LININGS - SCL | TECHNICAL
integrated construction input. The chapter highlights the importance of single design responsibility to avoid duplication and ensure clarity, and stresses the need for iterative design, coordination, and validation during construction.
● Excavation Sequencing: various ways to split a tunnel are discussed, such as full-face excavation, top heading/bench/invert, sidewall drift, and pilot tunnel enlargement. The guide provides guidance on selecting excavation methods based on ground conditions, stability, and project requirements.
● Material Parameters: this chapter details the properties and behaviour of sprayed concrete and fibre-reinforced concrete (FRC), including mechanical parameters, durability, and time-dependent behaviour. It highlights the importance of testing and specifying concrete mixes for strength, ductility, and durability.
● Waterproofing and Water Management: discusses the impact of waterproofing systems on tunnel design and construction, covering strategies for managing groundwater, including fully tanked solutions, sheet membranes, spray-applied membranes, and watertight concrete.
● Fire Safety: this chapter provides guidance on designing SCL tunnels for fire resistance, including fire suppression systems, cladding/coatings, and micro- synthetic fibres to prevent spalling.
● Instrumentation and Monitoring: emphasises the importance of monitoring during construction to manage risks and validate design assumptions, as covered by the ‘Monitoring Underground Construction - A Best Practice Guide’ also produced by the BTS and currently being updated to cover recent advances in technology.
● Sustainability: the guide encourages sustainable design practices, including optimising lining configurations, reducing waste, and minimising carbon footprint. It does not shy away from the comparatively high wastage factors found in SCL and highlights the need for efficient use of materials and innovative solutions to reduce environmental impact.
● Specifications and Assurance: this chapter provides guidance on preparing materials and workmanship specifications, defining roles and responsibilities, and ensuring quality assurance. It stresses the importance of independent checking and validation during design and construction.
● Numerical and Analytical Modelling: discusses the use of numerical and analytical solutions for SCL design, including 2D and 3D models, constitutive models, and methods for simulating construction processes. The guide highlights the importance of validation, calibration, and sensitivity analysis.
● Risk Management: building on the lessons from Heathrow, the guide advocates for rigorous risk management to include comparative risk assessments, trigger levels, and contingency planning. The chapter emphasises the importance of communication and understanding of the tunnelling process and residual risk among all project stakeholders.
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The team that authored the book has extensive
experience of design and construction of sprayed concrete linings and was supported by a fabulous team of reviewers. Due to the long period from Conception to Delivery, responsibility for leading the guide has passed hands a number of times, with everyone involved keen to see its eventual publication. All the contributors were volunteers, working in their spare time to complete the document, update and edit it. Some team members were simultaneously working on other projects for the BTS, including the Specification for Tunnelling 4th Edition, released in 2023. Participation in the BTS’s Technical Subcommittee does
not require election to the BTS Committee. If you have time, are willing and would like to contribute, please contact the BTS at
bts@ice.org.uk. As well as revising the ‘Monitoring Guide’, there are
currently Working Groups active on the following themes: segment production; impacts of biogenic corrosion on tunnels; and, implications of upgrades to the Eurocodes on fibre reinforcement design in tunnel applications. Future areas of knowledge sharing can be brought to the Subcommittee by any contributors. In order to access the SCL design guide, go to the BTS
website, or directly to the Emerald Publishing website where it can be selected for download as a complete pdf document by selecting open access. It is also possible to order a hard copy, for £55. There are alternative website pages that allow you to select individual chapters if that is your preference, but this is not essential. We hope that the guide proves useful and a good read
for anyone interested in the use of sprayed concrete for tunnelling applications.
Above: Cover of BTS SCL Guide, published at the end of 2025. Available in ebook and print. Go to
http://britishtunnelling.com
From the Guide (Below, top): Drawing of top heading and invert excavation sequence; and, (bottom): A comparison between a TBM and SCL cross section for a typical road tunnel.
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