Engineering Wonders of the World
Volume I

184 ENGINEERING WONDERS OF THE WORLD. SECTION OF TUNNEL. bricks, and commenced the permanent masonry, in which were built forty-eight 1-inch vertical iron bolts, extending from the lower wooden curb to another curb on the top of the caisson. The men worked rapidly, laying one thousand bricks each per day, so that the setting of the 900 tons of masonry was completed in three contact, and to draw the piles and let the caisson rest on the gravel. This having been man- aged without Sinking O a. i difficulty, a r . J ’ Caisson, staging was built over the caisson to carry the machinery for working an endless chain of buckets reaching down to the level of the lower curbs. Men stationed inside the caisson undermined its cir- cumference carefully, throw- ing the spoil to the centre, whence it was raised by the buckets. The substitution of steam power for hand labour at the windlasses enabled the latter part of the excavating to be done at a better speed than prevailed during the earlier stages. By May 16 the iron curb had sunk to within a couple of feet of the required depth, Then weeks, and the whole made snug by means of outside iron hoops and the vertical pull of the nutted ver- tical bolts. Dur- ing erection the caisson pene- trated the ground uni- formly to the depth of half an inch. The next op- eration was to strike out the wedges between the piles and the curb so as to bring these into BRICK CAISSON FOR SHAFT NO. 1. Part of wall broken away to show tie-rods.