Spheroid graphite iron lining repairs the collapse distorted section

Finishing works at the Bayswater transition portals

After consultation with international experts, project owner Metronet decided on the repair method for the damaged section of the 8km long twin bored tunnels of the Forrestfield and airport rail link in Perth, Western Australia. A 26m long section of one of the running tunnels was damaged in September 2018 when excavation of the first cross passage collapse and the groundwater pressure and associated loading caused localised distortion to the running tunnel and movement of its precast concrete segmental lining.

A spheroid graphite iron lining is being installed by main contractor Webuild (Impregilo) and its local JV partner NRW. The structure will correct the tunnel alignment and ensure the required strength, durability and water tightness of the tunnel. Excavation of the failed cross passage is complete and more than 50% of the new lining in the running tunnel has been installed. Rectification work is expected to be completed by the end of the year (2020). This will free up the last section of running tunnel for track laying.

Ground freezing has been applied to prepare for the excavation of the final cross passage. Once complete early next year, the twin tube alignment will have 12 cross passages and three emergency and maintenance access shafts.

At the tunnel portal at Bayswater, about 11m below ground level, works are ongoing on waterproofing the tunnel sealing rings. The sheeting of the lower roof of the portal building is complete and installation of the upper roof structure is ongoing.

Excavation of the TBM running tunnels was completed in April 2020 when the last TBM drive broke through at the Bayswater transition ramp to the surface rail alignment that connects services to the city central railway station.

Tunnelling on the Forrestfield-Airport Link in Perth is now complete after the final TBM broke through at the Bayswater dive structure on 20 April, 2020. The 8km-long twin-bored tunnels will link three new stations at Forrestfield, Airport Central and Redcliffe to the existing rail network in Western Australia.

The final breakthrough on April 20

A joint venture comprised of Salini Impregilo and NRW spent more than 900 days tunnelling through varied and sometimes challenging conditions, including under Perth Airport and the Swan River. The breakthrough at Bayswater meant the last of the 8km-long twin bored tunnels is now complete. It was achieved just nine weeks after the first TBM finished excavating the first bore on February 18, 2020. The two Herrenknecht 7m diameter variable density machines installed 9,000 tunnel rings comprising 54,000 locally fabricated concrete segments to form the tunnel walls, which were designed and built to have 120-year durability.

With tunnelling now complete, the TBM will be dismantled and hoisted out of the dive structure. Installation of the first stage of track slab is 50% complete and track laying is scheduled to begin in July. Construction and fit-out of the station buildings continues. The AU$1.86 billion rail line is scheduled to become operational in the second half of 2021. The line has capacity for 20,000 passengers a day and will cut the journey from the east to the central business district of Perth from 45 minutes by car to 20 minutes by underground rail.

The first TBM has broken through on the AU$1.86 billion Forrestfield-Airport Link project in Perth, Western Australia, after an 8km drive from Forrestfield towards Bayswater. The arrival marks the end of tunnelling for the TBM – one of two Herrenknecht 7m diameter variable density machines specifically designed for the project – with the second expected to finish tunnelling in May 2020.

Lead TBM being assembled for launch in 2017

Tunnelling started in late July 2017 but in February 2018 both TBMs were at a standstill while the causes of surface settlement of up to 1m were investigated by contractor Salini Impregilo-NRW JV. The project was held up again in September 2018 by the formation of a sinkhole about 200m north of the Forrestfield Station site. Rectification works were successful in repairing both the tunnel leak and sinkhole, allowing tunnelling works to continue, but the projected completion date for the project was delayed by one year.

The recent breakthrough for the lead TBM was the last of three, with intermediate breakthroughs at Airport Central and Redcliffe Station boxes. Half of the 54,000 locally fabricated concrete segments were installed during tunnelling, with the route going under Perth Airport, the Swan River and reaching depths of up to 26m through diverse ground conditions. The TBM will now be dismantled and craned out of the Bayswater dive structure in preparation for the arrival of the second machine.

The end of tunnelling will bring a number of changes for the project, such as the demobilisation of the slurry treatment plant in Forrestfield, as well as major progress in fit-out works within the tunnels. A number of lengths of rail have already been delivered to Forrestfield and Bayswater, and construction of the first stage of the track slab within the tunnels is more than 40% complete. The second stage of the track slab, including installation of the rail, will start mid-2020.

Trains are set to run on the new rail line in the second half of 2021, providing a 20-minute rail link between the eastern foothills, Perth Airport and the Perth central business district.

Relaunch of the two variable density TBMs working on the Perth Airport link in Western Australia is the latest milestone for the project following a problematic stoppage and successful introduction of work procedure changes to enable works to proceed safely in recent months.

Relaunched TBMs are heading to Airport West which is renamed Redcliffe Station

The two 7m diameter, multi-mode EPB machines completed the twin 1.9km long drives from Forrestfield to the airport in May and have relaunched from the box structures below Perth International Airport to complete the 2.8km long drives to Redcliffe Station.

The drives will progress under revised working procedures introduced by the Salini Impregilo-NRW JV in April after investigations of a ground surface disturbance of up to 1m deep over the cutterhead of the lead machine in mid-February. The trailing TBM was halted close behind in late March while investigations proceeded. The TBMs were underway again in April with changes introduced including continuous no-weekend stoppage operation, altering the excavation chamber pressures, and adjustment of the bentonite slurry density.

Beyond Redcliffe Station, the tunnels will pass under the Swan River before ending at Baywater Junction. Final breakthrough is anticipated by mid-2019 to complete the 8km long underground rail link.

The Salini Impregilo-NRW JV was awarded the lump sum design-build contract for the Forrestfield-Airport Link in April 2016 and launched the TBMs at Forrestfield in July and September 2017.

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.