There are several things to consider before undertaking a microtunnelling installation that influence accuracy, downtime and costs. Here, Stuart Harrison, Managing Director at Edge Underground, looks at five critical considerations that need to be taken into account to achieve a successful installation.
Without a highly accurate installation method, projects with tight tolerances are at risk of costly installation failures. When it comes to accuracy, microtunnelling is unrivalled in the industry as it utilises a laser guided system to achieve an accuracy of +/-25mm, with the Vermeer AXIS guided boring system commonly achieving +/-10mm accuracy on line and on grade.
“The simple reason that we can claim microtunnelling as the most accurate pit-launched installation technique is the fact that microtunnelling machines typically run high end guidance systems with live monitoring. The position of the target being at the extreme front allows our actual position to be know at all times precisely and as a result – and this is the critical part – the operator can correct in real time to ensure an accurate installation,” Mr Harrison said.
“For example, the Vermeer AXIS system uses a laser guide and camera that allows the operator to adjust the drill head for increased precision.”
The ability of an operator to hydraulically adjust the drill head maximises accuracy and allows for real-time adjustments.
The drill head of the AXIS system contains a camera connected to a monitor on the operator console. With the camera viewing the laser, the operator can accurately monitor the target grade and make adjustments if the drill head moves off course.
The AXIS drill head can also cut across, meaning potentially dangerous ground shaking is avoided and there is less chance that existing underground infrastructure will be affected.
“It is the extreme accuracy combined with our efficient extraction method that allows us to core our way through the ground with less chance of damaging existing infrastructure as we are not creating ground pressure in consolidated conditions.”
In order to be able to steer correctly, the cutter face needs to match the ground conditions. However, one of the challenges of tunnelling projects is the risk of ground conditions changing mid-installation or not matching the geotechnical information that has been provided.
“The problem most machines have is that once a microtunnel has begun, there’s no turning back. If there is a change in ground conditions and the cutting face needs to be changed, contractors have to stop the microtunnel and dig down to access the drill head to change the cutting face. This costs time and money,” Mr Harrison said.
“Unlike other microtunnelling machines, the Vermeer AXIS guided boring system gives contractors the ability to retract, access and change the machine head part way through drilling the pilot line. Contractors are able to configure the cutting tool to test for changing conditions before committing to jacking the final line.”
The aim for any installation should be to minimise downtime and extra costs. An effective way to manage any unforeseen complications is to ensure the quality and working order of the equipment prior to commencing the installation. In most cases, any issues found can be quickly corrected keeping the project on track and avoiding problems in the future.
Having personnel that are experienced in microtunnelling who can also handle a range of different ground conditions is a necessity to any project.
Often operators who lack experience or an understanding of ground conditions and cutting faces results in slow reaction times and poor interpretation of the way the head is steering or not steering. This inexperience can impact accuracy and increase the risk of unforeseen problems arising.
According to Mr Harrison, understanding prevailing ground conditions is paramount to achieving successful outcomes in microtunnelling.
“The more the contractor is experienced in tunnelling through a wide range of ground conditions, the less likely it is that unexpected complications will occur.
“Familiarity with equipment and how to best adapt cutter faces for different ground conditions is something that can only be gained from working on projects in a variety of locations.