Engineering Wonders of the World
Volume I

THE THAMES TUNNEL. 187 foot was then le- vered forward into its new position and the leg screwed down tight. The other foot having been treated in the same manner, the staves of the head were forced forward into the ground. It then only remained to drive the body of the frame towards the poling boards by means of large screws abutting on the brickwork top and bottom (see cut on this page), SECTIONAL VIEW OF TUNNEL AND SHIELD SHOWING TRAVELLING PLATFORM AND SCREWS FOR ADVANCING THE SHIELD. release the poling screws from the neighbouring frames, and return them to the notches in their own frame. When all the “ even ” frames had been moved, the “ odd ” could be taken in hand. The Ground DIAGRAM TO EXPLAIN METHOD OF HOLDING THE POLING BOARDS DURING THE ADVANCE OF A FRAME. extent of the advance was, in the earlier stages of the tunnel driving, only 41 inches— half the length of a brick ; but later on ampler movements were made, though not without proportionate risk. The various parts of the shield were so con- structed that any one of them could be re- moved without endangering adjacent parts, and this feature proved a most valuable asset on several occasions. By the 28th of October, 1825, all twelve frames were in place, and a month later the shield began its historic journey under the Thames. The strata to be penetrated soon gave signs of being very dif- ferent from the “ good blue Tunnelling commences, clay of the preliminary report. Water found its way in repeatedly in sufficient quantities to delay the work, until the men had become more handy and had learned how to meet emergencies. The 80-ton mass of iron advanced at an average rate of about a foot a day for several weeks. On May 22, 1826, the top plate of frame 1 gave way unexpectedly, and by the end of the month the shield had got so much out of line that Brunel had to move it bodily eastwards three feet before going farther. Some time earlier than this the chairman of the company had expressed the opinion that