A
world-class underground discharge channel has been constructed on the outskirts
of Tokyo, the capital of Japan. The tunnel, dug about 50 meters below ground,
extends 6.3 km in total. The underground construction, comprising vertical
shafts to store floodwater – which look more like gigantic tanks – and a
mammoth water tank supported by towering pillars weighing 500 tons each, is far
beyond anyone's wildest imagination. It was planned as an anti-flood scheme for
local residents and completed in 2006. The underground discharge channel,
having employed a variety of new technologies, is the very best of Japan's
state-of-the-art civil engineering technology.
Gigantic Underground Shaft
Japan is a long ribbon of islands
stretching north to south, divided by a spine of steep mountains occupying 75%
of the land. Steep rivers flowing into the ocean easily overflow when heavy
rain falls. So, Japan's effort to develop flood-control technology dates back
to the ancient past. The Metropolitan Area Outer Underground Discharge Channel,
an underground discharge channel constructed in Saitama Prefecture adjacent to
Tokyo, represents technological innovations accumulated over many centuries.
The discharge channel is a
mechanism to drain water from flooded residential areas into five gigantic
vertical shafts built below ground and then discharge it into rivers through an
underground tunnel connecting the shafts.
The cylindrical shafts are
about 70 meters tall. The large shafts measure about 30 meters in diameter,
spacious enough to park a space shuttle.
The connecting tunnel 50
meters below ground measures about 10 meters in diameter. The tunnel stretches
for 6.3 km, including a sharp curved line with a minimum radius of 250 meters.
Before being discharged
into rivers, the drained water is stored in a huge pressure-controlled tank.
The tank is designed to perform multiple functions, including abating the force
of running water and adjusting water pressure that could change sharply if a
water pump breaks down. Measuring 177 meters long and 78 meters wide, and lying
about 22 meters below ground, the water tank is larger than a soccer pitch. The
ceiling of the water tank is supported by 59 pillars which are 18 meters tall and
weigh 500 tons each. An inside look at the tank structure conjures up the image
of a “temple” below ground.
Developing New Construction Method
The underground tunnel for drainage,
dug with a gigantic shield tunneling machine, employed an improved segment
technology. Segments, which are concrete plates, used to be bolted together to
form the outer wall of a tunnel. It consumed a lot of time to bolt them.
In constructing the
underground channel, however, segments were joined together with a wedge method
developed on the principle of wedging instead of bolting. Unlike bolted walls
with an uneven surface, joined walls have a smooth surface and are instrumental
in greatly reducing construction time. The segment-joining method was jointly
developed by six Japanese companies, including general contractors, and it
became possible to reduce the time to build walls so much so that it has
emerged as a standard method for large tunnel construction work.
Rapid
& Massive Water Discharge
Flood water stored in the channel can
be discharged into rivers at a maximum rate of 200 cubic meters per second. In
other words, a 25-meter swimming pool full of water is drained in a second.
Under this mechanism, each drainage pump has a high-speed impeller that gives
flowing energy to water, discharging it rapidly.
The impeller is powered by
a gas turbine engine, a modified version of the high-performance engine used in
jetliners. Hot, strong wind generated by burning fuel revolves the impeller,
which in turn makes the water flow. The impeller is compact and generates less
vibration and noise, saving much needed space below ground.
After the underground
discharge channel was completed, a torrential rain hit the area in August 2008.
The facility was able to discharge into rivers about 12 million cubic meters of
water, a record high, or the equivalent of 25,000 25-meter swimming pools.
Flood control using an
underground discharge channel has attracted worldwide attention and draws
visits by flood control experts from many countries, including China and South
Korea. Japan has a small land area and cities are densely populated, making it difficult
to build large water discharge channels on the surface. Japan has solved this
dilemma with its civil engineering and flood-control technologies.
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