Volume Six - Pre-Victorian to the Present Day - Select Aspects - The subway under the river Ely story . . .

We now have a circular aperture of slightly greater diameter than the ring so the shield is now retracted into the tail section and the segments are assembled with nuts and bolts. Heavy gauge lead wire filament is fed into a groove machined in the flanged faces of each of the segments and at the vertical flanged faces. Tightening the nuts and bolts sequentially provides a watertight seal between the machined faces of the segments in a ring and between each ring. Another common sealing medium was 'rust cement', or rather iron filings mixed with sal ammoniac. The annular gap of 2" (5 cm) around the ring was pumped with concrete grout to fill the gap and provide additional sealing. The construction gang worked behind an air lock since the shield area was pressurised to counteract the hydrostatic (water) pressure outside and thus to prevent or limit the ingress of water to the workings. Air was supplied through pipelines leading to a steam powered compressor at the surface. The pressure inside the shields would be progressively increased as the depth of the tunnel increased and/or as the tide above rose. It had to be carefully balanced since the increased air pressure had implications for the health of the workers and there was always the risk of the compressed air pressure overcoming the resistance from the surrounding ground with a sudden blow-out resulting. To prevent this occurring, a surcharge of spoil was dumped into the river above the excavation much to the annoyance of local mariners!

Another view of the workers within the tunneling shield showing the cramped conditions.