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Boxed in at Nam Cheong

Nam Cheong Station will be the major terminus for Hong Kong's West Rail Line. David Bennett reports on the complex construction of the station being built at the meeting point of three separate rail networks and two motorways.

The HK$61. 2bn (£5. 1bn) West Rail line is the largest section of the ambitious programme of work being carried out by the Kowloon Canton Railway Corporation in Hong Kong.

The 30. 3km long railway, due to open in 2003, will link West Kowloon with Tuen Man in the north west New Territories. Much of the line passes through mountainous terrain and builtup areas, with more than 90% in tunnels, on viaducts or enclosed. Design and build contracts have now been let for the track and for the nine stations on the route.

Nam Cheong Station in West Kowloon will become the major terminus for the West Rail Link. It will be an important interchange with the Mass Transit Railway Tung Chung Line serving Lantau Island, while allowing the Airport Express Line to run through the middle unhindered and maintaining the integrity of the foundations of the elevated West Kowloon Expressway motorway above.

Contract CC402 for the £183M Nam Cheong Station was awarded by client Kowloon Canton Railway Corporation to the joint venture contractor Balfour Betty-Zen Pacific, based on an alternative design by civil engineering works engineer Robert Benaim & Associates. KCRC's consultant is Hyder Consulting, with consultant Tonkin & Taylor (Hong Kong) responsible for geotechnical aspects. Geotechnical contractor Bachy Soletanche is carrying out piling and groundwork.

The station covers an area of 400m by 90m, is typically 11m deep - requiring 400,000m 3of excavation - and includes a 'podium' structure rising 21m above ground level. The corridor created by the Airport Express line and the elevated expressway bisects the station, creating two box-like structures to the east and west.

The eastern box is 400m by 40m. It is the larger of the two underground structures and comprises the podium, the first-floor and concourse slabs; both platforms for the new West Rail Lines; one platform for the Tung Chung Line at ground level and the elevated P1 Highway at first-floor level (including sliproads).

The west box houses one elevated platform for the expressway, the plant rooms and the ground-level station entrance and access area.

The deepest parts of the station are the escalator pits between the two boxes, which are 15m below ground level and lie below the concourse slab.

'Above podium level on the east box side there is provision in the substructure design to support the construction of a 10-storey building, ' adds Robert Benaim & Associates project director Robert Cook. 'This will act as a noise screen to the residential developments in the vicinity. '

In the original design, the perimeter walls of the boxes were to be built by diaphragm walling. Where the diaphragm walls were next to the foundations of the elevated expressway viaduct piers, the loads were to be carried by underpinning the walls with minipiles.

Generally, the column and floor slab substructure were to be supported on end-bearing piled foundations. Adit construction for the concourse access between the two boxes was to be formed below the Airport Express line foundation slab using tunnelling methods.

The alternative design and construction approach aimed to reduce the quantity of materials used and the overall cost of construction. Both the original and alternative designs had to be built top-down to achieve an early handover of the track slab at ground level to KCRC and the Mass Transit Railway Corporation.

'Major time and cost savings were made by adopting an insitu structure for the east and west box basements within a sheet piled cofferdam, with the external walls supported by plunge columns, ' Cook explains.

'Integrating the 1m thick raft slab of the airport line enables the adits to be built bottom-up, under the slab, allowing the trains to operate at over 125km/h. '

Choosing shaft grouted barrettes also saved money, reducing deep excavation to an average depth of 55m instead of up to 90m for the original end bearing bored piles. The overall saving of the alternative design was estimated to be £29M.

Site investigations revealed some 25m of hydraulic sand overlying a variable thickness of alluvial silt and clay underlain by weathered granite. Formation depth for rock sockets for the end bearing piles and barrettes was generally between 50m and 100m below ground level.

'The water table is about 3m below ground and will be lowered by 12m within the excavation using deep multiple wells, ' says Cook. 'Settlement predictions have been analysed using Piglet pile modelling software and three-dimensional structural modelling to account for load redistribution. '

Predicted maximum settlement under the high-rise development over the east box has been estimated at 35mm.

The complexity of the geotechnical and engineering analysis of the foundation and substructure work was increased because new tracks have to be built either side of the airport line to allow the Tung Chung Line trains - which use the same tracks - to run into the new platforms below the elevated viaduct. Work on the new station also has to be carried out on a curved 12m grid to follow the plan of the viaduct.

Barrettes are being formed using a combination of grab and hydrofraise to excavate the trench slots under bentonite.

Grout is pumped into the shaft after the excavation has been concreted to enhance skin friction and reduce further excavation depth through difficult ground.

'Shaft grouting is carried out using tubes a manchette attached to the rebar cages spaced evenly around the perimeter of the barrettes, ' says Cook. 'Grout is pumped into each sleeve until either the design volume or pressure is achieved. '

In all, there are about 50 end bearing bored piles and 250 barrettes.

Detailed vibration studies were carried out to determine the effect ofdriving sheet piles for the cofferdam 28m below ground level close to sensitive structures.

'In general it was proven that pile driving could be carried out for the vast majority of the sheet piles except for localised sections immediately adjacent to the airport line ground support slab and expressway viaduct pier foundations, 'says Cook.

In these locations the cofferdam will be formed with prebored steel H piles encased in concrete.

Real time monitoring of sensitive structures during construction will allow rapid response if movements begin to exceed tolerance thresholds. At the end of the substructure work, after backfilling the insitu perimeter walls, it is intended to recover all the sheet piles.

The 1m deep raft slab supporting the airport line track will form a temporary soffit above the concourse link adit spanning 20m and will be integrated with the station structure. The slab will be supported on rows of plunge columns within the barrettes. The ground below will be contact grouted after backfilling to minimise any future settlement that may occur once the slab is disconnected from the adjacent structure.

'Staged excavation and dewatering cycles during top-down construction will reduce the settlement values by as much as 50%. This approach minimises the changes in loading history as the overburden is removed, ' says Cook.

Instead of tunnelling below the airport line track slab, the adit will be built bottom-up using the its side walls as deep beams spanning between the east and west box walls, removing the need for the minipiling specified in the original design.

Work on the station began on site in November 1999. Barrette and pile installation is now well under way, with sheet piling for the cofferdam and installation ofdewatering wells substantially complete. A thousand people are working on site with an assortment ofplant including four piling rigs, two hydrofraises, four kelly grabs,25 site investigation rigs and various heavy cranes.

Foundation work is due to finish early next year.

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