Engineering Wonders of the World
Volume I

306 ENGINEERING WONDERS OF THE WORLD. Fig. 11.—THE HOODED SHIELD AND CLAY-POCKET SYSTEM OF TUNNELLING THROUGH WATER-BEAR- ING STRATA. high - water, when the head was greatest, than at times of low-water, the explanation being that the additional weight of water so consolidated the sand and mud through which the tunnel was being driven as to negative largely’the effect of the increased head. The air losses through the face were correspond- ingly reduced'. CIRCUMVENTING DIFFICULTIES. Where the ground is so loose that it will not stand of itself, special measures are required to prevent its falling in at the working face and subsiding on to the tunnel lining behind the shield when the latter is advanced, and before the annular space left by the skin of the shield has been filled with grout. When water was encountered on the City and South London Railway, Greathead adopted what he termed his “ assisted shield system ” of operations. He drove a timbered heading at the roof of the tunnel, a short distance in advance of the shield. This heading was roofed in with poling boards and grout, under cover of which the face could The “ Assisted Shield ” Method of Tunnelling. be excavated A Difficulty in Tunnelling. But in the case of a tunnel it is obvious that the water pressure at the bottom is greater than that at the top of the tunnel. As, how- ever, the pressure of the air must be sufficient to balance the water pressure at the bot- tom, there is consequently a tendency for a large amount of air to escape at the upper part of the face, where, if the nature of the material is the same throughout its depth, as it is assumed to be in this hypo- thetical case, it would naturally take place. In practice, of course, the nature of the material varies greatly. In fact, Curious un(^er the Clyde, during the pac* construction of some tunnels in Glasgow, it was found that a smaller pressure was required at time of laterally and downwards, the space so exca- vated being supported by boards resting at one end on the shield, and at the other end on vertical timbers placed against the face of th© excavation. The excavation of a “ length ” being complete, the face timbering had to be secured independently of the shield before the latter could be moved forward to permit the insertion of another ring of lining. Two vertical timbers, called “ soldiers,” were laid against the boards of the face, and held tightly in position by struts projecting through the shield and wedged against a transverse beam jammed between the segments of the third ring from the end. (See Fig. 10.) A circular timber grouting-rib was now in- serted between the heads of the hydraulic rams and the leading face of the tunnel ring,