ANCHORS ON ROCK 

 Introduction 



The special anchoring requirements of rock seafloors are discussed 

 in this section. Rock will probably be encountered for some OTEC sites 

 in the Gulf Stream. Thus, the difficulties of anchoring in rock are 

 compounded with the problems imposed by large mooring loads. 



There is relatively little experience in rock anchorages, and 

 certainly no experience with the load magnitudes possible with OTEC. 

 Consequently, the anlysis below is conceptual. The designs cited are 

 basic ideas which will be refined, altered or combined to achieve optimum 

 performance for a particular situation. 



Environment 



A typical site in the Gulf Stream would have a water depth of about 

 460 meters. It would have a karst-like, possibly cavernous limestone 

 topography. The University of Miami has made several dredge hauls in 

 such areas on the Miami Terrace. Their samples were identified (X-ray 

 crystallography) as amorphous flour-apatite, a rock in which the carbonate 

 has been replaced by phosphate and flourine ions. The vertical extent 

 of the rock is unknown. Samples taken were finely laminated. They were 

 composed of alternating fossiliferous and barren layers. Foraminifera 

 were predominant, but not excessive. Deposition was probably Tertiary. 



Very little is known about the engineering properties of Gulf Stream 

 rock. This is unfortunate since knowledge of rock strength characteris- 

 tics is of primary importance in anchor design. Obtaining representative 

 data is a difficult and an expensive task. The engineering properties of 

 marine limestone may vary drastically within a small area (Yang, 1976). 

 For this reason a broad general sampling program would be of little value. 

 Sampling and testing should be done carefully at the actual site location. 

 Actual site topography and rock properties are essential for an optimum 

 anchor design. 



Basic Anchor Concepts in Rock 



The several basic anchor concepts described below are summarized in 

 Table 18 All anchors were designed assuming that horizontal and vertical 

 force components of 180 MN (40xl0 6 lbs) were applied at the anchor. 



The seafloor was assumed to be composed of uniform limestone with an 

 average compressive strength of 29 MPa (4200 psi) (Farmer, 1968). A 

 coefficient of friction between concrete and rock of 0.3 was assumed for 

 calculations involving anchor sliding. The bond strength in shear 

 between concrete and a natural rock surface was assumed as 69kPa (10 psi). 

 A bond strength in shear of 517 kPa (75 psi) between g»out and steel and 

 between grout and rock was used for piles and tendons in drilled holes. 



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