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Team tunnel

A close working relationship between operations teams and construction teams has helped MTR connect its new West Island Line with existing Island Line running tunnels.

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Cramped: The access shaft at Sai Woo lane

Work to build MTR’s £1.3bn West Island Line involved the first ever weekend closure of part of the Hong Kong mass transit railway. It has also involved huge amount of cooperation between MTR’s Projects and Operations divisions.

The unprecedented 54-hour closure of Sheung Wan Station in August 2011 was necessary for contractors to splice a new running tunnel with the existing overrun tunnel just beyond Sheung Wan station at the eastern end of the West Island Line.

When it opens in 2014 the West Island Line will extend MTR’s existing Island Line which runs along the northern edge of Hong Kong Island. At its eastern end West Island Line twin bore tunnels will join onto existing overrun tunnels located beyond the current terminus at Sheung Wan Station.

To enable this to happen, a 132m section of one of the overrun tunnels, where Island Line trains currently turn around and where broken down trains are temporarily stored, had to be cut through by the tunnel boring machine. But to do so the contractors had to remove the tunnel lining before the tunnel boring machine arrived for fear of cutter head damage from the reinforced precast concrete and cast iron linings.

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“We have weekly meetings with the operations team to plan works train deliveries”

Brendan Reilly, MTR West Island Line project manager

Track in the other bore was diverted into a new section of tunnel constructed by the West Island Line project, creating a new, temporary siding. These works were performed in advance of the tunnel dismantling operation which commenced in summer 2012 (see box).

The new siding had to be fully equipped with track, a set of points, signalling and overhead power lines before the southern bore could be closed off. This is why the line between Sheung Wan station and the Admiralty station of the Island Line was closed for 54 hours from 11.30pm on 5 August 2011.

The 54-hour weekend closure required more than four months of intense planning and coordination between the construction teams and the Island Line operators. Government approval for the shutdown had to be obtained as did approval from the Fire Services Department and various other statutory bodies. Meticulous planning was vital to ensure that the Island Line could be brought back into full operation at the end of the closure on 8 August 2011. At the same time, the public also had to be kept in the loop with information about bus relief services and walking routes to Admiralty, which was to become a temporary terminus during the closure.

To ease passenger congestion at Admiralty, where the Island Line interchanges with the Tsuen Wan line, Tsuen Wan line services were also stepped up. An army of MTR staff was also on hand at Sheung Wan, Central and Admiralty stations to direct commuters to buses and walking routes during the shutdown. The operation was a success and the Island Line resumed service on time.

More coordination between operators and construction teams will be needed for the delivery of materials and equipment to site as work on the West Island Line moves into the fit-out phase. This will be done using works trains running from the Island Line’s Chai Wan depot near its eastern terminus to the new station at Kennedy Town. “We have weekly meetings with the operations team to plan works train deliveries,” says MTR’s West Island Line project manager Brendan Reilly.

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Nearing completion: Tunnel lining work is well underway

Removable bulkheads have been fitted in the tunnels just west of Sheung Wan Station to separate the construction works from the operational tunnels. Permission to install these bulkheads requires a wide range of approvals, so that protection is provided from smoke/ fire and inundation from the construction worksites.

“We also have to ensure the works trains are fully operational as they can’t break down on the Island Line,” says MTR West Island Line and South Island Line electrical and mechanical project manager Dono Tong.

Deliveries are tightly scheduled as the works trains can only run at night when Island Line passenger services are finished and there is limited time for loading and unloading.

All this work has been going on as the main tunnelling and excavation phase of the project has almost come to an end. The West Island Line is now over 60% complete. The line is a mix of drill and blast and bored tunnels with the Hong Kong University station cavern 22.4m wide 16m high and 260m long. There is a section of soft ground bored tunnel at the eastern end near Sheung Wan station, excavated using a slurry tunnel boring machine and at the western end cut and cover methods have been used to construct the new Kennedy Town station.

Underground work is now focused on concrete lining of the running tunnels and station caverns, using rail-mountes mobile shutters in the tunnels and reusable formwork in the station caverns. Precast linings for the tunnels at the eastern end are already complete, installed by the tunnel boring machines.

Deep shafts have also been constructed at Hong Kong University Station to house lifts which will take passengers from station entrances down to concourse level.

“At Hong Kong University station we are at the stage where we have completed the excavation of the shafts,” says Reilly.

The cut and cover box for the Kennedy Town station terminus is now complete as is the overrun tunnel, which is currently being used to store two works trains and the rails which are ready to be installed.

Some drill and blast work remains, although this is confined to underground corridors to give passengers access to the platforms.

A journey around the project reveals shafts for underground works and station access points in often unlikely locations. Near Sai Ying Pun station, site traffic must negotiate a narrow alleyway between two dried seafood shops while crowds of pedestrians mill about. On the foreshore, another temporary access shaft is located at the end of the site of a new swimming pool, built to replace one which had to be demolished to make way for the Kennedy Town station site.

Great efforts were made to minimise the impacts of the project on the surrounding community. Vast amounts of spoil had to be removed from the excavations, as much as 70m below ground, and to keep trucks off the narrow, congested roads. Most material was crushed underground and removed by conveyor via a series of adits to a barging point on the seafront.

“At Hong Kong University station around 140,000m3 of rock was excavated for the caverns and adits. This couldn’t have been put onto trucks,” says MTR’s West Island Line civil construction manager Steve Hamill.

Splicing the tunnels

The southern tunnel bore of the West Island Line is designed to merge gradually with the existing Island Line bore.

Splicing them together was a complex task, as it involved dismantling 132m of the reinforced concrete tunnel lining and a section of cast iron lining, of the existing tunnel. Contractor Dragages Maeda Bachy brought in a purpose built tunnel dismantling machine (TDM) to remove the lining. The machine, manufactured in France by specialist company CSM Bessac is similar to a tunnel boring machine except that it is equipped with a powerful claw arm which levers out the lining segments as it works its way along the tunnel. These segments are then passed, one by one, through a chamber in the front of the machine which conveys the segments back through the machine where they are then removed. Grease-filled wire brush seals are mounted on the external body of the machine, pressing against the internals of the tunnel linings, maintaining the air pressure within the TDM excavation chamber in front.

Work was complicated by the fact that the tunnel is in soft, unstable ground, so the front of the machine during excavation was maintained under compressed air at a pressure of 2.8 bar. The surrounding ground was grouted and the existing tunnel linings back-grouted to improve the ground through which the TDM was to pass.

The tunnel wall was shotcreted after each ring of segments was removed, after which the complete tunnel section was filled with shotcrete, of sufficient strength to support the ground but also able to be cut through by the future slurry tunnel boring machine. “Controlling the permeability of the ground was a major risk mitigation measure, limiting settlement during pressurization and depressurization of the excavation chamber. Existing tramcar lines and multiple utilities are very close to the tunnel alignment” says MTR’s West Island Line project manager Brendan Reilly. The machine is restrained by hydraulic jacks, which react against steel thrust frames erected in the tunnel. The frames and jacks resist the backwards movement of the TDM resulting from the pressurized excavation chamber.

To reduce the learning curve on site, CSM Bessac set up a 15m section of mock-up tunnel at its factory in Toulouse, France to replicate the difficulties, and so understand the constraints, for building the TDM underground in the 5300 mm id tunnel. It also sent its own team of operators to erect and work the machine in Hong Kong.

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