Numerous other foundations have been overdesigned with what were 

 thought to be large factors of safety to ensure satisfactory performance. This 

 was typically a successful, but usually expensive, approach. All of the systems 

 were designed with what was thought to be an adequate factor of safety. In a 

 few cases, however, because all of the performance parameters were not thor- 

 oughly understood, one was neglected. In such cases, when poor performance 

 occurred in that parameter, the overall safety factor of the system became less 

 than one. 



As the national interest requires, and as the technology is developed, 

 the Navy is planning more numerous and larger installations for the ocean 

 bottom. With this increased activity on the seafloor, and with the increasing 

 sensitivity of many of these installations (such as manned installations, which 

 require a high degree of confidence in the design, since any unsatisfactory per- 

 formance may endanger human life), there is a need to (1 ) improve the capability 

 for designing seafloor foundations which will perform satisfactorily, (2) increase 

 the confidence level in these procedures, and (3) use designs which are econom- 

 ically consistent with safety. 



For these reasons, the Navy has undertaken research that will develop 

 design guidelines for seafloor foundations. The overall development of this 

 design capability can be significantly improved by the study of past successes 

 and failures. The results of such a study can be used directly as design guide- 

 lines (a strictly empirical approach); or, more appropriately, they can be used 

 to point out past problems (leading to the delineation and understanding of 

 additional design parameters) and to act as test cases against which various 

 proposed design rules may be compared and evaluated. 



EVALUATION OF FOUNDATION PERFORMANCE 



Satisfactory foundation performance can be defined in several ways. 

 However, satisfactory performance basically is performance that permits the 

 installation to complete its mission as intended. 



Specific performance parameters contribute to this overall behavior. 

 These parameters often include the following: (1 ) stability relative to bearing 

 capacity; (2) stability relative to overturning; (3) stability relative to lateral 

 motion; (4) tolerable differential settlements; (5) tolerable total settlements; 

 and (6) sufficient rigidity (stiffness) to prevent motion. These parameters 

 must consider dynamic (such as earthquake) as well as static (such as sub- 

 merged weight) situations and soil behavior (such as compression or rupture) 

 as well as other environmental influences (such as undermining, current scour- 

 ing action, or slope instability). 



