After the successful completion of Hong Kong’s West Island Line extension in 2014, the focus for the territory’s transport operator MTR has shifted to finishing its South Island Line.
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The £1.5bn project is one of five schemes that will together add an extra 56km to the MTR network by 2020, and has been underway for the last five years.
The West Island and South Island lines were developed as community railways, with the intention of regenerating historic districts and providing connectivity to built up areas, as well as relieving traffic congestion and stimulating economic activity. There is already evidence that this has been successful in the case of the West Island Line, which opened in December 2014.
“Journey times have been reduced by 50%, and we have seen a lot of rejuvenation and old buildings being refurbished,” explains MTR West Island Line/South Island Line general manager Mark Cuzner, adding that the South Island Line is intended to help increase tourism in the south of Hong Kong.
The South Island line is a 7km standalone medium capacity railway that will carry passengers between the Admiralty business district on Hong Kong Island and South Horizons, a housing area on Ap Lei Chau island, in just 11 minutes. The project includes five new stations, one of which is a very complex structure linking the new line to three other MTR lines at Admiralty.
Admiralty station Hong Kong
According to Cuzner, the project faces many challenges common to all large infrastructure and construction schemes in the territory. “Hong Kong Island has very high land values, so it is not possible to use private land; and space is sought after and hard to get hold of,” he explains.
“We are working in a densely populated urban environment, as a result of which there are a lot of old and unmarked utilities; it is difficult to get traffic management permits; and environmental issues are very high.”
In addition, this is just one of many major infrastructure projects underway in the region. MTR itself has five network expansion schemes on the go, and other projects under construction include the $10.6bn (£7.4bn) Hong Kong-Zhuhai-Macau bridge – all of which mean that construction resources are scarce and construction costs in the region are escalating.
South Island Line
Within these parameters, however, the South Island Line is progressing well, with all five stations and connecting tunnel and viaduct sections nearing completion.
In all, 4km of the line will run in tunnels that are being excavated using drill and blast techniques, and 2km will be on viaduct. In addition, there is one bridge, one section of cut and cover tunnel, and four ventilation buildings.
There are three below ground stations and two above ground, and the project also includes a new depot. Temporary facilities built to support construction include two barging points and an explosives magazine.
Lei Tung tunnel
Almost the entire section of the route on Ap Lei Chau is within tunnel, including both Lei Tung and South Horizons stations. This section – known as the Lei Tung Tunnel – is 1,092m long, of which 873m is being excavated using drill and blast, and 219m in cut and cover.
The excavation is mainly in volcanic tuff, a fairly soft and porous form of consolidated rock. The total volume of material excavated to form the Lei Tung Tunnel is 130,000m3, requiring 580 separate explosive blasts.
“One of the challenges is the density of population,” explains Cuzner. “Ap Lei Chau is one of the most densely populated islands in world.”
This was one of the factors the MTR team took into consideration when it opted to move the portal and a section of platform of the predominantly cut and cover South Horizons station into the rock face, excavating it by drill and blast rather than mechanical excavation. “It took a lot of explaining, but it did allow for the works to progress more quickly, which meant less disruption and less noise,” says Cuzner.
North of the Lei Tung Tunnel, the new line runs on a section of viaduct before heading into the 3.2km long Nam Fung Tunnel. Again, the predominant rock type is volcanic tuff, with short sections of granite at each portal. The rock cover is up to 320m, and the excavation has been affected by high water inflow at the portals and at faults throughout the length of the tunnel.
There are two major fault zones, so we have been grouting ahead of the excavation face for 50% of the tunnel, and have had a computer controlled jumbo to drill the blast holes
Mark Cuzner, MTR
“There are two major fault zones, so we have been grouting ahead of the excavation face for 50% of the tunnel, and have had a computer controlled jumbo to drill the blast holes,” says Cuzner.
Around 300,000m3 of rock has been excavated to create the tunnel, which has been achieved by 750 individual explosive blasts.
