduction of a new design method for flexible pipes as well as the introduction of new and updated guidance throughout.

Had the review of IGN 4-11-02 been undertaken prior to 2020, it could arguably have been necessary to retain it and its updating would have been relatively easy to do. The publishing of BS9295:2020, means that IGN 4-11-02 is no longer necessary and is potentially misleading as the bedding factors now apply to all rigid pipes, rather than purely applying to vitrified clay pipes.

It is a fundamental tenet of Water UK’s WIS and IGN standards’ development and maintenance that such standards may complement but should not duplicate or be inconsistent with the content of published BS, or BS EN standards. When a latter standard is published the WUK Standards Board then seeks amendment of the relevant WUK document. If this cannot be achieved then a decision to withdraw the document (to archive) is the likely outcome.

Following discussions with the Clay Pipe Development Association the WUK Standards Board has determined IGN 4-11-02 Issue 1 should be archived.