A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER CONSTRUCTION. I71 work, with numerous compartments formed by means of transverse and longi- tudinal ties. The compartments form réceptacles for stone rubble. From the crudeness of their build, these cribs can only be looked upon as of the nature of temporary structures. They are referred to in somewhat greater detail in Dock Engineering.1 Fascine Work. — Another form of construction adopted in certain localities for moles and breakwaters is known as fascine work, and consists of bundles of brushwood arranged as mattresses, which are sunk in position in successive layers and weighted with stone. Piles are then driven through the mattresses into the sandy bottom to prevent displacement. In process of time the interstices of the mattresses become filled with sand and drift, forming a solid mass. The system is more particularly characteristic of the low-lying coasts of Holland and Denmark, though it is also to be found on Prussian shores. Fascine mattresses are also described at some length in Dock Engineering? and they are alluded to in Chapter IX. of the present volume in connection with charme! training-works ; but, as in the previous instances, they have exhibited no great resisting powers to rough seas. The following is a description of some early breakwaters on the Baltic littoral :— “The fascine dams consisted, according to the depths of water, of one or several layers of fascines 3 feet to 4 feet thick, which were floated down from the inner harbour where they had been made, and sunk on the spot. The ripper layer was afterwards covered with a packing and witli a stratum of small stones and rubble, about 3 feet in height and rounded on the top. This cover was paved with large, approximately cubical, stones of granite. The capping, which had a width of about 13 feet and was slightly arched, was scarcely 6 feet above mean water level. The slope from the capping down to the outer edge was 3 to 1 on the sea side and 2 to I on the harbour side. The thickness of the stone paving was 3 feet in the capping and 2 feet elsewhere. The stone layer was further secured by strong oak piles from 7 to 10 feet long and G inches square, called caisson piles; they were driven at distances apart of 6 feet along the edge of the capping, and of 18 inches along the water-line. “These fascines were exposed to heavy damage, for every storm from the sea lifted the paving stones of the slope, especially at the head and on the sea side, from their seats, and carried them inland, or hrirled thenr up tire slope aird over the mole into the inner harbour. The reason for tlris was maiirly that the stones did not lie sufficiently close upon the flat slopes, and that they could be loosened separately and disturbed by the waves, lacking sufficient weight in themselves to resist this action. In order to make the surface of the slope as plane as possible, with a view to avoiding points of attack for the impinging waves, the stones had been placed with their roughly hewn, approximately square, heavy portions—that is to say with their bases—upwards, and with the tapering parts downwards. In this 1 pp. 286, 287. 2 pp. 282 et seq.