3. Responding to complications

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So far we have touched on how geotech and surveying investigation campaigns are used to forecast the excavation process. For example, drilling bore holes and extracting core samples from hard rock. This process can take a long time and be extremely expensive, as you need a large amount of samples to know what story the rocks are telling at the depth that the tunnel needs to go.

Going in blind is too risky from both a safety perspective and a costs/logistics perspective. Thorough investigations help contractors order the right equipment ahead of time. If they know what rock types they’ll encounter, they can bring in a specialised TBM that’s strong enough and has the right cutters for the job. 

That's neat! So it's smooth sailing from there, right? 

Not exactly.

Nature is unpredictable. In an ideal world, your tunnelling plan would look the same in the delivery phase as it did when you first put it together based on investigations. In reality, though, rock types change, and they change a lot. And sometimes, that change is unexpected.

Have a look at the figure below, which was developed by Underground Infrastructure Engineering (click here to learn more). It shows how uncertainties in tunnel construction can be divided into 3 broad categories: 1) known knowns; 2) known unknowns; and 3) unknown unknowns. 

Such curve balls can push your timeline back or increase costs. If the rock is changing, then you might need to change the machinery or construction plans to accommodate that. 

The news clip below will give you a glimpse into the real-world impacts and conflicts that unforeseen tunnelling complications can have – not just for construction workers, but the broader community – and the importance of resolving those issues effectively, within time and budget constraints. 

So how do we go about adjusting our plans? And what might some complications be for the Toowoomba tunnel? 

Your problem scenario

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