When it comes to microtunnelling projects, there are a number of factors that determine whether a pipeline installation is successfully installed the first time, including the experience of the contractor, pre-planning and having the right equipment.
But what we see most commonly in projects that run into trouble part way – or have trouble finding a contractor to take it on – is that there is a lack of accurate information to identify and confirm ground conditions at the site.
Know the ground conditions
Understanding the prevailing ground conditions at the site is extremely important on a microtunnelling project as it will help determine what drill head will be used.
However, while geotechnical information should be provided at the start, it is not uncommon for it to be incorrect.
This is through no fault of anyone – core samples and geotechnical inspections are like looking for a needle in a haystack.
This is because you are just looking at a small sample, and what you see may not reflect the conditions even a short distance away.
When you install a pipeline though, you pass every metre of ground from one point to another, so if there are any changes in the ground along that route, you’re going to find them.
The problem with this is that the drill head needs to be selected to best match the ground conditions, otherwise there’s a high chance that you will need to stop the drill part way through to change out the head if there is a significant change in ground conditions.
However, most microtunnelling machines don’t have the ability to retract, so a hole will need to be dug from above the drill head to swap it out, adding time and cost to the project – and that’s as long as no other problems occur.
If the area is known to have, or the geotechnical information shows, difficult ground conditions, project managers may also find it difficult to find a contractor willing to take on the job due to the high risk of problems occurring.
A retractable option
Having state-of-the-art equipment is one way to mitigate the risks of changing or difficult ground conditions. While most machines are designed to only move in a forward direction, the AXIS is designed to retract.
This allows the contractor to perform a pilot line, inspect the ground conditions and select the best drill head before committing to the final jacking.
This feature also means that if something does change part way through the drill, the head can be retracted and the contractor can reassess the next steps.
We have found this feature invaluable, allowing us to take on jobs that no one else can or wants to, and complete jobs successfully and with minimal risks.
In one instance, the geotechnical survey indicated the ground was sandy clay, but when drilling started, we found that it was actually wet sand with clay bits.
The original contractor had been unable to complete the installation because of this as they were using a vacuum style machine which isn’t suitable for wet ground as the drill head will drop.
This led to project delays. We were called in to help and after completing a pilot line, we were able to select the correct drill head, advise on the best pipe materials and complete the project successfully.
In another example, they were having trouble finding a microtunnelling contractor willing to do the job due to the ground conditions being known to be difficult and high risk.
Our pilot line showed us that there was more rock with seams of clay through it, as well as partial fracturing, then the geotechnical survey suggested. These conditions are particularly difficult due to the potential for wedging to occur.
But armed with all the information and knowing how the AXIS reacts to these ground conditions, we were confident we could install the pipeline successfully – and we did.
By making sure you have all the right information from the start and having equipment that can retract to change out the drill head if required, you’re mitigating a lot of the risks that will prevent you from having a successful installation the first time.
About Stuart Harrison
Global microtunnelling pioneer Stuart Harrison is the Managing Director of Edge Underground, where he specialises in on-grade microtunnelling installations with millimetre accuracy. Stuart is also the inventor of the Vermeer AXIS Guided Boring system, and he is constantly working to improve the effectiveness of this and other trenchless systems used in the installation of gravity sewers. To discuss your next microtunnelling installation, contact Stuart on 1300 JACKED or at [email protected].