Before you start one of the largest bored excavation projects in the world, you need to be very sure of the ground beneath your feet.
In preparation for the massive West Gate Tunnel project in Melbourne, Australia, it fell to geotechnical services firm Golder Associates to undertake a comprehensive investigation of the geological risk before work could begin.
Faced with a complex and often deceptive geology they employed the ground engineering 3D implicit modelling solution offered by Leapfrog Works to deliver an exhaustive assessment, then communicate its findings to a variety of stakeholders from a wide range of disciplines. What Leapfrog Works uncovered was to have a direct impact on the route the tunnels took, and the success of the contractors who used its data to shape their plans.
A story of two tunnels
The West Gate Tunnel project is the largest diameter bored excavation project ever undertaken in the southern hemisphere. Two huge boring machines will create twin tunnels between the West Gate Freeway and the Maribyrnong River, taking trucks and motorists underground en route to the port. They’ll provide a much-needed second river crossing, helping to remove thousands of service vehicles from residential streets.
Construction is expected to take four years at an estimated cost of A$5.5 bn. The work will generate 2 million cubic metres of spoil from the creation of 6km of tunnels and road upgrades. The investigation alone required a team 200 strong, including six drilling crews, and the entire undertaking will generate 6,000 jobs.
The challenge beneath the surface
As part of Golder’s study, Leapfrog Works had to make sense of geology that included two separate volcanic flows, a range of different sediments all sandwiched together and changing laterally with depth, a former creek converted to a storm drain, plus sedimentary and igneous deposits dating to the Cainozoic era.
The investigation unearthed problems such as a snaking creek that crisscrossed the tunnels as they went under the freeway, and ground so poor it made predicting the magnitude of movement a major challenge.
“This was the first project where we’d used Leapfrog Works,” said Golder’s Principal Geotechnical Engineer, Trevor O’Shannessy. “We started by building the geological model in 2D using a very standard method, and then we brought that 2D information into Leapfrog Works to build a 3D model. Going forward I’d say we’d start the project in Leapfrog Works.”
3D beats 2D at revealing the issues
Looking at those 3D outputs, Golder geologists quickly spotted a problem at West Gate’s Southern Portal with a creek that wound around the proposed route of the tunnels. “Getting a sense of how those geometries interacted was very difficult in 2D,” explained Golder’s Senior Environmental Engineer Scott Ambridge. “But in Leapfrog’s 3D environment we could see much more and determine the risks from a geotechnical point of view.”
Golder were able to use Leapfrog Viewer to let contractors freely see the geological model in 3D – and act accordingly. “The contractors were able to slice through the model, rotate and spin it around, and see where there was greater uncertainty. The successful contractor identified the problem with the Southern Portal and went deeper with their tunnel design to avoid the high-risk areas.”
Below the West Gate Freeway ground conditions were so poor that there was considerable uncertainty around the potential ground movement magnitude. “We therefore needed to visualise the subsurface conditions and be confident about our interpretation of the subsurface model,” explained Trevor O’Shannessy.
“As this area was targeted with numerous boreholes during the investigation, Leapfrog provided a good interpolation tool in the 3D space and enabled the construction of a model in a relatively short period. It also allowed us to rotate the model, interrogate it in 3D, and experiment with assumptions such as bed level and upstream/downstream gradient of the creek incisement.”
Golder Associates were even able to develop a 3D volume estimate for the spoil generated by the boring and identify material that could be used for other projects to enhance the scheme’s sustainability credentials.
Laying the ground for a four year push
Leapfrog Works played a key part in helping the West Gate project begin construction, on time, on January 31st this year. In 2019 the scheme’s two huge 15.6 diameter tunnel boring machines will begin cutting through the (thoroughly well mapped) earth at nine metres a day, installing a concrete lining behind them. The West Gate Tunnel will open in 2022 bringing a bold infrastructure project and a vital transport alternative to the city of Melbourne.
For a more detailed look at how Leapfrog Works was involved in the project see our February issue of Unearthed – Seequent’s new global briefing on the technology and innovation revolutionising the Civil Engineering and Environmental industries.