Tunnelling must be the only civil construction discipline that takes out more than it leaves behind. When building a skyscraper or bridge, the area to be used is in free space and the materials for construction have to be provided to fill that space. All that is needed from nature is the air to put it in and a suitable foundation to work on.
For the most part, successful surface civil construction is down to good initial design, organisation and competent workmanship. Coupled with the ability to change design ‘on the hoof’, or ‘fast track’ as it is known, and barring litigation or misfortune, the boundaries for successful project completion are more or less in place. Herein lies tunnelling’s uniquity.
Where skyscraper engineers have visible space to work in, tunnellers have solid ground. This is not to belittle the amazing achievements of ‘surface civil engineers’, the Hong Kong skyline and San Francisco’s Golden Gate Bridge are just two examples of their tenacity. But the truth is, the nature of the tunnelling work environment makes it something of a blind science in comparison. It is an amazing achievement that the majority of projects come in on time and within budget, lets not lose sight of this fact or underplay it. But in the news we do read reports of unforeseen ground conditions throwing projects off-schedule and contracts into the court. The importance of a thorough site investigation (SI) is never underestimated by tunnel designers and constructors. The problem is, these investigations are limited by the techniques available and the areas to which they can be applied. Tunnelling engineers have had to learn flexibility to offset SI’s short comings. More ‘fast fix’ than ‘fast track’.
Could a solution be on the horizon? The rapid evolution in seismic imaging technology looks set to change the way tunnellers predict and react to the conditions ahead. Although in it’s infancy, could the time be near where detailed 3D images of a complete tunnel alignment are available before construction starts? If computer technology continues to move at its current rate, this could become a reality sooner than we may think.
