to a grid pattern or according to the geometry of known deposits. Examples 

 of various sand bodies with offshore borrow potential are also shown in 

 Figure 5 (b to e) . Each sand body requires a different sampling scheme as 

 shown by the x's on the insert maps. These kinds of deposits in nature 

 might also be covered by an overburden of modern fine-grained sediments 

 and thus require seismic reflection or side-scan Sonar data to delineate 

 the bodies in three dimensions. 



Characterizing finger shoal sediments (Fig- 5,b} would require a len- 

 ticular sampling pattern that defines the boundaries of the body and surveys 

 the textural variations of the shoal sediments. From an interreef shoal 

 (Fig. 5,c) only those parts where a dredge could operate without damage to 

 the cutter head from the reef rock should be core-sampled. The distribution 

 of offshore ancestral river channel deposits would be controlled by the 

 ancient drainage network (Fig. 5,d),, whereas dipping sandy strata (Fig. 

 5,e) would only be sampled to the depth reachable by a dredge or where 

 undesirable overburden is not excessively thick. 



Cores rather than surface samples are usually collected to evaluate 

 the composite texture of a borrow source and to estimate the potential 

 volume of suitable sand in the body. Vibratory-type coring devices are 

 usually used and although cores up to 40 feet (12.2 meters) in length can 

 be recovered with this kind of equipment, the cores generally obtained 

 range from 10 to 20 feet (3.Q to 6.1 meters) in length; thus, all dredgable 

 parts of a sandy body might not be sampled. Seismic profiles showing 

 sand-body thickness can be used to obtain data for calculating the volume 

 of sediments in a body where dredge depth capability exceeds the cored 

 limit,, but the samples that are recovered must be used to estimate the 

 textural composite for the entire body. 



Once the cores have been obtained, the problem becomes the method of 

 sampling them. Here, as in most aspects of a beach-fill problem, engi- 

 neering judgment and experience become important. In a typical situation, 

 2 to 6 feet (0.6 to 1.8 meters) of fine silt and clay might overlie a sandy 

 deposit that contains thin lenses of fine-grained sediment. Physical prop- 

 erties of the sand indicate suitability for beach fill, and it is dredged 

 and placed by pumping onto the high foreshore of the project beach. The 

 engineer knows that the silt and clay will not be stable in the beach 

 environment, that much of the fine sediment will be re-suspended and lost 

 during dredging, and that most of the remaining fines could be washed 

 seaward beyond the active limit and lost during placement. For this case, 

 only those parts of the cores suitable as beach fill should be sampled 

 and analyzed. Care should be taken that the core samples are proportionally 

 representative of the suitable materials encountered in the cores. Several 

 sampling schemes might be chosen for this example. A composite sample 

 could be "mixed" from: (a) samples collected at equal intervals along the 

 core, (b) a channel or continuous sample, or (c) samples from each unique 

 layer within the core and then "weighted" as to the thickness each repre- 

 sents. Perhaps, the first scheme where subsamples are collected at regular 

 intervals would be preferable to a continuous sample because these individ- 

 ual samples could be easily stared and re-examined if necessary. 



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