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Crossing lines

Building an underground junction between new and existing track is one of the trickiest civil engineering challenges of the whole Tseung Kwan O project

The busy eastern end of the Eastern Harbour road and rail tunnel is the focus of one of the most critical parts of the Tseung Kwan O extension. It is here that Gammon China Harbour joint venture has to connect to an existing section of the MTR system, punching through the walls of a cut and cover Eastern Harbour Crossing approach tunnel used by the Kwun Tong line.

The Tseung Kwan O extension will take over this part of the track. Trains will travel from Quarry Bay on Hong Kong Island through the Eastern Harbour Crossing and onto the new Tseung Kwan O line.

At the start of the Tseung Kwan O line, trains will branch off the approach tunnel and run through a new cut and cover tunnel to the station at Yau Tong.

Kwun Tong line trains will be rerouted via an extension from Lam Tin to two level interchange stations at Yau Tong and Tiu Keng Leng where passengers can switch to the Tseung Kwan O line.

Preparations for the connection are under way. Before Christmas, Gammon China Harbour was removing backfill covering 87m of the approach box. With this removed, it will start work on a new side wall, following the alignment of the diversion. The new retaining wall will support a new roof slab to be cast on top of the existing box.

Constructing this junction presents some of toughest technical challenges.

Disruption to existing Kwun Tong line services has to be kept to a minimum and road traffic must also be allowed to flow freely through the Eastern Harbour Crossing.

As a result the design minimises demolition work inside the existing tunnel, restricting it to the removal of sections of outer and central wall. Roof slab above the areas of wall facing demolition will be retained, and tied into the new roof slab using hook bars.

The new roof slab will in turn be supported by dowels drilled into the retaining wall which lines the south side of the tunnel approach road. The contractor will erect bulkheads inside the diversion box to prevent disruption Kwun Tong line services.

Although as much of work as possible will be done around the outside of the Kwun Tong line tunnel, some night possession will be necessary. The contractor needs to insert 66 ground anchors into the tunnel floor to prepare for uplift forces caused by demolition of the walls.

Gammon China Harbour's contract also includes the 700m drill and blast tunnel for the extension of the Kwun Tong line from Lam Tim to Yau Tong.

This passes under the live motorway approaching the Eastern Harbour Crossing. Ahead of this, the contractor is casting a concrete slab above the line of the tunnel to protect the road from settlement.

At Yau Tong, the same contractor is also demolishing areas of steep-sided hill to create pads for 18, 50-storey blocks of flats, for the Housing Authority and to form a new road for the Highways Department. It is also blasting a site for the cut and cover, two level Yau Tong interchange out of the hillside. A covered enclosure ensures flying rock does not injure residents or damage their homes.

The area around the huge 15ha site is highly congested. Blasting work for Yau Tong station is under way immediately downhill of high rise blocks.

Below the station site, a local road has to be protected from flying rocks by mesh supported on a giant steel frame.

Elsewhere on site, huge steel framed cages and mesh frames dot the landscape, ready to be placed around blasting areas.

In all, Gammon China Harbour will remove 2M. m 3of material from site, mainly by barge, using a docking point in the harbour adjoining the site.

When the joint venture has finished site preparation at Yau Tong station, contractor Kumagai Gumi will move in.

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