Failure of a steel bracket is understood to be responsible for the collapse of the concrete ceiling in a Japanese road tunnel that killed nine people and injured two more this week.
Tunnel owner and operator the Central Nippon Expressway (Nexco) has launched an investigation into the cause of the collapse, with engineering experts citing failure of a steel bracket holding up a suspended precast concrete ceiling.
“It’s the failure of long steel bolts holding up the ceiling separating the roadway and air ducts,” said Halcrow global technical director for tunnelling Martin Knights. “It’s not a tunnel collapse.”
The twin bore, 4.7km long road tunnel is located on the Chuo Expressway about 80km west of Tokyo. It links the capital with the mountainous regions to the west of the country. It was built in 1977 and last inspected two months ago.
Around a 100m length of concrete ceiling panels fell from the roof of the Toyko-bound carriageway at about 1.7km from the eastern portal just after 8am local time on 2 December. Three vehicles were crushed by the falling concrete, with at least one catching fire as a result.
Each bore is around 7m high and 6m wide and has a suspended concrete ceiling about 5.5m above the dual carriageway road level.
The tunnel construction method was unclear as NCE went to press. But Nexco confirmed that the ceiling panels are 5m wide, 1.2m long and 100mm thick precast concrete slabs suspended from the top of the tunnel bore and supported on both sides by the tunnel wall (see drawing).
The panels separate the roadway from two ventilation ducts that also carry the tunnel’s lighting system.
Knights said the likely failure of one fitting caused a progressive collapse but said the exact cause of failure could not be established yet. “It could be corrosion or lack of durability,” he added.
Confidential Reporting on Structural Safety (Cross) director Alastair Soane told NCE the collapse bore very similar hallmarks to that of the Boston tunnel collapse (20 July 2006).
In that case the failure was down to incorrect resin being used to fix the panels in place.
Cross has been concerned about suspended ceilings for some time, and issued an alert on the issue in 2008. “There is often not enough attention put on this issue,” said Soane.
Soane said progressive collapse of a tunnel ceiling - as appears to have happened in Japan - should be avoided. “There should have been a risk assessment to ensure there was not an unzipping of the tunnel linings,” he said.
Rescue efforts were hampered by excess smoke build up in the tunnel.
Independent fire consultant and expert witness Fathi Tarada said this was in part down to the ventilation system used.
“In tunnels of this length - say over 3km - you would have to use a transverse ventilation system,” Tarada told NCE.
A transverse ventilation system has a centralised exhaust and supply system - often at the tunnel portals - with exhaust and fresh air ventilation ducts running the length of the tunnel.
As a result when the ceiling collapses - as in Japan - it also takes out the ventilation system.
Tarada said better co-ordination between structural and fire safety engineers would have reduced the risk.
The Japanese government has ordered the emergency inspection of 49 tunnels following Sunday’s collapse.
The Ministry of Land, Infrastructure and Transport ordered the inspections following fears of another collapse.
Japan’s Highway Public Corporation was privatised in 2005 and split into three companies - East Nippon Expressway Company, Central Nippon Expressway Company and West Nippon Expressway Company.