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Tunnels twins test contractors

Two major tunnels will take West Rail under busy northern Kowloon and through a steep sided mountain to Kam Tin

A need to get the main tunnelled sections of West Rail under way as early as possible led to a decision by Kowloon Canton Railway Corporation to opt for design and build instead of the traditional Federation Internationale des Ingenieurs Conseils-based contract form prevalent elsewhere on the project. 'We decided to use design and build on the basis that designs did not need to be so far advanced before we went out to tender, ' says KCRC West Rail construction manager Howard McKay.

The first contract to be awarded was the HK$1.79bn Tai Lam tunnel which went to Nishimatsu Dragages joint venture in September 1998. At 5.5km, this is the longest transportation tunnel in Hong Kong, taking West Rail from Tsuen Wan West, under mountainous Tai Lam Country Park to the Kam Tin Valley.

Hard on the heels of the Tai Lam tunnel is Dragages Zen Pacific's adjoining HK$ .9bn contract to build what are collectively known as the Kwai Tsing tunnels, linking Tsuen Wan West with Mei Foo.

At Tai Lam the French and Japanese split the work along national lines with Dragages working from the northern end and Nishimatsu from the south.

Both contractors are using drill and blast techniques to get through the hard, occasionally faulted, granite rock. But they are using different spoil removal methods. 'Dragages is using a crusher and a conveyor belt, ' explains McKay. 'Crushing produces good material for re-use, but if the conveyor breaks down, it's a big problem. However maintenance has been good and there has been very little down time in practice.' Progress is also dictated by the speed and capacity of the conveyor. But the advantage of using a conveyor is that it reduces the number of trucks in the tunnel, improving air quality.

At the Nishimatsu end, spoil is being removed by trucks, which take the material the short distance from the southern portal to barges. Excavation through the fault is necessarily slower as the contractor is having to install steel arch ribs to support the ground as it edges forward.

Less straightforward is Dragages Zen Pacific's Kwai Tsing tunnels contract. This is effectively two tunnelling jobs; one a 1.7km drill and blast tunnel though a steep sided hill just north of Mei Foo and the other a 1.8km bored tunnel linking a drill and blast section with Tsuen Wan West.

Dragages Zen won the contract with an alternative design.

KCRC had gone out to tender expecting the drill and blast section to connect with a stretch of cut and cover. McKay says this section was not thought long enough to justify the use of a tunnel boring machine.

During the tender process, two bidders proposed lowering the tunnel alignment to enable use of a TBM. Dragages Zen Project director Charles Etienne Perrier claims the area around the tunnel is too congested to make cut and cover effective.

This part of the project crosses three major roads and 12 high voltage cables. Cut and cover work would also have involved excavation in a nullah or flood drainage channel.

A shallow cut and cover tunnel would also have passed through poor quality ground which would have needed extensive stabilisation work before excavation could start, he adds.

Tough noise restrictions on excavation work as it passes near residential and office buildings would have added to the problem.

As a result the joint venture, working with designer Atkins China, came up with a bored tunnel solution based on an alignment 5m below that of the designed cut and cover tunnel.

The new alignment passes through a mixture of soft granite and soft ground mixed with granite boulders. As a result the 8.75m diameter machine made by Framatome subsidiary NFM has been designed to work in earth pressure balance shield mode in soft ground or in 'open' mode in the granite sections.

Bored tunnel work has not been easy. Dragages Zen's machine managed well with the hard rock, but has struggled to excavate the transition area from the granite to soft ground.

Problems have arisen because the screw conveyor used to remove spoil during closed mode operation has had trouble digesting chunks of granite boulder suspended in the soft ground. Tool holders housing cutter heads used in closed mode operation have also taken a lot of wear from the granite boulders with the result that time has been lost as tool holders are repaired or replaced.

Though the contract was awarded in October 1998, drive of the first TBM bore only began in April 2000. It took eight and a half months to complete the 1.8km long bore, which broke through in December. There is a strong hope that progress on the second bore will be quicker with lessons learned from the first.

Breakthrough of the second bore is expected in September.

Work has been more straightforward on the drill and blast section between Mei Foo and the access shaft at the start of the TBM section. Here the biggest issue facing Dragages Zen was tough restrictions on noise and tunnel blasting from the environmental protection department and the Hong Kong mines department.

Drill and blast work is now complete and the contractor is also nearing completion of the insitu tunnel lining using mobile steel shuttering.

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