Considered one of the most problematic soils found in the state of Victoria, Coode Island Silt is accorded a notoriety among town planners, builders and architects for the complexity it adds to ground construction, particularly tunnelling.
Prone to hazardous chemical changes when exposed to air, CIS is highly compressible, soft, and water-saturated soil inconveniently found in the largest urban development precincts of Melbourne.
Think Southbank, Fishermans Bend and the Docklands. It was at the latter location microtunnelling maestros Edge Underground in partnership with Able Drainage and SJE Consulting, were challenged with completing installation of a gravity sewer.
The project involved installing 345mm OD GRP through Coode Island silt, with blow counts as low as N=0 in some locations.
The ground conditions were extremely soft, saturated and unforgiving — the kind of environment most contractors like to avoid. The works were carried out beneath a road near a primary school, meaning the construction window was limited to the January school holidays to minimise disruption to the surrounding community.
For Able Drainage Director Anthony Marsh, success on a project like this starts long before the drilling begins.
“Edge Underground was involved from the very start of the design,” Marsh said.
“They’re one of the few companies in Australia who have successfully worked in Coode Island silt for gravity sewer installations where accuracy is critical.”
The project design was developed in collaboration with SJE Consulting, with Senior Engineer Chadia Chahoud leading the engineering design.
According to Marsh, the early collaboration between all involved was essential for a positive outcome.
“In this project the consultant, contractor and tunnelling contractor collaborated well to make sure the plans were right from the start for success,” he said.
“With input from Edge, plans were modified and the pipe size changed to ensure the project was possible.”
Customised solutions for super soft ground In these circumstances conventional tooling was, by and large, eschewed.
With a purpose-built solution deemed necessary, Edge Underground deployed a new super soft drill head, specifically designed to operate in ultra-soft, saturated ground conditions where standard heads struggle to perform.
The custom head was engineered to manage the unique challenges of CIS, reducing disturbance at the face while maintaining stability in material offering almost no natural resistance.
Combined with vacuum extraction technology, broken ground was immediately removed, helping keep the bore clean, stable and predictable throughout the installation.
“Super soft ground demands a completely different approach,” said Stuart Harrison, CEO of OptionX Group and Founder of Edge Underground.
“You need to be able to extract ground, yet it is very easy to over extract. We have a pressure balance bit which goes a long way to controlling this, giving the operator more options than ever.”
Maintaining line and level where others walk away In ground with a blow count approaching zero, achieving accuracy is often the greatest challenge.
The absence of natural resistance means even small disturbances can quickly translate into deviation.
Despite the saturated conditions, Edge Underground successfully delivered two precision drives measuring 54 metres and 36 metres.
Both bores were, crucially, undertaken on extremely flat grades. One was 1 in 160, while the second was installed at 1 in 180. Marsh said the final accuracy of the installation exceeded expectations.
“Above all, the shots came in within three and four millimetres with each bore, so we were very pleased with that,” he said. He also praised the professionalism of the drilling crew.
“Blade [Richards], the drill leader, was great and it was a pleasure working with all the guys. They were good to work with and very professional.”
Redefining what’s possible in trenchless The successful Docklands breakthrough reinforces Edge Underground’s reputation for tackling complex trenchless projects that push the limits of conventional practice.
Marsh said the decision to involve the team early in the design phase played a significant role in the outcome.
“We knew this was a very difficult project with challenging ground conditions,” he said.
“We knew Stuart and his team had successfully laser bored in Coode Island silt before, so we relied heavily on his expertise.”
Projects like this demonstrate how collaboration between consultant, contractor and tunnelling contractor can overcome even the most challenging ground conditions.
“Trenchless construction is about solving problems underground without disrupting what’s happening above,” said Harrison.
“When we take on complex ground conditions and deliver a clean, accurate result, it’s a win for our client and a win for the industry.”
As trenchless construction continues to expand across Australia’s cities and infrastructure corridors, the Docklands project highlights the importance of experience, innovation and precision when working in some of the country’s most difficult ground conditions.
For more information, visit optionxgroup.com.au
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