130 WAVE IMPACT ON 



Mr. Mallet on the sea-walls of the Dublin and Kingstown 

 railway 1 during a severe gale, in which masses of water, 

 deflected vertically upwards by the sea face of the wall, fell 

 heavily upon the pavement inside. In this case many of the 

 square granite pavement blocks were seen to jump vertically 

 out of their beds at the instant of the fall of the sea upon them 

 and were thrown landwards towards the line of way. 



The commonly accepted explanations of such a phenome- 

 non will be gathered from the following quotations from 

 authorities on harbour works. Mr. Vernon Harcourt notes : 



' The blocks at and below low water have open joints, 

 into which air penetrates on the recoil of a wave and also fills 

 any cavities behind ; and the succeeding wave, compressing 

 the air inside, leads to the gradual forcing out of a block 

 by the pressure from behind on the retreat of each wave 

 during a storm.' 2 In another passage the same writer 

 remarks, ' Waves striking against the outer blocks also 

 compress the air in the open joints, which aids in the dis- 

 placement of the inner blocks.' 3 



A somewhat similar but wider explanation is to the effect 

 that, ' The air or water confined within a joint, when struck 

 by a wave, is converted into a very destructive agent. The 

 air is compressed and forced along joints and seams, and, 

 immediately upon the wave receding it expands and tends 

 to loosen and push out pieces of the rock. When water is 

 confined in joints the force of the wave stroke is transmitted 

 by it, and is made to act over large areas on the same principle 

 as a hydraulic press.' 4 



Similar views were adopted by Captain D. D. Gaillard, 

 U.S.A., 5 who, as a result of experiments, came to the con- 

 clusion that the pressure due to the impact of an interrupted 



1 Proc. Inst. C.E., vol. xviii. p. 113. 



* Vernon Harcourt, Civil Engineering applied to Construction, p. 486. 



8 Vernon Harcourt, Harbours and Docks, p. 298. 



4 Wm. Shield, Harbour Construction. 



5 Wave Action in Relation to Engineering Structures, Capt. Gaillard, p. 181. 



