The G-Cans project is the largest flood protection in the world. Its official name is Metropolitan Area Outer Underground Discharge Channel, but it is most known as G-Cans Underground Temple because of its support pillars and interior that reminds the visitor of a giant cave. The purpose of this facility is to protect the city of Tokyo from floods. The northern edge of G-Cans touches Kasukabe, a city located in Saitama Prefecture, 30 km north of Tokyo. The necessity of building such facility originated from the danger of overflowing of the rivers and main waterways during typhoons and rain periods.
In fact, the G-Cans Project collects the flood water from several rivers around Tokyo, such as Oochi Kotone, Arakawa, Kuramatsu and Nagakawa, and transports it safely to the Edogawa River. This “exhaust river” begins as a split from the Tone River and has several cities on its way to Tokyo Bay. Thanks to this complex that cost $2 milliard, the Tokyo district settlements are saved from flooding. According to the official information, “The facility is capable of withstanding a once-in-200 years flood.”
The G-Cans complex was built for 17 years, between 1992 and 2009. It has six general points – 5 concrete silos with 32 m diameter and 65 m height, and the main facility known as “The Temple” – a huge water tank with 59 pillars with height of 20m. The Temple is more than twice shorter than the other silos – 25.4 m, but it takes several times larger area with its length of 177 m and width of 78 m. It weighs roughly 500 tons. The Underground Temple has connection to 78 10MW pumps and 14 000 hp turbines that are able to transfer around 200 tons of water per second to the nearby Edogawa River. This river is located at lower altitude than the Japanese capital, crossing the Kanto Plain though the middle just 20 meters above the sea level.
The Temple and the other silos are connected via 6.5 km long and 10.6 m diameter tunnel constructed 50 m below the surface that spans between Showa in Tokyo and Kasukabe in Saitama. When flood occurs the water is filling the silos consequently and the final point of the water “journey” is the Temple, from where it is pumped out to the river. The whole process is observed by a control room.
The underground canal was built by using shield tunneling technology. The essential idea of this technology is a reinforced solid frame to be pushed through a soft ground. This prevents the ground collapsing and allows for building a tunnel within the frame. Of course, mud and soil are excavated from the inside of the frame so that the building could start. The tunnel grows in length by repeating the process until the tunnel is completely built.
The G-Cans complex has no water inside during the dry months so it is used as a tourist attraction. Stairs fixed to the silo walls allow the access to the tunnel. However, if you’re a foreigner in Japan and want to see it, you’ll need a Japanese-speaking friend because all explanations are on the local language.
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