PART I 



DREDGING EQUIPMENT 



About 99% of the dredging volume in the United States is accomplished by 

 hydraulic dredges (Pequegnat et al . 1978). Hydraulic dredges mix sediments 

 with water to form a slurry which is pumped to the discharge point. Mechanical 

 dredges such as the bucket or dipper dredge (Figure 1) are seldom used in pro- 

 jects involving large volumes of material but are valuable for working in 

 small areas such as near docks or boat slips and for the cleanup of spills and 

 contaminants. Mechanical dredges are usually mounted on barges and move mate- 

 rial mechanically with some type of bucket. The dredged material is usually 

 transported by barges. Less water is incorporated into the material than 

 occurs with hydraulic dredging. Most of the information in this section is 

 derived from Pequegnat et al . (1978). Other general discussions are found in 

 Boyd et al . (1972) and Gren (1975, 1976). Common types of dredges are illus- 

 trated in Figure 1. Certain new types, such as pneumatic dredges, are not 

 discussed in our review because of the lack of documentation at the time of 

 writing, 



HYDRAULIC DREDGES 



Pipe! ine Dredges 



Pipeline dredges are usually cutterhead-equipped and work by hydraulic 

 suction. They remove both consolidated and unconsolidated material and pump 

 it through a pontoon-supported pipeline for discharge at a disposal site. 

 Discharge is a continuous operation as long as the dredging unit is operating. 

 Disposal is usually nearby, but the distance can be increased up to several 

 kilometers by the use of booster pumps. Disposal can be on land or in open 

 water. The use of a pipeline dredge is limited to relatively protected waters 

 because of problems with unstable pipeline units. 



Pipeline dredges make up the bulk of the equipment of the private dredg- 

 ing industry and are the type of dredge most commonly used in the United 

 States. Variations of pipeline dredges include suction dredges (without a 

 cutterhead) for use in soft material and the dustpan dredge which is used ex- 

 tensively on the Mississippi River. The dust pan dredge has a wide (up to 14m 

 or 45 ft) suction inlet and is especially efficient in removing sandbars. Dis- 

 charge is into the water adjacent to the dredge (Gren 1976). 



Hopper Dredges 



The hopper dredge is a self-propelled vessel, equipped with a hydraulic 

 suction dredge system and with hopper bins to contain and carry the dredged 

 material to a place of disposal. Most of the hopper dredges are owned and 

 operated by the Corps of Engineers and work in coastal waters and in the Great 

 Lakes; however, private industry is rapidly developing such a capability. Hop- 

 per dredges are used chiefly for maintenance dredging and usually transport 

 sediments to open water where they are dumped through bottom doors on the 

 hoppers. The hopper dredge has the advantage of being highly mobile, less 

 disruptive to vessel traffic, and can operate in waters too rough for a 

 pi pel ine dredge. 



