anchors/foundations may serve quite well in many situations with small 

 structures; however, for large structures, a deadweight foundation, to 

 provide stability or fixity, will have to be excessively heavy. 



EVALUATION 



Most of the rock foundation systems evaluated are subject to dis- 

 tinct microrelief limits. These rock foundations must accommodate ten 

 degree slopes (typical of basalt on seamounts) and microrelief of up to 

 three feet. Figure 7 depicts these conditions, assuming a spread of 

 13 feet across the foundation framework and assuming the upslope contact 

 point of the tripod bears on impenetrable microrelief three feet high. 

 For the conditions depicted the installation is on the verge of being 

 unstable but this is an extreme combination of circumstances; therefore, 

 the design is reasonable. If conditions more extreme than ten degrees 

 overall slope and one meter superimposed slope are expected, then tined 

 and crushable element foundation systems will not survive; instead dead- 

 weight or embedded anchor systems are required. 



For those environments where tined or crushable element foundations 

 are thought applicable, it appears best to select a rigid-jointed, 

 three-tined foundation system with tines tilted out 20 degrees and 

 equipped with sand bearing collars one foot above the tips. Tined 

 foundation systems with pods of tines at each of three corners are not 

 as desirable because of potential improper alignment of the flexible- 

 jointed pods. Crushable-element foundation systems are considered 

 less desirable because of their limited lateral load resistance 

 capability. Horizontal- lying, open-ended cylinders will have a diffi- 

 cult time engaging a smooth rock surface, even with cleats. The founda- 

 tion underlain by grout or plastic does have special merit in that it 

 does have potential to perform better than a tined foundation on pillow 

 basalts, etc.; however, this system will require considerable develop- 

 ment, such as a suitable grout or plastic, a containment system, a 

 shipboard mixing system, and techniques for filling the grout container 

 at sea. These development hurdles, when coupled with the fact that 

 emplacement delays of any sort may result in the material setting-up or 

 gelling before the foundation touches bottom, cause this system to lose 

 appeal. 



Because the selection of a foundation type is so highly dependent 

 on the microrelief magnitude, a potential site must first be surveyed. 

 Based on the survey data the percentage of microrelief less than three 

 feet can be estimated, and thereby the probability of success of a 

 tined or crushable element foundation determined. A deep-towed side- 

 scan sonar system can be used for this survey work. The side-scan data 

 can be used to develop a topographic map of the survey area; however, 

 the resolution of the available towed systems (three feet) 2" is barely 

 adequate for establishing the suitability of a site based on a three- 

 foot microrelief criteria. Bottom stereo-photography is capable of Pro- 

 viding improved resolution (per photograph, potentially to 0.3 inch); 



20 



