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.
The comments issue by PTA are interesting. One wonders whether the changes to face pressures are to increase or decrease them. The obvious answer to that would be to increase them, but in open non-cohesive granular material this would encourage even more losses into the face, which could lead to sink holes. Perhaps the geotechnical assessment of face pressures carried out before the drive has been revised. Were the DAUB Recommendations issued in 2016 used in the original drive?
More relevant in the granular material would be the adjustment of slurry density and quality. This is the benefit and reason for using a Variable Density TBM, which enables the density of slurry in the face to be increased in order to reduce losses into the face. This feature has been used and proven on previous contracts with the variable density machine. When the density is increased an increase in face pressure can then be tolerated to increase face support with reduced, or even no, slurry loss.
Continuous operation under sensitive structures should be standard practice, to apply this in sensitive ground would seem to be a sensible and prudent procedure.
The PTA statement refers to “minor ground disturbances”, a questionable description of a 1 metre depression, which was considered sufficient to stop both TBMs. Let us hope that the improved procedures enable the Variable Density machines to continue their drives unhindered and thus demonstrate the benefits in loose ground with low covers for which these machines were developed.