Two Herrenknecht variable density machines (VDMs) will be used by the Salini Impregilo/NRW joint venture for excavation of the 8km twin running tunnels of the Aust$1.86 billion Forrestfield Airport Link Project in Perth, Western Australia.

The pair of 7m diameter machines, procured at a cost of Aust$40 million, is selected for their capability in managing the variable ground conditions expected along the alignment. EPB mode will be used for excavation of the rocky geology under the Swan River while the slurry mode will be engaged for reaches through the sedimentary sands and materials expected under the airport itself.

A source close to the project confirmed to TunnelTalk that the choice of machine had been specified by the client, the Public Transport Authority of the Government of Western Australia.

The source said: “The client wanted a TBM that could work in both modes: EPB mode and slurry mode. The specification was also supported by the authority that operates the Perth international airport.”

Herrenknecht is currently manufacturing the VDMs at its facility in China, ahead of scheduled dismantling and transportation via sea to Fremantle port on the west coast of Australia in mid-2017.

Launch of the first machine, from a transition portal structure that is currently under construction at the Forrestfield end of the alignment (Fig 1), is scheduled for July 2017. The second machine will be launched two months later in September to minimise the impact of any ground movement and surface settlement. The drives are scheduled for completion in April and June 2019.

Construction of the diaphragm walls at the launch structure is underway and expected to be complete at the end of April. A slurry separation plant will be established on the job site along with a storage facility for some of the 60,000 concrete segments that will line the 6.2m i.d. running tunnels.

Early works also include preparation for excavation of the first of nine cross passages, three of which will incorporate emergency exit shafts to the surface. A program of jet grouting is underway at the Dundas Road site to establish the underground concrete block that the TBMs will tunnel through ahead of emergency shaft connection to the running tunnels.

Fig 2. Location of the new airport rail link station

In addition to a surface station at Forrestfield, which marks the terminus of the new 8.5km-long branch of the main Midland Line of the city’s commuter rail services, the new alignment incorporates two further underground stations. One under the airport adjacent to the international terminal building and immediately south of the control tower (Fig 2) and the other underground at Belmont.

The tunnels, which make up 94% of the new branch line, run to their maximum depth of 26m below the surface as they pass under the Swan River. Average depth of the alignment is 15m.

The project is fully funded by the Western Australia State Government. A single design-construct-maintain contract was awarded to Salini Impregilo/NRW in April of 2016 and for a contract sum of Aust$1.176 billion. The contract includes the first ten years of maintenance of the new infrastructure once it is completed in 2020.

The new link will provide an alternative to the current 45-minute journey by road between the airport and the city centre.

VDMs are a relatively new type of machine developed by Herrenknecht and MMC Gamuda, the JV contractor selected for the now-completed tunnelling section of Phase I of the Klang Valley MRT project in Kuala Lumpur, Malaysia. The variable density concept and technology was designed, originally, to deal with the challenges of mechanised tunnelling through karstic limestone. The machines, of which six were procured for Klang Valley MRT Phase I, can work in EPB mode and in a modified slurry mode with the screw conveyor in place to extract the material from the excavation chamber when using either operating mode.

The concept was applied most recently for a demanding metro drive for the new Shatin-Central MRT metro tunnel drive beneath shallow cover, under main roadways on the surface and through mixed geology of made-ground, fill-material, sedimentary deposits and decomposed granite, all beneath a high watertable close to Victoria Harbour on Hong Kong Island.

The Salini-Impregilo/NRW joint venture is selected as preferred bidder for $2 billion construction of the Forrestfield-Airport link in Perth, Australia.

Alignment of the 8.5km Perth/Forrestfield Airport Link

Following a 15-week period spent evaluating the proposals of three shortlisted JV teams, the Public Transport Authority of Western Australia (PTA), as project owner, will now enter into final negotiations with Salini-Impregilo/NRW.

“The final value of the contract is to be determined as part of these discussions and further information about the construction timeline and tunnel boring requirements will also be announced then,” David Hynes, for the PTA, told TunnelTalk.

He added: “In considering the selection of a preferred bidder, the PTA is required to take into account a range of factors, not just cost.”

The unsuccessful bidders were the John Holland/Leighton JV, and the Acciona/BAM/Ferrovial Agroman JV. Salini-Impregilo, which was involved in construction of the STEP project in Abu Dhabi, and in December 2014 completed a hard-fought and technically demanding TBM drive under Lake Mead in the USA, has been successful in securing a number of large underground construction contracts throughout Europe and the USA.

Until now, however, it has struggled to break into an Australian tunnelling market that has been extremely buoyant in recent years and for which there are a number of major projects still in the pipeline, including the mega-TBM WestConnex highway tunnel in Sydney and a 15km rail link that will include an undercrossing of Sydney Harbour.

Contract scope of the Perth Airport–Forrestfield Link includes construction of a junction between the new Airport Link and the existing Midland Line near Bayswater; a new tunnel portal to take the line underground; 8km of segmentally lined twin running tunnels of 6.2m i.d.; nine cross passages – three of them with exit shafts to ground level, and all of them to be constructed with a cast iron lining using the freezing method for ground stabilisation; and construction of two underground stations and a third one at grade. The alignment will require an undercrossing of the Swan River, and tunnelling underneath the busy Perth Airport.

Construction is expected to start later this year, with EPB or slurry machines likely to be specified for a geology that comprises soft calcerous sands and the calcernite cobbles of the Ascot Formation. The Forrestfield–Perth Airport link is fully funded by the Government of Western Australia.

The Government of Western Australia confirms it is on target to award the Aust$2 billion state-funded design-build Forrestfield–Perth Airport Link in the second quarter of this year (2016).

Thousands of pages of bid documentation

Five construction joint ventures expressed interest in construction of a new 8.5km link between Perth’s eastern suburbs, via the airport and under Swan River, for connection at Bayswater with the existing Midland Line into the city centre. Three have now submitted detailed bids. They are:

• Forrestfield Connect (Acciona/BAM/Ferrovial Agroman)
• JHL JV (John Holland/Leighton)
• SI-NRW JV (Salini Impregilo/NRW)

Connecting Forrestfield, a JV of Lend Lease and Ghella, and the CRCC-BGC-VDM JV, a joint venture comprising China Railway Construction Corporation, BGC and VDM, expressed interest but were not shortlisted to bid.

A project spokeswoman told TunnelTalk that at this stage it is not known how many TBMs will be required to complete the job, but that slurry or EPBM machines will be required to negotiate a geology that comprises the soft calcerous sands and calcernite cobbles of the Ascot Formation. More than 400 boreholes have been drilled along the alignment, and a barge drilling program under Swan River has been carried out, ahead of the bidding process.

Construction is expected to get under way in the second half of 2016, with rail services scheduled to start operation in 2020. All environmental and government permissions have been secured.

Advancement of Perth’s Airport Link Project coincides with a current boom in underground construction in Australia. In Sydney, TBM progress continues on the 23km-long North West Rail Link (NWRL), 15km of which is in twin running tunnels; planning continues on a 29km extension from the southeastern terminus of the NWRL that will incorporate 12.5km of twin running tunnels and a new undercrossing of Sydney Harbour; excavation contracts are already awarded for the first two phases of Sydney’s WestConnex highway project, both of which require a significant element of underground construction; and roadheader excavation of the city’s 9km NorthConnex highway tunnels is under way. In Melbourne, which cancelled the East-West Link highway mega-project in controversial circumstances just weeks after awarding the Aust$5.3 billion public-private partnership contract, planning continues on the preferred alternative: the 9km Melbourne Metro Rail Project.