Engineering Wonders of the World
Volume I

THE STORY OF THE FORTH BRIDGE. 327 they bit into the ground easily, it was of the utmost importance to centre them correctly before sinking, and this proved a matter of no small difficulty, owing to the strong currents, violent waves, and high winds that vex the Forth. Foreign workmen—Italians, French- men, Belgians, Austrians, and Germans—were engaged to excavate the foundation, being ex- perienced hands brought over by M. Coiseau, the sub-contractor for the work, and able to stand the highly-compressed air needed at the greatest depths. Each of these four caissons had three shafts, 3 feet 7 inches in diameter, connecting the air-chamber with the platforms above— two for the removal of debris, one for the ingress and egress of the men. Fitted with its temporary caisson, stagings, machinery, and a ballasting of concrete, a caisson had a launching weight of over 3,000 tons ; yet so great was its bulk that it floated high in the water while being towed to its position in one of the four open- ings left for it at the north end of the Queens- ferry temporary jetty. On reaching its des- tination it was moored into place and weighted very carefully, so as not to press too heavily at low tide on the mud and silt. For the removal of the last the engineers employed an ejector of a simple kind—a pipe leading The Caissons floated into Position. end, through A Mud Ejector. from the air-chamber, where it had a flexible the sides of the caisson above water level. The method of using the ejector may be thus briefly described. Water was forced into the chamber and mixed with the mud to reduce it to a semi- fluid condition. One man then opened the cock of the ejector pipe, while another manip- ulated the hose end, dipping it periodically into the “ sump ” of mud. The velocity of the air gave it sufficient momentum to catch up and carry out some of the liquid. The operator learned by practice how long and how deep to submerge the nozzle to get the best results. Under the soft upper stratum lay hard boulder clay, of such a consistency as to render ordinary spade work very laborious and ineffective. Owing to its somewhat elastic nature it could not be blasted satis- Hydraulic Spade for factorily with, dynamite. Ex- (jutting- Clay, cavation had come almost to a standstill, when Mr. (now Sir) William Arrol, the contractor for the bridge, devised a hydraulic digger, consisting of a ram work- ing in a long cylinder and ending in a large spade. On the top of the cylinder was a headpiece to rest against any projection on the roof of the air-chamber which would hold it fast while the water forced the spade to its full depth down into the clay. By means of this device a series of trenches 18 inches deep were dug all over the area covered by the caisson, the edge of which was also under- cut with, the help of the same instrument. An accident overtook the north-west caisson while it was being sunk into position. An unusually low tide caused one side of it to settle immovably in the mud, and tilt to such a degree that it was filled by the next tide and driven yet deeper into its bed. Additional rings of plates, were added An Accident to a Caisson. with all possible speed to the top of the iron- work. Unfortunately, the men in charge of the pumps emptied the caisson more quickly than the carpenters could place the internal stiffening struts, with, the result that the plates gave way under the pressure of the water outside for a distance of 30 feet. Here was a disaster indeed, though not sufficient to dishearten the workers. After mature consideration it was decided to surround the caisson with a timber barrel reaching some distance above high-water level. At the top this was supported by a heavy circular timber frame, at the bottom by the iron plates of