To counter this tendency, the net downward force on the sediment must be 

 increased to increase the effective normal stress across the potential 

 shear zone. Raising the normal stress raises the shear strength of the 

 sand and increases the bearing capacity. In other words, the weight of 

 the block must be increased to prevent a bearing capacity type failure. 

 This means a large increase in the material and installation cost. of the 

 anchor. Further work to be done in the next phase of the CEL QTEC effort 

 will determine the consequences of such a failure. Model tests and 

 refinements of the analysis procedure could indicate that a bearing opacity 

 type failure would not adversely affect anchor performance in a uni- 

 directional loading environment (Gulf Stream). Such a result could mean 

 substantial savings in hardware and installation costs. 



The use of deadweights on exposed or shallowly buried rock surfaces, 

 such as found on the floor of the Gulf Stream, offers some special prob- 

 lems. The rock surface is relatively unyielding so it is difficult for 

 cutting edges to bite into it. Nor will the rock surface conform to the 

 deadweight bearing surface. As a result, it is difficult to achieve 

 sufficient lateral load resistance with a deadweight. Effective coeffi- 

 cients of friction on rock are about 0.3, leading to a lateral holding 

 capacity to weight ratio for a deadweight of 1/6 to 1 (1/3 to 1 in terms 

 of submerged weight). On rock, it appears prudent to provide additional 

 techniques for developing lateral load resistance. Short stub piles 

 acting in shear appear excellent candidates for this purpose. Installa- 

 tion of the piles would require some means of developing sufficient thrust 

 on the drill bit in order to start the drill hole, but otherwise should be 

 state-of-the-art. 



Pile Anchors 



Pile anchors are ^ery efficient carriers of axial load. Thus, those 

 piles installed vertically as from a drillship can economically resist 

 very high vertical components of mooring line force. However, piles are 

 not efficient in resisting large lateral loads in the almost normally con- 

 solidated soil profiles of the deep ocean. Soft surface sediments are not 

 able to provide sufficient lateral load resistance to the deflecting pile. 

 Thus, the load is transferred to lower elevations in the pile, and bending 

 moments are increased. These bending moments dictate structural design of 

 the pile. The weight of hollow cylindrical steel piles in about 20 percent 

 of the lateral force being resisted (lateral holding capacity to weight 

 ratio of 5 to 1). The steel pile sections must be pressure grouted in the 

 bored holes, especially in the predominantly calcareous ooze sediments. 

 Given the sophisticated construction techniques required, and given the 

 resulting lateral load to weight ratio, pile anchors in typical unconsoli- 

 dated seafloor sediments appear of marginal value to the OTEC mooring pro- 

 gram. This conclusion presumes that low mooring line angles will prevail 

 for OTEC mooring systems. 



122 



