Engineering Wonders of the World
Volume I

THE ROTHERHITHE TUNNEL. 61 consisted of a hollow cylinder, 30 feet 8 inches in diameter and 18 feet long. The front part, including the cutting edge, was built up of cast-steel segments, and the The Shield, rear part of steel plates riveted together. The shield was stiff- ened internally by vertical and horizontal partitions, which divided it up into sixteen compartments each about 6 feet square. It was forced forward by hydraulic jacks, forty in number, housed in the cast-steel segments all round, close to the outside of the shield, and whose rams pressed against the last ring of tunnel lining erected. On the back of the partitions were fixed two hydraulic “ erectors ” for lifting up the lining plates and swinging them round to any point in the ring of lining for erection. In order to start the shield on its journey across the river, it was necessary to remove the “ plug ” which temporarily blocked the opening in the side of the Starting shaft. This was done piece by piece, the earth outside being Shield. 1 supported by timbers. As soon as the plug had been all taken out, the shield was moved up to the face, and excava- tion was begun. In order to move the shield, several rings of the tunnel lining were tempo- rarily built up behind it, resting on a timber cradle specially prepared for them, and heavily shored to the opposite side of the shaft. Against the flange of the foremost of these rings the hydraulic ranis pressed, causing the shield to advance. The temporary rings were removed when a few permanent rings Advancing the Shield. of the tunnel had been built in. After tunnelling had fairly started, the method .of working was as follows. A ring of lining having been erected, the miners and excavators re- moved the earth from the front of the shield, supporting the with timber where required, thrown out through the back exposed face The spoil was of the shield and filled into small wagons on the temporary rails laid in the tunnel. As soon as sufficient ground had been removed, excavation, was suspended and pressure ad- mitted to the hydraulic jacks. The rams of these, pressing against the flanges of the last completed ring of lining, forced the shield forward. Under favourable conditions the shield would advance the whole distance necessary for a ring—namely, 2 feet 6 inches —in a few minutes, a total pressure of 2,000 to 3,000 tons being exerted. As soon as this was done the rams were drawn in again, and the erection of the next ring commenced. The shield being 8 inches larger in diameter than the tunnel lining, an annular space of 4 inches all round the tunnel was left void after the shield had passed. This was filled up with grout in the following manner. A mixture Gtouting of ground lime and water was Lining filled into a cylindrical steel vessel, known as a grouting-pan, and fitted for withstanding high internal pressure. The grouting-pan was provided with, a horizontal shaft, on which iron blades were fixed, and as soon as the lime and water were introduced the shaft was rotated so that its blades, or paddles, should keep the mixture stirred. When the pan was full, an air-tight cover was fixed over the opening and compressed air of about 80 lbs. per square inch pressure ad- mitted to the top of the grouting-pan. At the bottom there was an outlet, fitted with a tap and hose-pipe. The hose ended in,.a piece of iron pipe which was applied to the grouting holes, of which there was one in each plate of the lining. The tap at the bottom of the grouting-pan was then opened, and the air pressure forced the grout through the hose into the 4-inch space round the tunnel. This process was repeated until the whole space was full of lime, which set in a short time. The grouting was most important when tun- nelling under the land, as by its means damage