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DISCUSSION FORUM Possible causes of Japan's fatal tunnel failure Mar 2012
Shani Wallis, TunnelTalk
All five bodies of workers drowned by the catastrophic failure of the subsea tunnel drive at the Mizushima refinery in Japan in early February have been found; the last being recovered from the destroyed heading by divers earlier this month.
At the same time, an official review into the safety of shield tunnelling in Japan has been launched. Possible causes of the disaster, however, remain the source of speculation and there appears to be a reluctance on the part of the main contractor Kajima Construction to release specific information about the event.
The 5m diameter, 800m long tunnel was being excavated by an EPB TBM to link Factory A and Factory B of the refinery, under a sea channel. It was 140-160m into the drive from the 30m deep access shaft when the fatal accident occurred. The EPBM being used was of Kajima's own design and fabrication.

800m subsea alignment

A call to JX Nippon Oil and Energy Corporation, owner of the refinery, revealed that the project was being developed by Kajima and its local tunnelling subcontractor subsidiary Koshin Kensetsu as a turnkey type of procurement. "We gave an order for the project to Kajima and asked for a tunnel," said the spokesperson, adding that JX Nippon had "no influence" over the design or method of excavation. He did confirm however, that the project "will continue" but that it is not known if they will recover and "continue" the same heading "or make a new tunnel". The tunnel is to house a pipeline that would allow exchange of petrochemicals between both sides of the refinery.
Calls to Kajima and an invitation to submit a list of questions is yet to produce a reply. Conversations with others in the industry, as well as with contacts in Japan, have led to educated speculation about the possible causes of the disaster, and newspaper reports in Japan reveal further clues.
As one TunnelTalk reader said: "It is a 5m diameter tunnel, so plenty of room for the workers to make it out if the inrush had been through the screw; even if the discharge gate had been fully open or somehow the screw was retracted from the bulkhead."
Suggestions that the tail seal might have failed met with a similar response. Still the inflow would not be catastrophic for the tunnel-heading workers. A catastrophic failure of the bulkhead or other parts of the machine were also dismissed by other manufacturers. "It just could not happen that such a mistake in manufacture could happen," was one comment. Equally dismissed were suggestions of failures in the shaft, either the tunnel eye failing in the soft ground near the sea or the invert giving way under high upward water pressure.
"No, I do not believe the problem was with the machine," said another representative of the manufacturing industry, who added: "More likely the lining failed or the shield 'dropped' into a huge void or fault in the ground and the tail can separated from the lining."
It seems the latter scenario is being reported as the unofficial cause. Newspaper reports in Japan have published information that police divers, who recovered the bodies of the workers from within the failed tunnel heading, also found a sinkhole on the sea bed of some 19m diameter. The location of the sinkhole also centres the failure behind the face of the TBM.

Divers search for five lost workers

More damning, newspaper reports say that the thickness of the concrete segmental lining is 160mm and that a broken segment within the lining, one or two rings behind the tail seal, had been reported in just days before the accident on 7 February. The segment was reported, in translation, as being "broken into half". The unsubstantiated evidence assumes that this damaged segment collapsed and caused the disaster.
Details of the lining design are not known but the reported 160mm thickness of the segments is thin compared to other lining designs for a tunnel of this size through soft ground conditions and for a subaqueous tunnel project.
The Minister of Land, Infrastructure, Transport and Tourism in Japan, Takeshi Maeda, is reported as saying that the Ministry will set up a discussion session with experts in the wake of the accident. This forum will examine safety of design, construction and technology of the shield tunnelling method with a view to preventing reoccurrences of similar accidents. But this falls far short of an official safety investigation or any suggestion of prosecutions for failure to comply with any national safety legislation. One comment was that because the project was an agreement between one private company and another private construction company, laws, regulations and minimum design specifications that would govern projects by government agencies do not apply.
Another anomaly, as reported to TunnelTalk by a JX Nippon spokesman, is that "excavation [of the 800m long tunnel] started in August 2010 and was scheduled for completion in March 2014". This suggests that there was a long period of delay in advancing the heading by only 140-160m in some 18 months.
A possible explanation for this, from a contact in Japan, is that work on many construction projects was suspended after the devastating consequences of the earthquake and tsunami that hit the east coast of Japan in March 2011. A contact explained: "Power was in very short supply after the disaster, especially over the winter. All nuclear power stations in Japan were closed down following the failure of the Fukushima Daiichi nuclear plant and some are still off-line. Some projects are also still on suspension while others are only just starting again." To put this in some form of context, Japan is one of a very few countries in the world that relies heavily on nuclear power to meet domestic power demand.
In the aftermath of the tunnel failure disaster at Okayama, many shield TBM tunnelling projects in Japan were also closed down, with government authorities asking contractors, and in turn, manufacturers, to be sure of the safety of the TBMs and their systems.
The outcome of the disaster may lead to changes in the law in Japan but the whole incident may also be allowed to pass without significant impact. The evidence of the one survivor of the tragedy might influence the outcome and as another contact said: "It is a very sad story and should remind us all that tunnelling is a potentially dangerous game. That is why engineers should be in control and not accountants or executives."
References
Five feared dead in Japanese tunnel collapse - TunnelTalk, February 2012

           

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