Deadweight - Sinkhole Combination . Sinkholes large enough to be 

 explored by submersible occur frequently on the terrace off southeastern 

 Florida. Placing a deadweight into a singkhole would alter the mechanism 

 for resisting the lateral load component. Much or all of the lateral load 

 component would be transferred directly from the mooring line to the rock 

 at the lip of the sinkhole. The required deadweight size could be decreased 

 accordingly. For example, the anchor in Table 18 was designed to resist a 

 maximum line tension of 180 MN x 1.41 = 255 MN. 



Two major difficulties with utilizing this concept are: 1) there is no 

 guarantee that a suitable sinkhole will be near an acceptable OTEC site, 

 and, 2) line abrasion at the lip of the sinkhole would be severe. 



Driven or Drilled Piles . The behavior of piles in rock is not clearly 

 defined. Information dealing with pile systems in rock in the marine 

 environment is limited. The amount of practical installation experience is 

 likewise limited. However, pile systems remain a strong contender for the 

 OTEC anchor in rock because: (1) pile systems are much less dependent on 

 seafloor surface irregularities than a simple or grouted deadweight and 

 (2) piles also substantially reduce the required dimension and material 

 weights as shown in Table 18. 



An OTEC anchor cmprised of a group of bored and grouted piles in rock 

 could potentially fail in three simple mechanisms: (see Figure 58) 



1. The individual piles will exert high bearing loads against the 

 restraining near-surface rock and could cause crushing of that rock. 

 Crushing of the near-surface rock would allow pile lateral deflection with 

 accompanying increase in bending moment in the pile, and a shifting of 

 restraining load to lower rock layers. The piles could fail in bending or 

 the crushing of rock could progress downward until the piles pulled out. 



2. Axial load in a pile could exceed the capacity of the grout to 

 steel bond, the grout to rock bond, or the grout shear strength. Thus, 

 excessive axial load could cause axial pullout of a grouted pile. 



3. The rock mass into which the piles are grouted may be weak or 

 fractured. Uplift failure of the entire rock mass could occur. If we 

 assume a weathered, layered limestone, as appears from the limited dredge 

 hauls, along with some jointing, this assumption is likely the case. To 

 reduce the likelihood of such failure, the site survey must identify the 

 probable rock strength and joint and fault systems, and the pile system 

 must be designed to entrain a sufficient mass of rock to provide the nec- 

 essary vertical restraint. 



The OTEC anchor could also fail in combinations of the above mechanisms 

 depending primarily on the system of discontinuities in the rock The 

 following analysis considers only the above three simplistic mechanisms. 



118 



