DOCKS each block being formed wedge-shaped so as to be easily adjusted to any required height. There is considerable variety in the shape and arrangement of the side walls of graving docks, but they must be strong enough to bear any pressure that may be applied to them as training walls. The faces are constructed in steps, or altars as they are termed (Fig. 5), on which rest the shores which support a vessel in the dock. Steps are constructed in suitable positions for access to the bottom, also slides down which timber and other materials can be lowered. These side walls are usually of concrete faced with stone; in some docks the facing is formed of concrete of extra quality, and in others of blue bricks with copings of stone. The entrances are constructed with side walls and sills to suit the method adopted for closing the dock, gates requiring a different arrangement from that provided for a caisson. Culverts fitted with penstocks are built in the side walls near the entrances, for filling the dock with water. For emptying, centrifugal pumps seem on the whole the most convenient and the least liable to be affected by the passage of small chips of wood, which cannot always be avoided even with the greatest care. When arrangements have to be provided for keeping the entrance clear of mud occasional dredging is generally resorted to. At Liverpool an extensive system of sluicing is adopted, the head of water retained in the dock or closed basin as the tide falls being utilized to produce a strong current which carries off the mud. At Tilbury jets of water under pressure, combined Avith an arrangement of harrows, are applied at ebb tide to the bottom of the dock and entrance, from an apparatus attached to a floating craft which can be moved to any part required; the mud stirred up by the harrows is set in motion by the jets, and carried away by the ebb tide. Hitherto most of the dry docks for the United States Government have been constructed of timber, those of inasonr Tirnb r y having been considered too expensive. docks, t*16 nayy yards of Charlestown, Brooklyn, and Norfolk there are masonry docks, the sills of wdiich are 25 feet below high-water spring tides; in the timber docks at the various yards the sills are from 15 to 22 feet below high water. A report on the relative value of timber and masonry for dry docks by Commodore Endicott, U.S. Navy, chief of the Bureau of Yards and Docks, states that timber docks are temporary structures, and that the wood of which they are built requires extensive renewal at the end of 25 years, practically amounting in some instances to rebuilding; also that accidents have occurred from the partial collapse and bursting of the floor and sides of some of the timber docks. He considers that, in view of the increased depth required in new docks to accommodate vessels of large draught, the risk to their stability will be largely increased, as they will be subjected to more unfavourable conditions, in particular to a much greater hydrostatic pressure, and that freedom from such risk can only be obtained from a masonry structure designed to resist by its own weight the dangerous force to which it is subjected. He recommends that any new docks should be constructed, not of timber, but of masonry and concrete combined; and that instead of being constructed by day labour and by Government employees, they should be carried out by contract, which would be the means of greatly reducing their cost. Dock gates are constructed of timber or iron. The two doors are exactly similar, each being rather longer than Dock gates th® width of the entrance, so that when ‘ they meet in closing their faces form two sides of a triangle whose base is the straight line drawn from one heel post to the other (Fig. 6). The distance from the apex or meeting-point to the base, which is termed the rise of the gates, is a matter of considerable importance,
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as affecting the strains on the gates themselves and on the masonry, to which the strains are transmitted at the heel posts; in practice it varies from one-sixth to one-fourth of the width of entrance. The heel post is vertical, and shaped to form a tight joint with the closing face of the
holloAv quoin; at the bottom is fixed a cast-iron socket which rests upon a cast-iron pivot firmly secured to the masonry of the gate platform, the top being held by an iron strap passing round it and securely anchored to the masonry of the side walls. The main beams of each gate are built into and secured to the heel post at one end and to the mitre post at the other end; the two mitre posts constitute the meeting face of the pair of gates, and should form a tight and even joint from top to bottom. The timber generally used for gates is greenheart, which is durable and not liable in Britain to the attacks of seaworms. . When gates are made of iron the closing faces at sills, mitre posts, and heel posts are formed of timber to ensure close-fitting joints. The gates when open fit back into recesses formed in the side walls. Timber gates are less liable to injury or twisting, and are more easily fitted together than iron gates ; on the other hand, the latter can be easily rendered buoyant by the introduction of air chambers, which reduce the strain on the hinges and the weight on the rollers supporting the outer end. These rollers are fixed so that they can be adjusted as required to support the gate, and Tork on a circular iron way attached to the masonry of the gate platform ; they are not necessary for gates where the entrance is less than 60 feet wide. The pressure on a gate is due to the difference in level of water on the inside and outside surfaces; this has to be withstood and conveyed to the side walls by the structure of the gates. The strains in any gate are those of an arch with the load applied in radial lines and equally distributed (Fig. 6). A pair of gates should form part of a circular dam, and should be so designed that the line of pressure approximates to a line forming the arc of a circle passing through the centres of the heel posts and the Centre of the meeting face of the mitre posts, to ensure that the gates are subject to compressive stress only. For convenience a pair of gates are usually shaped so that their outside faces when closed form a continuous curve, with the inside faces straight. The subject of strains in dock gates has been most fully dealt with in a paper by Mr A. F. Blandy (Proc. Inst. C. E. vol. Iviii.), and by Mr J. M. Moncrieff (ih. vol. cxvii.), to which the reader is referred for information. For moving dock gates an opening and a closing chain is attached to the outer and inner faces respectively of each gate as low down as can be arranged. The chains are conducted to the back of the side walls through openings, and are worked mostly by hydraulic power. At Barry, Leith, and other ports a more rapid and simple arrangement has been introduced by attaching the piston of a hydraulic engine direct to the back of each gate (Fig. 7). The gate thus closed is held up firm against any undulation of the water outside, and the chains are done away with, together with the necessary guiding sheaves. Instead of gates caissons S. III. —62
