A Treatise On The Principles And Practice Of Harbour Engineering

134 HARBOUR ENGINEERING. and forming the bulk of, ehe breakwater. Darger and special material for protecting the surface slopes will run to -6s. and 7s. per cubic yard. Possibly 4s. or 5s. per cubic yard might be taken as an average all-round cost for the whole. For dressed masonry or concrete work, either in the form of blocks or in bulk, set mainly below the water level, it is difficult to assess a rate without a full knowledge of the circumstances and resources at disposai ; yet it would be unjustifiable to imagine the work as capable of being carried out at a lower rate than 2s. a cubic foot, and it might easily attain a very much higher figure.1 Even this minimum rate is from eleven to thirteen times that of rubble work, so that, ceteris paribus, the bulk of the mound would have to exceed the bulk of the wall in something like the same ratio before it ceased to be the more economical method. The cost of composite breakwaters combining a foundation mound with an upper wall will, of course, lie between both extremes, and probably, in the majority of cases, it will prove to be rather more than half the cost of an equivalent upright wall. Actual examples affording any degree of serviceable comparison are difficult to quote, as so much depends upon the particular circumstances and con- ditions of each case. It would, in fact, be necessary to go very minutely into detail in order to estimate the relative value of each variation from its funda- mental type, and, apart from this, no effective comparison could be made. All that can be said is that breakwaters have cost anything from £50 to £400 per lineal foot. The lower limit appertains to minor structures only. Among those of greater importance may be cited the following. Portland breakwater cost approximately £130 per foot run; Holyhead, £160; Colombo, £170; Alderney, £235; Plymouth, £300; Peterhead, £3002; and Dover,’ £370.2 Other instances will be found in connection with their detailed descriptions. Cost of Maintenance.—A comparison of the expenditure upon upkeep of the wall and the mound admits of only one conclusion. The wall, provided it be carefully and properly constructed in the first instance, calls for no further attention save for such rare and occasional damage as results from some storm of exceptional severity. Ihe mound, on the other hånd, is peculiarly susceptible to the constant fretting and attritional action of waves. Concussion and back-draught, or suction, constitute two alternating forces continuously and incessantly at work, even in times of moderate and calm weather. Rough rubble is smoothed and rounded by repeated movement, until it is easily sucked out of position and rolled away. The surface slopes thus become gradually less steep, while the flattening correspondingly inereases the power of the waves, converting them more and more from the.oscillatory into the translatory variety. The 1 Pai tieulars of ashlar work at Holyhead breakwater : Runcorn sandstone below zéro 2s. Ild. per cubic foot. Anglesea limestone below zéro, 3s. 5’d. per cubic foot. Runcorn limostone above zero, is. 9d. per cubic foot. Anglesea limestone above zero 2s 3'd ner cubic foot. ’ ■ - • 1 2 Incomplète ; estimates only.