A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER CONSTRUCTION. 179 Upon the foundation course thus prepared was raised the body of the jetty, which comprised three courses of concrete blocks, each 16J feet long, 8| feet broad, and 6| feet high, weighing 55 tons. The concrete of which they were composed was rather richer in cement than that previously described, in that it contained 14J Ibs. of cement per cubic foot instead of 12| Ibs. Breakwater at Cette.—New spurs extending the existing breakwater at this port at each extremity of its length were constructed between the years 1881 and 1895. The foundation is of a very shifty nature, being sand exposed to considér- able scour. Such action inevitably tends to settlement and dislocation in any struc- ture built upon it. Still, the problem had to be faced, and the system adopted was as follows. Upon the treacherous base was deposited a mass of small riprap and rubble, the pieces ranging up to a weight of 440 Ibs. and forming a core about 80 feet wide by 13 feet thick (fig. 153). Above this there is a layer, one-half that thickness, of larger rubble, the largest lumps of which attain a weight of 4 tons each. This layer extends seaward of the riprap core for a distance of about 60 feet, and thereon is laid in regular horizontal courses artificial blocks having a volume of 700 cubic feet each. The blocks were laid by floating der- ricks, and were so placed as to have their longitudinal axes perpendicular to the line of the breakwater. They are not in actual contact with one another, but the spaces of 24 or 30 inches between them have been filled up with masonry and concrete so as to form an unbroken front. Fig. 153. —Section of Cette Breakwater. Sandy Foundation The lowermost two courses of blocks, however, on the sea side were simply tipped into position, the upper surface being roughly levelled by means of rubble filling. Altogether, the blocks were not adjusted with the precision which is characteristic of similar breakwaters elsewhere — at Genoa, for instance. At first it was intended to surmount the whole structure with a blockwork parapet, but this idea was abandoned, as the addition would probably have