Segment design code of practice 18 Apr 2016

Surprising as it may seem, there is no universal standard for the design of segments for precast concrete segmental tunnel linings. There are many local or private codes for the guidance on the quality and testing of materials and elements but nothing that presents an industry consensus on best practice or guidance recommendations for the design of segments through all stages of their production and through their design life.

Design best practice for segment tunnel linings

To address this shortfall, the tunnelling and ground engineering industry in the UK has collaborated on the preparation of a new code of practice written specifically for the design of segment linings with the intent of establishing specification recommendations of these structures from project inception to the end of the service life of the tunnel.

The document, titled PAS 8810, standing for ‘publically available specification’, is the dedicated work of engineers and professionals with more than 20 companies and organisations and was sponsored by the UK High Speed 2 (HS2) delivery company and the British Tunnelling Society (BTS), and facilitated by the BSi, British Standards Institution. It was published in April 2016 and details about the need and development of the document were explained at a public launch of the publication on Monday 18 April at the offices of Arup in London. Arup won the contract to author the document and Hyuk-Il Jung was lead author and Laura Cordell of BSi was project coordinator and editor.

At the presentation Matt Sykes explained the need for the document. “Design of segmental linings has been something of a dark art with application of many different guidelines and recommendations,” he said. “This has caused conflict and confusion as well as delay and additional cost in the design process.”

This was illustrated most recently on the Crossrail project in London with initial designs being rejected or requiring extra work and revision as a result of the design checking phase, causing delay in project development and cost increases. It was recognised that a coordinated, collaborative effort was required to develop a single, usable standardisation document that aims to reduce unnecessary administration and delay by streamlining, clarifying and standardising the design process for segmental lining design. 

PAS 8810 the new segment lining design guide

Colin Rawlings of HS2 as a sponsor of the document explained that the PAS is needed to improve infrastructure delivery through better use of standards. “It is not designed to stifle innovation but rather to introduce some standardisation and consensus of design requirements to maximise efficiencies in the delivery of major public infrastructure projects. It sets out detailed recommendations by referring to existing national and international industry standards, including Eurocodes and includes specific design recommendations not available in any other available standards.”

For the BTS as a co-sponsor, Chairman Roger Bridge said at the official launch that up to now the UK underground construction industry has adopted and adapted other codes, notably design codes for buildings and bridges, to the design of segments. "It was time for a widely accepted guidance document to be developed to bring benefits of efficiency and savings and it was better for the industry to work on this ourselves rather than have the work progressed by others imposed upon us. BTS as a sponsor and collaborator was able to bring to the development process a depth of knowledge from the BTS membership. The code of practice will be a useful tool in progressing current tunnelling projects in the UK and the aim is for it to be used also by others beyond our shores.”

As lead author of PAS 8810, Hyuk-Il Jung explained that there was no intention to impose strict barriers. “The objective was to avoid being too prescriptive and to avoid also any conflict or contradiction with existing codes of practice. The PAS is to be used in conjunction with other existing codes, including the Eurocodes, and there are frequent references to other standards including the Health & Safety Executive’s British Standard BS 6164 that addresses health and safety requirements in the tunnel construction industry and BSEN 16191 that covers tunnel machinery safety.”

The PAS is also more wide ranging than other codes of practice pertaining to segment linings. “It begins with consideration from conceptual design at project inception to technical specification to instrumentation and monitoring through the design life of installed segmental linings. For example,” explained Jung, “the PAS begins with lining design concerns from the approval in principle(AIP) of new tunnel projects. Chapter 8 discusses durability and from a performance based design as well as a limit state design. Chapter 10 is dedicated to design requirements for the handling and transportation of segments while another chapter deals with numerical analysis, not necessarily presenting the numbers but offering best practice for following the necessary procedures."

BSi project coordinator and editor Laura Cordell provided details of the development process. “This PAS document took longer than most to develop and is longer in its content than most other PAS publications. The steering group comprised engineers from 20 companies and projects and we had a peer review panel of nine experts. These stakeholders presented more than 750 comments on a first draft of the document and a further 150 comments were submitted following the public review process. It illustrated how passionate and engaged members of the tunnelling industry are in their profession.”

At the start, PAS 8810 was to cover also sprayed concrete and cast insitu linings but once into the process, it became apparent that the scope was too wide and that in fact three PAS documents are needed. This first PAS on the design of segment linings provides a starting point on which new PAS codes of practice for sprayed concrete linings and cast insitu linings can and will be developed.

The recommendations in the PAS are not mandatory or part of legislation or subject to litigation if not followed. Project owners however can insist that the recommendations of the PAS be followed and it is also the stage before the recommendations are upgraded to BSI Standard.

Copies of PAS 8810 are available from the bsi bookshop for £113 per copy.

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