The Government of Western Australia Public Transport Authority (PTA) has said that the Forrestfield-Airport Link project is runing late as a result of work to repair a ground collapse caused by tunnelling.
In a statement reported by New Civil Engineer’s sister title Ground Engineering, the PTA said: “The Forrestfield-Airport Link is continuing to move forward, with significant progress made on the project and rectification measures identified for a damaged 26m section of one tunnel.”
According to the PTA, the project remains on budget but the opening date has been moved back from late 2020 to the second half of 2021.
The collapse occurred on 22 September when a leak developed in a cross passage between the tunnel boring machine bores 200m to the north of the Forrestfield Station site. The leak led to the movement of 16 concrete rings, damaged a 26m section of the tunnel and led to a ground collapse at surface near Dundas Road.
In a statement, Western Australia transport minister Rita Saffioti said that investigations indicate a number of potential causes, including construction defects in the grout block and failure of the join between the tunnel lining and grout.
“Tunnelling through the grout block and/or vibration from excavation of the cross passage may also have contributed,” she said.
“New measures will be put in place for the construction of future cross passages to reduce the risk of a similar event occurring. The construction of the next cross passage is due to start in January.”
The PTA said that the damaged section of tunnel 1 has been stabilised and made safe with solid temporary supports. Cement grouting has sealed the leaks and the voids created by ring movement.
Permanent repair options include fixing the damage from inside the tunnel or rebuilding the damaged tunnel section by removing the old rings and casting the new tunnel lining insitu. This could be done from the surface down, or from within the tunnel. The chosen solution will have to ensure that the tunnel has a design life of 120 years.
Ground stabilisation around Dundas Road is also ongoing, with two traffic lanes to be opened later this month while compaction work continues.
Despite the issues on the project, Saffioti said that work has now reached the half way stage. “This Forrestfield-Airport Link is the biggest tunnelling project ever conducted in the Perth metropolitan area,” she said.
“The two tunnel boring machines are now halfway through their journey, and only recently finished digging under the runways at Perth Airport. This is a massive milestone, considering tunnelling under an operating airport has only ever been done a few times across the world.
“Our primary focus is on safety and quality, and that is why today we have set a revised timeframe for the project. It is also to make sure we inform the public of the proposed timeframe as soon as practicable.
“While the delay is unfortunate, this project is being built to last 120 years. I believe the time added to the construction schedule is a small price to pay to ensure we safely deliver this asset which will serve the people of Western Australia for generations.”