Every day in Admiralty, next to the United Centre, a large crane lifts and lowers a small metal cage at least 100 times into a deep shaft. Inside, workers, mostly men from Nepal, are transported to and from their workplace some 50 metres underground.
The cage can hold no more than six and it's the only available access, save for the precarious-looking stepladder bolted to the granite running down one side of the shaft wall, available to those fit and brave enough to use it.
"It gets very busy at lunchtime," said Stephen Jones, the MTR's senior construction engineer for the Admiralty site.
The actual construction work is being carried out by Kier-Laing O'Rourke-Kaden, a consortium of three contractors. Below, a crew of 60 works around the clock to excavate thousands of tonnes of rock, making way for the new Admiralty station, Hong Kong's first four-line interchange.
When it is completed in 2015, the MTR Corporation's South Island East line will arrive deeper underground than the current Tseun Wan and Island lines. The seven-kilometre line will be the first to extend metro service to the city's southern district, serving the 121,000 people who live there and shuttling an estimated 46,000 there for work. And the three-kilometre West Island Line, due to be finished at the end of this year, will provide service to 200,000 people who live or work in the area.
By 2020, when the link between Sha Tin and Central is due to be completed, Admiralty will be a major transport hub for the city, an interchange of four railway lines with four new platforms connected by a new escalator network, a new concourse and a new entrance, said Michael Bezzano , the MTR's engineering planning manager for the site.
In the heart of Hong Kong's business district, the station is surrounded by high-end stores, offices and the central government headquarters.
"Everything we're doing has required very careful planning and co-ordination to make sure we have no impact on the general public and also the operating railway. That's been our main key challenge on the project," Bezzano said.
The changes appear to be desperately needed as the MTR faces growing pressure from the government to ease crowding on rush-hour trains, most of which run at maximum capacity or close to it during peak hours. Transport capacity has become an urgent concern as officials project that tourist numbers will double during the next decade in an already overcrowded city. In February, the Transport and Housing Bureau urged the MTR Corp to study measures to reduce overcrowding, and said the opening of new lines would help.
To get there, the MTR has embarked on four years of complicated construction work involving deep drilling, daily explosions and massive excavation. Tens of thousands of people pass through Admiralty every day, most of them unaware that hundreds of thousands of kilograms of explosives are routinely detonated beneath their feet, usually around three or four in the afternoon.
The "drill and blast" construction method is the only way to work with Hong Kong Island's rocky terrain. The Admiralty area has two main kinds of rock: granite and volcanic tuff, says Chan Lung-sang, an earth sciences professor at Hong Kong University and expert on the geology of the region.
"The granite is very old, dating back to the Jurassic time, formed around 160 million years ago. Often, you need to resort to powerful methods to excavate the tunnels," Chan said.
Hundreds of kilograms of ammonium nitrate, a volatile chemical compound often used in improvised explosive devices, are detonated daily, blowing out the hard rock to create the new station.
The intricate procedure is carried out with surgical precision. Up to 100 holes are drilled into the rock face, evenly spaced in a spiral formation. Small, sausage-like packets of ammonium nitrate, weighing about 200 grams each, are inserted into each hole. The staff evacuates using the metal cage and the switch is flipped. They explode each hole individually, but in quick succession. A blast of the explosives takes around five seconds and after the dust has settled, about 15 minutes later, the workmen ride the cage back down to clean up.
Using a small, diesel-powered vehicle fitted with a large scoop, tonnes of rock are transported to an 18-tonne skip at the shaft entrance, which is hauled out by a gantry crane. The debris is packed into trucks and transported to various "fill banks" around Hong Kong. Bezzano says it's likely that much of Admiralty's excavated rock will be used in the ongoing bridge-and-tunnel project connecting Hong Kong, Macau and Zhuhai on the mainland.
When the rubble is clear, the cycle begins again. More than half the space has been cleared for a new concourse, extensions to the current platforms and two new platforms. Progress is measured blast by blast, often no more than a metre at a time.
Unlike a similar expansion in London, where teams are digging up Eastbourne Terrace and diverting traffic, the Hong Kong engineers are working without dismantling the United Centre and parts of Pacific Place. Two major roadways, Connaught Road and Queensway, edge the site's northern and southern border. The Hong Kong Police Headquarters to the east and Admiralty and United Centres on the west close in the box, restricting access to the site and further complicating the work.
The entire subterranean and ground-level operation, with 400 construction workers, is based in an area about the size of a football field where Harcourt Garden once stood. Five cranes and a management office now occupy the site.
"It's a very small site for the size of project we are doing", says Bezzano.
Subterranean structures encircle the work as well, including the two current MTR lines and two car parks. So the work requires the utmost care.
A variety of measures to minimise noise pollution are enforced by the Environmental Protection Department. Acoustic shields, sound blankets, machinery mufflers and silencers in the fan ventilation system keep the noise down. "You might hear a rumble," Bezzano said.
When the new station opens in 2015, passengers will be able to travel via Admiralty from the New Territories to the Southern district, and from Chai Wan to Kennedy Town.
Just three kilometres west of Admiralty, construction continues on the West Island Line extension. Sai Ying Pun presents another set of geologic conundrums: soft soil, flowing underground water and old buildings with poor foundations. To dig a 100-metre pedestrian tunnel connecting the Queen's Road West entrance to the station via First Street, engineers use artificial ground freezing.
The technique is straight out of a sci-fi novel. Two massive, 260 kilowatt refrigerators run 24 hours a day, cooling brine to minus-30 degrees Celsius.
The freezing brine travels 30 metres below the surface through insulated pipes. It then goes into 25 non-insulated pipes drilled in a circle around the tunnel work, circulating back to the coolers at the end of its journey.
As the brine travels 100 metres to First Street, each finger-width pipe freezes a 50cm diameter ring of soil around itself, reducing the temperature to minus-10 degrees Celsius from its usual 25 degrees. The freezing process stabilises the ground around the work, which proceeds inside the ice-walled tunnel.
On First Street, just west of the Centre Street junction, a peek behind the MTR-branded balustrade reveals a tangle of pipes caked thick with ice, even during the spring's heat. When finished, the tunnel will measure six metres across, about the length of a minibus.
At Hong Kong University, the next new stop along the line extension, there is another distinct set of challenges. A dizzying distance of 80 metres separates the platform from the ground level entrance. Escalators were ruled out (too big, too slow). The solution was to install high-speed, high-capacity lifts. Double-sided doors will allow 28 passengers at a time to be catapulted towards the surface at a speed of 3.6km/h.
As the city's work day winds down, slumped office workers and parcel-laden shoppers spew forth from Admiralty's office buildings and malls. The tunnel project's night shift clocks in.
Joanna Chiu contributed reporting.