A Treatise On The Principles And Practice Of Harbour Engineering

I/O HARBOUR ENGINEERING. pressure from a ram or piston, fluid concrete is forced into all the adjacent cavities. To prevent the escape of the concrete, however, all the face joints must necessarily be closely caulked. This has been done in certain cases by forming slightly dovetailed grooves near the outer edges of the blocks and ramming them with rolls of canvas containing neat cement, as shown in Fig. 145. —Joint Packing. sketch (fig. 145). These were allowed to harden, and the joints packed with shingle before grouting. In the case of wide joints, the apertures may be faced with brickwork in cement, or with small bags, containing neat cement, stacked compactly. The concrete for grouting purposes should not be too fluid. Other materials are also used, such as clay worked up with hydraulic lime, and sand mixed with iron filings and sal ammoniac; but Portland cement concrete, on the whole, is préférable. Minor Breakwaters.—All breakwaters are not planned on the same scale. Massive construction may be both necessary and possible in the case of leading harbours and ports. But there are also small harbours, where any great outlay on protection works is out of the question, and where some expédient must be contrived for affording reasonable protection at moderate cost. It is both interesting and instructive, therefore, to consider the steps which have been and may be taken to meet these cases. Crib and Box Breakwaters.—The submersible caisson of steel and concrete has had its prototype in a long, wooden box, floated out into position and filled with rubble. Such was the form of breakwater adopted in many early instances, and still practised at some ports on the Baltic seaboard. These boxes or cribs were braced, at intervals of 8 to 10 feet, by transverse partitions, and, in wide boxes, there were often one or two longitudinal partitions as well. Floors of planking were arranged in several—about three —tiers, with a charge of stone incased in eacli. Both the outer casing and the inner partitions were constructed in solid tiniber. “ Stone cribs of this type of construction constantly required repairs, since, apart from scouring, the terminal boxes were damaged by every strong sea; the planks and the upper balks were torn off, although secured by means of strong iron bolts, and the stones were hurled about.”1 The dams were strengthened, as far as possible, by driving piles through the inclosures, in two rows with cross ties, and by depositing a mound of huge stones in front of the seaward face. But no measures proved com- pletely satisfactory. Breakwaters such as these could only be employed in comparatively shallow and but moderately exposed positions. Depths of 15 feet of water probably mark the limit to which they may be advantageously applied Somewhat similar to the foregoing are the timber cribs used on the North American Lakes. They are box-shaped frames of timber constructed in open 1 Anderson on Breakwaters, Proc. Int. Nav. Cong. Milan, 1905.