One of the most challenging parts of MTR’s major projects programme is taking place at the heart of Hong Kong’s busy financial district, as part of the HK$12.4bn (£1bn) South Island Line (East) project.
The underground station at Admiralty is already a busy interchange between the Island Line and Tsuen Wan Line. Two more lines are to be fed into it by the end of the decade, with new platforms to be excavated below the level of the existing ones. The new extension of the station is expected to be opened for use upon completion of the SIL(E).
“The Admiralty Station site is surrounded by underground structures on all sides”
Mark Cuzner, MTR South Island Line project manager
When the work is complete, Admiralty will be Hong Kong’s first four line interchange, housing platforms for the South Island Line (East) terminus and the Shatin to Central Link. The South Island Line (East) terminus is to be excavated as a single cavern housing two platforms, while the Shatin to Central Link platforms will be in two parallel tunnels along side the South Island Line (East) cavern with both lines being excavated as part of the South Island Line (East) project. The integrated nature of the new ADM Station and the deep excavation required beneath the existing railway tunnels necessitated the construction of the fully extended South Island Line (East) and Shatin to Central Link ADM Station in one single phase. The new integrated ADM Station is being constructed under the South island Line (East) project as this is due for completion in 2015 which is earlier than the planned opening of the Shatin to Central Link North-South Line in 2020 (see feature p12).
The new cavern and platform tunnels will extend to the south of the existing station, but their northern ends will be excavated beneath the Tsuen Wan Line and Island Line platforms which must be kept in operation throughout the construction period.
Contractor Kier-Laing O’Rourke- Kaden is the main contractor for the civil works which are being carried out from a tightly constrained 80m by 80m site just to the east of the existing station. Drill and blast methods are being used for the rock excavation of the caverns and tunnels which are being built via a small access shaft located within the site at Harcourt Garden.
The work within Harcourt Garden also involves building the main body of the new station using a top down method which will allow the permanent structural slabs to strut the existing Island Line and Tsuen Wan Line either side of the excavation. Above there will be a new entrance which will be incorporated into a new landscaped garden located directly above the new concourses with connections to the existing footbridge system, allowing easy access from the station to buildings nearby.
The work is extremely challenging, given the location of the site, the 40m depth of the excavation, and the scale of blasting works to be carried out close to the operational railway tunnels. There are some similarities to the work carried out to expand London’s Tottenham Court Road Underground station to accommodate Crossrail, in the shadow of the Centrepoint office block.
“The site is surrounded by underground structures on all sides,” says South Island Line project manager Mark Cuzner. There are also existing ventilation shafts for the operating railway within the site which have to remain in use throughout the construction period and will eventually be adapted and incorporated into the new station structure. There are also two major dual carriageways on each side of the site restricting access and egress and a high rise building that houses the Hong Kong Police Headquarters overlooking the site.
At the western end there is also one of Hong Kong’s premier shopping centres and a busy pedestrian footbridge which overlooks the site. The project also faces intense scrutiny from Hong Kong’s political elite, as Admiralty station serves the new Legislative Council building, a stone’s thrown away. “There is a lot of political and public interest,” says Cuzner.
And then there is the site’s proximity to the Victoria Harbour. “There is a lot of monitoring, we are already excavating below the water table,” says Admiralty Station construction manager Alan Boden.
Before the contractor can get down to excavating the station tunnels, it has to underpin the operating Tsuen Wan Line and Island Line tunnels, which converge about 20m below ground. Beams and columns will be inserted beneath the existing insitu concrete box structures for the Tsuen Wan Line and Island Line to carry the load away from the new station tunnels. Close work with MTR’s railway protection team is vital to ensure that work can proceed without affecting the rail service above.
Recognising the challenges of the project, MTR is using a target cost contract rather than tying the contractor down to a fixed price. “We are using target cost due to the complexity of the work, the high level of risk and the amount of method related work,” says Cuzner. “Target cost requires more resources from us during tendering and construction. However, the contractor is getting paid his costs so there is more emphasis on risk mitigation and value engineering,” says Cuzner.
“Every stakeholder is different. We have had very detailed discussions with all the stakeholders and spent time to explain how we are going to carry out the work”
Mark Cuzner, MTR South Island Line project manager
But while the Admiralty station extension is extremely sensitive and complex, it is only part of the South Island Line (East). When complete in 2015, the line will carry a three car train service into Admiralty from the densely populated Ap Lei Chau (an island near Aberdeen) via Wong Chuk Hang, on the southern side of Hong Kong Island, relieving the overloaded commuter roads.
From Admiralty, the line runs through the approximately 3.2km drill and blast Nam Fung tunnel and then onto an approximately 2km viaduct, which is now taking shape south of the mountains overlooking Aberdeen. The viaduct is being constructed by Australian contractor Leighton as a series of balanced cantilevers. Most are made from precast segments but at one point, where it crosses a highway intersection it runs on 21m long precast beams. This elevated section incorporates stations at the Ocean Park theme park and at Wong Chuk Hang as it snakes past Aberdeen. Local contractor Hsin Chong Construction Company Limited is building the depot for the South Island Line (East) trains next to Wong Chuk Hang station.
The viaduct crosses the Aberdeen Channel on a 250m long cast insitu prestressed concrete balanced cantilever bridge which mirrors the profile of the adjacent road bridge. From there, a short section of cut and cover tunnel leads into a 1km section of drill and blast rock tunnel which includes a cavern station at Lei Tung. The line then emerges into a cut and cover terminus wedged tightly between the 34 storey high rise buildings that form the South Horizons housing estate.
Strong relationships with the local community are essential to the success of the project.
The elevated section threads through high rise estates and runs close to schools, a police training college, a temple, hospitals and an old peoples’ home.
“Every stakeholder is different. We have had very detailed discussions with all the stakeholders and spent time to explain how we are going to carry out the construction work - especially for the viaduct,” says Cuzner.
At the southern end of the project, the contractors have limited the amount of explosives they were using for tunnel blasting because of the proximity to the blocks of flats. Normally a charge weight of 3kg to 4kg per delay is used for a single blast, but here it was necessary to limit this to 0.1kg to 1.5kg per delay for the blasting in sensitive areas.
To mitigate the impact of the noise from the operating railway on noise-sensitive residents and other stakeholders, the viaduct will be partly enclosed by noise barriers, which will also support the overhead electrical cables which will power the new line’s three car trains.
At one point, part of a multi-storey indoor food market will have to be modified to make way for the viaduct.
Another challenge was the need to build some of the viaduct piers for Wong Chuk Hang station within a nullah or flood drainage channel. In Hong Kong, nullahs are large canals which can fill up rapidly during the summer rainy season. Building structures in the nullah can increase the risk of flooding so MTR had to widen the channel to compensate for the obstruction caused by the new piers.
This work had to be carefully timed to avoid the rainy season and once the contractor started work in the nullah there was pressure to finish promptly thus avoiding the risk of over running construction work causing flood risk.
Work on Nam Fung tunnel has progressed steadily since blasting commenced at the southern end in late August 2012.
Around 350m of the tunnel has been excavated by the contractor Nishimatsu Construction Ltd and the first section of poor quality faulted rock with high water ingress will be encountered early in 2013. “We expect to have to grout ahead of the tunnel face to control ground water” says Cuzner.
Viaduct construction is well underway with most of the structure now visible along the route. The work is expected to finish ahead of the tunnelling. “One of our long term strategies is to start the dynamic testing of the rolling stock in the middle section of the railway as soon as possible. This will help to de-risk the overall testing and commissioning programme,” says Cuzner.