The West Gate Tunnel is one of the largest diameter bored excavation projects in the world and the biggest ever undertaken in the southern hemisphere. Two huge boring machines will create twin tunnels between the West Gate Freeway and the Maribyrnong River, providing a much needed second river crossing, removing thousands of trucks from residential streets.
An exhaustive investigation of the geological risk is underway, led by geotechnical services firms Golder Associates. For the first time Golder has used the ground engineering 3D implicit modelling solution offer by Leapfrog Works to assess geological risk and communicate that to a variety of stakeholders. Construction will begin following ministerial planning approval, expected soon.
The tunnel passes through a complex geological and geomorphological region, and Golder Associates were tasked to undertake an impact assessment for the project.
“This was the first project where we’d used Leapfrog Works and our use has evolved since,” 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 we’d start the project in Leapfrog Works.”
Multiple layers… two different volcanic flows… varying properties, some hard, some wet… different sediments all sandwiched together changing laterally with depth… a former creek converted to a storm drain… sedimentary and igneous deposits from the Cainozoic era.
Leapfrog’s contribution to the project
- Highlighting uncertainty around the Southern Portal
- 3D spoil volume modelling
- Supporting the tender process by providing contractors with
interactive 3D models
- 3D modelling of the aquifer geology
- Generating 2D geological sections for geotechnical ground
- Use as a key communication tool with the client and at the State
2 million – cubic metres of spoil from the construction of 6km of tunnels and road upgrades. Using Leapfrog, Golder Associates were able to develop a 3D volume estimate for the spoil, and also spot contaminated areas as well as identifying material that could potentially be used for other projects.
The problem at the Southern Portal
Looking at the 3D outputs from Leapfrog Works, Golder geologists immediately spotted an issue with a snaking creek that crisscrossed the tunnels as they went under the freeway. “Getting a sense of how those geometries interacted was very difficult in 2D,” said 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 was able to use the Leapfrog Viewer to let contractors see the geological models – even without access to the software – 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.”
Conditions below the West Gate Freeway
Ground conditions were considered poor here, with greater 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.”