Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 239 Excavating the “ Face.” strength immediately the iron segments of a ring have been bolted up and the space outside them grouted. The Greathead system of tunnelling in the clay consists of driving a heading by hand labour in front of a shield, as Tunnelling shown in several of our illus- yvith t e trations, and utilizing for pur- Greathead & r Shield in Poses of excavation the hy- London Clay, draulic power by which the shield is propelled, so that in advancing it causes the face of the excava- tion to be broken up by a series of short timber piles placed between the shield and the face of the excavation, the materials so dislodged being removed by hand labour after the shield has come to rest. Owing to the extremely limited space directly in front of the shield, between its diaphragm and the cutting edge—namely, some 15 inches —at certain stages of the work it is not possible for more than one miner, or two at most, to excavate. Conse- quently, in order to get more men “ into the face,” the heading referred to above, repre- senting about twenty-eight per cent, of the total area of the face, is always driven for- ward of the work, while the advancing shield shortens the heading at the back. In order to utilize the pressure of the rams to the fullest advantage for the purpose of excavation, the timber piles mentioned are placed round the entire circumference of the bull-head casting, slanting slightly upwards. When all is ready, the two nearest supports at the shield end of the heading are removed, or in some cases only slackened, so as to allow the face to collapse towards it; and the in- stant that hydraulic pressure is applied to the rams the shield advances, causing the piles to penetrate the face and so detach from it large lumps of clay. In this manner the material is completely broken up by the piles and falls into the “ length.” It is subsequently re- The Greathead Shield. moved by the miners and cast on to the stage at the back of the shield, and thence shovelled into skips for removal. Enough has been said to show that the piles form an extremely valu- able adjunct to the shield when tunnelling in the London clay. Clay stones frequently occur in the London clay in beds of varying thickness or in isolated lumps, and these have to be removed by hand, as the piles cannot break them up. The maximum speed of advance attained is generally about 10 feet, or a little over, per day of twenty-four hours in the case of the small 11 feet 8| inch or 12 feet 7 inch diameter tunnels between the stations. Referring to the design of the Greatliead shield : It consists of an outer cylindrical shell of steel two inches larger in diameter than the external diameter of the tun- nel, the total length of the shield from cutting edge to tail being 7 feet. There are seven 7-inch hydraulic rams placed within a cylindrical casting, to which they are con- nected by stout bolts. The stroke of the rams is 22 inches, or 2 inches longer than the length of a ring of tunnel lining. Each ram is cap- able of exerting a thrust of about 34 tons. In the old type of shields, as used on the Waterloo and City Railway and the City and South London lines, the hydraulic pressure in the rams was produced by two hand-pumps, one on each side of the shield. For the Water- loo and City Railway, in the compressed air portion of the work, electrically driven hy- draulic pumps were used with great advan- tage to the men but in all the most recent shields a very admirable and compact type of compressed-air-driven pump is used. This, although noisy in action, is of the greatest advantage to the men, as it relieves them of what used to be a period of very hard work every time they had to “ pump up the shield ” by working the long pump handles. The piston rods of the rams fit into solid