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Japanese tunnel collapse: defects found in similar structures

Engineers have discovered defects in almost a quarter of Japanese road tunnels.

Japan tunnel collapse

Accident site: At the tunnel portal

The defects were found during safety inspections which followed last month’s fatal tunnel roof collapse 80km west of Tokyo.

Japan’s Ministry of Land, Infrastructure, Transport and Tourism (MILT) said it had inspected 57 out of 59 tunnels with a similar construction to the Sasago tunnel in which a section of roof collapsed killing nine and injuring two last month. Fourteen were found to have defects.

Emergency inspections were ordered by MILT minister Akihiro Ota after a bolt failure caused a 130m length of suspended precast ceiling to collapse, crushing three vehicles (NCE 6 December 2012).

Among defects found in the subsequent inspections of other tunnels were missing or corroded anchor bolts and concrete degradation.

Repairs have been ordered, but no timescale for completion has been announced.

Arup principal and tunnelling expert David Caiden he had long held concerns about this type of construction, which is common in road tunnels longer than 2km.

“It is shame when a safe compression structure like a tunnel is made into a risky tension structure by using steel hangers or bars to hang heavy panels from the crown,” he said.

The Sasago tunnel reopened on 29 December after repairs

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