June 1981 and was 80 percent completed by December 1981. During this phase a 

 hopper dredge and a hydraulic pipeline dredge were employed. 



Originally, it was intended to obtain beach-fill material from borrow 

 areas located in back of the barrier beach in Biscayne Bay. Prior to 

 beginning construction, the borrow area was relocated to the offshore areas to 

 avoid possible adverse environmental impacts on the Key Biscayne estuary. 



A variety of geological investigations were made to locate and define 

 several borrow areas seaward of Miami Beach. The borrow areas consisted of 

 trenches that ran parallel to the shoreline 1,800 to 3,700 meters (6,000 to 

 12,000 feet) offshore between submerged ancient cemented sand dunes. These 

 trenches, filled with sand composed of quartz, shell, and coral fragments, 

 vary up to 300 meters (1,000 feet) or more in width and from 1 meter to more 

 than 12 meters in depth. The borrow sands generally have a high carbonate 

 (shell) content. The sand size ranges from fine to coarse, with some silty 

 fines generally present. Shells and coral fragments (gravel size to cobble 

 size) are relatively common. The bulk, of the sand was in the fine- to medium- 

 size range. The silty fines form a small percent of the total and are within 

 acceptable limits. The quartz present is usually of fine-grain size while the 

 larger sizes are composed of locally derived shell and coral fragments. The 

 sand sizes generally are finer grained in the deposits that lie farther from 

 shore and in deeper water. The dredged sand is equal to or coarser than the 

 beach sand. 



The water depth in the borrow area is 12 to 18 meters (40 to 60 feet) , 

 and the excavation was accomplished primarily by either 70-centimeter (27- 

 inch) diesel-electric dredges or by an 80-centimeter (32 inch) electric dredge 

 running off land-based power. These large dredges excavate material at depths 

 greater than 27 meters. The average daily yield was about 19,000 cubic meters 

 (25,000 cubic yards), with a maximum of 32,000 cubic meters (42,000 cubic 

 yards) being obtained for a 24-hour period. 



When wave conditions exceeded 1 to 2 meters, the operations had to be 

 curtailed due to the breaking up of the floating pipeline and possibility of 

 damaging the cutterhead and ladder. For these reasons, dredging was conducted 

 only during the calm season from the end of May to mid-October. 



One problem area encountered during the project was the existence of a 

 small percentage (usually less than 5 percent) of stones in the beach-fill 

 material. Until the phase IV contract, the elimination of all stones had been 

 considered impractical. Therefore, removal of stones greater than 5 centi- 

 meters (2 inches) in diameter was required only in the upper 30 centimeters 

 (12 inches) of the surface. This was accomplished using a machine originally 

 designed for clearing stones, roots, and other debris from farmland. Dade 

 County has purchased one of these machines and also two smaller versions for 

 conducting an active beach maintenance program. 



The phase IV contract requirement to remove all stones larger than 2.5 

 centimeters in diameter was prompted by the problems involved in removing 

 stones deposited subaqueously , which tend to concentrate in the nearshore 

 trough. Several methods are being used to relieve this problem. This was not 

 a problem in the phase IV and phase V contract areas. 



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