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How the sinkhole formed
On Saturday, 22 September 2018, a leak developed while crews excavated the first cross passage connecting the twin tunnels near Dundas Road, roughly 200 metres north of the future Forrestfield Station. Water and silt flowed into Tunnel One. A sinkhole formed beside the road early the following morning.
Cross passages connect the two running tunnels. The Western Australian Government described their intended role as “a throughway to connect the two tunnels and will be used as an emergency exit once the rail line is operational.” The leak occurred during construction of that connection, rather than as an unrelated surface event. (Government of Western Australia, 24 September 2018; METRONET project information)
Damage and immediate response
The leak and the pressure and loading from groundwater distorted a 26-metre section of Tunnel One and displaced 16 lining rings, according to the project fact sheet. The parliamentary account reported that Tunnel Two was unaffected and about one metre of Dundas Road had been eroded. Crews injected grout to stabilize the area and stop the inflow; the fact sheet says groundwater inflow had stopped by 3 October 2018. (METRONET project information; Parliament of Western Australia, 9 October 2018)
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Dundas Road was closed during the response, and both tunnel-boring machines were stopped as a precaution. These were measures taken in 2018, not an indication of current road conditions. (PerthNow, 23 September 2018; Parliament of Western Australia, 9 October 2018)
What the investigation said about the cause
The project fact sheet did not identify one proven defect as the definitive cause. Instead, it listed several probable grout or interface failure mechanisms associated with the cross-passage works:
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- Defects in the jet grouting between the tunnels.
- Defects in the annulus grouting around the tunnel lining.
- Failure between the annulus grout and jet grout.
- Failure at the interface between the segmental tunnel lining and the annulus grout.
The fact sheet also said tunnelling through the grout block and vibration from cross-passage excavation may have contributed. These are investigation possibilities, not confirmed findings that a single mechanism caused the sinkhole. (METRONET, December 2018 project fact sheet)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What is known about the road reopening
In a project update dated 18 December 2018, the State Government said two traffic lanes were expected to open later that month while compaction works continued. That was a forecast, not confirmation of the exact reopening date. The cited 2018 sources do not establish Dundas Road’s present-day traffic status. (State Government update, 18 December 2018)
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