280 



HARBOUR CONSTRUCTION. 



concrete, though sometimes of slate or other suitable stone. 

 They ought to be somewhat smaller than the holes they are 

 intended to occupy; and after having been deposited in the 

 holes, they should be well grouted. 



An efficient method of forming joggles in under- water work 

 is to place a canvas bag in the joggle hole, and fill it with fine 

 Portland cement concrete (say 3 parts of fine gravel to 1 of 

 cement). 1 The bag should have a properly shaped flat bottom, 

 and it should be slightly larger than the hole. 



An elongated tripping bag will be found a convenient means 

 of lowering the concrete through the water and depositing it. 



When under-water grouting is resorted to, ordinary joggles, 

 formed so as to allow of the grout passing around them, may be 

 inserted in the holes before grouting commences. If, however, 

 the joggle holes are small, the grout alone will suffice to fill them. 



FIG. 92. 



Sometimes small bags of concrete are rammed into the holes ; 

 but these do not make such close or satisfactory work as the 

 methods above described. Loose joggles, if exposed to wave-action 

 through open joints, may become a source of danger to work. 



In fissures amongst rocks it is common to find cavities which 

 have been formed and gradually enlarged by the rolling about 

 of boulders within them. A boulder probably thrown into a 

 fissure by the waves becomes lodged in some place from which 

 it cannot escape, and by constant movement, due to the waves, 

 it erodes the rock, and so forms a cavity, which, as it enlarges 

 and the boulder gains more freedom, becomes a veritable mill. 



In the same way a loose joggle may enlarge the hole which 

 it occupies, and by degrees be itself completely worn away, the 



1 For small joggles, say up to 3 inches square, cement mortar, composed of 

 equal proportions of Portland cement and sand, thoroughly well mixed, so as to 

 form a thin slimy paste, is preferable to fine concrete. 



