Underconsolidated sediments are almost always found in active 

 river deltas such as that of the Mississippi River. If deposition is fast 

 enough, there may be almost no buildup of strength with subbottom 

 depth. If one is operating near the mouth of a river, such as the 

 Mississippi, Amazon, or Nile, one should be prepared for unusually 

 weak sediments and never rely on typical property profiles. Other 

 areas of underconsolidated sediments include embayments that exhibit 

 high depositional characteristics with low water velocity. 



2.3.4 Sediment Property Selection 



With this brief background as a basis, it is recommended that the 

 designer use the following procedure for selecting typical sediment 

 strength, density, and sensitivity data. 



(1) If the site is on the continental shelf or slope, the sediment 

 is assumed terrigenous. Available National Ocean Survey 

 charts are consulted to determine whether the sediment is 

 primarily sandy or cohesive ("mud")- If the sediment is 

 cohesive, Figure 2-4, which gives a lower bound for the 

 strength distribution for a normally consolidated sediment, is 

 referred to. A search for strong indications of overconsolida- 

 tion is made: recorded outcrops of older sediments, exposed 

 location (rise top, high recorded bottom currents). If suffi- 

 cient evidence exists to suspect overconsolidated soils, it 

 would be prudent to drop some penetrometers or short 

 gravity corers. However, the engineer should be aware that 

 both of these devices will not penetrate deeply into highly 

 overconsolidated sediment. Nonpenetration or slight penetra- 

 tion with attainment of minimal sample length can add cre- 

 dence to the suspicion that overconsolidated sediment does 

 indeed exist. Typical sand properties are given in 

 Figure 2-5. If the location is near a large active river delta, 

 the site must be surveyed directly. 



23 



