also that they may have been much thicker in the past. In addition the 

 age of types B and C sediments indicates they were more likely subjected 

 to cyclic loading in the past, due to seismic events, than type A sedi- 

 ments. There is evidence that a past history of cyclic loading reduces 

 liquefaction potential which tends to increase the density (Seed, Arango, 

 and Chan, 1976). Finally, a very rudimentary cementation was noted in 

 several samples of types B and C sediment, particularly type B, due to 

 partial leeching and redeposition of calcium carbonate particles. 

 Although these sediments are not consolidated, the slight cementing in 

 many places may lessen liquefaction potential. Though fine type A and 

 types B and C sands are almost identical in size characteristics, they 

 can be readily identified by their distinctive microfauna, even where 

 one overlies the other (Sec. IV, 1) or by the absence of microfauna as 

 in barren types B or C. 



Slope failure by downslope sliding, slumping, and creeping of soil 

 masses often occurs in the submarine environment as a result of gravita- 

 tional forces or cyclic loading by waves and seismic events. In general, 

 the bottom slope, the thickness of the unconsolidated sediment blanket, 

 the soil strength, and the nature of the load applied dictate the type 

 and magnitude of displacement. Evidence of prior downslope soil dis- 

 placement may be indicated in bathymetric and high-resolution seismic 

 reflection data, but the absence of such evidence does not preclude the 

 existence of displaced soil masses or the potential for such occurrences. 



Most of the study area is occupied by a shelf floor with an extremely 

 gentle gradient of about 1:3000. Although gravitational downslope forces 

 are small on the gentle slopes, soil displacements occur under highly 

 favorable conditions, such as liquefaction of the soil mass or of an 

 unstable sublayer. 



Considerably steeper slopes occur in the shoreface zone, with 

 gradients as steep as 1:60, and in the large shoal fields where slopes 

 of 1:20 occur locally. Because of the steeper gradients in these areas 

 and because some are areas of active deposition where soil density is 

 probably low, they are considered more prone to slides and sliimps than 

 shelf floor deposits. 



Landward-facing rock scarps up to 5 meters (16 feet) high occur in 

 extensive carbonate rock outcrops inshore of the 15-meter depth off New 

 River Inlet (Crowson and Riggs, 1976). While these scarps are probably 

 relatively stable, the highly irregular topography would affect engineer- 

 ing activities such as pipeline construction and dredging. 



3. Soil Properties . 



Knowledge of the engineering properties of submarine sediments and 

 rock deposits is needed to determine bearing capacity for various types 

 of foundations, side-slope stability in dredged channels, and the uplift 

 resistance of embedded footings and anchors. Because of the unsuitabil- 

 ity of available vibracore samples for standard engineering soil tests. 



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