• Sun
  • Oct 26, 2014
  • Updated: 4:46pm

The link they built without any fireworks

PUBLISHED : Thursday, 01 May, 1997, 12:00am
UPDATED : Thursday, 01 May, 1997, 12:00am
 

It may not be as picturesque as the soaring Tsing Ma Bridge but the man in charge of building the Western Harbour Crossing tunnel, opened yesterday, believes it is an even more impressive engineering achievement.


Two kilometres long, 1,300 metres of that length under the sea, the tunnel runs from the tip of sandy Western Kowloon Reclamation to Sai Ying Pun, on Hong Kong Island, where a complex new road junction to disperse tunnel traffic is crammed on a narrow site between high-rise buildings and the harbour.


'This is one of the largest immersed-tube tunnels in Southeast Asia, so it was no mean feat,' project manager John Mundy said proudly, in his dusty office in a prefabricated building near the gaping West Kowloon portal.


The figures are mind-boggling: more than 12 million man-hours to turn 470,000 cubic metres of concrete and 84,000 tonnes of reinforcing steel into a tunnel. The airport core project can handle 180,000 cars each day, directed to more than 10 kilometres of approach roads and 17 bridges by more than 300 road signs across the territory. Construction accidents cost four workers their lives.


As an immersed tube tunnel, the Western Harbour Crossing runs through a giant concrete pipe buried just under the seabed rather than through a hole bored deep in the bedrock.


Crowded Hong Kong is ideal for this technique because it does not take up much space. Now five of the world's 100 immersed-tube tunnels are located in the territory, says Mr Mundy, including Eastern Harbour Crossing. 'You choose to go for an immersed tube because you can keep the tunnel shallow, and therefore that reduces the lengthy approaches,' he explains.


For the 56-year-old Londoner, who has lived in Hong Kong since 1982, Western Harbour Crossing is the latest in a string of tricky projects in the territory, including Kwai Chung Container Terminal No 8, Tait's Cairn Tunnel, Plover Cove Reservoir and Sai Wan Ho MTR station.


Sinking a tunnel in the world's busiest harbour was always going to be difficult, but marine traffic was not the only problem.


The cramped Sai Ying Pun site was on an old unstable reclamation filled with rubbish and mud. Before they could start building, the construction team had to compact the land and drain off trapped water to make the ground solid.


All the while, heavy traffic had to be kept moving in the area, and the team had to work around a dangerous bank of high-voltage cables that straddle the site and supply electricity to the whole of Central.


They were on firmer ground with the brand new West Kowloon Reclamation - and no cables or pipes ran through it - but workers had to watch out for unexploded World War II bombs. Several, sucked up with marine sand from the bottom of the harbour by dredgers collecting material for the reclamation, were found during excavation work.


In addition to these difficulties, speed was crucial. The clock started ticking on the 30-year 'build, operate and transfer' franchise awarded by the Government to Western Harbour Tunnel Company the moment the agreement was signed in July 1993.


The longer it took to build the link, the less time the operators would have to collect revenue from paying users before handing the tunnel over to the Government in 2023.


Mr Mundy managed the project for a joint venture by two Japanese construction companies, Nishimatsu and Kumagai Gumi. The $5.7 billion contract bound Nishimatsu-Kumagai to complete the link within 47 months. They finished it in 44.


Unlike conventional projects which can be on the drawing board for several years for detailed blueprints to be completed before building even begins, construction on the tunnel started within two months, with just rough plans for how the tunnel would eventually look, according to Mr Mundy.


Work began on land, but soon dredgers were out in the harbour clearing a trench through the seabed to place the tunnel in. The water above the tunnel had to be at least 13.5 metres deep to allow ships in and out of the harbour.


'We've probably gone down 25 metres in some places,' Mr Mundy says. 'If you look at the geology of the harbour it's got marine mud, then alluvial deposits, and then you get down to harder material.' Dredging was complicated by the heavy marine traffic. As it progressed over the months, the Central Fairway, a marked passage for ships through the harbour, had to be moved five times.


Work on another immersed-tube tunnel, for the MTR's new Lantau line, was going on nearby so, to minimise disruption, the project managements synchronised their progress across the harbour.


At the same time the first of the 12 huge concrete tunnel sections - each 113 metres long, 33.5 metres wide and 8.5 metres deep - was being made at Shek O quarry, chosen because it was next to the sea and remote enough to minimise environmental concerns. The sections, produced in batches of four, were sealed at each end with a steel bulkhead on completion. The quarry was then flooded and the sections were floated out to sea and towed to Junk Bay to be fitted with pontoons. One by one they were pulled through Victoria Harbour into position, a journey that took about four hours.


'I must admit harbour traffic was a concern I had initially,' Mr Mundy recalls, 'but I think people have a fairly healthy respect for something as large as a tunnel unit. They were escorted by tugs and patrol boats, so anybody who was getting too close to them would have been chased away.' Once a section was manhandled to a point above its correct position, ballast water was slowly pumped into internal tanks to sink it. Each section then came to rest on brackets projecting from the end of the previous section and against two hydraulic rams resting on concrete pads that had been strategically placed on the seabed to nudge it into position. Eight divers swam round checking that each section was perfectly in place.


The water between the bulkheads of the previous section and the next one was then pumped out, and sand pumped through pipes in the section walls, filling the void beneath.


Eventually, then, each section rested on a base of sand. The water occupying the ballast tanks was replaced with concrete.


It took 16 hours to lower the first section to the seabed, but with practice the crew halved the time this process took.


The bulkheads between sections could then be removed, although to guard against a catastrophic failure the construction team preferred to keep two steel bulkheads between the workers and the sea, he said.


By last October all the tunnel sections were in place, and it was possible to walk straight from Sai Ying Pun to West Kowloon.


Two metres of rubble was piled on top of the tunnel as 'armour rock', to protect it against anchors. Within a couple of years, natural silt will cover the 'armour rock' and the tunnel is expected to be no longer visible from the seabed.


With the finishing touches of lighting and ventilation completed, the construction team handed the crossing to the Western Harbour Tunnel Company last Monday week.


The end of the project spells unemployment for Mr Mundy, who will leave his drab site office in the next few weeks. He plans to take a holiday before questing after his next construction project in the territory.


'I keep promising myself I'll retire,' he says, 'but that seems to be getting pushed further and further back into the distance.'

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