Microtunnelling in difficult conditions

Undertaking a pipeline installation in a tight access area can be logistically challenging, especially where there are risks from surrounding infrastructure. When a project in Maribyrnong in Melbourne’s inner west needed a sewer installed in such conditions, no subcontractor wanted to take in on except for Edge Underground, whose experience and confidence in its equipment made it confident it could complete the project on time and in budget.

The project required a sewer pipeline to be installed for a high-rise development in a very tight access area in the middle of a road, close to tram tracks.

Stuart Harrison, Managing Director at Edge Underground, said the area the subcontractor needed to be able to work in was between two road lanes in a very slow road reserve.

“The high-rise development was nearing completion and the builders were having a lot of difficulty finding a contractor that was willing or able to install the sewer due to the access constraints,” Mr Harrison said.

“The trams also posed a major risk as there were a lot of overhead power issues, which prevented the use of excavation equipment because of the close proximity to high voltage power. So there were a lot of poles that were suspending the overhead network, as well as a significant underground network.

“Managing excavated material was also a key challenge as the subcontractor would not be able to load it into trucks, take it away, and bring it back for reinstatement. This made that job near impossible for most contractors to take on. 

“All of the work also needed to be done in typical business hours, so there was no additional noise disturbance to the neighbours in the area.”

Working with mixed ground conditions

Along with the site constraints above ground, the prevailing ground conditions also posed a challenge.

“The ground conditions on site was a mixture of clay and rock, with the rock probably less than 120 MPA. 

“Whenever there in a combination of hard and soft materials there is always some difficulty working with it because when you try to use a lot of force to break through the rock, there’ll always naturally be a shift of the drill heads towards the softer condition,” Mr Harrison said.

“When you’ve got all these limitations and you need to get a pipeline installed through that route, you start to run out of options pretty quick as to how that can be achieved.”

Taking on jobs no one else will 

Knowing the site constraints, ground conditions and challenges facing the install, Edge Underground presented a package, stating it would be able to complete the job within two weeks.

“We were working with Eden Nominees on the project. They’d already seen some of our past work and they’d heard from a number of companies that we were offering a service that would be able to complete the job,” Mr Harrison said.

“We were confident we could take on the job because we use the Vermeer AXIS laser guided boring system.

“The drill has been designed to be able to counteract and manage the different pressures in the ground, so it can drill through rock and clay while maintaining line and grade.”

Meeting the challenges 

Mr Harrison said Eden Nominees was great to work with, and the company’s openness to innovation allowed an unusual solution to be found to manage the challenges of the project.

“The window in which we had to work in was so tight that it would’ve been difficult to put shoring into place. However, after a few meetings and assessing the conditions, we were able to come up with a solution; we essentially shotcreted the tight access shafts, and we managed to only take out one lane of traffic whilst working.

“This type of innovative thinking and flexible approach that Eden Nominees took gave us full scope to achieve a fantastic outcome.”

Variations in rock strength and ground conditions also posed a challenge once drilling started.

“At one point the rock strength got close to going over 120 MPA and in other parts it broke down and got closer to around 50 MPA. Some of the clays got a little bit wet in places as well, making it less consolidated, which can create further issues,” Mr Harrison said. 

“We were monitoring the ground conditions all the time, allowing us to reassess and manage conditions as we were going. 

“We had one cutter change during the job, from a solid rock to a mixed cutter face, part way through one of drives.” 

Edge Underground was able to change out the cutter face with minimal disruption to the project due to the unique retracting capabilities of the AXIS system.

“In ground conditions such as this, where there is a mixture of hard and soft rock as well as wet clays, there is a real problem that the weight of that setup and gravity creates a lot of a downwards pressure so the drill head will drop under its own accord,” Mr Harrison said.

“Any other microtunnelling machine would’ve required us to dig out the drill head from above in order to change it, adding time and cost to the project. But with the AXIS we were able to retract the drill head and change it out to a special mixed ground cutter, which we’ve developed over the years or trialling and testing, giving us some unique capabilities to manage some really tough conditions.”

A successful outcome

Edge Underground worked predominantly during business hours to complete the project during low traffic flow times, reducing the noise and road impacts on the community.

“We completed the project in about seven days of work using two different drives in the 60-70m bracket to install the 300m PVC pipeline on grade,” Mr Harrison said.

Lauren brings a fresh approach to content. While she’s previously written for publications as diverse as Australian Geographic, The Border Watch and Girlfriend, she’s found her true passion in her current role as an editor in the world of energy and infrastructure trade magazines.

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