A Treatise On The Principles And Practice Of Dock Engineering

282 DOCK ENGINEERING. 25 to 30 tons weight at the surnmit. Owing to the difficulty of quarrying such blocks, concrete monolitlis are now generally adopted for the upper- most layer. No benefit is derived from any attempted consolidation of the work by intermixing large and small pieces. On the contrary, the result is likely to be harmful, since the dislocation of the larger blocks will Be facilitated in consequence of the small pieces getting under and between them. Blocking the interstices with cement concrete, in bags or otherwise is a much more satisfactory course. As an illustration of the combined system of construction, we may take the North Jetty at Algiers (fig. 214). The bottom hearting, 16 feet in height, consists of rubble from 30 to 200 Ibs. per piece. Overlying this there are two layers, each 10 feet high, of natural Blocks, ranging in the lower layer from ^ to 1| tons, and in the upper layer from 1| to 8 tons in weight. The remaining distance of 32^ feet to low-water level is occupied by artificial blocks containing about 550 cubic feet. The superstructure is carried to a height of 16 feet. Timber Piers are less substantial than those of masonry or concrete, but they possess certain advantages as regards economy and rapidity of execu- tion. Where the ground is suitable for the reception of piling, and in localities where storms are infrequent and of no great severity, timber jetties and piers can be constructed at a cost much less than that of more massive structures. In ice-bound ports, too, such as those in the Baltic, the prosecution of the work of piling is independent of the season and can be carried on uninterruptedly through the winter, which is an important considération. The simplest, and certainly the most primitive, system of timber jetty work is that inaugurated by the Dutch, who build their quays very largely with the aid of fascines (Dutch, ryshoot), or bundles of brushwood derived from copses of willows, osiers, &c. Mattrasses of this material, weighted with stone, are sunk in position in successive courses, the whole structure being secured by rows of vertical and inclined piling. The advantages claimed for the use of brushwood are (1) its elasticity, which renders it less liable to injury from the impact of waves, and (2) its solidification under the accumulation of sand and drift in the interstices. To these may be added its convenience and cheapness. The following particulars relate to the piers at the Hook of Holland, near Rotterdam (see figs. 215, 216, and 217) : — The piers were constructed of successive layers of zinkstukken, or mattrasses, 54’7 yards long by 26'2 yards broad, and 1 foot 8 inches thick,. constructed as follows :—Two stakes were driven into the ground, about 2 feet 6 inches apart, to which a cross stick was secured about 2 feet 3 inches from the ground. A series of these frames were erected, 2 feet apart, the number depending on the size of the zinkstuk. The fascines were then placed on the cross sticks, being drawn out lengthways, so that each bundle overlapped and bonded well into the next. They were