TOKYO’s extensive network of underground train stations is in peril from the
city’s rising water table. Japanese engineers are trying to prevent some
stations from essentially pushing their way upwards like submerged bubbles.
“This was a unique problem, and it needed a unique solution,” says a senior
Japanese railway engineer, who asked not to be identified.
The most pressing problem at the moment is the city’s main terminus, Tokyo
station, whose lowest platforms are now 12 metres below the water table.
Engineers believe that the station’s platforms and tracks could buckle if the
water table rises only another 70 centimetres.
The problem dates back to the 1960s and 70s when Japan’s expanding urban
stations had to extended downwards. At that time the water level was low, after
decades of industrial growth. But the falling water table caused so much land
subsidence in Tokyo that the Japanese government banned the practice of drawing
underground water in the 1970s.
Advertisement
At Ueno station in Tokyo, the water table was 38 metres below ground level
when new underground bullet train lines were designed. By the time construction
was complete in 1985 the water level was just 18 metres down—10 metres
above the level of the new lines.
According to Japan Railways East, which operates Ueno station, upward
pressure at the lowest level of the complex is around 16 tonnes per square
metre. To counteract the pressure a ballast of steel slabs, each 13 by 0.8 by
0.24 metres, was laid out between the bullet train tracks. Together they weigh
around 30 000 tonnes.
But that was not an option at Tokyo station, in the heart of the city where
space is at a premium. Instead of using ballast, engineers decided to anchor the
structure to stable rock 18 metres below the tracks. Each anchor is made of nine
12.5-millimetre twisted steel cables that can take a load of 100 tonnes.
Mike Gellatley, infrastructure systems engineer at London Underground, says
that London’s tube network is also affected by a rising water table. “This
hasn’t taken London by surprise. New structures have flotation consideration
incorporated into the design and the buildings themselves act as dead weight,”
he says.
Tokyo’s water table hasn’t risen now for three years. Japan Railways East is
keeping its fingers crossed that this is not just a temporary reprieve.
