SITE SURVEY REQUIREMENTS 



Before any surveys are conducted, the site requirements for each 

 specific structure must be defined to establish the proper scope of the survey. 

 The following are the major items which should be defined: purpose of the 

 installation, operational requirements, size of structure, location of structure 

 (on or in bottom), and degree of precision required in the survey. 



The purpose of the installation has to be known so that the proper 

 site parameters are measured in a degree commensurate with the importance 

 of the structure. For example, the measurement of the foundation-supporting 

 characteristics for an unmanned Submersible Test Unit (STU) (Figure 2) will 

 be far less extensive than for a manned underwater structure (a model of which 

 is shown in Figure 3). If the bottom structure is for biological studies, a site 

 considered to be a dead area similar to some basins off of Southern California 

 may not be acceptable. Thus, the purpose of the bottom structure determines 

 the extent and the type of survey parameters to be measured. 



The operational requirements of the bottom structure influence the 

 selection of proposed sites for survey. For example, the use of surface support 

 power sources for a manned station on the seafloor requires that the surface 

 vessel be in relatively quiet and shielded waters (safety considerations). If the 

 bottom installation is a telemetering station, it would be desirable to locate 

 this structure on a topographic high where it can be monitored over the widest 

 area. On the other hand, if the bottom structure is for defense purposes, it 

 may be desirable to conceal this structure in a depressed area on the seafloor 

 or possibly in the bottom sediments or rock. Thus, the operational require- 

 ments of the structure should be defined so that the appropriate survey can 

 be conducted. 



The size of the structure has a direct bearing on the extent and depth 

 of the foundation engineering survey. With respect to Boussinesq's theory 17 

 of stress distribution in an elastic, homogeneous, isotropic, and semi-infinite 

 medium, it is noted that the "bulb of influence" becomes larger and extends 

 deeper as the footing size increases. The effect of this increase in footing size 

 is illustrated in Figure 4 in which isobars of equal stress intensity are plotted. 

 For the same structure unit loading, it is seen that a given stress isobar is 

 larger and extends deeper into the sediment column for the larger footing. 

 The effect of this increase in size of a given isobar for the same structure 

 unit loading is to increase the total settlement. Figure 4 also illustrates the 

 importance of investigating deeper into the sediment column for larger 

 structures to insure that no weak layer exists which may influence the 

 stability of the structure. Thus, the size of the structure must be defined 

 in order that the appropriate survey be conducted. 



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