ABSTRACT
The virtue of unmanned in-caisson excavation systems (Shiraishi, 1993), is that men are relieved from hard digging in the hazardous compressed air environment of caissons. However, occasional man entries under very high air pressure are still required for inspection or minor repairs of the excavating machines and appliances installed in the compressed-air chamber. Because the incidence of such harmful disorders as decompression sickness, nitrogen narcosis, dyspnoea or oxygen toxicity, is higher under air pressures exceeding 4 bar (5 ATA, kg/cm2g) than under lower air pressures, the limiting pressure had been considered to be 4 bar. In order to overcome this pressure barrier, an experimental research programme was sponsored by Shiraishi Corporation with the cooperation of the Tokyo Medical and Dental University (TM&DU) and the Japan Marine Science and Technology Centre (JAMSTEC) aiming to establish a system which would enable men to enter into compressed air at very high pressures, up to 7 bar, by applying deep-sea diving technology to the compressed-air work. In this system, a mixture of oxygen, helium and nitrogen (Trimix) is used for breathing through masks at the same pressure as the air in the working chamber.
