Intermediate breakthroughs for Perth TBMs 24 May 2018

After resuming their journeys last month following a forced stoppage, the two TBMs on the Forrestfield rail link project in Perth, Western Australia, have broken through into the Airport Central station box. The two 1.9km long TBM drives started nine months ago in July and September 2017 and the lead TBM broke through into the first of two underground station boxes on the 8km twin-tube underground rail link on 8 May with the second TBM breakthrough 11 days later.

Lead TBM breakthrough on 8 May

The drives included a forced stop to investigate and implement tunnel operation changes to avoid a set of ground movement incidences that caused ground loss surface settlement above the cutterhead of the leading TBM. The lead TBM was on hold for two months from 14 February while the trailing TBM waited once within 40m and until the lead TBM restarted.

Part of the new operating practice was to keep the TBMs on the move, operating 24hr/day, 7 days/week and eliminate the initial TBM downtime over weekends. Both TBMs were underway again by mid-April and the lead TBM completed the final 270m of its drive within 20 days.

The TBMs entered the Airport Central Station through mass concrete blocks installed to stabilise the walls and reduce water ingress at breakthrough. The TBMs are to undergo a month-long maintenance program before being moved across the box to continue their journeys from the opposite end-wall to Redcliffe Station some 2km further along the rail link route (Fig 1).

Breakthrough for TBM 2 achieved 11 days later

As the TBMs undergo their maintenance, project contractor, Salini Impregilo-NRW JV is also progressing works at other sites on the route.

At Redcliffe Station excavation of the box from within the diaphragm wall supports is complete and pouring of the concrete base slab will continue into late August. Once complete, dewatering operations at the site will be turned off.

At the Bayswater transition portal where the underground alignment junctions with surface tracks into the Perth City Station, work reached a significant milestone in early May with the completion of all diaphragm wall works on the project and the last of the 133 panels for the transition structure. Attention has since turned to excavation of the 400m-long transition structure. All dewatering recharge and monitoring wells have been installed within the portal and along the transition structure.

Work is also underway to construct the three emergency egress shafts that connect to the running tunnels. With the emergency egress shafts there are another nine cross passages that will link the running tunnels along their 8km route.

The AUS$1.176 billion lump sum design-build construction contract with the Salini Impregilo-NRW JV for the AUS$1.86 billion Forrestfield-Airport Link project was awarded in April 2016 and includes a ten-year maintenance period after project inauguration scheduled for 2020. TBM tunnelling is scheduled to take two years and be complete with breakthroughs at the Bayswater Junction transition ramp in April and June 2019.