Volume Ten - Pre-Victorian to the Present Day - Even more aspects - The Ely River Subway 1900 - 1963 . . .

There are few estuaries with a maximum ebb and flow as great as that experienced in the Severn Estuary, and these changes would have produced considerable fluctuations of pressure. At high tide the pressure in the tunnel at its lowest point would have been more than double normal atmospheric pressure. The control of pressure in the tunnel was therefore a delicate operation. Too low a pressure might fail to hold back the unstable ground - there were, in fact, two inundations, one in November 1897 and the second in December 1898.

If pressure in the tunnel is too high there is a danger that instead of water leaking in, air would leak out, with the possibility of a destructive blow-out, as happened on several occasions during the construction of the Glasgow District Subway under the River Clyde (1892/95).

In order to reduce the possibility of leakage or inundation, when the tunnel reached its lowest point Mr Sibbering used a well tried technique. Several hundred tons of clay were dumped into the river along the line of the Subway, and action which provoked a strong protest to the Board of Trade by the Wharfingers holding rights in the Harbour.

Tunnelling was carried out using a tunnelling shield. The first shield was patented by Sir Mark Brunel in 1818, for use in the construction of the first Thames Tunnel at Rotherhithe. The original design was greatly modified by civil engineers during the century, and it was James Henry Greathead (1844-1896) who adapted it to permit the use of compressed air, and introduced modifications to reduce the danger from flooding. The use of the shield enabled miners to work almost continually in supported ground, both during excavation and when erecting the cast iron lining.

The first cylindrical section was laid on July 5th, 1897, the last on September 25th, 1899, the task occupying 813 days out of a total of 1140. The tunnel was 975 feet long. The remaining days were occupied in excavating the trenches, laying the concrete footway, providing services - the Subway was lit electrically and regularly pumped dry, and building the toll houses, gates and turnstiles at each end. The opening ceremony was performed by Mrs Beasley on Monday, May 14th, 1900.