Perth Airport Link TBMs intermediate break through

1 June 2018

Australia - The two TBMs boring the Forrestfield-Airport Link in Perth have broken through to the Airport Central Station. Salini Impregilo-NRW was chosen to build the 8.5km line, of which 8km is underground, with a 7m diameter. So far, some 2km have been bored and the TBMs will undergo servicing for a month before continuing.

The successful breakthrough a quarter of the way along the route follows TBM stoppage earlier this year. Ground disturbance was detected prior to the TBM entering an area of ‘critical airport infrastructure’ so the TBMs halted as a temporary safeguard. At the time, a spokesperson for the project said, “Perth presents challenging geological conditions for tunnelling, with some elements of uncertainty to be expected in adapting the TBMs to varying ground conditions. The Ascot Formation, through which the TBMs were tunnelling when disturbance occurred, comprises non-cohesive granular material that makes tunnelling more complex.”

To deal with the situation, monitoring was stepped up.

The TBMs are Herrenknecht Variable Density machines capable of operating in EPB or slurry mode. The lining is a precast concrete segmental ring, in a 5+key configuration with a length of 1,600mm. The concrete is class f’c = 60 MPa, reinforced with 40 kg/m3 of steel fibres and a frame of steel bars.

The tunnel runs through an alluvial plain and ranges in depth from 7 to 25m (at present it is 17m below the airport). It will pass through clay, sand and weak rock. Towards the end of the bore there is an estuary, which prior to tunnelling the contractor said was the most critical point, besides the airport which is built on sandstone.

The project began in mid-2017 and is due to last until 2020. The EUR 791.7M (USD 854M) rail link will connect with the existing Midland line near Bayswater Station and will run to Forrestfield through underground tunnels, to ensure minimal impact on the existing land and road network.