A relatively short but critical drive in Hong Kong for the expansion of its metro network has marked another milestone for the variable density (VD) TBM technology developed by Herrenknecht. A 7.41m diameter VD machine was used successfully by the Dragages-Bouygues JV to excavate one of two tunnels on part of the southern section of the Shatin-Central Link (SCL) project, boring 680m through complex coastal geology and below busy roads on Hong Kong Island.
Launched in mid-August 2016 from the works site at Causeway Bay Typhoon Shelter zone, the VD TBM successfully completed its delicate drive for the down-track running tunnel at the end of November to reach the site of the future Exhibition station at the southern end of the project alignment (Fig 1).
“The VD TBM outperformed our expectations,” said Thomas Barrett, Construction Manager for metro owner MTR Corporation. He told TunnelTalk the machine built “almost 400 x 6.5m i.d. rings of lining, comprising six segments and a key, during a period of about 100 days.”
On its journey, the VDM passed above the up-track running tunnel that had been built only a few months earlier by a 7.41m diameter Herrenknecht slurry Mixshield. That tunnel drive started in March 2016 and was completed in July 2016, explained Barrett.
The Mixshield drive was at sufficient depth between the Causeway Bay Typhoon Shelter site and Exhibition Station to pass through mainly rock with some decomposed granite. The second drive however, presented a number of sensitive challenges in terms of both geology and boundary conditions, calling for a different solution to re-using the Mixshield, said Barrett.
The second drive involved excavating close to and over the first tunnel, in a stacked arrangement and close to the ground surface below an area of busy traffic. Adding to the shallower alignment, the geology along the short reach comprised a complex mix of materials including land reclamation fill, marine sediments, some rock, with decomposed granite, and some alluvium deposits. The reach of new metro tunnelling also runs under the alignment of the new Central-Wan Chai bypass express roadway which is built underground in cut-and-cover excavations and a length of complex rock tunnelling at the point where the projects cross each other underground.
With a need for careful negotiation through the mixed ground and at less than 6m beneath busy roads, the JV opted for a VD machine even though it would drive only this single, relatively short drive.
“The variable density TBM bears the merit of changing its boring modes to adapt to different ground conditions along a complex tunnel alignment,” said Barrett. “Slurry mode was used on the drive for the initial stages, then high density/EPB mode for the marine deposits. As a result, the VD concept reduced the risk of leakage of foam or slurry to the surface roads in the busy Causeway Bay area.”
The VD system is designed and described by Herrenknecht as an all-rounder TBM for loose soils of all kinds, offering the capability to vary the density of the EPB foam conditioning or slurry support medium depending on ground conditions, and do so without major equipment modifications.
Initial use of the VD technology was in Malaysia, on the Klang Valley MRT project in Kuala Lumpur. The tunnel alignment passed through karstic limestone where the rockhead was variable and within which high groundwater levels could change quickly.
The new Shatin-Central metro link is a 17km long, twin-track addition to the Hong Kong metro network. The project has 10 underground stations, six of which are interchange stations with other MTR lines, and a new immersed tube metro crossing of the harbour (Fig 1).
To the north, the 11km long Hung Hom to Tai Wai Station section extends the existing Ma On Shah Line to the west and helps alleviate metro travel on the East West Corridor. Main construction on the section began in 2012, and, in September 2016, MTR announced final tunnel breakthrough on the section’s running tunnels towards overall scheduled finish in 2019.
To the south, the 6km long Hung Hom to Admiralty section extends the existing East Rail Line onto Hong Kong Island and assists metro transport on the North South Corridor. In September 2016, construction was about 40% complete and on schedule for commissioning in 2021.
Station construction works on Link have had some challenges. In mid-November 2016, MTR re-iterated its position as set out in half-yearly results issued in August, that project costs are likely to rise due to a number of factors, including:
Across the project, a variety of construction methods have been employed, including open excavation, cut-and-cover, open-face sequential excavaation, drill+blast and TBMs.
At the end of 2016, with the demanding VD TBM drive complete, two final drives remained to excavate - the twin tunnels between the Fenwick Pier works site and Admiralty Station. The Dragages-Bouygues JV is re-deploying the Mixshield for both drives. Reassembly of the TBM is nearing completion for relaunch this month (March 2017), said Barrett. Geology along the route is mainly rock, with also decomposed granite to be encountered, and cover to the tunnel will be at least one TBM diameter. The TBM is expected to finish the two drives by early 2018.