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Edge Underground: microtunnelling through very hard rock

Edge Underground's Air Hammer

Installing pipelines via trenchless methods such as microtunnelling in areas where there has been volcanic activity presents a number of challenges.

Depending on the conditions, and pipeline route and specifications, there could be few options available and may result in higher costs for the client. However, a new, innovative advancement by Edge Underground is allowing for easier drilling in difficult hard rock conditions.

Stuart Harrison, Managing Director at Edge Underground, says that there are a number of conditions that can be found in areas where volcanic activity has taken place, including intersecting lava flows, quenching, lava tunnels and extreme rock hardness.

“Due to the nature of lava flows, drilling in areas of volcanic activity can be particularly challenging as ground conditions can vary a lot along the pipeline route. While geotechnical information will be provided prior to drilling, it’s not uncommon for it to be inaccurate in such areas,” Harrison says.

“This is because it doesn’t provide the details of the ground along the entire length of the pipeline route; all it shows are the conditions at the location of the core sample, which could be vastly different to the conditions a short distance away.

“However, where areas are notorious for having difficult ground conditions, and if you’re accepting a project in those areas, you know there is a high chance of challenges arising.”

Donnybrook in Victoria is notorious for its difficult ground conditions, consisting of clays and boulders from the underlying Quaternary Age Newer Volcanics basalt and multiple basalt flow boundaries.

Heading into a project in Donnybrook that comprised 2.4 km of DN700 GPR pipe – 1 km to be completed by trenchless with depths from 10-14 m over ten bore lengths – Edge Underground knew the job would not be easy. 

But when ground conditions presented as more challenging than expected, it provided an ideal opportunity to prove the capabilities of its innovative pneumatic rock hammer.

“We knew the area was made up of intersected lava flows from different volcanoes that were located in different directions, and we were prepared to adapt based on how the drill performed,” Harrison says.

“The thing to keep in mind is that if you drill at the intersection of these flows, you will find abnormalities; you may find the drill will go well in some directions but not others.

“In Donnybrook, when we hit those intersections, we found there were elements of water quenching in the rock – a condition that is rarely seen and wasn’t in the geotech we were provided with. 

“We found the ground in these sections was .5 Mohs and was formed from two layers of lava flow meeting with a gaseous layer which created a lava tunnel, so when we tried to drill there was no support, which put the drill head at risk of snapping.

“On top of that, we also found high water flows where water quenching had occurred, adding the extra challenge of the drill head changing structure where hot steel met the water. This caused further issues as the water flows had caused a crust to form and there were areas of rock over 250 MPa, which hadn’t been identified in the geotechnical information.

“So we had extreme hard rock, what we thought at the time to be basalt up to 800 MPa, intersecting lava flows, voids, and lava tunnels filled with water.”

Edge Underground - Donnybrook
The project required the installation of 2.4 km of DN700 GPR pipe – 1 km to be completed by trenchless with depths from 10-14 m over ten bore lengths.

Harrison says that, in this situation, usually the only option would be to upsize from DN700 to DN1200 and install concrete casing pipe, which would incur a greater cost to the client. 

However, Edge Underground proposed using a new innovative and pioneering advancement based on its new pneumatic rock hammer that its team had just developed. 

The technology uses air-driven hammers (similar to vertical DTH heads) on Edge Underground’s AXIS laser guided system. This system was previously untried and is a new technology to the microtunnelling market, enabling drilling in very high strength rock, a ground condition that has traditionally been a challenge for the industry to work with.

“It can be operated via the next generation AXIS drill rods with a power enhancing driver. The system uses high pressure air hammers in sequence – exhausting cuttings through a custom removal system – and is ideal for very high strength rock,” Harrison says. 

“The new system also has environmental and workplace health and safety benefits, enabling small to medium-size pipe diameters to be installed in hard rock faster, with a significant reduction in spoil removal and truck movements compared to open cut methods.”

For more information visit Edge Underground.

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This article appeared in the October edition of Trenchless Australasia. Access the digital copy of the magazine here.

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