New procedures get Perth TBMs restarted 19 Apr 2018

Shani Wallis, TunnelTalk

Changes to the excavation chamber pressures, continuous no weekend stoppage operation, and adjustment of the bentonite slurry to reduce losses through loose granular ground conditions are the principle changes applied to get the TBM drives for the Forrestfield-Airport rail link in Perth resumed.

New operating practices have allowed excavation to resume
New operating practices have allowed excavation to resume
From the project’s media relations department

The changes are implemented following a thorough investigation of two incidences of ground disturbance that caused depressions on the surface of up to 1m above the cutterhead which brought excavation by the lead TBM in the twin tube alignment under the airport property to a halt on 14 February and the second, at a safe distance of 40m behind, to a stop on 28 March.

The lead TBM was working through the loose granular soils of the Ascot formation when the incidences of ground disturbance were reported over the cutterhead of the lead TBM. Before proceeding further under the airport towards the project’s station under the new central terminal of Perth international airport specialist advisers from the TBM manufacturer and other TBM experts came together to review the operation of the multi-mode slurry-EPB variable density TBMs, to work with the contractor, the owner of the project and the operators of the airport and to recommend improvements to avoid similar ground movement situations for the continuation of the drives under the airport.

Contractor Salini Impregilo-NRW JV procured two 7m diameter multi-mode variable density slurry-EPB TBMs from Herrenkencht for the 8km twin tube rail tunnel drives to extend the rail system from the city centre to the new airport terminal and on to the eastern suburbs of Perth. The TBMs were designed specifically to operate in ground conditions that were predicted to be challenging for TBM excavation. The slurry circulation based TBMs are capable of operating with variable densities of bentonite slurry as well as operating in modified EPB mode with material excavated via a screw conveyor and passed through a rotatory crusher and into slurrifier box for transport out of the tunnel in the slurry suspension circulation system to the recycling treatment plant on the surface.

The TBMs are completing 8km long drives to connect Perth city centre with the new airport terminal and the eastern suburbs of the city
The TBMs are completing 8km long drives to connect Perth city centre with the new airport terminal and the eastern suburbs of the city

The implemented changed in operation of the TBMs are listed as:

  • Changes to the pressure in the excavation chamber when excavating in the Ascot formation.
  • Continuous operation (no weekend stoppages) especially when tunnelling beneath airport infrastructure.
  • Slurry and bentonite quality to be adjusted to reduce potential slurry losses
Segments back to being built into the drives
Segments back to being built into the drives

Following restart in mid-April of the lead TBM, it has advanced to a total of 1,034 x 1.6m long rings installed to the 6.770m o.d.. The trailing TBM is to restart operations prior to the end of April and is currently 1,590m into its 8km long drive. Works at the Airport Central Station are progressing to prepare for arrival of the TBMs in May where they will breakthrough and undergo several weeks for maintenance before being walked across the station box to continue their journeys to the Radcliffe Station, under the Swan River and through to final breakthrough at the Bayswater Junction transition portal (Fig 1).

Works have also begun to connect the first of three emergency egress shafts to the tunnels. At the first of these at Abernethy Road, a 2.1m circular saw is being used to cut through diaphragm-walls ahead of excavation of the cross passage. At the shaft at Wright Crescent jet grouting is continuing to stabilise the ground ahead of cross passage excavation. For the Airport West egress shaft, excavation is progressing to its required 34m depth.

The AUS$1.176 billion lump sum design-build construction contract with the Salini Impregilo-NRW JV for the AUS$1.86 billion Forrestfield-Airport Link project was awarded in April 2016 and includes a ten year maintenance period from once the project is inaugurated, which is scheduled for 2020. TBM tunnelling is scheduled to take two years and be complete with breakthroughs at the Bayswater Junction transition ramp in April and June 2019.

References

Both TBMs remain on hold in Perth 12 Apr 2018

Shani Wallis, TunnelTalk

Both TBMs working on the 8km long Forrestfield-Airport link in Perth, Western Australia remain on hold while investigation into two incidences of ground disturbance that created a 1m depression on the surface above the cutterhead of the leading TBM continue.

Fig 1. Details of the mulit-mode TBMs procured by the JV for the project
Fig 1. Details of the mulit-mode TBMs procured by the JV for the project
From the project’s media relations department

News from Perth from the client, is that both machines are under Perth Airport Estate property and came to a stop at 1,630m and 1,590m into their respective drives on 14 February and 28 March and after being launched by the Salini Impregilo-NRW JV in July and September 2017 respectively.

The TBMs were procured by the contracting JV from Herrenknecht and are described as multi-mode Mixshields capable of operating in EPB and slurry mode with material from the excavation chamber being delivered by the screw conveyor into a rotary crusher for break down into a mixture for transfer from the slurry box via the slurry transport circuit to the slurry treatment plant on the surface (Fig 1).

Both machines are confirmed as being launched from Forrestfield in the slurry mode and operating in the slurry mode for passing through the non-cohesive sand and gravel materials of the Ascot Formation funder Perth Airport. The EPM mode will be used through for clay and alluvial muds that are to be encountered as the drives pass under the Swan Review ahead of final breakthroughs at the Bayswater Junction transition ramp where services will join surface tracks for the link to Perth’s central railway station.

An independent TBM tunnelling expert and specialist advisers from Herrenknecht are working with the JV and the client’s project team to review tunnelling procedures and incorporate revised TBM operations to ensure secure progress through the complex geological conditions under the airport. The Ascot Formation was predicted during the project development stages as presenting challenging conditions.

The trailing TBM experienced no ground disturbance issues and dates for resuming advance is still as yet unknown after reports last week that it is anticipated within the coming weeks.

References

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.

References

           

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