Tunnel Boring Machine begins digging under Perth city

25/10/05

Excavation of the first of two 770m tunnels under Perth city for the New MetroRail project has started.

Planning and Infrastructure Minister Alannah MacTiernan announced today that the Tunnel Boring Machine (TBM) had begun its first pass under the city.

"Tunneling will operate 24 hours a day and will progress at an average of 10m per day before the TBM emerges in the new William Street Station box and then continues on to Roe Street," Ms MacTiernan said.

"Once the first tunnel is completed the TBM will be dismantled and reassembled for its second pass under the city next year, also starting from the southern end of the city project where the new Esplanade Railway Station will be situated.

"This is a very complex and demanding part of the project with about 100,000 cubic metres of earth to be removed.

"A special concrete grouting program has been under way to underpin city buildings above the tunnel, prior to this operation.

"The TBM is digging through soft soil such as sand and clay so large concrete rings that will line the tunnel are being installed as it progresses.

"The TBM literally leaves a completed concrete tunnel in its wake."

The 60m-long TBM was specifically made for this project by Mitsubishi Heavy Industries in Japan.

The tunneling program was slightly delayed by technical issues associated with the commissioning of the TBM.

These included the installation of jacking rings, required to position the TBM before tunneling could commence, and installation and calibration of the conveyor belt that will remove excavated soil.

"The TBM operates by applying pressure to the excavation face ahead of the machine while soil is removed at a carefully controlled rate," Ms MacTiernan said.

"This balance between pressure at the face and the rate at which soil is removed allows ground settlement - and hence the impact on buildings above - to be minimised."