Mats have been designed with a streamlined configuration to reduce scour 

 effects. Mats currently in use range up to 185 by 200 feet in size. Spud piles 

 are sometimes built in to increase lateral stability; peripherial scour curtains 

 help in this regard also. These foundations have been exposed to tropical 

 storms and hurricanes in at least 82 instances; and in only three cases were 

 horizontal displacements detectable. 



ANALYSIS OF CASE STUDIES 

 Foundation Performance Problems 



The seaf loor structures discussed in the Case Studies section have 

 encountered performance problems in the following three areas: soil behav- 

 ior, environmental conditions, and deployment techniques. Unsatisfactory 

 performance in each of these areas has been of sufficient magnitude to impair 

 the performance of an entire structure. In several cases, a minor initial perfor- 

 mance problem generated other, more serious performance difficulties. In 

 almost all cases the unsatisfactory performance could have been prevented 

 or minimized if environmental parameters had been properly measured and 

 effectively used before design or during deployment. It is hoped that a sum- 

 marization of the major problems encountered by existing seafloor structures 

 will be helpful in reducing the number and degree of future unsatisfactory 

 performances. 



In almost three-fourths of the situations involving foundation 

 problems, the structure or object was placed on the seafloor before an ade- 

 quate investigation of the sediment properties had been performed. In many 

 cases, no sediment samples were taken at the site. The design engineer, conse- 

 quently, did not know whether the foundation was being placed on, for example, 

 soft cohesive clay, medium dense sand, or fractured rock. The resulting founda- 

 tion design reflected the obvious lack in data. 



When sediment samples were taken at a site before the foundation was 

 designed, the percentage of successful performances increased. However, foun- 

 dation difficulties such as excessive settlement or tilting were still experienced. 

 These difficulties were attributed to either the failure of data obtained from 

 the soil sample to represent conditions at the entire site or the inability of 

 analytical techniques to predict performance. 



The soil sample fails to represent conditions at the entire site when 

 (1 ) the soil properties vary vertically and laterally from the point of investi- 

 gation, (2) methods for obtaining the samples alter the properties of the 

 material, or (3) laboratory testing techniques cannot adequately reproduce 



