Tunnelling and underground work are now the main focus of civil engineering work on MTR’s 7km long South Island Line (East) project, which will link Hong Kong Island’s busy business district in Admiralty with the Southern District, terminating at South Horizons on Ap Lei Chau.
Hemmed in by skyscrapers and busy dual carriageways, a tiny 80m by 80m park is the only visible evidence of the vast amount of underground work taking place at Admiralty as part of MTR’s HK$12.4bn (£1bn) South Island Line (East) project.
Here, MTR and contractor Kier Laing O’Rourke Kaden JV, is extending and upgrading the existing MTR station at Admiralty which houses the interchange between the Island Line and Tsuen Wan Line.
When complete in 2015 it will accommodate the new South Island Line (East) terminus and platforms for phase two of the Shatin to Central Link, which is scheduled to open in 2020 creating the first four line interchange in Hong Kong.
Over 40m below ground a vast 26m wide drill and blast cavern for the South Island Line (East) platforms has been opened up, flanked by the Shatin to Central Link platform tunnels.
Already the cavern crown is excavated down to what will be the concourse slab level, and the two Shatin to Central Link platform tunnels are taking shape.
Benching in the cavern has already commenced for the next phase which is to blast down to South Island Line (East) platform level.
At Admiralty we have put in heavy plant to increase productivity, but the rock is really hard and it is a continual challenge to achieve the production targets
Just below the surface, the adjoining 40m deep underground station box is also being excavated in ground comprising of mostly granite. This is being excavated within the Island Line and Tsuen Wan Line platform structures to the south and north and the existing Admiralty Station to the west.
At the east of the site, a new diaphragm wall and secant pile wall forms the cut-off. The box will contain a new entrance plus extended circulation space for the existing Tsuen Wan Line and Island Line platforms, and deep escalators to and from the new platforms and intermediate concourses.
The station box is a mixture of top down and bottom up construction. The ground floor, concourse, and upper track slabs have been completed as excavation progressed as part of the top down works. These have been supported on 50m long plunge columns, with additional support to the existing perimeter structures and diaphragm wall/secant pile cut off walls being provided by large steel raking props.
Part of the landscape deck has also been completed to provide early plant room access for electrical and mechanical installation to commence in the second quarter of this year. Since the excavation began in earnest in November 2012, progress at the site has been rapid despite the complex matrix of constraints affecting construction in such a built up area.
“Sixty per cent of the excavation work of the station box is complete,” says MTR South Island Line civil project manager Ken Wong. A total of 110,000m3 of granite will have been removed from the site to create the station box, with a further 60,000m3 for the platforms and cavern by the time work is complete.
Among the most significant constraints are those imposed by the existing Island Line and Tsuen Wan Line which interchange underground at Admiralty. These run in stacked tunnels directly adjacent to the excavation work and the Island Line tunnels run across the top of the site.
In addition, the site is formed around plant rooms and a ventilation shaft serving both the operating railways. Both must remain in operation during the extensive station expansion programme.
One of the particular challenges with drill and blast works at Admiralty is managing the resultant vibrations. These must not exceed 25mm/s on any part of the tunnel structures, and in practice the blasts are programmed to produce smaller vibrations to limit the risk of damage to any part of the operating railway.
Careful consultation was required with surrounding residents to keep them updated on the blasting times as the impacts could be felt in the areas above the new tunnels and cavern
The cramped site at the surface is the only exit point for the excavated material from the cavern and platform tunnels. There is a 40m deep, 144m2, fully enclosed access shaft with a huge 800t capacity hopper into which material from the deep cavern excavation is lifted and stored during night time mucking operations.
The excavated material is removed by truck during the daytime, to keep within the statutory noise limits, and taken to barging points at Hong Kong’s nearby Victoria Harbour.
Work on the station box has presented challenges, not least because of the nature of the material to be removed. The rock is extremely hard, pushing huge 85t breakers to the limits of what they can achieve.
“The key challenge here is to advance the progress of the rock breakout,” says Wong. “We have put in heavy plant to increase productivity, but the rock is really hard and it is a continual challenge to achieve the production targets.”
As a result, the contractor has introduced blasting in the station box to supplement the mechanical excavation, with the blasts being carefully controlled outside the operating hours of the railway, with these typically taking place between 1am and 2am.
“We are managing vibration to reduce the risk of causing problems to the operating railway,” explains Wong.
South of Admiralty, contractor Nishimatsu Construction is building the 3.2km Nam Fung tunnel, using a combination of mixed-ground excavation and drill and blast techniques from the south portal.
The tunnel will take the line south and onto the viaduct section in Aberdeen. Worse than expected ground conditions have affected work on the Nam Fung tunnel, as fractured, water bearing rock has reduced the productivity of the drill and blast excavation.
In some areas the rock has been so fractured that the contractor has mechanically excavated the tunnel, with the face stabilised by one or more rounds of grout fans inserted ahead of the work. Where the ground is more stable, drill and blast has been used.
So far 2km of the tunnel has been excavated since work began in late August 2012 and the contractor is preparing to encounter another faulted section and has grouting equipment and steel rib supports ready to keep work progressing safely.
The viaduct section further south is almost complete, comprising a mixture of precast balanced cantilever sections with a 21m section supported by precast beams and in-situ span-by- span post-tensioning sections, plus a 250m three span in-situ balanced cantilever bridge crossing of the Aberdeen Channel.
The elevated Wong Chuk Hang Station and Ocean Park Station are structurally complete and are now being fitted out.
Final viaduct section
The last section of viaduct to be completed is a short precast balanced cantilever section connecting it with the western end of Wong Chuk Hang Station. Here, the contractor has had to remove part of the roof structure of a municipal cooked food market building to make way for the railway structure. This section of viaduct work is expected to be completed this month.
At the southern end of the line beyond the Aberdeen Channel, Leighton John Holland JV has been working on a relatively short 1km drill and blast tunnelled section, which includes an underground station at Lei Tung. Main tunnelling works and station cavern excavation are now complete and station structure works are underway.
This work was not as constrained as that at Admiralty as the tunnel passes further beneath the residential buildings that are above and around the railway alignment. However, careful consultation was required with surrounding residents to keep them updated on the blasting times as the impacts could be felt in the areas above the new tunnels and cavern.
The South Island Line (East)’s southern terminus at South Horizons is a cut and cover station squeezed among several 34 storey blocks of flats.
The station box excavation is now well advanced after a painstaking period of pipe pile and temporary road deck installation which has required careful coordination and consultation with residents to mitigate the disruption caused by the construction works.
Similar to all the other areas affected by the SIL(E) railway construction works, community support has been strong and has greatly assisted the implementation of the works.