Nexco Central officials speculated that bolts used to connect the concrete slabs, each weighing up to 1.5 tons, to the tunnel’s inner wall and ceiling could have deteriorated with age. One theory is that the bolts, which have never been renewed, may have been loosened by seismic activity.
Ryoichi Yoshikawa, a Nexco Central official in charge of maintenance, said damaged bolts had been found at the site, adding that they did not appear to have been replaced in 35 years. "There is no record that shows repair work was carried out in the past,” he told reporters.
The company said that no structural faults had been found when routine tests were carried out in September, but admitted the inspection did not include acoustic tests on the section of ceiling that caved in. "That is something we need to reflect on,” Mr. Yoshikawa says. “I offer my profound apologies.”
The collapse sent almost 400 tons of concrete cascading on to vehicles about 1 mile inside the Tokyo-bound lane of the tunnel.
The Sasago tunnel is part of a nationwide network of more than 1,500 tunnels that dot Japan’s mountainous terrain. About a quarter of them were built during Japan’s dramatic postwar growth more than 30 years ago.
Shinzo Abe, who is expected to become Japan’s seventh prime minister in six years, promised his Liberal Democratic party would invest heavily in public works to boost the economy. The cost of repairing and maintaining roads, bridges and other infrastructure is expected to soar in the next two decades.