A gas explosion occurred May 24 inside a tunnel under construction in Niigata Prefecture, killing four workers. It is known that the workers were not carrying explosive gas detectors. The police must carry out a thorough investigation of the contractor, Sato Kogyo Co.

They also should examine whether the infrastructure and transport ministry had properly advised the company and its subcontractors concerning the prevention of an explosion.

The construction of the 2.8 km-long Hakka-toge (Hakka mountain pass) Tunnel started in the spring of 2007 as part of National Highway No. 253 linking Minami Uonuma City and Tokamachi City, both in Niigata Prefecture.

Sato Kogyo contracted to construct the 1.4 km section of the tunnel on the Minami Uonuma side.

After the construction work was halted in December 2011 due to the coming of winter, preliminary work started May 18 to prepare for full resumption of the construction. The excavation operation had been completed by then.

The explosion occurred around 10:30 a.m. on May 24, trapping four workers scheduled to check a blower at a point about 1.2 km from the tunnel entrance. Three other workers outside the tunnel were injured by an explosion blast. Rescuers found the four workers’ bodies at a point about 1.3 km from the entrance around 12:15 a.m. on May 27. A high concentration of explosive gases hampered the rescue work. The bodies were taken out of the tunnel shortly after 6 a.m. — about 67 hours after the explosion. On the morning of May 26, a 30 percent gas concentration was detected at a point some 900 meters from the entrance.

Despite this high concentration of explosive gasses, the circuit of the blower in the tunnel was not covered by insulation.

A key question is whether the infrastructure and transport ministry’s Hokuriku development bureau — which placed the order for the tunnel construction and was aware of the geological conditions — clearly warned Sato Kyogo to take precautions against explosive gases and whether the company accurately conveyed such instructions to its subcontractors.

An official of one subcontractor firm said it was not told about the dangers of explosive gasses, and that if it had known of the dangers, it would have requested the contractor to authorize the use of an explosion-proof blower.

Sato Kogyo’s planning document failed to make it obligatory to measure the concentration of explosive gases in work that does not involve excavation. If the ministry’ regional bureau officials and Sato Kogyo had been conscientious about the safety of workers, the tragedy could have been avoided. The ministry should launch an in-house investigation to reveal what missteps led to this disaster.

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