On April 17th 2018, the Salini-Impregilo (SI-NRW) JV restarted TBM Grace on Perth’s Forrestfield-Airport Link, with the project’s second TBM, Sandy due to follow shortly ‘at a safe distance behind Grace’. Operation of the two Herrenknecht Variable Density TBMs being used to construct the project’s 8km long, 7m diameter twin tube tunnels was ceased temporarily on 14thFebruary and 28thMarch, 2018 respectively following ‘ground disturbance issues’ associated with the leading TBM.
Though Sandy has not experienced the same ground disturbance issues, it is widely-accepted best practice that two TBMs should not be positioned side-by-side during tunnelling, so she was halted 40m behind the first TBM.
The temporary suspension was to allow for the processes associated with the tunnelling to be independently reviewed and validated. The review is now complete with the issues having been, “comprehensively investigated and changes made to reduce the risk of further disturbances,” according to the Public Transport Authority (PTA) of Western Australia.
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A statement from the PTA says, “The Ascot formation (through which boring was taking place), comprises non-cohesive granular material that makes tunnelling more complex. And this presented issues for the project team.
“Since the minor ground disturbances and subsequent investigations a number of changes have been made including;
- Changes to the pressure in the excavation chamber when excavating in the Ascot formation.
- Continuous operation (no weekend stoppages) especially when tunnelling beneath airport infrastructure.
- Slurry and bentonite quality to be adjusted to reduce potential slurry losses.”
The $1.86bn Forrestfield-Airport Link is jointly funded by the Australian and Western Australian governments and will deliver a new rail service to the eastern suburbs of Perth – with three new stations at Redcliffe, Airport Central and Forrestfield via 8km long twin tunnels.
In April 2016 the PTA awarded the $1.176bn single-package, lump sum design, construct and maintenance contract to the Salini Impregilo – NRW Joint Venture.
TBM Grace began tunnelling in July 2017, with TBM Sandy following in September. Their underground journey is scheduled to take almost two years with breakthroughs slated for April 2019 and June 2019 respectively.