New sprayed concrete lining guide reshapes soft-ground tunnelling standards after decades of lessons

The publication of a new best-practice design guide for sprayed concrete linings (SCL) in soft ground marks an important technical development for underground construction, consolidating lessons from major British tunnelling projects into a single modern framework. Developed for designers, contractors and project managers, the guide reflects how SCL has evolved into a widely used engineering method for complex underground spaces such as stations, cross passages and non-circular tunnels, where tunnel boring machines are often impractical.

Sprayed concrete lining, closely associated internationally with the Sequential Excavation Method, provides a flexible structural shell that can be applied rapidly to freshly excavated ground, allowing engineers to adapt excavation sequences to variable geology while helping to limit settlement. The new guidance builds on lessons from historic failures such as Heathrow’s NATM collapse, while incorporating delivery experience from major infrastructure works including urban rail expansions and station caverns. Key areas of emphasis include excavation staging, groundwater management, fibre-reinforced concrete, fire resistance, waterproofing and real-time instrumentation.

A central theme of the guide is integrated risk-based design, where geotechnical understanding, construction methodology and monitoring are considered together rather than as separate disciplines. Modern SCL practice places emphasis on predictive modelling, independent design checks, trigger-based movement thresholds and workforce safety measures that can reduce exposure at the excavation face. Sustainability is also addressed, with guidance covering waste reduction, lining efficiency and embodied carbon considerations.

The guide provides a structured reference for soft-ground tunnelling by bringing together project-specific experience, current recommended practice and recent technical developments. It reinforces the importance of excavation sequencing, geotechnical assessment, monitoring and disciplined risk management in the successful design and construction of sprayed concrete linings.