Land space is at a premium in Hong Kong, so building a major new rail project within the confines of its heavily massed infrastructure is a tricky undertaking. The Kowloon Canton Railway Corporation’s (KCRC) new West Rail Mass Transit project is designed to link the Kowloon Peninsula with the Northwest New Territories.
Along its route, the US$83.5M, DD400-Sham Shui Po tunnel passes through the heart of the busy Nam Cheong and Lai Chi Kok districts of Kowloon and includes two interchange stations and 2km of cut and cover connecting tunnel. It also includes large sections of newly developed reclaimed land, which form part of the West Kowloon reclamation.
Alignment
Prior detailed studies on the preferred locations of the two stations meant that Hyder Consulting’s design team was faced with a fixed horizontal route. Nam Cheong Station, which is the size of five football pitches over four levels, provides the interchange to the Mass Transit Railway’s (MTR) Tung Chung line, while the similar scale Mei Foo Station links to the MTR Tsuen Wan line.
By fixing the horizontal alignment, the vertical alignment was defined by the need to weave the track over or under existing infrastructure. Principal among these were the existing Drainage Services Department culverts. The Kowloon Peninsula is an area that is susceptible to flooding and the network of culverts that crosses the tunnelling area is critical to the control of this flooding. The design team had to ensure that the tunnel didn’t reduce the capacity of the existing culvert network, but also had to take into account the drainage authority’s plans for future construction and maintenance.
The second major constraint on the design team was the West Kowloon Expressway (WKE). WKE structures run along and across the alignment of the DD400 tunnel section. These included a major twin-box girder segmental viaduct, lying parallel to the West Rail tunnels for over 1km. Two pedestrian and four highway routes also passed along the tunnel alignment. The tunnel design and detailing was therefore developed to accommodate the existing features of these highway structures and minimise both permanent and construction effects on the structures.
Further constraints were the proximity of the Airport Express Line which, at one point, is only separated from West Rail by 20m and the existing MTR, which actually passes under Mei Foo Station.
Initial investigations showed that the answer to the various alignment issues was a route weaving above and below the existing ground level. The possibility of an open track section was considered in the very early concept designs. However, the alignment passes through both the Nam Cheong and Lai Chi Kok Parks, which are important public spaces for the local community, most of whom live in high rise apartment blocks.
Since KCRC had placed environmental and community issues at the top of its agenda, this drove the decision to fully enclose the railway within the park areas to mitigate the visual and noise impact. The most striking outcome of this design approach is the tunnel and Mei Foo Station, which have formed new features of Lai Chi Kok Park.
Although there was no way to avoid disruption to the park during construction, the tunnel and station have effectively been modelled into features of the park, with soft and hard landscaping over and around both tunnel structures.
Similarly, at Nam Cheong Park, which provides one of the main pedestrian access routes to the local fish market, detailing of the tunnel box has minimised environmental impact and has created a number of pedestrian features such as stairs and seating.
Tunnel design
The tunnel has been designed to satisfy West Rail’s design criteria and comply with the limits imposed by the particular site constraints. To achieve this the design:
The resulting tunnel is mainly twin-celled. Each cell is on average 6.3m wide and 7m in depth; sufficient to allow for the KCRC Structure Gauge envelope. All walls and both the roof and base slabs are an average of 600mm thick. The design was carried out to KCRC’s project specific design criteria, which is mainly based around Hong Kong and UK standards.
The tunnel is fully supported on piles, to avoid imposing additional loads on the existing structures and to minimise the effects of the long-term movement anticipated in the reclamation zone. The piling consists of large diameter bored and driven piles. The bored piles range in size from 1.5m to 2.5m diameter and the driven piles are H section piles, all are founded on slightly decomposed granite at depths varying from 20m to 70m below ground level.
This approach enables the tunnel to span over the tops of all of the large drainage culverts in the area. It also allows for the foundations of the tunnel to be taken down to similar foundation levels as the existing structures of the West Kowloon Expressway, minimising potential long term effects on the existing structures.
Particular attention was given to the interface between the relatively lightly loaded tunnel section and the two stations. Nam Cheong Station is earmarked for future high-rise developments, while Mei Foo Station will be subject to further park development with significant additional landscaping features due to be added in the future.
