A Treatise On The Principles And Practice Of Dock Engineering

THE CONSTRUCTION OF GRAVING DOCKS. 477 It has already been remarked that, where the natural foundation is sound hard rock, the necessity for an artificial floor to withstand hydrostatic pressure disappears. At the same time, care must be taken to see that there is no possible infiltration of water under the sill. The slightest film can transmit all the pressure of the external head. To prevent any such contingency—one inevitably producing disaster—it will be well to have numerous ground drains communicating with the surface of the floor, so that the water may have free vent, and the worst effect of infiltration will be some leakage or a possible flooding. There is yet another side to the question. When a ship is dry-docked, her weight is transmitted through the keel-blocks to the floor, the centre line of which consequently undergoes a shear on each side of the blocks equivalent to this weight. And, as the imposed stress due to the vessel is best taken in the form of compression, it will be advisable to design the floor so that it may possess a second real or virtual arch, in this case upright, not inverted. A slight camber in the upper surface is useful for draining the water to the side channels. The Construction of Graving Docks. Masonry at one time constituted the material most in favour for the construction of graving docks, but of late years Portland-cement concrete has superseded it to a very large extent. Either material is extremely durable,* but concrete has the advantage of greater economy in most cases, Timber has been, and is, largely used in the United States. It is, however, much inferior to stone or concrete in durability, and there are indications that the desirability of a more permanent form of construction is becoming recognised. One advantage of wood is stated to be that it is safer than stone to work upon in frosty weather, ice being less likely to form and remain upon its surface. The claim is of dubious validity. Another contention, that timber-work is injured less than masonry by the severity of North American winters, strikes one as being untenable and even absurd, if any analogy exist in the behaviour of the two materials in this country. Timber docks are certainly much cheaper to construct, and herein, apparently, lies their most effective recommendation. The methods adopted in building stone or concrete docks are identical with those in building locks, and the general features of these having already been discussed in Ohapter vi., the subject need not be further considered. In the construction of timber docks, the most prévalent practice is as follows : _The site of the floor is first enclosed within continuous sheet piling, formed of half-timbers having tongued and grooved joints, and the whole area is then studded with bearing piles of whole timber, driven at * The deterioration of concrete work in certain graving docks, as at Belfast and Aberdeen, has been the subject of an inquiry in the Chapter on “ Materials,” to which the reader is referred for an explanation of the phenomena.