488
DOCKS
and cast-iron weights amounting to 300 or 400 tons were generally required to force them down. At Brest, where the range of spring tides is 20 feet, the foundations of the quay walls were constructed in the deep water without a cofferdam. A bank of rubble was formed up to a level of 15 feet below low water ; on this large artificial blocks were placed in two courses reaching to low-water level, the part of the wall above low water being constructed in the ordinary manner by tide work. The blocks, about 100 tons in weight, and constructed of rubble masonry, were built on platforms on a slipway, and when ready for setting were lowered down the slipway sufficiently far to admit of a barge being floated over them ; each was then suspended under a barge, and, as the tide, rose, was lifted up and taken to the position where it was required to be set, and deposited in its place as the tide fell. In favourable circumstances this method answers well, and with slight modifications has been adopted at other ports. At Quebec, where the time for constructing harbour works was limited to the summer months, the quay walls were formed of cribwork filled with concrete. The cribs were constructed during the winter, and in the spring were floated into position, where they rested on a foundation of bearing piles ; they were then filled with concrete deposited under water. The cribs were in lengths of 40 feet, being 27 feet high, 33 feet wide at base, and 23 feet wide at top. During the improvements and extensions executed at the port of Antwerp, it was found necessary to build on one side of the river an entirely new line of quay walls in advance of the old line, so as to secure a depth of 26 feet of water at low water and prevent the silting up caused by the irregular and broken line of the old wall. The new quay wall, which was constructed by a special system of movable cofferdams, was built of brickwork faced with stone, and was carried out in lengths of 82 feet. First an iron caisson for compressed air was provided, which was 82 feet long and 29J feet wide, and which varied in height from 8J feet to 16J feet according to the depth required below the bed of the river, the footings of the wall being always 265 feet below low water. This caisson, after serving for the removal of the soil, was filled with concrete and became an integral part of the foundation. A movable iron cofferdam 40 feet high was fixed by bolts on the top of the caisson and well strutted inside ; within this the wall from the top of the foundation to low-water level could be built in the dry and in the open air, the struts being removed and replaced in the ordinary way as the work advanced. Iron tubes provided with air-locks were fixed to the top of the caisson, the wall being built round these tubes so that they could be removed when the caisson had reached the bottom and was filled in. The cofferdam was suspended complete by chains from an iron framework carried on two iron barges, and was raised or lowered by hoisting - gear. The site for the caisson was first prepared by dredging. The caisson with cofferdam attached was then fixed in place by means of the floating barges, and sunk by the weight of the work being carried on above and within it. The narrow spaces between the lengths of wall thus built were filled in afterwards by an arrangement which effectually tied together the whole wall. Dock walls are ■usually now constructed of Portland cement concrete, mixed in proportions varying from 5 to Walls ^ parts of shingle and sand to 1 part of cement, the sand being in the proportion of from 1| to 2J parts to 1 part of cement according to the quality required. The interstices of ordinary shingle without sand form about 33 per cent, of its bulk, and they should be completely filled by the cement and sand when mixed. To ensure good work the cement must be of the highest quality, and the other materials must be perfectly clean. Broken stone is sometimes substituted for shingle, and some engineers use large fragments of stone bedded in the mass of the concrete. Care should be taken to make the concrete as compact and solid as possible, in order to prevent the percolation of water, since concrete is liable to damage from sea-water in course of time if this precaution is not observed. The upper portion of the wall, where it is exposed to the rubbing of ships and boats lying alongside, should be faced with masonry or timber fendering. Some quay walls are constructed entirely of timber, the filling at the back being retained by sheet piling up to about low water, and above that level by a paved slope. A slight batter or slope—from 1 in 8 to 1 in 12 is that usually given—to the face of a wall adds to its strength and appearance. Where the
range of tide is great, the walls should be vertical or with a very slight batter, since the fall of the tide will otherwise cause the side of the ship to be an inconvenient distance away from the line of the coping. Later improvements in the design of graving docks consist chiefly in the increased dimensions, the facilities for moving the gates and caissons at their entrances, and the arrangements for pumping docks^ out the water. With the increasing size of vessels, it has become necessary to provide larger dry docks in which repairs to such ships may be carried out. In all the most important ports new and extensive graving docks have been constructed. The following are the lengths of some of the largest: Liverpool, 1000 feet; Southampton, 750 feet; Glasgow, 880 feet; Belfast, 850 feet; Barry, 740 feet; Tilbury, 875 feet. Some of the longest graving docks are provided with a caisson or gates, dividing them into two lengths, so that for a short vessel only a part of the dock need be used. The construction of a graving dock requires great care, especially if the ground is of a soft or treacherous nature. If soft ground, such as sand or silt, extends to some depth below the bottom of the dock, and the depth to hard ground is so great as to render the removal of the soft material impracticable, it will be necessary to enclose the site by sheet piling to prevent any movement of the subsoil through scour. This should be done in addition to any piling that may be required to carry the floor and side walls, and is necessary under the entrances of both wet and graving docks; for if water finds its way under the entrance with the pressure due to the head of water on either side of the gates, it will cause a scour, and sooner or later the destruction of the entrance will follow. When the site has been excavated and prepared, and suitable drains have been provided for drawing off any water that may be met with, the foundations for the floor and side walls have to be constructed. For this class of work Portland cement concrete has almost entirely superseded brickwork where suitable materials for concrete can be obtained. The floor of a graving dock is in principle an inverted arch, which has to resist the upward pressure of any water that may penetrate to the under-side, and as such water will also be equally at the back of the walls, they will be subjected to a similar pressure in addition to the lateral pressure due to the filling. This fact should be regarded in putting in the concrete, which should he constructed in sections to the full depth of the floor, with the longitudinal vertical joints radiating as if the surface of the invert were curved (Fig. 5). When the concrete is if in contact with rock, the joint should be broken by trenches cut into the surface of the rock, to which Fig■5the concrete is thus firmly keyed; this is of especial importance if the The surface of rock is of a water-bearing nature, the floor is generally formed of granite masonry bedded on the concrete, deeper stones being placed along the centre to carry the blocks, while the sides are sometimes paved with granite setts, or blue bricks, or, as in the case of a dock at Halifax, FT.S., with pitch-pine planking fixed to longitudinals bedded in the concrete. Usually the centre of the floor is kept a few inches higher than the sides, to allow the water to draw off. The blocks on which the keel of the vessel rests when in dock are fixed along the centre of the floor, and are constructed of hard wood, or of cast-iron and wood combined, the parts of
