A Treatise On The Principles And Practice Of Dock Engineering

284 DOCK ENGINEERING. through the mass into the sand below. The outer slopes and edges of the mattrasses were covered with a coating of stone, averaging 13 cubic feet per lineal foot of pier. The part above water was covered with larger stones, retained by rows of small oak piles, the ends of which project above the level of the work, with a view to breaking the force of the waves. A cross-section of the north pier is given in fig. 218. It has a width of 29 feet 6 inches between the main piles. The crown of the south pier is 26 feet 3 inches wide, rounded on the upper surface, which attains the level of ordinary high water. The piles connecting the mattrasses are carried to a height of 9 feet 10 inches above this level. A timber roadway, carrying two lines of rails, is attached to the piles. Open Timber Frames are very often employed for piers and wharfs where the water is tolerably quiescent and but moderately deep. The frames may be either lixed or movable. In the first instance, the verticals consist of whole timber piles, generally greenheart or creosoted pitch pine, driven down to a solid stratum and connected transversely above the water level by cross pieces and inclined struts, as at Hull (fig. 246). In the second case the verticals are tenoned into and rest upon a timber sole-plate, set upon a naturally hard bottom, as at Blyth (figs. 219 and 220). In both cases, the frames are erected at distances apart, usually Figs. 219 and 220.—Jetties at Blyth. from 10 to 15 feet, and the bays thus formed are faced with horizontal walings and fenderings. The movable frames have necessarily to be weighted down with heavy stone filling, and this is frequently added in the case of fixed frames, in order to stiffen the work. A foundation of concrete is occasionally to be found, as at Liverpool, and exemplified in three instances (figs. 221, 222, and 223), especially when it can be utilised in the formation of culverts with sluice openings to maintain the required depth of water in situations where there is a tendency to silting. A concrete apron must then be added to the structure, or it will inevitably be undermined by the current. Piled timber jetties have also been constructed upon a rock bottom. At Newcastle, for the uprights of coaling staiths, holes, 3 inches in diameter, were drilled into the rock and into these the pile shoes, which had 4-inch square spikes, 6 feet long at their ends, were driven. At Liverpool, similar but larger holes were drilled for the Prince’s jetty, the holes being 25 inches diameter, and