HO 



HARBOUR CONSTRUCTION. 



SECTION II. STONE. 



Stone for sea- works should have high specific gravity Basalt, granites, and syenites 

 List of stones Indications of decay Chalk, limestones, and sandstones 

 Pholas dactylus Quartzite Weathering qualities of stones Iron-furnace slag 

 used for constructing breakwaters. 



It has already been stated (p. 105) that, in selecting materials 

 for sea-works other things being equal preference should be 

 given to those of high specific gravity. The reason for this is 

 evident ; inasmuch as, comparing blocks of equal weight, their 

 displacement is inversely proportionate to their specific gravity, 

 as is also the area exposed to the wave-stroke. 



For example, a block of cast iron weighing one ton would 

 contain 5 cubic feet, and would thus be represented by a cube 

 the side of which would have an area of, approximately, 2*9 

 square feet. This block, immersed iri sea-water, would weigh 

 about 1920 Ibs., and a wave striking the face of it with a force 

 of, say, 1 ton per square foot, would exert a pressure of 2*9 tons. 



Now compare this with a block of concrete weighing 1 ton, 

 which would contain about 16 cubic feet, and be represented by 

 a cube one side of which would have an area of, approximately, 

 6J square feet. Such a block, immersed in sea-water, would 

 weigh only 1214 Ibs., and a wave striking the face of it with a 

 force of 1 ton per square foot, as before, would exert a pressure of 

 6J tons. 



Thus, although the two blocks weigh the same in the air, the 

 one possessing high specific gravity has an effective weight of 

 1920 Ibs. to resist a wave-stroke of 2*9 tons, whereas the one of 

 low specific gravity has an effective weight of only 1214 Ibs. to 

 resist a wave-stroke of 6 J tons. 



The following is a list of some of the stones in common use, 

 with their relative weights : 



Name of stone. 

 BASALTIC ROCK- 



Scotch 



Greenstone ... 

 Welsh 



CHALK 



Weight of 

 a cubic foot 

 in pounds. 



... 184 

 ... 181 

 ... 172 



from 115 to 170 



Name of stone. 



GRANITE AND SYENITE 

 Aberdeen, grey 

 Peterhead, red 

 Cornish 

 Mount Sojrrel 

 Guernsey ... 



Weight of 

 a cubic foot 

 in pounds. 



... 165 

 ... 165 

 ... 166 

 ... 167 



185 



