CHAPTER XIII 

 HYDRAULIC DREDGES GENERAL DISCUSSION 



IN working with ladder dredges of large capacity through 

 different materials, European engineers discovered that it w r as more 

 difficult to dredge through very loose material than in soils that 

 were hard and compact. In fine sand and mud the impact of the 

 buckets scraping the bottom caused the stirring up of the fine 

 particles of the soil, to such an extent that it floated away, thus 

 the buckets were mostly filled with water. In the harbor of Cette, 

 France, a ladder dredge that used to excavate 180 cu.m. per hour 

 of coarse sand and gravel could not excavate more than 45 cu.m. 

 of fine sand. In the year 1867 M. Basin, a French engineer, presented 

 to the Paris Exposition a model of a new type of dredge. M. Bazin 

 proposed to raise to the surface the loose materials by means of a 

 suction pump. The same method was suggested again in 1870 by 

 Mr. C. Randolph, and in the following year it was used by Gen. 

 Gillmore, U. S. A. in dredging the channel over the bar, at the 

 mouth of the St. John's River, Florida. 



Hydraulic dredges are of very simple construction. They consist 

 of a pipe which reaches the bottom and is attached to a powerful 

 centrifugal pump discharging the materials into another pipe, 

 the whole being mounted on a float. Owing to the simplicity of 

 construction hydraulic dredges can be built at a smaller cost than 

 ladder dredges, and since they are more efficient on sandbars or at 

 the estuary of wide rivers, they have become quite common and 

 have been a success from their first appearance. 



The efficiency of the hydraulic dredge has been continuously 

 increased and to-day we have machines of over 3000 cu.yds. capacity 

 per hour. The cost of construction of these machines has :ilso 

 increased in proportion. To give continuous employment to the 

 hydraulic dredges they are now built to work through L;ml and 

 compact soils. For such a purpose the lower end of the suction 

 pipe is provided with different devices, designed to dislodge ili<* 



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