Densely packed, narrow streets and a shortage of access points are some of the challenges facing MTR Corporation on Hong Kong’s West Island Line project. Andy Bolton reports.
Western District on Hong Kong Island is a mass of densely built-up residential areas set on steep, narrow streets. It is relatively close to downtown Hong Kong, where many of the city’s bankers and civil servants work, but it lacks a connection to MTR’s subway system. Commuters must therefore pack themselves onto buses and trams to get to work.
This is about to change, with the construction of the HK$15.4bn (£1.3bn) West Island Line extension to the existing Island Line, the subway which runs along the northern edge of Hong Kong Island.
Compared with MTR projects like the Express Rail Link (Hong Kong) Section or the Shatin to Central Link, the West Island line is relatively modest.
It is a 3km extension with three stations and running tunnels linking the existing Island Line terminus at Sheung Wan to Kennedy Town in the Western District on Hong Kong Island.
Despite its size, the project faces numerous constraints which have dictated the route alignment, station entrance locations and spoil removal points.
Ground conditions have determined that much of the tunnel route hugs the slopes of the steep hills which descend to reclaimed shoreline in this area of Hong Kong Island.
Putting the tunnel into the hillside has the advantage of giving tunnellers relatively stable rock along most of the route, although there is a soft ground section at the eastern end.
Here contractor Dragages/ Maeda/Bachy Soletanche will use a tunnel boring machine (TBM) with slurry shield to bore through a mixture of silt, reclamation material, boulders and completely decomposed granite between Sheung Wan and the new station at Sai Ying Pun.
The other advantage of this alignment is that it gives better access to more passengers than a route further downhill through more difficult to tunnel reclaimed shoreline.
Dragages/Maeda/Bachy is now on site having won the contract for the bored tunnel section which will dovetail the West Island Line with the overrun tunnels at Sheung Wan last summer. Advance works are well underway and the contractor is preparing to order the TBM which will make both drives on this section.
Among the challenges facing the contractor on this section is the connection between the existing segmentally lined, bored Island Line overrun tunnels at Sheung Wan with the new ones. This involves pushing the TBM through the existing tunnel wall to splice the tunnels together.
“The contractor has developed what he calls a TDM, a tunnel dismantling machine, or segment erector in reverse to remove the existing lining segments,” says MTR West Island Line project manager Julian Saunders.
The plan is to remove the lining segments and replace them with a spray concrete lining, prior to backfilling this existing tunnel section with foam concrete, which is more easily chewed through by the TBM.
Soft ground and ageing buildings with shallow foundations present another major challenge to the project team.
“Contractors are going to have to be extremely aware of ground settlements.”
Julian Saunders, MTR
This part of Hong Kong Island’s north shore has some of the oldest residential buildings in Hong Kong. These often rest on relatively shallow friction piles while more modern buildings rest on piles which have been taken down to rock.
“As there are older buildings in this area, the contractors are going to have to be extremely aware of ground settlements and temporary works designs have been developed with this in mind. One specific building above the mixed ground TBM tunnels requires some of its piles to be removed as they are blocking the tunnel alignment. This building will need to be strengthened before the TBM gets there,” says Saunders.
Strengthening will likely involve adding steel plates to beams and columns and possibly some underpinning work.
In addition, there are four steel sets or steel I-section portal frames in the ground on the tunnel alignment within the TBM tunnel section.
These were installed as temporary works during the original Island Line construction in the 1980s but are now redundant and must be removed so as not to damage the TBM.
Client: MTR Corporation
Contractor: Sheung Wan to Sai Ying Pun tunnel Dragages/ Maeda/Bachy Joint venture
Contractor Kennedy Town station: Gammon Construction
Contractor for magazine: Gammon Construction
Design consultants: Arup/Atkins joint venture and Meinhardt China
The stations and remaining running tunnels pass predominantly through granite, and will be constructed using drill and blast methods. This work is thought to be relatively straightforward as there are few major obstacles in the ground.
But to maintain progress, MTR is building an underground magazine in the hillside near Kennedy Town to store a two day supply of explosives to allow two blasts at up to 13 faces a day and avoid disruption in supply caused by adverse weather, as all explosives must be delivered daily by sea to Hong Kong Island.
The magazine is a horseshoe shaped 350m long tunnel big enough for trucks to drive into. Explosives and detonators will be stored in 9 separate niches connected by this tunnel.
Contractor Gammon Construction started work on this recently, excavating short 40m to 50m entrance sections through grout fan strengthened soft ground under a road cut into the hillside before getting on with the drill and blast work later this year.
Much of the route is deep underground and Sai Ying Pun and Hong Kong University stations will be in drill and blast underground caverns.
“The contractor developed a segment erector in reverse to remove existing lining.”
Gammon has the contract to build the other station which is the Kennedy Town terminus. This will be a cut and cover structure constructed across an existing playground and swimming pool site.
The playground will be reinstated after construction is complete, but the swimming pool will be relocated and rebuilt to a more modern design under a contract awarded to Paul Y Construction in July 2009.
As a community railway, establishing and maintaining good community relations is vital to the success of the WIL project. MTR and its contractors have regular community liaison meetings with residents and local groups to inform them of progress and maintain close contact with the community.
Lack of space for access shafts has made arrangements for spoil excavation a challenge as there is scant space for these. The steep narrow streets along the tunnel alignment limit the scope for removing spoil by truck.
As a result the project team is having to excavate long construction adits linking the tunnels with surface access points close to the north shore, from which they can remove spoil to barging points with minimum impacts to the road traffic and the environment.
For the tunnels between Sai Ying Pun and Kennedy Town stations as well as for the station caverns themselves, all of the excavated material must be removed from just two of these adits.
Material to line the tunnels and fit out the two underground cavern stations as well as all the manpower for the underground works also make use of the same two adits.
“This is a major constraint,” says Saunders. At Hong Kong University station, one of the adits will house a spoil conveyor which runs for 400m before emerging at the foreshore near a barging point.
A tight squeeze
At the same time MTR is having to occupy small public spaces as close to the tunnel alignment as possible for station entrances at Sai Ying Pun and the University of Hong Kong.
The densely packed streets and steep sloping topography also mean that access to platforms at the University of Hong Kong and Sai Ying Pun is either via long corridors or via deep high speed lift shafts.
One of the access points to Hong Kong University station is a 64m deep shaft which emerges into a 50m high above ground structure which provides access to a building further uphill.
At Sai Ying Pun station, at the soft ground end of the line, two of the entrances and the platforms are linked via a 20m deep, 80m long adit which will be excavated during a ground freezing operation.
Ground support using compressed air at 3.5 bar was considered, but ruled out because of the need to comply with tough compressed air working regulations and minimise ground settlements.
The line is due for completion in 2014.