GROUP CAPACITY 



Factors Affecting Group CaDacity 



Introduction. Data available from which to determine qroun canacity 

 is extremely limited; thus, maximum use of related data was made. The 

 factors affectina arouD caDacity are not well understood. DeDendina on 

 the theory or test results one chooses, the following aDDear to have an 

 effect: 



1) SDacing 



2) Relative embedment deDth 



3) Soil characteristics 



4) Anchor type and characteristics 



5) Anchor number and arranoement 



6) Method of installation 



7) Loading geometry (axial, lateral) 



The only factor which stands out enough to be considered exolicity is 

 spacing. All theory, model results, and the limited field data available 

 for soft seafloors indicate that decreasing anchor SDacina decreases group 

 caDacity. Recommended SDacings will be discussed and categorized accordinq 

 to anchor type and soil type. Suggested procedures must be considered 

 interim due to the limited data; designs based upon these must use suffi- 

 cient factors of safety. Once the primary OTEC anchoring choices are 

 defined then the needed data will have to be gathered in order to justify 

 reduced design safety factors. 



Clay Seafloor . After olacing anchor flukes, either throuqh draaaina, 

 Dile driving, augerinq, ballistic driving, etc., their oerformance as a 

 function of spacing should be similar. An approach for determining desired 

 SDacina in clay was derived from data presented for the prediction of anchor 

 holding capacity by Taylor and Lee, 1972. These data were reolotted in 

 Figure 55 as relative anchor spacinq versus soil strenqths. Minimum 

 required embedment depth is also indicated by the abscissa. For example, 

 if two anchors of width B in 28 kPa (4 nsi) shear strenqth soil are to 

 exhibit maximum efficiency then they must be placed at least 5R center to 

 center. 



As the anchors are placed closer toqether, the efficiency of the 

 qrouo is reduced because the individual anchors behave as a single unit. 

 The failure mode chanqe may be of three types. First, if embedded near the 

 soil surface, the failure mode could chanqe from the deeo plate mode for a 

 single embedded plate at Z; to the shallow plate mode for a arouD of 

 closelv snaced plates all at denth 7. A qroun of three or more plates, 

 embedded to a soil deoth near their individual critical embedment depths, 

 would act as one large olate at the same depth Z and the shallow failure 

 mode would occur (see Taylor and Lee, 1972). This failure mode chanqe 

 could reduce the averaqe holdina canacity of each anchor in the grouD to 40 

 nercent of that of the oriainal , uncombined, oriqinally-spaced anchor capacity. 



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