Japan team improves tunnel inspections


A research team has succeeded in improving a tunnel inspection method that uses lasers, detecting cracks and other flaws under the concrete surface far more rapidly than existing systems.

The team, including researchers at the Japan Atomic Energy Agency and the Institute for Laser Technology, announced the improvement Sunday in Nagoya at an annual meeting of the Laser Society of Japan.

Under their method, inspectors can check tunnel wall conditions at the frequency of 25 times per second, compared with twice a second with the current method.

The improvement will make it easier to introduce machine-based inspections to replace the current manual inspections, in which workers sound tunnel concrete with hammers to discover flaws under the concrete.

“We want to verify (our method) using tunnels within two years” as there are various sizes and shapes of cracks, holes and other flaws inside tunnel walls, said Tetsuya Kawachi of the Japan Atomic Energy Agency.

They will also have to ensure compatibility between this method and tunnel sounding inspection data accumulated by tunnel managers, he said.

Laser radiation causes a sudden rise in the temperature on the surface of tunnel wall concrete, generating shock waves as a result.

Inspectors can measure vibrations caused by the shock waves through separate laser radiation. Flaws inside the walls can be detected through computer analyses of the frequencies of reflected laser light.

The performance of a laser generator deteriorates due to the heat that the equipment generates. The team solved this problem by introducing a water-cooling system and a special lens.

The improved inspection method can be used to check tunnel wall conditions up to 10 cm under the surface. Deeper locations are checked with ultrasonic flaw detectors.