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  Saturation Techniques in Support of Pressurized Tunnels

  Types of Exposures Hyperbaric conditions allow for access to the components of tunnel boring machines which are subjected to pressure from the surrounding soils and groundwater. In relation to saturation diving, there are two distinct types of hyperbaric exposures are bounce and saturation. Bounce mode refers to a hyperbaric exposure followed immediately by decompression. In this mode, the amount of time spent in the excavation chamber is based on the pressures encountered. The higher the pressure, the shorter the work time will be. Decompression may be either “constant slide” or “staged”. Constant slide is the continuous reduction of pressure until the exposed worker is at atmospheric pressure. Staged decompression is a reduction in pressure to various “stops”. Each stop is timed and typically involves the inhalation of pure oxygen. As an alternative to this, saturation may be used. In saturation, workers are compressed in a hyperbaric chamber and their body tissue can equilibrate with the pressurized gas in the system. At the point of equilibrium, the decompression duration is fxed. Any additional time spent at pressure will not require additional decompression time. This allows for work shifts which are not determined by pressure. Cutter head maintenance or repair can be done around the clock with two or three crews working for several hours each. In saturation, workers live in a pressurized chamber, travel to and from the TBM in a pressurized shuttle, and work on the TBM itself at the same pressure. Work cycles of up to 28 days are possible with this technique. Decompression is typically a constant slide and takes place at the end of the work cycle. Decompression from saturation typically takes several days but is usually above ground and restarting of the TBM will not interfere with it.

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