1956 Start of Construction of Kurobegawa No. 4 Power Station Omachi Tunnel
As Japan entered a period of high growth, it faced a serious shortage of electric power. In 1955, Kansai Electric Power Co., Inc., which single-handedly provided power to the Kansai Region, decided to build the Kurobegawa No. 4 Power Station. Kumagai Gumi accepted a contract to work on construction of the Omachi Tunnel, which would extend 5,400 meters through Japan's Northern Alps to enable the transportation of dam construction materials to the difficult terrain of the remote Kurobe Valley. (Kumagai Gumi handled the 2,700 meters from the Omachi side.) The work proceeded smoothly at first, but at about 1,700 meters they hit a major fault and a fracture zone which had amassed a large amount of water. In the shaft, there were areas at which water and soil poured in at rates of up to 660 liters per second, creating the danger of a cave-in.
Every method was tried, but the tremendous flow of cold, 4°C water hindered construction, and changing the route was even considered.
Seven months after reaching the fracture zone, the impact of the drainage tunnels that were repeatedly dug combined with Kurobe's bitter cold to reduce the water flowing into the shaft, and they were ultimately able to push through the fracture zone, something that had been considered impossible.
The two ends of the tunnel met in February 1958, a year and ten months after work on the tunnel had started.
In addition, the project was documented in the film Kurobe no Taiyo ("Kurobe's Sun"), which was released in 1968. Kumagai Gumi gave full cooperation to the filming, building an enormous set at the Toyokawa Plant to recreate scenes of work in the tunnel and the outflow of water.