The British Tunnelling Society (BTS) is pleased to share that the latest free-to-download guidance produced by the Technical Subcommittee – ‘Sprayed Concrete Linings in Soft Ground: A Best Practice Design Guide’ – is now available to read. It has been published and made available by Emerald Publishing.
The guide aims to provides comprehensive guidance on the design of sprayed concrete linings (SCL) in soft ground, particularly in London Clay. It is intended for designers, project managers, contractors, and clients involved in SCL infrastructure projects.
Published at the end of 2025, the guide covers industry standards, excavation sequencing, material parameters, sustainability, fire safety, and monitoring. While the HSE guidance produced in the wake of the collapse of the tunnels at Heathrow (Safety of the New Austrian Tunnelling Method (NATM) tunnels), established many of the tunnel management practices employed by the industry in the years since, subsequent lessons learnt, from successful projects, had not been gathered together and shared.
The document seeks to build on the ‘Tunnel Lining Guide’ produced by the BTS in 2004, folding in the knowledge gained from a number of projects, including: the Elizabeth Line (nee Crossrail); London Underground’s series of station upgrades; utility projects; the A3 Hindhead tunnels; and, the ongoing HS2 works.
For the task, the BTS first established a Working Group to share best practice from all aspects of SCL back in 2011, when the Crossrail project was in full swing. Some delays due to the global pandemic and the authoring team being particularly busy meant that the document was finally completed at the close of 2025.
These delays meant that developments in design and materials have been able to be incorporated over the years, including: lessons learnt from the transition to using fibres to provide flexural capacity in primary linings (avoiding the need for lattice girder and rebar reinforcement in many circumstances); and. methods to reduce the workforce needing to enter the face.
The guide can be used as a checklist for designers, contractors and clients to ensure they are using the most up-to-date methodologies and maximising the efficiency of their sprayed concrete linings.
While the guide is written based on projects in the UK, and undertaken in London in particular, the lessons will be relevant to any projects using sprayed concrete linings (also known as the Sequential Excavation Method (SEM), or Conventional Tunnelling Method in other countries) around the world.
By making the guidance free to access, the BTS aims to fulfil its role in promoting best practices and encouraging tunnel projects to be as sustainable and safe as possible. It emphasises collaboration, risk management, and continuous improvement in design and construction practices.
The document also guides those needing further information with a detailed list of standards, codes, and references for further reading and guidance.
Comprehensive knowledge of the relevant geotechnical and structural standards will not be replaceable for those with design responsibilities, but the guide can help ensure they have considered practicable permutations, while also leading those less involved in the design details and analysis to ask the right questions.
Key chapters include:
- Design Process: the guide emphasises a risk-based approach, progressive design development, and 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.
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.
