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Caverns and congestion: Kwun Tong Line Extension

In a vast excavation site at Ho Man Tin a major new interchange between two of MTR’s newest subway lines is taking shape.

The site is part of the Kwun Tong Line Extension which will take the Kwun Tong Line south of its terminus at Yau Ma Tei to a new one at Whampoa.

Ho Man Tin is the station site at which the Kwun Tong Line Extension will intersect with the massive Shatin to Central Link, and at the moment contractors are in the final phases of blasting out the cruciform shape station into which the intersecting platform tunnels will fit.

The site has been excavated into a steep hillside, through which the almost complete drill and blast running tunnels for the Kwun Tong Line Extension from Yau Ma Tei will emerge. The whole of the side of the hill has beenremoved to facilitate excavation of the station caverns and stabilised using soil nails and mesh.

The Ho Man Tin excavation is 93% complete with over 640,000m3 out of 690,000m3 of material removed

Last month contractor Nishimatsu had excavated the station down to platform track level for the Shatin to Central Link platforms and was casting the track slab at the
northern ends.

The Ho Man Tin site is divided into three sections, with two shallower Shatin to Central Link platforms to the north and south of the deeper excavation where the central portions of the Kwun Tong Line Extension platforms will merge from blasted rock caverns at right angles in the east and west directions.

At present, excavation of the central section is still underway. When complete a slab will be cast across it to support the mid-sections of the Shatin to Central Link platforms.

“The Ho Man Tin excavation is 93% complete with over 640,000m3 out of 690,000m3 of material removed,” says MTR Kwun Tong Line Extension general manager James Chow.

Over the last year, blasting work has gone from surface operation to deep cut below ground level. The site is now draped with a huge protection net to prevent blast debris from accidentally flying out of the site. Noise caused by blasting is less noticeable now that excavation work is so far below surface level.

“We are down to 20m below ground, so noise from blasting is not such an issue with local residents,” says Chow. There is still another 20m of drill and blast excavation to go before Kwun Tong Line Extension track level is reached.

Tunnelling progress

Meanwhile tunnelling from either end of the extension is approaching the interchange site. From the east, drill and blast work has halted 3m from the breakthrough point, as the contractor waits for the station excavation work to finish.

On the other side of the station, Nishimatsu is carrying out drill and blast work on two fronts, working from an access shaft at Wylie Road. The tunnel towards Ho Man Tin is 45% complete, with another 360m to go before breaking through at the interchange site.

Heading the other way, the contractor is 40m away from breaking through into two enlarged cable tunnels which will form the connection to the existing live tunnel south of Yau Ma Tei Station. This is expected to be complete in time for the start of track laying work this summer.

“We are now about 60% complete in terms of drill and blast tunnelling,” says Chow.

Among the biggest challenges at the moment is the work to build the Whampoa terminus station, which is now being constructed under the main road serving the densely populated Whampoa district of Kowloon, which is also lined with shops.

Progress of the work on the street is painstakingly slow, as MTR must maintain the same number of traffic lanes open while working within touching distance of neighbouring tall buildings and minimising disruption to pedestrians

The station is split into two underground concourses, which will be built within a cofferdam of 610mm diameter pipe piles driven into soft reclamation fill within an old dock yard site.

Although the running tunnels are in rock, the station concourses are being constructed in unpredictable reclaimed land complete with rubble voids and uncharted obstacles. At some locations, the concourse sites are in rock, but drill and blast cannot be used in the excavation because it is too close to the surface and the busy, densely built up street above.

In addition, and as is often the case in urban areas, uncharted utilities have been discovered. “Some of these utilities have been there for more than 40 years and are in a poor state. We have to replace them before we can support them insitu,” says Chow.

Progress of the work on the street is painstakingly slow, as MTR must maintain the same number of traffic lanes open while working within touching distance of neighbouring tall buildings and minimising disruption to pedestrians moving around what is a busy shopping area.

Pipe piling takes place first on one side of the road and then on the other, with temporary traffic management arrangements altered accordingly. “For the western concourse, 95% of the pipe piling is complete,” says Chow. Excavation work is now underway, beneath a temporary deck which carries traffic above the concourse site.

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