DOCKS. waggon on leaving the cradle runs down an incline to a second weigh - bridge, and thence to the sidings provided for empty waggons. A similar expeditious system is adopted at other South Wales ports and at Hull, variations in the details of the tips being introduced to meet local requirements. A somewhat different method is practised on the Tyne and at other north-country ports, where there are no hoists, but the waggons, which carry about 4 tons and discharge through a door at the bottom, are brought on to the tipping staiths at a level high enough to suit any vessel. Each staith is provided with two or three spouts so as to load at two or three hatchways of a vessel simultaneously, the spouts being arranged to swing to the extent of about 15 feet on either side of their centres. Modifications of these systems are adopted at most of the various coal-ports. The speed of the coaling depends greatly upon the rapidity with which the loaded and empty waggons can be brought to and from the staiths or tips, and also upon the trimming of coal in the hold of the vessel. For loading and discharging ordinary cargoes steam or hydraulic travelling cranes are provided by some dock companies ; at other docks the cargoes are worked by steam winches and derricks on the ships themselves ; while at others, again, the quays are leased to various steam shipping companies, who provide their own cranes or other gear for working their cargoes. Dock walls may be considered as generally partaking of the nature of retaining walls. Theoretically the lateral pressure exerted by a bank of earth or other material of the height as the wall is that due to the wedgeDock walls. same spape^ mass, obc (Fig. 2), included between the vertical back of the wall ab and a line be bisecting the angle between the vertical ab and bd, the slope of repose of j the material. This, however, cannot always be entirely relied on in practice, especially in some ground, such as clay, of which the slope of repose is found to vary considerably in different circumstances. An important point in the design of dock and quay walls is the nature of the foundation, as is shovra by the instances of failure which could be attributed to its defects. If the foundations are insecure, a wall is liable either to slide bodily forward at the base when the pressure of the filling is applied at the back, or the yielding of the ground under the front toe of the wall causes the top to fall forward. Conditions of the • subsoil, which it is difficult, if not impossible, to foresee, have sometimes caused the failure of walls which theoretically were of more than sufficient strength. The experience of some engineers is that a dock wall should be constructed of sufficient strength to support a head of water equal to its height when the dock is empty, but it would appear that such a wall is only required in exceptional cases, and even then would fail if the foundation were not secure. In ordinary dock walls a margin of strength should always be provided to meet any weight which may be placed on the quays at the back, and this, if the foundation is good, should be sufficient for any pressure due to backing. Docks are usually constructed in estuaries and rivers where the circumstances are rarely favourable; the engineer must therefore be guided by practical experience and his own judgment, rather than rely on the ordinary calculations which might be set forth as generally applicable to retaining walls. As regards the foundations of walls in soft ground, if the depth of mud or silt is not excessive, and a firm foundation can be obtained beneath it, the wall tions^3 should be carried down well into the solid ground. The usual course is to enclose the site of the works by a cofferdam so as to exclude the tide, and enable the foundations and other vyork to be constructed dry, any leakage being kept down by pumping. Where the head of water is not great and the space not limited, effectual dams can be made by embankments.
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When, however, a considerable head of water has to be dealt with, it becomes necessary to use timber piles driven either in a single continuous row with the joints caulked, or in a double row, with clay puddle between, and well secured by struts at the back. Timber cofferdams admit of much variety of design to suit local circumstances and to meet the varying strains to which they may be subjected. In some instances the cofferdam is partly or wholly formed by a permanent embankment or sea-wall reclaiming ground from the foreshore and enclosing the site of the works; such a method has been adopted at Leith. If the nature of the ground will not admit of a trench being excavated deep enough to afford a firm footing for the wall, this must be obtained by some other method, and the expedient most frequently adopted is to drive timber piles to carry the weight of the wall. The timber chiefly used for this purpose is pitch-pine from the states bordering the Gulf of Mexico; in comparison with the Baltic timbers it is stronger, quite as cheap, and can be obtained in longer lengths. Elm is very tough and durable; so also is beech, which has been largely used, especially when always under water or embedded in the ground. The durability of timber piles depends greatly on their situation; those which are alternately wet and dry do not last so well as those which are wholly in the ground or under water, while some soils have a more injurious effect on timber than others. Many piles have been found in good condition after being as long as 500 years in the ground. If practicable, it is advisable to drive piles until they reach hard ground. Failing this the only satisfactory way of arriving at an approximation of what they will carry is to test the ground by driving one or two piles to what is considered a sufficient depth and loading them with dead weight, from which may be determined the final set for the remainder. It has been conclusively proved that a light hammer with a high fall causes more injury, with less effective work, than a heavy hammer with a short fall. In driving piles in sand a water jet under pressure, in a pipe sunk down so that it delivers the water just below the point of the pile, loosens the sand and greatly facilitates the operation. Cylinders constructed of brickwork have for a long time been used in India and elsewhere for foundations. At Glasgow, where they formed the foundation of part of the quay walls, they were 12 feet external and 7 ft. 4 in. internal diameter, and were tongued and grooved together (Fig. 3). The ground in which they were sunk consisted chiefly of sand and waterbearing gravel. They were 35 feet in length, Line oF Quoy Wall and were sunk until their tops were about 2 feet Fig 3 above low-water level; they were then filled with concrete, and upon this foundation the upper wall was constructed. Concrete cylinders were afterwards adopted, built into groups of three, and placed as shown in plan (Fig. 4). They were made in rings 2 ft. 6 in. deep formed alternately of three and four segments so as to break bond when built together, the inside diameters being 5 ft. 9| in. and the outside 9 ft. 7^ in. Under the bottom ring a castiron shoe was secured of the same external size and shape as the ring. This was made in six parts bolted together, and shaped Line of quay wall. to form a cutting edge. A/y 4. As the rings were built up, the sand and gravel were removed from within the cylinders by means of grabs or excavators. The total height of these cylinders was 28 feet, and when sunk their tops were about 3 feet below the level of low water ; the average rate of sinking was about 12 inches an hour,
