The American system gives the greatest possible rapidity of erection of the bridge on its piers. A span of 518 feet, weighing 1000 tons, was erected at Cairo on the Mississippi in six days. The parts were not assembled until they were put upon the false works. European engineers have sometimes ordered a bridge to be riveted together complete in the maker’s yard, and then taken apart.
The adoption of American work in such bridges as the Atbara in South Africa, the Gokteik viaduct in Burmah, 320 feet high, and others, was due to low cost, quick delivery and erection, as well as excellence of material and construction.
FOUNDATIONS, ETC.
Bridges must have foundations for their piers. Up to the middle of the nineteenth century engineers knew no better way of making them than by laying bare the bed of the river by a pumped-out cofferdam, or by driving piles into the sand, as Julius Cæsar did. About the middle of the century, M. Triger, a French engineer, conceived the first plan of a pneumatic foundation, which led to the present system of compressing air by pumping it into an inverted box, called a caisson, with air locks on top to enable men and materials to go in and out. After the soft materials were removed, and the caisson sunk by its own weight to the proper depth, it was filled with concrete. The limit of depth is that in which men can work in compressed air without injury, and this is not much over one hundred feet.
The foundations of the Brooklyn and St. Louis bridges were put down in this manner.
In the construction of the Poughkeepsie bridge over the Hudson in 1887–88, it became necessary to go down 135 feet below tide-level before hard bottom was reached. Another process was invented to take the place of compressed air. Timber caissons were built, having double sides, and the spaces between them filled with stone to give weight. Their tops were left open and the American single-bucket dredge was used. This bucket was lowered and lifted by a very long wire rope worked by the engine, and with it the soft material was removed. By moving this bucket to different parts of the caisson its sinking was perfectly controlled, and the caisson finally placed in its exact position, and perfectly vertical. The internal space was then filled with concrete laid under water by the same bucket, and levelled by divers when necessary.
While this work was going on, the government of New South Wales, in Australia, called for both designs and tenders for a bridge over an estuary of the sea called Hawkesbury. The conditions were the same as at Poughkeepsie, except that the soft mud reached to a depth of 160 feet below tide-level.
The designs of the engineers of the Poughkeepsie bridge were accepted, and the same method of sinking open caissons (in this case made of iron) was carried out with perfect success.
The erection of this bridge involved another difficult problem. The mud was too soft and deep for piles and staging, and the cantilever system in this site would have increased the cost.
A staging was built on a large pontoon at the shore, and the span erected upon it. The whole was then towed out to the bridge site at high tide. As the tide fell, the pontoon was lowered and the steel girder was placed gently on its piers. The whole operation was completed within six hours. The other five spans were placed in the same manner.