Engineering Wonders of the World
Volume I

 THE TUBE RAILWAYS OF LONDON. 307 and made tight by ramming clay into any cracks between it and the skin of the shield. The grout was then forced into the cavity through holes made for the purpose in the segments. This system of tunnelling required the for- cavating a series of holes, 12 inches high and wide and 23 inches long, in advance of the cutting edge, and filling them with puddle clay. (Fig. 11.) In this manner three-quarters of an annular ring of clay was formed. Meanwhile an excavation had been made for the lowest VIEW SHOWING EFFECTS IN THE THAMES OF A “BLOW-OUT” OF COMPRESSED AIR FROM TUNNEL WORKINGS THROUGH THE BED OF THE RIVER. {Photo, F. Milner.) The “ Clay- Pocket ” System. Waterloo and mation of a timbered “ length ” practically all round the shield before the apparatus could be moved forward; and the poling boards had to be left permanently between the iron lining and the ground. Dur- ing the construction of the City Tube a novel method of tunnelling through water-logged strata was tried. This, known as the “ hooded shield and clay-pocket system,” consists of first ex- quarter of the shield to advance into. The clay was penetrated easily by the cutting edge. Be it noted that a sufficient quantity of clay was put into the pockets to ensure a layer two inches thick being left between the shield and the ground. When the shield advanced, the ground at the tail end was deprived of the support afforded by the shield ; but it would not fall in, as the air- pressure in the tunnel, acting against the impervious clay, afforded sufficient support.