Understanding the prevailing ground conditions is paramount to achieving successful outcomes from microtunnelling. Planning and preparation before arriving at the job site are also key. But what happens when you arrive on site, and the ground conditions are not what you expected? Microtunnelling expert Stuart Harrison shares his tips and tricks for a successful installation when the ground conditions aren’t what they seem.

There are many factors that need to be taken into account when it comes to presumed ground conditions. Is the ground hard or soft? Is clay present, and if so, what type of clay is it? What is the plasticity like? Is it silty and/or sandy? Is the water table high?

In general, the more the project owner, contractor and subcontractor know about the ground conditions, the less likely it is that unexpected complications will occur.

However, it’s not uncommon for contractors and subcontractors to arrive on site and find the ground conditions are somewhat different to the geotechnical information provided for a particular job.

“Unfortunately it’s not uncommon for this to happen,” said Stuart Harrison, Owner and Managing Director of microtunnelling contractor Edge Underground. “I’ve lost count of the amount of times I’ve arrived on site and the presumed ground conditions don’t match up with what’s actually there.

Fortunately, Mr Harrison has learned a thing or two in his time about how to handle unexpected ground conditions – here’s how he gets around ground conditions that differ against what was initially advised.

Figure it out early

According to Mr Harrison, to have the best chance of success, you’ll need to identify the difference in the geotechnical information versus what is actually there as early as possible.

“If we are lucky, the difference from the presumed conditions will be picked up at the earliest stage – ideally when excavating the shafts,” said Mr Harrison. “This gives us the opportunity to change our approach to the microtunnel before we launch, giving us the best chance of a successful drive from the outset.”

Unfortunately, things don’t always work out this way.

“If we are unlucky, the difference won’t be established until during the microtunnel. Worst case scenario is discovering a difference in ground conditions directly under the very asset that prompted the use of a microtunnel to begin with.”

Pilot lines are key

Traditional microtunnelling techniques rely on the jacking of a product pipe to propel the microtunnel head. According to Mr Harrison, when the ground substantially changes mid-installation – to the point where the contractor can no longer proceed with the line – inevitability there will be a need to dig up the head.

For this reason, there is a lot to be said for installing a pilot line that can be retracted prior to jacking. This gives the contractor the ability to test the ground conditions before they are committed to jacking pipe, which is essentially the point of no return.

Extraction over displacement

According to Mr Harrison, in the past, pilot lines were predominantly installed via a displacement method. For this method to be successful, the ground being tunnelled through must be displaceable. When utilising this method, contractors also need to be weary of the effect of the displacement of the ground on the surrounding assets.

Now however, installation technologies are available which extracts the ground being tunnelled through, rather than displacing it. One such example is the Vermeer AXIS machine, which is designed to cut and extract the ground as it proceeds, and in doing so, has little to no influence on the ground directly surrounding the installation. By extracting the ground, rather than displacing it, microtunnellers can visually inspect the condition that is being excavated at the face.

In addition, because the pilot line can be retracted, it allows for different bits to be effectively trialled during the pilot stage. This allows the contractor to assess the best bit for the prevailing ground conditions. Once the pilot is complete, the microtunneller can commence jacking the final product pipe with a far greater likelihood of a successful final installation, as the ground conditions are now predominantly known.

Risk minimisation

According to Mr Harrison, his tips for successful microtunnels in changing ground conditions essentially come down to one simple concept: risk minimisation.

“When it’s all said and done, we are all looking to reduce the risk on our trenchless projects,” said Mr Harrison. “Before your next microtunnelling project, please consider these tips – particularly the use of a pilot shot to confirm the presumed ground conditions – and minimise your risk early, before it’s too late.”