Engineering Wonders of the World
Volume I

THE STORY OF THE FORTH BRIDGE. 325 yards, and workshops of all descriptions. Here, too, rose a large drawing-loft with a blackened floor 200 by 60 feet, Work on which all parts of the steel- Shore work could be designed full size. The workmen also busied themselves with the construction of a jetty 2,100 feet long running out from the South Queensferry shore through shallow water to the and thereby converted into huge diving-bells in which the men could attack the ground drysliod. The caissons were made up of two parts—the cylindrical, and the taper. The first, which varies in height, has an external diameter of 70 feet. The tapering part is 24 feet high in all cases, and decreases to 60 feet in diameter at the top, situated at low- water level. The Queensferry Jetty. site of the Queensferry piers. Simultaneously, jetties were built at Inchgarvie, and the whole area enclosed by the central piers was covered by platform of timber and iron the actual sites of the piers a substantial girders, while were being levelled and otherwise prepared. On shore masonry gangs pushed forward the piers for the approaches ; so that from the beginning of 1883 the work was being prose- cuted at many points at the same time. The twelve circular piers carrying the main towers have—with the exception of the north-east and north-west Fife piers—a top diameter of 49 feet. They increase their diameter down- wards to 55 feet at low-water level, below which point, in the case of the Queensferry and Inchgarvie piers, material changes from granite to mass concrete, extending to the natural foundation on which the pier rests. As the Fife piers were built on practically dry ground, we need not concern ourselves with them, reserving our space for a brief description of the building of the other eight, sitated the use steel caissons, to water from the of which they Soundings for the Inch» garvie Foundations. The Circular Piers. Use of Open and Hydraulic Caissons. This neces- of circular exclude the foundations, formed the permanent exterior. The caissons employed for the Inchgarvie north piers were open at the top ; while the other six were provided with an air-tight deck seven feet above the bottom, The foundations for the Inchgarvie north piers were surveyed by means of a floating circular stage, from which soundings were taken every six inches round the circumference. In accord- ance with the readings ob- . tained the bottom of the caisson was shaped, so as to fit fairly accurately into an annular groove cut into the rock. All existing gaps and cracks were then staunched with cement and clay, after which the water was pumped out and the caisson used as a half- tide dam—that is, work could proceed inside till the tide had reached half its full height, when, to preserve the stability of the caisson, valves At a valves again. The water, were opened to allow water to flow in. corresponding point in the ebb the were closed, and the interior emptied the floating When Sinking an Inchgarvie South Caisson. Inchgarvie south piers being in deeper the procedure was different. After the preliminary survey with stage, to ascertain the contour of the rock, a level foundation was built up with sandbags, to give the caissons an even bearing all round. floated across from the Queensferry shore, on which they had been constructed, the caissons were moored in their exact positions, and gradually loaded with concrete till they grounded at low water. As soon as the cutting edge had obtained a satisfactory bearing, blasting of the rock commenced. Holes were bored with hand and pneumatic