Page:Walker (1888) The Severn Tunnel.djvu/155

88 It is continually getting out of repair, on account of the quantity of grit and dirt to be raised with the water.

A chain-pump can be used only for low lifts, say of 30 or 40 feet. Even the ordinary bucket-pump, which is the only one that can be trusted for working to depths of 100 to 200 feet, is subject to be constantly damaged and broken by the blasting in the pit. It is necessary, also, to have the holes of the suction-piece or wind-bore of the bucket-pump close to the bottom of the pump, or the miners would be standing in a considerable depth of water, and the pumps rapidly wear out their buckets from the quantity of sand and grit raised with the water.

In four of the shafts we sank at the Severn Tunnel, after we had a first shaft down and pumps fixed in it, we escaped this pumping difficulty by driving a heading from the existing pumping-shaft to the shaft we were sinking, or rather to the spot where the shaft would eventually be when it should be sunk its full depth. From this heading we sunk the bottom part of the shaft and lined it with brickwork, and then put down a bore-hole through the upper strata till it reached the completed work below; the water then drained through this bore-hole, as the men sunk the shaft, into the heading beneath, and so into the other shaft alongside from which it was pumped. In two of the shafts, by driving this cross-heading before the bore-hole was made, and accurately setting out the centre of the shaft