Forrestfield-Airport Link sinkhole stops work

26 September 2018

Australia – On 22 September a leak developed in a cross passage between the two Forrestfield-Airport Link tunnels, causing a sinkhole to form at the surface. No one was hurt in the incident. Safety managers are onsite and measures have been put in place to ensure worker safety.

The leak occurred around a cross passage adjacent to Dundas Road, which led to the formation of a sinkhole early on Sunday morning and closure of the road.

It was the first tunnel-to-tunnel cross passage to be constructed on the project. The first cross passages were completed at an emergency egress shaft, connecting to the tunnels.

A jet grout block had been installed around the cross passage which is about 9.4m below the ground at the crown. The cross passage in question was 4.8m from the centreline edges of the tunnel and 8.2m measured from the crown and invert. The passage is about 5.7m high and 4.7m wide at the widest points (oval shaped) and had been mostly excavated.
The inflow appears to be from the bottom corner of the cross passage at the cross passage/tunnel interface. No equipment was lost and concrete poured in the sinkhole has not flowed in to the tunnel or cross passage.
The removal of segments in both tunnels for the cross passage openings was completed prior to the incident.  
TBM Grace was approximately 200m north of Forrestfield Station and 1.4km from Redcliffe Station at the time of the incident. The other TBM, Sandy, was 2km away.

A statement from the state government said: “On Saturday afternoon, a small leak occurred in tunnel one (TBM Grace tunnel) of the Forrestfield-Airport Link project, approximately 200m north of Forrestfield Station.

“The TBMs have now stopped as a precautionary measure and work is continuing to stop ground water from leaking into tunnel one.

“While that work is ongoing, a survey of the ground along Dundas Road and tunnel connecting to the cross passage is also underway.

“Due to the leak there has been some movement of approximately 10 tunnel rings near the point of the cross passage. Additional internal bracing is now in place as work continues to stop the leak and resolve the sinkhole damage.

“TBM Grace is currently about 3km ahead of this cross passage site and TBM Sandy is located about 2.5km ahead of the site.”

Transport Minister Rita Saffioti said, "The State Government is monitoring this very closely and working with Salini-Impregilo - NRW Joint Venture. We are hopeful a resolution can be found in the very near future. My number one priority is to ensure work is carried out safely. I have asked the Public Transport Authority to review the timeline of the project, to ensure worker safety is paramount.

"In addition, there will be a review into the method used to create future cross passages and surveys are now being undertaken of the ground and the two tunnels in the vicinity of this particular cross passage."