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

To prevent water leaking into the subway, each segment and then each ring upon assembly had all mating surfaces jointed and sealed using lead wire caulking. As the bolts were tightened the lead wire deformed and filled any remaining gaps caused by misalignment. The engineer for the project was Mr. G. Sibberling of the TVR who conducted bore hole investigations to confirm the geological conditions they would experience on the path intended for the subway and found alluvial clays and sand.

	The images, left and below, show the first sections of the subway on the northern side of the river Ely. Work commenced from the Ferry Road side of the river during April 1897 where the initial sections were installed within an open cut trench. The space between the outer diameter of the cast iron rings and the surrounding earthwork was grouted with a concrete mix. A tunneling shield was delivered to site in July 1897 and work proceeded using conventional tunneling techniques. The shield was basically a cylinder which was 4" (10 cm) larger in diameter than the cast iron rings. Around the rims of the shield was a series of "cutting edges" which were forced forward into the ground using the hydraulic power of rams which used the previously installed ring to push against.

When the shield was extended by the length of a ring, the hydraulic cylinders were retracted and the soil and gravel dug out. The process continued in this manner installing one ring at a time. A railway was installed within the subway to deliver the cast iron segments to the face and to transport spoil out.

Working in the shield the procedure was to manually excavate ahead the width of one ring, in this case 1' - 6" about 0.5 of a metre. The "cutting edge" at the front of the shield is extended using the hydraulic cylinders to move it into the excavation i.e. the same distance as the width of the ring. The edge didn't actually cut the tunnel but seemed to act as a circular gauge for the excavation.