The tunnel design uses transition tunnel units spanning from the tunnel section to the stations, to allow for the potential relative movement of the station to the tunnel.
To ensure waterproof tunnel construction, the design included two different types of waterproofing. Continuously keyed sheet Voltex composite has been used for pilecaps and the base slab up to the construction joint on the walls. Spray applied waterproofing membrane (2mm thickness of Conipur 225) has been used for all external walls and roof slabs buried below ground.
The design of the permanent ventilation to the tunnels was a system wide contract with provision for civil works made within each construction contract. On the Sham Shui Po stretch, the Prince Edward ventilation building in Nam Cheong Park provides for both the initial passenger demand and for the future expansion as passenger numbers increase to the full design load.
Tunnel construction
Tunnelling work began in March 2000 and the first base slab of the tunnel box was cast on 1 April 2000. The main method of construction was by cut and cover, with the tunnel divided into bays of no more than 14.5m in length, separated by construction/movement joints. The construction of a typical tunnel section was divided into a base slab pour, and a single pour casting of the walls and roof slab. The typical works sequence was:
1. Construction of piles
2. Excavate to pile cut-off levels
3. Construction of pilecaps / cap beams
4. Backfill to tunnel formation level
5. Blinding concrete and base slab waterproofing installation
6. Base slab construction
7. Erection of mould and construction for walls and roof slab
8. Application of spray-on waterproofing membrane
9. Backfilling and compaction to soil surrounding the tunnel box
Steel and timber moulds were used for wall and roof slab construction. Once the roof slab and walls had gained sufficient strength, the moulds were stripped and slid along the top of the already cast base slab of the adjacent bay. After the formwork had set, the reinforcement for the walls and roof would be placed and the next stage of concreting carried out.
The average rate of progress was approximately seven days per bay for the base slab and 14 days per bay of wall and roof slab, which included the placing of reinforcement, casting of the slabs, dismantling the formwork and moving on to the next bay. The waterproofing operations followed up at a later stage as sufficiently large sections of the tunnel became available.
Excavated materials mostly comprised reclamation sand fills and these were largely used as backfilling subsoil on site as the completing tunnel was to be earth mounted in some locations. Additional materials were shipped out by the contractor to another of its sites that required imported soil.
Construction waste was removed to designated landfill sites while chemical waste was collected by an approved company for disposal. To control the potential for illegal dumping, disposal of materials was controlled by a trip ticketing system, to monitor the volumes exported off site and then compare them with the volumes delivered to the disposal locations.
Working in a constrained site on reclaimed land has inevitably presented some problems. Ground water was a particular issue necessitating extensive dewatering. The construction team also found temporary works sometimes in conflict with permanent works. This meant that some sections of the tunnel walls were cast with box outs around the struts and grouted when the struts were removed.
The final finishing works to the tunnel consisted of significant re-instatement works for the roads and park areas excavated during the construction. The intention of these works was to blend the tunnel box into the surrounding landscape of the area.
Planning for the future
Ultimately, the plan is for West Rail to link to KCRC East Rail around the Kowloon Peninsula, by extending south towards Tsim Sha Tsui. To accommodate this, the tunnel includes provision for the anticipated structures and equipment necessary. The current construction of the tunnel ends within Nam Cheong Park with a ‘temporary’ closure wall at the end of the tunnel.
The tunnel also allows for the future expansion of the two ancillary buildings supplying power, communications and ventilation to the tunnel system. The Prince Edward ventilation building, located in Nam Cheong Park, can be expanded around and under the tunnel box to cater for the potential increased ventilation requirements for the proposed Southern Link. Similarly, at Lai Wan interchange, the tunnel design allows for the expansion of the small communications building into a much larger communications and cooling plant building, supplying chilled water to cool the tunnels as demand increases.
West Rail is designed to transport a maximum capacity of 100,000 passengers per hour in each direction, adding to an already impressive array of transport options available to residents and visitors.
With mainland China continuing to open up and surface space at a premium, future developments may well be forced underground. The DD400 project illustrates some of the labyrinthine challenges ahead for tunnel designers.
Related Files
Map of Hong Kong showing the West Rail route and future connections
