Pressurized tunneling used when the tunnel constructed within the groundwater. The ingress of water to the tunnel construction area will make the constructing of a tunnel impossible. Pressurized tunneling method will use air under high pressure to stop the water flow and increase the ground stability below the groundwater table.
Originally the whole tunnel shaft was pressurized from the face to the pressure chamber, which was often near the shaft start. The pressurizing the entire shaft is no longer used. Nowadays, pressure bulkhead (airlock) has been placed to the rear of the tunnel face. The disadvantage of pressurized tunneling is that the materials and workers have to pass through the airlock system.
The maximum working pressure and the time spend by workers under compressed air should be controlled strictly. The normal air pressure at the sea level is around 1 bar (100KN/m2). The critical level of working begins with an atmospheric overpressure of approximately 1.0 bar. The soluble nitrogen (N2) will increase in the body of anyone who works in these conditions.
Then the nitrogen will come out when the outside pressure reduced. This process will form bubbles in the blood that can block the arteries. Also, the excess nitrogen can settle in the joint causing pains. In order to prevent endangering the tunnels work. The time of exposure to pressurized air should be controlled strictly. The figure no:2 showing the maximum recommended exposure time.
Pressurized air is compressed by compressors installed on the ground surface until the required overpressure is achieved. To reach the required overpressure, the amount of air should be determined before and during the tunneling process. Blow out of air should be reduced as much as possible.
In a blowout, the pressurized air finds a path and depart the tunnel suddenly. The pressure at the face will drop and can no longer be maintained. This can increase the porosity of the ground enormously, and the pressure will drop suddenly within the tunnel, which may impose a risk on tunnellers.