Update from Forrestfield

5 March 2019

There is some late-breaking news as Tunnels and Tunnelling International goes to press. On 19 February the Forrestfi eld-Airport Link’s client announced that its two stopped TBMs should resume tunnelling in mid- and late-March respectively, good news for a project that was previously held up by a sinkhole last September.

The project, which comprises a tunnelled link to the airport in Perth, Western Australia, brought the first of its two TBMs (named ‘Grace’) to a halt on 18 January after workers reported potential mechanical issues with the machine. Investigations by manufacturer Herrenknecht and contractor Salini Impregilo-NRW subsequently identifi ed damage to the screw conveyor. However, a spokesperson for the client said that the actual cause of the damage is still under investigation. Repairs will include replacing a 3m section of the 16m-long screw conveyor and repairing cracks that have a been identified.

On 4 February the other TBM, Sandy, was also stopped as a precaution so that the machine’s screw conveyor could be inspected for similar faults. Inspectors confi rmed the presence of cracks and the machine will remain stopped until repairs are completed.

The client has said these issues pose no risk to worker safety and that no ground disturbance issues are expected. Two new screw conveyors are being manufactured by Herrenknecht and will be installed while the machines are at Redcliffe Station later this year.

The approximately USD 850M Forrestfield-Airport Link is an 8.5km-long, 7m-diameter twin-tube line. The TBMs are Herrenknecht Variable Density machines capable of operating in EPB or slurry mode. The lining is a precast concrete segmental ring, in a 5+key confi guration with a length of 1,600mm. The concrete is class f’c = 60 MPa, reinforced with 40kg/m3 of steel fi bres and a frame of steel bars. The tunnel runs through an alluvial plain and ranges in depth from 7 to 25m. The alignment passes through clay, sand and weak rock.

At the time of the shutdown, TBM Grace had tunnelled 3,944m and installed 2,363 rings. Sandy was at 3,839m tunnelled and 2,297 rings.

Earlier in February, a project update was released: “It has been a busy start to 2019, with works continuing across nine construction sites, our stations starting to take shape, segment production reaching 91 per cent and the TBMs beginning their ascent towards Redcliffe Station.”

As an iconic project in a region not known for an excess of tunnelling, I wish them well.