MTR’s £1.3bn West Island Line is in its critical final stages as its project team prepares for completion at the
end of this year.
“The biggest challenge now is overcoming the logistics,” says MTR West Island Line project manager Stephen Hamill.
Most of the West Island Line construction work is now complete and mechanical and electrical work is well underway at the project’s three stations. This is highly intensive, keeping around 2,500 people occupied underground at any one time.
The West Island Line is a 3km extension of the existing Island Line, running west from Sheung Wan via stations at Sai Ying Pun and the University of Hong Kong to a terminus at Kennedy Town. The line runs deep underground, cutting under a steep sided hill packed with high rise buildings.
Hamill says that in a sense, the tunnelling phase of the project was relatively straightforward. Most was drill and blast aside from a section of soft ground tunnel between Sheung Wan and Sai Ying Pun, built with a tunnel boring machine.
Mucking out could be done from shafts and using conveyors set up in adits, which were later to become passenger access tunnels. Deep access shafts, later to house lifts, were additional points at which concrete could be brought in.
The fit out phase is more logistically complex, as a wide range of materials and equipment from floor tiles and cladding to escalators and platform screen doors has had to be fed into the underground stations where access is tight and storage space is limited.
Larger items like escalators and platform screen doors have been delivered via Kennedy Town, which is a shallow cut and cover station
Delivery routes were also constrained by the need to install lifts into the deep access shafts at Sai Ying Pun and HKU stations. Installing the honeycomb lift and ventilation duct structures within the shafts placed a limitation on the sizes of delivery they could handle.
“Now we are sharing the sites, we have got to be better coordinated,” says Hamill.
Storage space underground is limited to the platforms, where work is also underway to install anything from platform screen doors to wall panels and electrical equipment. This means that constant advance planning and a just in time delivery philosophy are vital to efforts to ensure no space is wasted.
Larger items like escalators and platform screen doors have been delivered via Kennedy Town, which is a shallow cut and cover station. Here, an opening had been left in the roof slab and intermediate slabs below it so that large items could be craned down to the platforms and onto a diesel powered works train for delivery to other stations.
So much material had to be delivered through this site that the opening had to be kept open for longer than originally anticipated with the result that some building services work at Kennedy Town had to be resequenced.
“Despite the logistical challenges, work remains largely on schedule,” says MTR West Island Line electrical and mechanical project manager Dono Tong. Aside from the stations, the tunnels between them are lined and track laid.
Some deliveries can also be taken from works trains which run along the Island Line during night time engineering hours.
These can be moved onto the West Island Line track via a bulkhead, which now separates it from the live Island Line running tunnel section.
But using this delivery method is limited as the works trains must fit in with timetables for works trains serving other parts of the Island Line. As a result, only one train is allowed through on any given night.
One section of passenger access tunnel at Sai Ying Pun is still underway as it involves an extremely delicate piece of tunneling.
This section of 100m long tunnel in water bearing decomposed granite varies from 25m to 35m below ground, passing below several buildings including an ageing multi storey block of flats built more than 50 years ago supported on H-piles founded within 3m of the tunnel being constructed.
To limit the risk of causing subsidence damage so close to the alignment, MTR decided to undertake the biggest ground freezing operation ever carried out in Hong Kong to stabilise the ground during excavation work.
Ground freezing is now underway, but it is a slow process, even after all the necessary building permits have been obtained.
It took 110 days for the ground to freeze sufficiently to allow excavation to take place
Directional drilling to tolerances of 300mm has been undertaken by specialist subcontractor Intrafor, whose team inserted 25 horizontal freezing pipes arranged in a circle around the outer edge of the 6.5m diameter excavation.
These which will carry brine at temperatures averaging -30˚C, along the outside of the alignment from a soundproofed freezing plant, squeezed onto a site close to the tunnel entrance. The average temperature of the frozen ground developed around the pipe network is -10˚C
It took 110 days for the ground to freeze sufficiently to allow excavation to take place. This work is being carried out in a heading and benching sequence using breakers and a road header, which chip out a ring into which supporting steel ribs are inserted before a sprayed concrete lining is applied.
Progress is slow but steady, at around 1m per day, and the work is energy intensive, with the 260kW freezing plant costing around £63,000 a month to keep in operation.
In addition to freezing the ground for the adit excavation, additional freezing is required at a point where the tunnel makes a diagonal turn. Here, a 40m deep frozen plug will be created to prevent water ingress and further limit the risk of settlement.