To prevent blows resulting from a leakage of compressed air and oxygen deficiency in the air by mixing a tracer gas in the compressed air, feeding the mixture into a tunnel, and checking the presence/absence of the tracer gas by a tracer gas detector at the ground surface.
When a tunnel is built, compressed air of a predetermined pressure with helium gas mixed is sealed from an upper air lock 3 into a working chamber 2 and the ground is excavated downward in the working chamber 2. In this case, it is enough to mix the helium gas with the compressed air approximately by 1%. A gas in the periphery of a caisson body 1 is sucked at the ground surface by a helium leak detector 6 which is loaded on a cart 7 and thus freely movable. The presence/absence of the helium gas is thus detected. If the compressed air with the helium gas mixed leaks in a stratum 5, the helium gas penetrates the stratum 5 faster than the air and scatters at the ground surface. Therefore, by checking the presence/absence of the helium gas by the helium leak detector 6, a leakage of the compressed air from the tunnel can be detected before the compressed air leaks to the ground surface.
TOMIYAMA YASUO
HIBI HIROYUKI
SATO HISAHIRO
KOBAYASHI SHIGERU
HASEGAWA KO
SUGIURA TOSHIHIKO
MEIJI CONSULTANT KK
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