Tunnel projects begin long before the first rock is exposed. Designs are often finalised  based on limited site data, evolving conditions and shifting timelines. From excavation strategy to concrete lining design and performance, many decisions are made based on expectations, which may differ from the actual conditions that will ultimately be encountered.

When reality deviates substantially from expectations, consequences can be significant. Globally, geotechnical uncertainty remains a major source of risks and the cause of delays, budget overruns, and contractual disputes. In some cases, uncertainty extends beyond ground conditions and behaviour during excavation; it also influences how materials respond under extreme conditions like fire. More than 400 tunnel fires have been reported across Switzerland, Germany and Austria in the past decade alone, highlighting the importance of validating thermal performance beyond simple compliance checklists (International Fire Academy, 2023). With increasing climate volatility, deeper infrastructure, and tighter construction programs, the pressure to get it right has never been greater.

Rather than chasing perfect predictions, many engineers are now asking a better question: how can we deliver tunnel projects that perform even when ground conditions differ from expectations?

This is the question our Geotech and Tunnels team at SMEC has been exploring. Through a series of technical papers, they have shared insights drawn from real project experience, including work on the Borumba Pumped Hydro Energy Storage project (Queensland, Australia) and West – Western Tunnelling Package (New South Wales, Australia).

At Borumba, the team delivered early-stage design in a remote and environmentally sensitive setting with limited geotechnical site data. Instead of waiting for certainty, they created flexibility within the design, applying observational design methods and establishing a fair risk-allocation, followed by continuous monitoring during construction to inform decisions as ground conditions became better understood.

Borumba Pumped Hydro Energy Scheme, Australia

On Sydney Metro West, fire modelling demonstrated that even small variations in moisture content, fire curve type or exposure geometry can influence the performance of concrete tunnel linings. These findings challenged the assumption that standard models alone are sufficient to define resilience.

Sydney Metro West – Western Tunnelling Package, Australia

The team also analysed tunnel boring machine (TBM) breakthroughs in Sydney’s layered sandstone and shale, revealing that traditional modelling often underestimates support loads when defect-driven rock behaviour is not properly accounted for.

This approach reflects how we work at SMEC. From the outset, our teams bring together geotechnical and tunnel specialists to collaborate closely, combining ground knowledge with advanced modelling and delivery expertise. Whether it involves shafts, caverns or TBM tunnelling, we design with the understanding that conditions will change and what matters is how the design performs when they do. Additionally, fire resilience is incorporated into the concrete tunnel linings design for all projects, ensuring that the design performs effectively as conditions change.

These papers don’t offer a single solution. Instead, they elevate thinking in a way that is becoming essential;  treating uncertainty not as a problem to eliminate, but as a reality that can be adequately managed.

In tunnelling, risk is not the exception, it’s an inherent condition.

The full papers are available to download below.