Compressed Air Tunnelling
This method is possibly the most modern method of tunnelling. The compressed air, which has a pressure of about 1 kg/cm2, is forced into the enclosed space within the tunnel so that the sides and top of the tunnel do not collapse and remain in their position. The equipment for tunnelling consists of a bulk head, which is an airtight diaphragm with an airlock. The airlock is an airtight cylindrical steel chamber with a door at each end opening inwards.
Tunnelling by means of compressed air is quite a difficult process because of the following reasons.
(a) The pressure inside the earth varies from the bottom to the top of the tunnel.
(b) It is not possible to ascertain the pressure on the floor of the tunnel as it depends upon the nature of the strata.
(c) The pressure varies from strata to strata depending upon the moisture content, which is difficult to ascertain.
(d) The compressed air normally escapes through the pores and the air pressure diminishes continuously. The application of air pressure has to be varied from time to time in order to achieve a balanced value. The determination of this value depends more on experience than on technical considerations.