Volume Ten - Pre-Victorian to the Present Day - Even more aspects - Investigation into the Condition of the Ely River Subway 1991 . . .

• April 1898 - Steam powered air compressor on the surface. + View of air lock and bulkhead within tunnel.

The tunnelling technique adopted for the construction of the Subway would have been similar to that used today, however the problems of maintaining a dry excavation using compressed air must have been considerable. The pressure of the compressed air is necessary to balance the hydrostatic pressure due to the height of the water table above the tunnel. However the air pressure is constant over the height of the excavation whereas the hydrostatic pressure increases with depth; this would have resulted in air escaping from the crown and water ingress at the invert. Additionally the water table level would have varied considerably with the ebb and rise of tide. The balancing of the air pressure and hydrostatic pressure was therefore very important a reduced air pressure would result in significant water ingress or even an unstable excavated face and an excess of pressure would have resulted in unacceptable air loss from the face or, much more significant, a 'blow-out'.

A 'blow-out' occurs due to a sudden escape of compressed air to the surface, resulting in a dangerous decompression and large inundation of water and soil into the tunnel. The Engineer and Contractor were obviously aware of these dangers; the archive photographs clearly show significant deposition of material above the line of the tunnel on the banks of the river, a practice which is still carried out today. It is also apparent that material was deposited on the river bed to the obvious protests of Harbour users.

• August 1898 - Surcharge placed above the line of the Subway in order to prevent a compressed air blow-out from the tunnel.