DREDGING AND DREDGES 3 



Dredging usually involves other operations, which are : The hy- 

 draulic survey of the locality to be improved, the knowledge of the 

 quality and quantity of the soil to be excavated, and finally the 

 work of the machines used in the excavation of the materials from 

 the bottom and of those employed in the transportation of the 

 debris. 



The hydraulic survey is easily obtained by any one of the well- 

 known methods. From this survey a chart may be made indicating 

 the depth of the various points along the line of the proposed improve- 

 ment. Knowing the depth of each point as required by the improve- 

 ment, while from the survey is learned its present depth, it is evident 

 that their difference will represent the height of the excavation at 

 such a point. The total difference of all the points will give the 

 total amount of material to be excavated in order to obtain the 

 required improvement. 



The soils, considered from the point of view of the cohesive 

 force of their particles, may be broadly divided into rocks and 

 loose soils. Rocks are those in which the force of cohesion is so 

 great that great power is necessary in order to separate its particles. 

 Loose soils are those having the particles united by such slight 

 cohesion that it can be easily overcome. 



In dredging, different kinds of machines are used, depending 

 upon the quality of soils. Solid rock must first be reduced to 

 small fragments, so as to be readily raised to the surface. The 

 rock is usually broken in two different ways, by hammering and 

 by blasting. The hammering consists in attacking the rock by 

 means of successive blows struck, with great violence, while in 

 blasting the rock is shattered by the mighty force of dynamite 

 or other explosive. It is in the excavation of loose soils as well 

 as for picking up the fragments of rock broken by any one of the 

 methods to be indicated, that the dredging machines are employed. 

 These are divided into continuous and intermittent types. Contin- 

 uous are those that remove continuously the material from the 

 bottom, while intermittent are those that engage the material 

 at intervals. 



Dredges. Continuous dredges are of four different types the 

 ladder, the hydraulic, the stirring, and the pneumatic dredges. 

 The ladder dredge excavates the bottom by means of a series of 

 buckets running with great velocity along a ladder. The buckets 

 scrape the soil at the bottom, raise the debris to the surface and 



