The impact of a landslide on a critical access road in New Caledonia’s rugged terrain could have been disastrous without the right technology.
By Paul Gorman
Constructing infrastructure in tropical regions requires particular expertise and care, especially if the country has tricky topography and there are ecological sensitivities.
New Caledonia ticks both boxes. Highly weathered rocks and unstable tropical soils, with frequent heavy rainfall and occasional tropical cyclones, provide major technical challenges, both for modelling and for mitigating against during construction.
Road-building costs can be high due to the remoteness of locations and the logistical difficulties of transporting equipment into isolated communities. Road construction also has to take into account protected flora and fauna, and avoid disturbing sensitive rainforest ecosystems.
Drone photograph of failure during aerial survey.
Source: Supplied
Hatch was the engineering and construction manager for a 2.2km critical access road on the island through relatively steep terrain, with several high-cut slopes.
In October 2021, a deep-seated landslide along a partly completed section of the road shifted about 15,000 cu m of material across a 50m section of the road. Luckily the landslide occurred after hours and there were no injuries or losses of equipment.
Work on the road stopped immediately, awaiting a comprehensive stability assessment of the zone and a review of adjacent cut slopes.
A secondary slide within 10 days, from the crest of the original failure, contributed to a total of 25,000 cu m of soil and rock being moved. The trigger for the slides was puzzling, as at that point of the road the cut slope was not as high or steep as other cut sections.
The landslide remediation nearing completion
Source: Supplied
Working out what to do next
The lack of warning, the size of the landslides and concerns about instability leading to more slope failures were uppermost in the minds of Hatch contractors and stakeholders.
Two options immediately came to mind: Either abandon the current partially completed road and construct an alternative road alignment, or flatten the existing cut to a lesser angle. Both would have cost more and taken longer for the project to be achieved.
As well as the geographical and geological challenges, Hatch personnel had to respect significant language, political and cultural differences, with French the dominant language, and there was the added complication of the Covid-19 pandemic and restricted travel and site access.
Hatch used Seequent and Bentley software to develop, model and assess the stability of the landslide area and cut slopes, develop the remediation design, and monitor the construction progress to manage future risk of slope failures.
Principal geotechnical engineer Simon Hope says the software Hatch used in the $US50 million project “allowed for the landslide to be stabilised and slope conditions assessed so that the project could continue”.
A detailed sequence of the landslide lifecycle, showcasing the progression from construction commencement, through slope failures, geotechnical investigations, model development, remedial design, and culminating in successful remediation works.
Source: Supplied
Benefits of 3D over 2D
Three crucial pieces of Seequent’s geotechnical software were invaluable in the remediation work.
Hatch used:
- OpenGround , for the remote collection and collation of site geotechnical data in the cloud;
- Leapfrog Works, for developing a 3D-geotechnical model for the complex conditions; and
The landslide was originally modelled using 2D sections of critical slope sections, Hope says.
‘This was generally okay where there was geotechnical data available. However, trying to interpolate between these sections was very difficult due to the meandering road geometry and variable geological conditions. A 3D Leapfrog model was therefore adopted to best model and assess the complex project conditions.
‘The geotechnical team chose to use Leapfrog to build the ground model due to its user-friendly platform, excellent visual presentation, (OpenGround) cloud-based storage for review control and remote work, and integration with geotechnical analysis software.’
Having a cloud database within OpenGround allowed near-simultaneous assessment and verification from team members around the globe, including in Calgary, Perth and Brisbane, as well as in New Caledonia. The data could then be directly imported into the live Leapfrog model, which allowed for real-time decision making once construction recommenced.
The 3D model identified a zone of very weak material at the toe of the cut slope. Back analysis showed that was likely to be what caused the failure higher up.
The compatibility between OpenGround, Leapfrog and GeoStudio software also allowed for routine geotechnical analysis of the slope’s constantly changing geometry during construction.
Halved
Landslide remediation costs using Seequent tools
Avoided
Road realignment at double the cost and up to a year’s delay
Saved
17,000 tonnes of carbon dioxide through remote work and reduced earthworks
Bringing a global team together
With a team working in New Caledonia, Australia and Canada, the ability to quickly visualise the problems was essential, Hope says.
‘Add in the decision-makers not always in the same room as the engineering team and it becomes critical to have a tool that allows people from very different technical backgrounds to quickly seize the evolution of the earthworks being completed versus the geological and geotechnical model.
‘Leapfrog made the consolidation and visualisation of the site survey results and geological and geotechnical data a very powerful and important tool in the quick turnaround and progress of our project.’
It also helped with the language barrier.
‘The stakeholders and the independent verifier were French speaking, so trying to discuss relatively complex geotechnical topics was extremely challenging. The model was routinely presented in meetings to explain a concept or present technical data.
‘As they say, a picture speaks a thousand words.’
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