Shani Wallis, TunnelTalk
- Fixtures holding the ceilings of transverse ventilation systems in the many road and highway tunnels across Japan will come under intense scrutiny following collapse of a substantial section of concrete ceiling panels in a tunnel on a major highway in Japan. The sudden and catastrophic failure trapped several vehicles and caused nine deaths.
Deadly collapse of the ventilation duct ceiling panels
- Emergency services searched through the rubble for survivors among those suspected as still being trapped.
- The twin-tube two-lane Sasago tunnel, about 100km west of Tokyo, is on the main Chuo Expressway to the west of the country. When heavy concrete panels covering about 100m of the ceiling fell onto moving traffic, one car caught fire, killing its occupants. The driver of a trapped truck, who managed to make a call for help on his mobile phone, was later found dead in his cab.
- The highway and its 4.5km-long twin-tube Sasago tunnel was built in the mid-1970s. It has a transverse ventilation system with two ceiling ducts for drawing in fresh air and exhausting polluted air (Fig 1). Longitudinal ventilation systems, with high-powered jet fans fixed at intervals directly to the tunnel crown to ventilate the tunnel via the portals and eliminating the need for ducts under the road deck or false ceilings above, are becoming increasingly the norm.
- A report to TunnelTalk from Japan explains that more than 300 ceiling panels, weighing about 1,200kg each, collapsed in a section of tunnel about 1.7km from the exit of the eastbound tube, crushing three vehicles and their occupants and damaging several other vehicles. Further reports describe the ceiling panels as being 80mm thick precast concrete combined with steel beams with a vertical concrete panel of 100mm thick to create the two ducts, and fixed by steel hangers and turn buckles anchored to the tunnel crown by the two fixing bolts at 1.2m intervals (Fig 2).
- An early suspected cause of the collapse is failure of the anchor bolts due to the ageing and deterioration of the concrete and other ceiling materials.
- The ceiling collapse situation in Japan is similar to the fall of transverse ceiling panels in a section of the Big Dig highway tunnels in Boston, Massachusetts, in July 2006. On that occasion a passenger in a moving car was killed. An official report cited deficiencies in the anchor bolts and epoxy used to hold 4,600lb suspended ceiling panels in the cut-and-cover tunnel. The design of the transverse ceiling in the tunnel in Japan, and the cause of its failure, is yet to be investigated and explained. One reporter stated that a mudslide had triggered the ceiling collapse but there is no substantive evidence that the tunnel itself suffered a geological or tunnel lining or primary support failure. Photos and video footage indicate that it is the false ceiling and its connections to the tunnel walls that have failed.
- The tunnel is owned and operated by NEXCO, a private highway company. It stated the tunnel was inspected and had passed an inspection two months ago. But these inspections, claim reports, did not include inspection or maintenance over the life of the tunnel of the precast concrete of the panels or the steel hangers and their anchors. Other opinions also suggest that there was insufficient knowledge during design of the tunnels in the 1960s-70s about the aerodynamic loading in the ventilation ducts of compressed air in the fresh air duct and vacuum pressure in the exhaust duct. It is also thought that the potential impact of pressure variations caused by heavy goods vehicles passing within 700mm of the ceiling surface might not have been considered.
- Reports state that this is the worst such accident in Japan since 1996, when a tunnel collapsed and falling rocks crushed cars and a bus, killing 20 people. Following the current incident, the Japanese Government ordered inspections of 49 tunnels with a similar ventilation system.
- The country has an ageing infrastructure, the maintenance of which is becoming increasingly costly to the public finances. The deadly accident this week elevated tunnel safety as a key issue ahead of national government elections in Japan next week.
- Osamu Fujimura, Japan's Chief Cabinet Secretary called for a substantial review of the country's ageing infrastructure, saying there was a need for "huge investment" to maintain its safety. Yoshihiko Noda, the Prime Minister, said in his official blog that Japan should now focus on inspection and maintenance work that would really protect people's lives, rather than elect the opposition on its ticket of a huge new debt-funded construction spending programme.
- The incident also has highway agencies around the world reviewing the interior ceilings of older tunnel assets and rethinking the criteria and procedures for inspection and maintenance programmes.
Exit spacing influence on ventilation choices - TunnelTalk, May 2012
Sprinkler limitations for tunnel fire fighting - TunnelTalk, September 2011
Mobile furnace used for fire safety testing - TunnelCast, August 2012
Fatal accident raises demand for Big Dig accountability - TunnelTalk, September 2006
Fire damage rebuild of Mont Blanc road link - TunnelTalk, June 2001
Apparently, in addition to failing bolts, some of them were missing. This is a perfect example of failure resulting from failing to consider 'imminent' and 'eminent' dangers.
Peter J. Tarkoy, Geotechnical and Underground Construction Consultant
From the Editor: An article to expand on this topic is to be published on TunnelTalk.
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