Page:Walker (1888) The Severn Tunnel.djvu/151

86 is loose, especially if it is full of water, a different system must be followed to secure the top.

Before mining to place a head-tree, or a crown-bar, polling-boards must be driven by mauls as piles; and where the ground is wet, this is one of the most difficult operations miners have to perform, for the dirty water streams over the end of the piles, and at every blow of the maul is spattered on all the men that are near.

A considerable length of the Severn Tunnel, on the Gloucestershire side, was in loose gravel, full of water, and required this operation.

In the same ground the crown-bars had to be placed entirely outside the tunnel, and the brick-work of the arch completed under them, the space between the crown-bars being filled up to the polling-boards with rough brickwork or rubble.

In shaft-sinking, I have known cases more than thirty years ago, where, when a depth, say 20 or 30 feet, had been sunk from the top, a curb was placed, carried by iron rods, and in some cases by chains from timbers laid across the top of the shaft, and the brickwork for lining the shaft was built upon this curb; and in some cases the brickwork has been built continuously on the top of the shaft, and the lining lowered away till it reached the required level. In other cases short lengths have been taken out in the sinking, and the brickwork added below the first curb on other curbs placed from time to time.