244 HARBOUR CONSTRUCTION. 



destruction of the piles passing through the concrete blocks by sea-worms, 

 and by the subsidence of the blocks beneath the superstructure of 

 homolithic concrete; 3rd, the disintegration of the surface of the 

 concrete by the chemical action of the salt water on the Portland 

 cement ; 4th, excessive pressure on points of the concrete blocks, due 

 to contact of the surfaces without mortar, splitting the blocks; 5th, 

 the abrasion of the sides near the beach by stones, and the smashing 

 of the roadway, parapets, and stairs at the lighthouse tower by the 

 water from spent waves falling off the tower. The most destructive 

 force was that of compressed air, the other destructive influences being 

 dangerous in the degree to which they laid the breakwater open to the 

 compression of air by the waves." 1 



It is not always necessary or desirable that staging should 

 extend from the shore to the end of the work, inasmuch as 

 lengths of it may be " fleeted " or taken up and used over and 

 over again as the work advances, provided that facilities are 

 afforded for running back, or transporting in some other way, 

 the cranes, travellers, and other plant employed on the staging. 

 This may be done either by means of ramps, or by suitably 

 constructed carriages. The latter were employed both at 

 Alderney, and at the Aberdeen south breakwater- works, and 

 were found to work satisfactorily. 



Where rubble mounds are of large proportions, as in the 

 Holyhead and Portland breakwaters, the system of depositing 

 the stone from a stage will generally be found the best to adopt ; 

 but where a mound is of moderate proportions, and other circum- 

 stances favourable, barges may often be employed with advantage, 

 especially in cases where the top of the mound can be kept at a 

 sufficient depth to allow of the barges passing over it at all times 

 of the tide. 



Where large numbers of men are employed getting and 

 supplying rubble for a breakwater, it is evident that, should a 

 spell of rough weather put a stop to its being tipped into the 

 sea, the quarry would soon become congested, and the men and 

 plant employed would be thrown idle. In some localities this 

 would mean a complete stoppage of work during the winter 

 months. With a properly constructed staging, having its road- 

 way well above the waves, the chances of such interruptions 

 would be far less than if barges were employed. The question 



1 Min. Proc. Inst. C.E., vol. Ixxxvii. 



