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Groundwater levels rising in Tokyo

Due to rising amount of groundwater, underground structures have faced a sharp increase in troubles, including a delay in a Metropolitan Expressway extension project. -Japan News/ANN

Sun, Apr 28, 2013
Japan News/Asia News Network

TOKYO - Groundwater levels in Tokyo have risen by about 60 meters compared to 40 years ago, according to surveys by the Tokyo metropolitan government and others.

The phenomenon has been attributed to years of controlled groundwater pumping to stop ground subsidence resulting from mass groundwater consumption by factories since sometime after World War II but mostly during the rapid economic growth period.

Due to the rising amount of groundwater, underground structures and construction sites have faced a sharp increase in troubles, including a delay in a Metropolitan Expressway extension project.

In addition to prohibiting large-scale groundwater pumping under the Industrial Water Law, the metropolitan government ordinance set an average daily limit of 30 cubic meters for small pumps. Currently, the Tokyo government permits an average of 10 cubic meters a day.

In the 1950s and 60s, there were areas in Edogawa and Adachi wards and elsewhere where ground subsidence advanced by more than 20 centimeters a year. After strict regulations, the subsidence level has been held down to less than 2 centimeters a year for almost all of Tokyo.

The Civil Engineering Center of the metropolitan government has been investigating the groundwater levels at its observation wells at 48 locations in Tokyo's 23 wards. The deepest well is about 350 meters.

Comparisons of measurements at 19 locations between 1970 and 2010 found that the levels rose in all the observation wells by 15 meters or more. Records from 1970, when the pumping regulation was enforced, exist for only those 19 locations.

The largest rise, about 60 meters, was recorded in the Fujimicho district of Itabashi Ward. In the Tachibana district of Sumida Ward, the rise was about 45 meters, while it was about 39 meters in the Hyakunicho district of Shinjuku Ward.

"Places like Itabashi Ward had many factories in the past and accordingly consumed a massive amount of groundwater. It means that the groundwater level is returning to its levels in the old days, rather than increasing," an official of the metropolitan government said.

Impact on expressway and subway

With the rise in groundwater levels came unexpected results.

During construction on the Metropolitan Expressway's Central Circular Shinagawa Route, a large amount of groundwater flooded underground construction sites, including a section in Nishi-Gotanda in Shinagawa Ward. As a result, completion of the Shinagawa route has been delayed by a year.

"Water came out from locations different from predictions made based on boring surveys. The groundwater flow isn't easy to get a handle on," an official of the metropolitan government's Construction Bureau said.

Groundwater is affecting not only construction sites, but also existing underground facilities.

On the Mita Line of the metropolitan subway, sections of tunnel walls have fallen off at four locations since March. In each case, leaked groundwater caused internal steel reinforcements to corrode. The spaces created by the corrosion caused the concrete of the walls to crumble, according to the metropolitan government's Transportation Bureau.

After the first such incident occurred in the metropolitan subway, the Transportation Bureau began conducting an emergency survey in March. It has found groundwater leaks at 2,100 locations.

"As groundwater is increasing, the costs of dealing with it are also swelling," an official of the bureau said.

Underground platforms may rise

There are fears that underground facilities may begin to rise due to the water pressure increase caused by the rising groundwater level.

About 37,000 tons of iron blocks were placed beneath JR Ueno Station's Shinkansen line platforms, which are located about 30 meters underground, by East Japan Railway Co. But because the weight was considered insufficient, about 650 piles were also wedged into the ground.

About 130 piles have been driven beneath the underground platforms of the Sobu Line in JR Tokyo Station.

Rising groundwater levels may also be related to the increasing number of underground structures in the centre of Tokyo.

"An increase in underground structures would hamper the flow of underground water, raising the levels of water," said Hiroki Takamura, professor emeritus at Rissho University and an environmental scientist specialising in water issues. He is also well versed in underground structures.

"From now on, pumping groundwater will be the only way to decrease the water level," said Atsunao Marui, a groundwater expert at the National Institute of Advanced Industrial Science and Technology.

However, groundwater currents differ from location to location, making it difficult to judge how much pumping is needed.

The Environment Bureau of the Tokyo metropolitan government has already started to tackle the situation, but an official of the bureau said: "Once the ground subsidence occurs, the ground cannot return to its original state. For the time being, we just have to keep a close eye on the water levels."

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