Cohesive Soil 



An anchor plate in soft clay changes from a shallow to a "deep" 

 anchor at a D/B of from 2 to 3 (Ali , 1968). A relative depth, D/B, 

 of 3 was used for the rectangular plan flukes. At values of D/B >3, 

 holding capacity was calculated according to Equation B-l, where 

 N = 1 for a cohesive ( <j> = o) soil. The resulting equation is as 

 follows : 



F T = cN c Ap + y b D Aj, (B-3) 



where c = undrained shear strength 



N, = breakout factor 

 a 



A^ = fluke area in plan 

 y, = buoyant unit weight 



D = depth of embedment 



Previous researchers, Mackenzie (1955) and Hanson (1953), have 

 shown that "deep" anchor blocks in clay exhibit breakout factors, 

 N , of 11 to 12 which roughly correspond to bearing capacity factors 

 for deep foundations, Skempton (1959). Hanson's results are of 

 particular interest because he showed that N c increases by up to 

 25 percent when going from a smooth to a rough plate. The breakout 

 factor, N c , used in the calculations was N = 11. Figure B-4 

 presents anchor holding capacity versus depth for the sand anchor- 

 projectile_f or a ratio of undrained shear strength to vertical effective 

 stress, c/p = 0.5. Seafloor soils are normally consolidated and can 

 be classified by a constant c/p ratio for the depths of interest 

 (6-30 feet). The results were plotted to separate the cohesive (F c ) 

 and the overburden (F ) components of the total holding capacity (Frp) 

 to permit calculation of holding capacity for clays of various c/p 

 ratios. Only c/p = 5 was used to calculate the results shown in 

 Figure B-4. 



Summary 



The preceding paragraphs illustrate techniques for developing 

 the relationships between long-term static anchor holding capacity 

 and depth for two ideal soil types. These are a clean sand, c = o, 

 and a normally consolidated clay. The plotted curves are used after 

 first determining anchor penetration depth and then estimating, from 

 previous results, keying distance to determine the correct embedment 

 depth from which to determine holding capacity. 



37 



