Engineering Wonders of the World
Volume III

76 ENGINEERING WONDERS OF THE WORLD. account of their huge size and weight they could not be transported complete to their final positions, nor was it convenient to mould them in situ. The engineers therefore adopted the following procedure : In the inner harbour iron caissons of the same dimensions as the blocks to be were put together. They had an inner skin some feet distant from the outer one, the two skins being brought to- gether at the bottom to form a cutting edge. The space between the skins having been filled up with sufficient concrete to give sta- bility, a caisson was towed out and sunk in its place by adding some more concrete, ui the cutting edge had sunk well down into the clayey sea bottom. The central space was then filled in with, concrete lowered by means of cranes and skips. For the Newhaven and La Guaira Har- bours the “ sack block ” system was em- ployed. For this a special barge, with hinged bottom, is used. The bottom hav- ing been closed, a large sheet of stout jute sacking is arranged over it and up the sides of the central well. The Concrete is deposited on the Sack Block , , n . ... _ . canvas and levelled until a System. sufficient thickness—from two to three feet—is attained, when th© edges of the sacking are brought over the top of the mass and laced together. The vessel is moved to the dumping spot, and, on the bottom being opened, the sack and its contents are deposited. The concrete soon hardens. At La Guaira courses of 180, 130, and 70 ton blocks were laid, the largest blocks being at the base, and the size decreasing upwards. Sack blocks were also used for th© founda- tions of the new south breakwater at Aber- deen. This breakwater is an interesting example of mass concrete Aberdeen. work. From the single course of sacks on the sea bed to low-water level at neap tides the structure is composed of large concrete blocks. These are capped by monoliths of concrete formed in place in wooden frames, and weighing from 335 to 1,300 tons each, according to their length along the line of the breakwater. Each mono- lith extends right across the breakwater. At Vera Cruz, on the Gulf of Mexico, is one of the greatest artificial harbours in the New World. The coast - line here faces north- north-east, and originally was ° J . Vera Cruz. exposed to the furious “ Northers,” which did great damage to any shipping anchored in the port. Even slight breezes hampered seriously the trans- blocks in courses ; D, concrete cap. feren.ee of cargo from ship to lighter, or vice, versa. In 1882, James B. Eads, the designer and engineer of the St. Louis Bridge (Vol. II., p. 163 foil.), submitted plans fqr utilizing the coral reefs near the port as foundations for breakwaters which would create a secure harbour. Between that date and 1895 a small part of the total work required was done. In the latter year the contractors handed over the enterprise to Messrs. S. Pearson and Son of London, who completed it successfully during the following seven years. Three separate breakwaters had to be built on the north-west, north-east, and south-east respectively. The first of these was formed by depositing a rubble mound from a trestle, and capping it with a wall of 35-ton concrete, blocks laid by a crane. On the seaward side this breakwater is partly- protected by random blocks placed by a pre- vious contractor. For the north-east mole a