Both TBM drives for Perth airport link on hold 05 Apr 2018

Shani Wallis, TunnelTalk

Both TBMs working on the 8km long Forrestfield-Airport link in Perth, Western Australia, are on hold. The trailing TBM in the parallel drive of the new rail link was stopped on 28 March as it came within 40m of the lead TBM which has been on hold since 14 February to investigate what has been described by project authorities as two incidences of ground disturbance.

The lead TBM had entered the airport precinct and was passing through the Ascot formation of non-cohesive granular material when two incidences of detected ground disturbance caused excavation to be suspended. According to information from the project owner, the disturbances were reported by project management staff to the contractor and took the form of surface depressions of about a metre deep in the vicinity of the cutterhead.

Both machines are multi-mode variable density TBMs and were specified by the client for the project
Both machines are multi-mode variable density TBMs and were specified by the client for the project
Photo supplied by WA Government

Although confirmed as distant from critical infrastructure and of no risk to the public, the incidences were cause for the lead TBM operation to be stopped by the contractor and for the airport and project owner authorities to suspend further advance until the occurrences had been thoroughly investigated.

An independent TBM tunnelling expert has been engaged and specialist advisers from the TBM manufacturer are on site to review tunnelling procedures and ensure any lessons to be learned are incorporated into operations.

The trailing TBM had not experienced the same ground disturbance issues but was stopped in accordance with widely-accepted best practice that two TBMs should not overtake each other or work in parallel or be halted side-by-side.

Owner of the project, the Public Transport Authority (PTA), together with the contractor, the Salini Impregilio-NRW JV, and the Perth Airport operator, are working together to allow full tunnelling operations to resume as soon as possible.

The two TBMs on the project are supplied by Herrenknecht and are of the variable density design. TunnelTalk reported when the TBMs were ordered in early 2017 that variable density machines (VD TBMs) were specified for the project by the owner PTA after seeing the technology in operation through complex ground conditions on the running tunnels of the Kuala Lumpur metro project in Malaysia.

The lead TBM was stopped as it started its passage under Perth airport property from the east
The lead TBM was stopped as it started its passage under Perth airport property from the east

The VD multi-mode TBM system was developed in Kuala Lumpur to operate in various combinations of EPB and slurry excavation techniques. Fitted with both slurry systems and to mix and feed a higher density bentonite slurry through the circuit, as well as an EPB screw conveyor and the availability to use a continuous conveyor to haul material through the tunnel in the EPB mode, was developed to cope with complex and variable geological conditions experiences on TBM tunnelling projects beneath Kuala Lumpur.

The higher density bentonite slurry provides a more secure cake of benonite face-support and helps prevent slurry loss from the excavation chamber of the pressurised TBM operation, using the slurry pipeline circuit to convey excavated material back to the slurry separation plant and recycling units on the surface. The technology further allows for ready conversion to EPB mode in more consolidated ground, with the excavated material passing through the screw conveyor and onto the continuous conveyor for transport to the surface for onward disposal.

In Perth, the two machines are described as multi-mode TBMs with variable-density technology and are required to operate in the slurry mode for reaches through sedimentary sands and granular materials expected on the alignment under the airport and in EPB mode through the rocky geology under the Swan River. The two machines were launched from the Forrestfield working site in July and September 2017 to advance under the airport to an underground station under the new central airport terminal and up to 26m deep under the Swan River towards a breakthrough at the transition ramp at Bayswater Junction (Fig 2). Segments are transported into the TBMs via wheel-bound mulit-service vehicles.

As well as successful operation of the VDM technology in Kuala Lumpur, a Herrenknecht VD machine was recently used successfully for a shallow metro drive over the top of the previously Mixshield excavated parallel tunnel on the Shatin Central Metro Line project in Hong Kong.

The parallel TBM drives pass under the airport to a station beneath the new central terminal
The parallel TBM drives pass under the airport to a station beneath the new central terminal

The two TBMs in Perth "have been specifically designed for the Forrestfield project by Herrenknecht,” said Public Transport Authority spokesman David Hynes. “Perth presents challenging geological conditions for tunnelling, with some elements of uncertainty to be expected in adapting the TBMs to varying ground conditions. The non-cohesive granular material of the Ascot Formation, through which the TBMs were tunnelling when the disturbances occurred, makes tunnelling more complex. This is the first time tunnelling has occurred in the Ascot Formation and it was always known that this would present challenges.”

TBM tunnelling is expected to resume in the coming weeks once a number of investigations into the incidences are complete. In the meantime, work continues on construction of the project’s Airport Central underground station and on the transition portal structure at Bayswater Junction to join existing surface rail tracks into the city's central railway station.

The AUS$1.86 billion Forrestfield-Airport Link will provide a new rail service from the city centre to the new airport terminal and on to eastern suburbs of Perth with new stations at Redcliffe, Airport Central and Forrestfield (Fig 1). The AUS$1.176 billion lump sum design-build construction contract, which includes a ten year maintenance period from once the project is complete in 2020, was awarded to the Salini Impregilo-NRW JV in April 2016. TBM tunnelling was schedule to take two years and be complete with breakthroughs at the Bayswater Junction transition ramp in April and June 2019.

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