Growing pressure on Lo Wu station at the border with mainland China is making construction of KCRC's third East Rail extension project an increasingly urgent priority.
Hong Kong's return to Chinese control in 1997 resulted in a massive increase in cross border traffic. This has pushed the capacity of KCRC's Lo Wu station, at the border with the mainland, to the limit with the result that the corporation is preparing to build a spur line to a new border crossing station at Lok Ma Chau.
At Lo Wu, East Rail, passengers travelling to and from Hong Kong's neighbouring Chinese city of Shenzhen must disembark to go through passport control. On a normal day, around 250,000 people cross the border here, while at weekends and on public holidays, the number climbs as high as 335,000.
At peak times, East Rail trains carrying 3,500 passengers each arrive at the Lo Wu terminus every five minutes, putting enormous pressure on the station, which is unable to expand to handle such large numbers. As queues spill out onto platforms, East Rail services face delays as trains are held back until backlogs clear. Such delays can have a knock on effect on the rest of the busy line, disrupting the whole service.
The solution is to build a spur line running 7.4km west from Sheung Shui station, to a new border station at Lok Ma Chau.
This will be capable of handling 150,000 passengers per day.
Construction of the spur was originally planned to start a year ago. But plans to put the line on viaduct across the environmentally sensitive Long Valley wetlands were thrown out by the Hong Kong government's Environmental Protection Department (EPD).
As a result, KCRC has developed plans for a 4.3km twin bore 8.75m diameter tunnel which will take the line under Long Valley before completing the link to Lok Ma Chau on a 2.1km viaduct designed by consultant Binnie Black & Veatch. The plan received conditional approval from the EPD last month, and it is hoped the project will go out to tender in May.
KCRC has drafted its West Rail tunnelling team on to the project to develop the tunnel design with consultant Arup.
The twin bore structure will run at depths between 10m and 12m through waterlogged terrain.
As a result, KCRC plans to build it with an earth pressure balance shield tunnel boring machine (TBM) similar to the one used on the West Rail's Kwai Tsing tunnels. 'The tunnel will have to traverse rock, mixed and soft ground, ' says KCRC West Rail general manager, construction Jaya Jesudason. It will also pass through waterlogged ground.
Tough environmental controls on work under Long Valley will also heavily restrict work.
'One of the prime criteria when we go across the most environmentally sensitive area is that there will be no access to the surface. We must not disturb the groundwater, pollute the soil or contaminate the groundwater, ' says Jesudason. 'We'll be looking for zero water ingress.'
Work will be complicated by the need to construct three cross passages under Long Valley.
Environmental restrictions could rule out the use of grout curtains to stabilise the ground at these points, because of the risk that this would pollute the ground water. Ground freezing is the other option.
The tunnel will in fact extend beyond Long Valley, to prevent the viaduct impacting on a built up area to the west. As a result, contractors will have to build a cut and cover box at the tunnel's mid-point. This will be built within a diaphragm wall and could act as a servicing point for the TBM.