CHAPTER XII. 



Dredging Clnss of Appliances Grabs Pomp-dredgers Bucket-dredgers 

 Stationary Dredgers rcrsus Seagoing Hopper-dredgers Variation of quantities 

 of dredged material as compared with sections Time actually employed in 

 dredging. 



SUBAQUEOUS excavation is frequently a matter of considerable 

 importance in connection with dock construction, either in 

 forming the leading channel from the open water to the 

 entrance, the maintenance of a sufficient depth in the basins, 

 locks, and approaches, alongside wharf walls, or in excavating 

 large basin areas after the wharf walls have been constructed. 



The appliances range from the primitive bag and spoon, 

 worked by hand, to the elaborate and powerful machines costing 

 twenty to thirty thousand pounds. 



The class of machines to be used will very greatly depend 

 upon the magnitude of the operations, the niture of the material 

 to be operated upon, the distance to which the dredged material 

 has to be removed, the facilities for disposal, and the conditions 

 generally under which the work has to be carried on. 



Generally dredging appliances may be classed under three 

 types, i.e. grab-dredgers, pump-dredgers, and bucket- dredgers, 

 each having their special uses. 



Grabs are most efficient when soft material has to be 

 removed in confined spaces, small docks, basins, and entrances, 

 and for lifting blasted or broken-up rock. 



Pump-dredgers render good service when large quantities of 

 sand have to be removed. In open situations, they have been 

 found to work efficiently in a slightly chopping sea. At Dun- 

 kirk, a two-foot swell did not impede their action. 



The bucket-dredger is undoubtedly the most efficient 

 appliance, when large quantities of material extending over 

 considerable areas have to be removed, ensuring continuous 

 work, and when a practically uniform bottom level is required. 



