Second, if the Dlates are auite deep but still at the same depth Z, then 

 when brought close toqether, their failure zones would interact to reduce 

 their caDacity. For instance, if we were dealing with eaual sauare plates 

 and one edge of each were to touch to form a rectanqle, then the group 

 holding capacity would be reduced to 92 percent accordinq to the shaoe 

 factor. 



S p = 0.84 + 0.16 R/L . (25) 



Third, if the olates were stacked one atop the other like the multiple 

 helices of a screw anchor or stacked one behind the other like multiple 

 draq anchors in series on one line, then close spacing of the flukes will 

 result in shearinq the soil on the surface of a cylinder or prism encom- 

 passinq the flukes. This latter mechanism can drastically reduce qroup 

 efficiency. 



The relationship of Figure 55 might be auestioned for individual 

 anchors spaced perpendicular to the direction of oull, because the rela- 

 tionship is based primarily upon the effect of the individual anchors 

 being spaced parallel to the direction of load. However, the results 

 of Lanqley (1967) show that for qrouos of bulbous piles in soils of shear 

 strenqth 24 to 124 kPa (3.5 to 18 osi), a spacinq of 5 diameters was 

 sufficient to develop the full oull out resistance of individual piles. 

 These data agree well with the data of Figure 55. 



Sand Seafloor . Minimum spacing for two anchors for 100 percent 

 efficiency is plotted in Figure 56. This data was also taken from Taylor 

 and Lee, 1972 and reolotted for simplicity. Typical friction angles of a 

 seafloor sand will range from 0.52 to 0.61 rad (30 to 35 deg); thus minimum 

 reauired soacings will be 4 to 5 times anchor width or diameter. The same 

 limitations regarding performance of anchors in clav spaced closer than 

 their minimum holds for anchors in sand. 



Anchor Spacing Recommendations 



Conventional (Draq Burial ) Anchor Spacinq. Conventional anchors 

 can be used singly, or multiply (in tandem or in parallel). The soacinas 

 for 100 percent efficiency in Figures 55 and 56 are recommended. When 

 multiple anchors are used, their group capacity will depend not only upon 

 their spacing and depth of embedment, but also upon how they are connected 

 and installed. 



To achieve proper spacing, anchor drag distance for an efficient 

 burial anchor is about six times embedment depth (embedment depth estimated 

 from Figure 5). For anchors attached in Darallel (Figure 57a) the anchors 

 must be spotted (before setting) a sufficient distance apart to insure that 

 their final spacing (after setting) will be adequate. A somewhat different 

 problem occurs with anchors placed in tandem (see Figure 57b). If the 

 anchors are not efficient burial anchors, for example, Doris mud anchors 

 (refer to section on conventional anchor extrapolation), then the minimum 

 desired spacing should be increased to prevent the "following" anchors 



111 



