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

The history of the tunneling method employed

Mariners of all nations were familiar with the "teredo", the ship-worm. Isambard Kingdom Brunel, the builder of the first tunnel under the Thames, employed a simple shield to enable the tunnelers to work in relative safety which, some say, was derived from watching the worm, which digs its way into the wood by means of a boring apparatus in its head, and as it advances, lines the tunnel behind it with the secretions from its own body. He patented the worm idea in 1818 in the form of a tunneling device consisting of iron cylinders which had an auger like cutter at its leading edge. As it advanced, the bore of the tunnel was lined with sheet iron plating in the form of a spiral which was finally faced internally with masonry.

Mr. B's idea was subsequently developed and improved upon by P. W. Barlow and later again by J. H. Greathead, whose shield design was employed in the creation of the early London tube systems. The Greathead shield consisted of three sections, the front, a body and a tail. The shield is circular and made from countersunk riveted plates which ensures that the outer surface is smooth. To stiffen the cylinder, a bulkhead, or wall, is included with a doorway. The front edge extends forward of this door to the "cutting edge" which is formed of cast iron ring segments onto which steel knives are mounted which may be adjusted so as to cut a hole of slightly greater diameter that the outer diameter of the shield.

At the rear is the body of the shield in which the hydraulic jacks, pumps, motors and valve gear for manipulating the shield are installed. At the back end is a powerful cast-iron ring to which are attached at regular intervals around the circumference the hydraulic jacks or rams for moving the shield forward. The combined power of these jacks is immense as even in stiff and stable clays a pressure of 4 or 5 tons for every square yard of the external surface of the shield is necessary and in sticky material the power must be increased to 18 to 24 tons. Each jack may be used independently of the others to steer the shield in the correct direction.

The requirement for an air lock is dependent upon ground conditions. In very stable ground where caving and water inroads are unlikely the bulkhead may be omitted, but in treacherous water-logged conditions the means to close the shield entirely and converting the front end into an air-tight chamber accessible through air locks.