Engineering Wonders of the World
Volume I

DOCKS. 183 excavated “ spoil ” for embankments, pro- tected on the outside by rubble or some other suitable material dumped at their bases. The difficulty of reclaiming part of the ground from the sea has to be overcome in many different ways, as each case calls for special treatment. The foundations of dock quay walls naturally depend on the nature of the subsoil. As a rule the ground on which a dock is to be built is sound enough to permit the Foundations erection of a quay on a broader for Dock ' Walls base °* concre^e- Occasion- ally, however, a difficult stra- tum is struck, and special measures have to be taken. In these circumstances two expedi- ents are open to the engineer. He may drive bearing piles, and, after connecting them suit- ably at the top by a layer of concrete, build the wall on that foundation ; or he may sink concrete monoliths to the necessary depth. These are made up of conveniently-shaped blocks, with two or sometimes three apertures in them. On their lower edges they have what are called shoes. These shoes are gen- erally of timber, but lately—at Rosyth, for instance—they have been made of steel. Their use is to grip the ground as the block sinks, ordinary grabs driven by steam assisting the downward progress by excavating the soil through the apertures. When the monolith is securely founded, the holes are filled with con- crete, and the walls are laid on top. An alter- native to this method is to confine difficult soil between rows of sheet piling. The fine sand or silt is then partially excavated from between the piles, the space is filled with con- crete, and the wall built up. This system of sheet piling has been used largely in the con- struction of river quays on the Clyde, with very satisfactory results. Sometimes monoliths form part of the base of the wall as well as the foundation, as at Havre, and, to a smaller extent, at Bordeaux. In a few instances none of the expedients named have been successful in securing the foundations of quay walls. Those of a part of Glasgow harbour at the , ,, Monoliths. Broomielaw once gave endless trouble, and subsided in spite of every kind of treatment the engineers could think of. Ulti- mately it was decided to sink caissons to the necessary depth and fill them with concrete. These caissons were large oblong steel struc- tures, with cutting shoes at each end. Men SINKING A CONCRETE MONOLITH FOR DOCK WALL FOUNDATIONS. The ground is removed by grabs through shafts left in the monolith, which settles by its own weight. When sinking is completed the shafts are filled up with concrete. working under air pressure excavated the soil from below a caisson, enabling it to sink, and the soil was passed out through air locks. The workings were protected from the river by sheet piles. Six of these caissons filled with con- crete were sunk, and the spaces between them were also filled with concrete. The long stretch of quay has been immovable ever since. Another interesting quay foundation is to be seen on the Liffey at Dublin. Here men in a diving bell levelled a part of the bed of the river. On this large blocks of rubble con- crete, weighing about 360 tons in all, were carefully deposited in their designed places by