A Treatise On The Principles And Practice Of Dock Engineering

2/2 DOCK ENGINEERING. diminishing to zéro at the crest, and to one-half the maximum at the bottom. But like wind-pressure data, results on small areas are no guide to stresses over extensive surfaces. If a wave, before breaking, reaches a wall or other obstruction having an abrupt, vertical face, it is reflected in the following manner: ____The particles of water in contact with the wall move up and down through a height double the height of the original wave. At a distance away from the wall equal to a quarter of the length of the wave, the particles move horizontally backwards and forwards. Between these two points the motion of the partiales is a compound one, and movement takes place at various angles. Consequently, the action of waves upon a pier or jetty must be taken as resulting in the création of four distinct forces : — 1. A direct horizontal force exerting compression. 2. A deflected vertical force, acting upwards and tending to shear any projections beyond the surface of contact. 3. A vertical downward force upon the base of the wall, due to the collapse of the wave. 4. The suction of the back draught upon the foundation. Apart from the hydrostatic pressure, augmented to a very considérable degree by the force of impact, the following subsidiary results will take place, viz. :— 1. A vibration of the structure tending to weaken the connection of the various parts. 2. A series of impulses imparted to partiales of water contained in the pores and joints of the structure, producing internal pressure in various directions. 3. The condensation and expansion of air confined in cavities and inter- stices, causing disruption. It is impossible on any purely theoretical basis to determine with the least degree of accuracy and precision the magnitude of these various stresses. Practical observation must therefore be called in to supply the deficiency, by providing data as to the maximum stresses likely to be encountered. Investigations have been made in several instances with the aid of a marine dynamometer, devised by Stevenson, with the result that in the most exposed cases, the pressure registered did not exceed 3J tons per square foot. With waves 10 feet high, a mean pressure of 1'36 tons was indicated. Other instances are as follows : — At Skerryvore, from 21 to 2| tons per square foot. At Bell Rock (German Ocean), 11 tons per square foot. At Dunbar (East Lothian), 3^ „ „ ,, At Buckie (Banffshire), 3 „ „ „ Experiments by Mr. Frank Latham, at Penzance, showed a pressure of 18 to 20 cwts. per square foot, at right angles to a sea wall, in 10 feet of water, with a wind pressure of 15 to 18 Ibs. per square foot.