TS = total settlement 

 n = number of layers 



It should be noted that if only the above equations are used, a gross 

 simplification is being made of the complex problem of design and analysis of 

 a foundation system. In a structure of significant importance, such as in a 

 manned structure, other considerations and more exacting design methods are 

 required. For example, shear and plastic deformations of the sediment con- 

 tributing to the settlement of the structure should be considered. As another 

 example, the rigidity of the structure and foundation elements has a direct 

 influence on the settlement behavior. An analysis to take this factor into 

 account is very complex but may be necessary where the safety of personnel 

 is of prime importance. As a further example, in granular sediments vibration 

 effects can cause compaction of the sediments to a relative density of 75%, 

 and this could result in considerable settlement and/or damage to the structure 

 if the sediments are initially loose. 



For noncohesive sediments, the serious problem exists of not being 

 able to obtain a suitable sample for laboratory testing. Because of the remote- 

 ness of the bottom beyond diver depths, the required in-situ density and other 

 values cannot be measured accurately. Thus, to be on the conservative side, 

 it would be safer to assume local shear failure conditions rather than general 

 shear failure conditions as illustrated by Figure 1 , which was developed by 

 Vesic 6 for terrestrial sands. Punching shear will most likely not be encoun- 

 tered, since the footing size will be relatively large to minimize settlement. 



The sediment thickness and bedding characteristics must be known 

 for each proposed site. If point-bearing piles are to be used, the depth to 

 bedrock is required. Bedding characteristics are also required if friction piles 

 are to be utilized. The various strata need to be delineated to insure that no 

 weak beds exist which might be the seat of settlement or through which a 

 failure plane could develop. Information on sediment thickness and bedding 

 is also required in bearing capacity and settlement analysis as discussed in 

 previous paragraphs. 



Data on the compaction characteristics of the sediments are also 

 required. (The definition of compaction as used here is making the sediments 

 more dense by some artificial means.) In many cases, the site will be accept- 

 able in respects other than bearing capacity and settlement. To improve these 

 characteristics, compaction of the sediment may have to be undertaken. 

 Although the techniques of the compaction of marine sediments have not 

 been developed, compaction appears feasible and will be required, particularly 

 when a location has been preselected in which the sediments will have to be 

 modified for the site to be acceptable. 



