Engineering Wonders of the World
Volume I

236 ENGINEERING WONDERS OF THE WORLD. A shield is steered vertically by means of a plumb line and a graduated “ plumb stick ” held against the shield. extent it may be “ out of plumb.” In order to determine whether the level of the shield be correct, two inverted T-shaped rods are suspended from the roof of the tunnel, the cross bars of the T’s being fixed at a pre- determined height. When the shield is at its correct level a mark on the diaphragm of the shield is in line with the line joining the upper surfaces of the T-pieces. SHAFT-SINKING. The shafts at the stations for the lifts and stairs are in all cases formed of cast-iron circular rings of a pattern similar to those used for the tunnels. The shafts to contain two lifts have an internal diameter of 23 feet, and those for three lifts one of 30 feet. The stair shafts vary in diameter from 16 feet to 18 feet. In one or two cases lift shafts of 16 feet and 20 feet internal diameter have been adopted for single lifts. The method of sinking the shafts is entirely governed by the nature of the ground and the probability of encountering water. Where the ground is good—that is, where it contains no water—the method of “ underpinning ” is applied. This consists of excavating below the lowest ring of segments in position, and bolting to the under side of that ring, in the space so excavated, a new ring of segments, and so executing the work downwards ring by ring. The exca vation always has a diameter some- “ Under- pinning’ Method. between the what larger than that of the shaft, so that the segments may be easily erected in posi- tion. This results in a small annular space being formed outside the ring, and it be- comes necessary to fill with lime grout the space so formed outside of the shaft and the ground. Where, however, the ground is heavily charged with water, as in the lower portions of the gravels overlying the clay, one or other of two further methods is generally adopted. By one method, a cutting edge is securely bolted to the lowest ring in position. The ground is then excavated over the general area of the shaft, and to make it sink into the ground large tim- ber baulks are placed across the top of the shaft and heavily loaded, so that the additional overcome the friction between the sides of the shaft and the ground, as well as the cutting and wedging resistances offered by the shaft in its descent. Use of a Cutting Edge. weight may This method is slow, and necessitates the removal of the load every time a fresh ring has to be erected on the top of the shaft, after it has sunk a sufficient depth to need an addition. Moreover, if the shaft does not sink evenly, it is often necessary to weight it more heavily on one side than on the other. This often results in the segments being fractured. Such water as finds its way into the shaft is pumped out. The other method—the one now usually adopted when the influx of water is not too great—is to construct the shaft by under- pinning. The ground outside the shaft is supported by poling boards, and the space between the boards and the ground is filled in with Portland cement grout forced in under pressure. Then the iron segments are placed one by one, and the space between them and the poling boards is similarly grouted. Note that