c. Disposal of Dredged Material. Most materials excavated in the dry can be used 

 directly for backfill, for raising ground levels, and for various other beneficial purposes; 

 other materials excavated by dredging below the water surface are difficult to dispose of 

 beneficially. Without careful planning, the disposal of dredged materials may be detrimental 

 to the environment, yet dredged sand and coral may be highly useful. Because such material 

 settles out of suspension quickly, it regains the same characteristics it had before being 

 dredged. It can be used to replenish or build new beaches by controlling the discharge from 

 the pipeline. By manipulating the runnoff waters properly, it can also be used to raise 

 ground levels over large areas. 



Silts, clays, and loams are often transformed by the mixing process of a hydraulic dredge 

 into substances that present numerous problems in disposal. Extremely fine particles with 

 certain chemical properties that occur in many soils often form colloids that remain in 

 suspension indefinitely in the waters into which they are discharged. Fine silts settle out 

 slowly and may be diffused over a wide area of the water body into which they are released 

 before finally dropping to the bottom. Clays form in balls during their passage through the 

 disposal pipe and create undesirable amorphous pockets wherever they are discharged. 

 Nonsandy material contained by dikes or training walls in a landfill after being discharged 

 tends to segregate with the coarsest materials settling out near the end of the discharge line 

 and with the fines being carried toward the spillway end of the enclosure. The fine silts and 

 colloids remaining in suspension at the spillway are then discharged into the receiving waters 

 where they present further problems . 



There are no simple solutions to the disposal problem, but some of the more successful 

 efforts have been as foUows: 



(1) In firm, clayey material, use a clamshell dredge in lieu of a hydraulic dredge, and 

 transport the material to a well-drained drying area. Spread it in a 2- or 3-foot layer, and 

 when reasonably dry, rehandle it to some beneficial disposal area such as a grade-raising 

 project, the core of a dam or dike, or a sanitary landfill. 



(2) Discharge the material from a hydraulic dredge into a diked area and spiU the 

 effluent into another basin for holding until clarified. This system may have to be expanded 

 to include two or more clarifying basins if the settling process is exceptionally slow. As the 

 diked area fUls, the good material may push a deposit of sludge toward the spillway. This 

 sludge may have to be removed and disposed of at times, but the disposal of these minor 

 amounts of poor material is not difficult. After the diked area is filled with reasonably good 

 material to about 110 percent of the design -fill thickness, it can be dried in any one of 

 several ways. Ditching at intervals to draw off the water that seeps out will help. Running 

 over the area with heavy equipment will speed the process. RehandUng the material into 

 high stockpiles will also help it to drain faster. If the base material over which the material is 

 spread is sandy, well points may be used to extract the water that leaches down into it from 

 the new fUl. The dried-out result should shrink to about the desired grade. If not, vary the 

 amount of initial overfill until it does. 



43 



