10 A TREATISE ON DREDGES AND DREDGING 



not excavate more than 45 cu.m. per hour when very fine sand 

 was encountered. 



The hydraulic dredge is the machine used to the best advantage 

 in dredging through sand. The dipper and grapple dredges are 

 objectionable owing to the fact that the fine material will easily 

 escape through the seams of the bucket and consequently only a 

 very limited amount of material will be removed at each lift of 

 the bucket. The buckets of the ladder dredges, in passing over 

 the bank of sand at great speed, stir up the material to such an 

 extent that a large quantity of water will be taken up and only a 

 small percentage of sand will be found in the buckets. The raising 

 to the surface of such a large amount of water decreases the efficiency 

 of the machine and increases the cost of the work per unit of volume. 

 It is obvious that the finer the particles of sand are the larger the 

 quantity of water will be in the buckets of the ladder dredges. 

 Although the hydraulic dredges are considered the best-adapted 

 machines for dredging through sand, yet in order to obtain the 

 proper efficiency from these machines, the end of the suction pipe 

 must be provided with some device that facilitates the separation 

 of the coarse sand from its bank, so that when well mixed with 

 water will be easily raised through the suction tube, and may also 

 prevent the floating away of the fine sand under the stirring action 

 of the pump. 



Mud or Silt. Another soil which is very extensively encountered 

 in dredging is mud or silt. This is composed of a finely powdered 

 mineral matter mixed with organic particles derived from animal 

 or vegetable sources, the powdered minerals being the result 

 of the disintegration of soft rock by water and other natural agents. 

 The particles of mud are so infinitesimally small and so finely divided 

 they are carried in suspension by the running water, and are deposited 

 in places where the water is quieter. Such a material is usually 

 encountered as forming the bottom of the mouth of rivers, bays 

 and harbors. The particles of mud are so finely divided they do 

 not run away as easily as sands, but stick together in such a way 

 that they can be removed with greater facility. 



Dredges of any description may be used with advantage for 

 the removal of mud. The dipper and grab dredges are found very 

 useful for dredging through mud, especially when buckets of lar.ire 

 capacity are employed. The ladder dredge also is considered a 

 very efficient machine in this kind of soil. The rapid movement 