At its northern end, the new South Island Line joins the rest of the MTR network at Admiralty station. Its lines feed into the station alongside a cavern housing platforms for the terminus for another new line - the Shatin to Central line, which is being constructed at the same time (see box). The new lines are at right angles to, and at a lower level that the existing Island Line tracks, which have had to be underpinned to enable the new running tunnels and platforms to be constructed. And a new station extension is being built alongside the existing Admiralty station, extending a further three levels below the existing.
Contractors at Admiralty are working adjacent to the operating railway and on a confined site surrounded by major highways, busy pedestrian areas, and existing structures. This has provided a logistical challenge for rock excavation, spoil disposal and material deliveries, in addition to the technical challenges of the project itself (see box).
Construction at Admiralty, as on the rest of the South Island Line, is reaching its final stages, with the entire line set to be finished by the end of this year.
Admiralty station is set to become Hong Kong’s first fully integrated interchange between four MTR lines: the new South Island Line, the existing Island Line and Tsuen Wan Line running east-west, and the Shatin to Central Link, due to open in 2020.
“Admiralty is one of the most demanding parts of the project,” says Cuzner. “It is surrounded by high rise buildings, roads and structures. And, although the current waterfront is a long way away, we encountered a 19th century sea wall during the excavation.
“The existing station has three levels below ground, and our station is extending three levels below that – including installing 37 additional escalators.”
At a depth of 40m below ground, contractor Kier, Laing O’Rourke, Kaden JV has excavated a new 26m diameter cavern to accommodate the platforms for the new South Island Line and to give access to the Shatin to Central Link lines. Around 300,000m3 of rock has been excavated to form this new cavern, the platform tunnels and passengers adits between them, more than half of it by drill and blast. The proximity of existing buildings and the operating railways means that, for many of the 571 individual blasts, the maximum instantaneous charge that could be used was just 1.25kg.
“We were excavating beneath a car park, a shopping centre and passenger adits in the existing station, so we had to have very careful control of the blasting operations,” says Cuzner.
New station box
To reach these lines, passengers will enter through a new station box being constructed in the rock adjacent to the existing Admiralty station. This box – which extends for the equivalent of six storeys below ground – was originally intended to be formed using “bottom up” construction, but MTR felt that “top down” methods would be better suited to the site constraints.
“The box is surrounded by two [MTR] lines, an underground car park and the existing station,” says Cuzner. “We chose top down because it gives better stability to the existing structures, and an opportunity to carry out blasting for the bulk excavation.”
Groundwater cut off
Cuzner says the key challenges for the box excavation were achieving groundwater cut-off prior to excavation; extending the existing station concourse above the existing Island Line platforms, and ensuring the stability of the adjacent structures.
These challenges have been addressed by using a combination of diaphragm wall, secant piles and curtain grouting to make the excavation watertight; making use of existing sheet piles and caissons for the concourse construction; and combining top down construction with plunge columns and partial permanent slabs for the main box excavation.
In addition, the construction team has had to excavate below a “finger platform” of the existing Island Line running tunnels without disrupting services on the line. “Trains run with a 112 second headway, which equates to around 50,000 passengers per hour; so there really could be no disruption to the existing line,” says Cuzner.
The contractor used underpinning to create a 26m deep excavation beneath a 58m length of the insitu concrete platform box, with the excavation done in three stages, using an incremental top down sequence with staged load transfer. Steel underpinning columns were installed in sections, all fitted with computer controlled jacks that have built in redundancy and failsafe systems, as well as extensive instrumentation with real time monitoring of load and movement.
26 underpinning columns
“The underpinning was fairly conservative and simple, but there was a lot of control,” says Cuzner. ‘We installed 26 steel underpinning columns in alternate slots, which extended as the excavation went down.”
This system included 116 individual jacks, which were linked to a computer controlled system that had five separate monitoring systems. It was so effective at controlling movement that the total movement measured on the existing railway was 5mm, compared with an allowable value of 7mm.
Once the excavation was complete, some of the steel underpinning columns were filled with concrete and incorporated into the permanent works.