A Treatise On The Principles And Practice Of Harbour Engineering

140 HARBOUR ENGINEERING. Uplifting ’ is an action which takes place through the application of wave force to the underside of a mass. Obviously the dead (effective) weight of the mass is the resisting element, and the problem is a simple case of the equi- librium of two opposing forces, each of which has already been defined and described. The fracture, or shattering, of a homogeneous block rarely results from the direct impact of the wave. When it does take place, it is probably caused by a prior dislocation, resulting in collision with other parts of the structure. The fracture of joints has been considered under the heading of resistance to shear. Blocks may also be fractured by unequal subsidence in the wall. This possibility applies more particularly to composite break- waters, where the rubble foundation mound is subject to irregular settlement. The results can only be guarded against by avoiding the use of bond in the building of the wall, or by the adoption of what is termed “ sloping ” bond, as exemplified at Kurrachee and Colombo (p. 169). Milan Conference; Report on Breakwaters. — The subject of Breakwater Design formed one of the topics of discussion at the International Maritime Congress of 1905. Papers, some of which have already been noticed, were presented by eminent engineers of various countries, and a general report was submitted to the Congress. This report was drawn up by Professor lo Gatto, and a transcript of his conclusions cannot fail to be of interest. They were as follows:— “ Breakwaters built of rubble, although expensive in upkeep, are suitable for very sheltered sites in shallow water, provided good and cheap material is procurable. This type is not affected by the muddy or soft nature of the sea bottom. “ When the structure is exposed to very heavy seas, the rubble type of mole can still be adopted, under the conditions mentioned above, provided a revetment of concrete blocks is added outside down to a certain depth. The method of depositing these blocks at random appears the best as regards resistance and maintenance, on condition that the profile of the protected slope is so designed that it will shear the waves at sea-level. On the other hand, the method of setting the blocks in regular courses offers serious objections, as they are liable to be disturbed by the settlement of the rubble base and to be completely destroyed during gales, and, in any case, they cannot be maintained in good condition without abandoning the principle of the system itself. “ Breakwaters with a rubble hearting and a double revetment of protecting blocks, laid in regular courses, are not at all reliable in very heavy seas, but they can render very useful service in sheltered sites and in waters of moderate depth, especially if the works are not of very great importance. “ Breakwaters with vertical, or almost vertical sides, are very suitable for moderate depths and hard sea-beds, where there is no fear of the undermining effect of the backwash and currents. They are very expensive and conse- quently inapplicable to unimportant works. “ The composite type of breakwater, consisting of a base formed by a loose