"Supporting the
foundations of the nation" Fujita is a construction company with a history stretching back more than 100 years. Against a backdrop of in which more and more of the country's infrastructure is aging, they took on the task of removing Japan's first ever concrete dam.

The clear-flowing Kuma River runs through southern Kumamoto Prefecture. Along its banks, can be seen fishermen, their fishing rods out in pursuit of the river's silver ayu sweetfish. This was once the site of the Arase Dam that utilized the river's abundant water to generate electric power. Completed in 1955, the dam played its part in economic development and river management for more than five decades until its water rights expired. The expiry meant the removal of the dam, starting in 2012, something that had never before been attempted in Japan for a concrete dam.

The project was commissioned by the Kumamoto Prefecture Enterprise Bureau and the work was contracted to a joint venture of Fujita Corporation and Nakayama Construction Inc. Toshimune Miyachi of Fujita was appointed as site manager. While his past experience encompassed a wide variety of different structures and construction work, including tunnels and bridges, overhead wires for the Shinkansen, and water storage and drainage systems, he had never previously worked on dams or demolition.

Arase Dam is 25.0m high and 210.8m along its crest. On first visiting the site and seeing the dam and its nine structural pillars, Miyachi was overwhelmed by the sight and left wondering whether removal was really feasible.

The pillars weren't the only challenge. The Kuma River's ayu fish make their way upstream each spring to spawn in the fall. This meant that river work could not start until early November after the hatchlings had made their way to the sea and could only continue through to mid-March. Moreover, work in the river's catchment was only permitted up to the end of February, meaning that only three and a half months were available in practice. It was on these terms that Japan's first ever large-scale concrete dam removal project got underway in April 2012.

The removal of the Arase Dam was done as follows: (1) holes were made in the main body of the dam to lower the water level on the upstream side; (2) the steel gates located between the pillars were removed; (3) the spillway on the right bank that carried the main flow of the river was removed and the flow partially restored; and (4) the pillars and control bridge were demolished using blasting and heavy machinery and the main body of the dam removed to complete the job.

One difficulty was the blasting of the pillars. Because the concrete used in the original construction was reinforced with different material than what is used today, it was impossible to predict how it would respond to blasting. The specialist blasting company was hesitant to proceed. Miyachi's Fujita team and their joint venture partners consulted the literature to look for a safer alternative.

During that process, Miyachi came up with idea that perhaps they could cut the pillars down like trees. In a previous project, he had once used a chainsaw to cut down trees for a road as part of site preparation work. If a V-shaped incision was cut into the base of the pillars prior to blasting they should fall over as desired. He consulted with other companies that had done similar work in the past. A test blast was conducted on a bank-side pillar during year one of the removal project. While it didn't go the way they had planned, it did show the way forward to a successful conclusion.

In the winter of the second year, changes were made to the geometry of the V-shaped incision in which the blasting dynamite was positioned and preliminary work was done to cut the reinforcing in the body of the pillar to ensure it would collapse the right way. Finally, the day of the blast arrived. Watched over by a large audience that included people from the Enterprise Bureau, a television crew, and local residents, Miyachi commenced the countdown.

"5, 4, 3, 2, 1,... Blast!"

The explosion boomed out, dust billowed up, and the giant pillar fell gracefully to the ground. "In that moment, all my fretting and worrying was rewarded," said Miyachi. Hugs and handshakes followed with his jubilant team.

Nevertheless, their work was only just getting started. They continued making improvements with each pillar they brought down, making the process more reliable and precise and giving it their seal of approval: "This is the right way to do it."

Demolition of pillar by blasting

Site of pillar after removal