Page:Walker (1888) The Severn Tunnel.djvu/154

Rh I have, however, adopted the system of completing the whole of the sinking of the shaft before commencing any of the brickwork. In doing this, strong curbs are made similar to the centres of the tunnel; those for an 18-ft. shaft being 21 feet in external diameter; those for a 15-ft. shaft being 18 feet in external diameter; and as we ultimately sunk a large pumping-shaft, 29 feet in diameter, with 3 feet of brickwork, the curbs in that case were 35 feet in external diameter.

These curbs are placed from 3 to 5 feet apart as the sinking of the shaft progresses, with polling-boards behind them to support the ground; and when the shaft is completed to the bottom the brickwork is commenced, and each curb taken out as the brickwork built up from the bottom reaches it.

In sinking a shaft at any place on the Severn Tunnel, the principal expense was incurred in keeping the shaft free from water. We have tried all kinds of pumps for this purpose, and there are objections and difficulties with all.

The pulsometer-pump, which can be slung upon chains and lowered as the work progresses, with a flexible rubber hose at the bottom of the pump, can only be used to a depth of about 50 feet, and even at that depth consumes so much steam as to be very expensive.

A direct-acting steam-pump, of which we had many patterns, made by a variety of makers, could also be lowered in the shaft as the work progressed.