INTRODUCTION 



Many potential seafloor structure locations consist of exposed 

 rock or rock with a thin sediment cover. The design of a fixed, 

 bottom-resting structure in such an environment poses a substantial 

 problem because of the unevenness of the bearing surface and the usual 

 accompanying strong currents. 



It is the purpose of this report to: (1) identify probable loca- 

 tions of exposed and "near-surface" seafloor rock; (2) describe the 

 rock surface microrelief or roughness and the rock mass integrity; and 

 (3) examine and criticize rock foundation concepts. The first two 

 purposes were fulfilled via a literature review, the third via prelim- 

 inary analytical work and qualitative examination. 



This report assumes a general knowledge of seafloor topographical 

 terminology; when clarification is required the reader is referred to 

 more complete texts such as References 1, 2, and 3. Reference 4 con- 

 tains a good terminology summary oriented toward foundation engineering 



BACKGROUND 



Usually, rock appears on ridges, topographic highs, and upper 

 surfaces of seamounts; in trenches, canyons, and fault scarps; and on 

 nearshore shelves, banks and terraces. But it is probable that out- 

 croppings of nearly all magnitudes occur throughout the entire range 

 of underwater environments. Rock relief above existing sediment sur- 

 face varies in height from a few centimeters to thousands of meters. 

 The Navy has placed some structures on these rock surfaces and has 

 conducted studies to delineate their microrelief. 



Case Histories 



The Navy has used foundations on seafloor rock when strategically 

 necessary. For example, structures are sited atop seamounts instead 

 of the adjacent abyssal plain either because of more favorable handling 

 and working conditions at the shallower water depths or because of the 

 more favorable acoustic view. Similarly, structures near a seacoast 

 are often established on rock simply because of functional geographic 

 location requirements or because of constraints imposed by support 

 facility location. 



References 5 and 6 describe seafloor foundation case histories, 

 some of which were on rock. For instance: 



(1) The hydrophones of the Barking Sands Tactical Underwater 

 Range (BARSTUR) , Hawaiian waters , cover a 5- by 10-mile area with 



MBL/WHOI 



D D301 0040237 b 



