METHODS OF CONSTRUCTING BREAKWATERS. 241 



of 8 or 10 feet above high water, a series of parallel longitudinal 

 banks or ridges being formed under the several lines of railway. 

 These were levelled down, and the mound consolidated by the 

 action of the sea, additional rubble being deposited from time to 

 time, as was found to be necessary. After a sufficient time 

 (usually from four to five winters) had elapsed to admit of the 

 thorough consolidation of the mound, a trench was excavated 

 along it, down to the level of low water of spring tides, to receive 

 the masonry of the superstructure. 



In the Alderney breakwater, the stone forming the mound 

 was, with the exception of a short length at the commencement, 

 deposited from hopper barges, varying in capacity from about 

 60 tons to 140 tons each ; but a staging was erected upon the 

 mound for use in building the superstructure. On account of 

 the exposure of the site, this staging had to be removed at the 

 end of each working season, and re-erected at the commencement 

 of the next. In general design it resembled that illustrated on 

 Fig. 61, p. 239. The bays each measured 30 feet, and the piles 

 were furnished with flat stone bases, each weighing about 15 cwt. 

 The piles were " pitched " from an outrigger travelling-carriage, 

 and they were surrounded at their bases with rubble stone as 

 soon as possible after they were placed in position. The great 

 depth of water at Alderney, and the exceptionally heavy storms 

 experienced there, afforded good grounds for the subsequent 

 adoption of barges, in preference to staging, for depositing the 

 rubble. 



One of the most important considerations in designing a sea- 

 staging is to allow as little material as possible to be within 

 reach of the highest waves. Its roadway should therefore be at 

 such a level as to place it altogether out of the reach of such 

 waves. Experience has shown that this should not be less than 

 from 18 feet to 20 feet above the highest tide in exposed situa- 

 tions around the British coasts. Other considerations in con- 

 nection with the work to be executed may, of course, necessitate 

 its being placed much higher. 



The roadway of the staging adopted by Mr. Dyce Cay, M.I.C.E., 

 in constructing the south breakwater at Aberdeen, was 30 feet 

 above high water of ordinary spring tides, the piles or uprights 

 being of round timbers. The fact that this staging sustained no 

 injury during the whole time that the work was in progress was 

 probably due in a great measure to these features in its design. 



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