A Treatise On The Principles And Practice Of Harbour Engineering

1 02 HARBOUR ENGINEERING. In all cases, it is essential to remove the surface coating of mud, ooze, and weed, which covers the sea floor. This may be done by dredging or with the aid of divers. If the stratum thus exposed be sufficiently firm for the purpose, the breakwater pier may be laid upon it forthwith. Otherwise, it will be necessary to excavate further until a satisfactory base is obtained. In the event of the desirable stratum lying at a great depth, as revealed by borings, shafts may be sunk and the work built up in the form of piers inside them (fig. 131). These shafts may consist of steel plating with walings of steel tees and angles, strutted with similar sections or with timber balks. They are usually so built, in lengths of convenient dimensions, such as can be handled by a crane or other lifting appliance. The lowermost length is fitted with a V- shaped cutting edge of hard steel. A sufficient number of lengths are bolted together to bring the shaft above the water level when resting on the sea bottom. Excavation is then carried on in the interior by grab buckets with frequent inspection by divers. As the shaft sinks under its own weight, combined with that of kentledge, additional lengths are added at the top. When the solid stratum is reached, the interior of the shaft is filled with conerete. The spaces lying between successive piers are arched over at or about the level of the sea floor (fig. 131). Another method of transmitting the weight of a breakwater to a lower stratum, is by means of timber-piling driven at short intervals over the whole area of the site. The required depth must, of course, lie within the range of ordinary logs, say from 40 to 50 feet. Piles of greater length are expensive and difficult to obtain. When driven to their utmost extent, the heads of the piles are cut off by divers and cased in rich conerete (say 3 to 1) to a depth of at least 2 feet below the mud level in order to secure immunity from vermicular attack. A foundation layer of conerete may then be distributed over the whole area. In certain circumstances it may suffice to inclose the site within sheet- piling; remove an upper layer of material, a foot or two in thickness, and deposit conerete.