(clays), the behavior may be both cohesive and frictional, depending upon 

 the time factor. If loading is slow or long-term, cohesive soils behave 

 in a frictional manner somewhat similar to sands. However, when loading 

 is rapid, the behavior is quite different in that the frictional element 

 disappears (0 appears equal to 0) and the cohesive element becomes 

 jequal to the shear strength. For this case, the holding capacity factor 

 N (which is the frictional factor) reduces to 1.0 and the cohesion, c, 

 becomes the measured short-term shear strength. In a later section, 

 methods for estimating and averaging soil strength properties will be 

 given. For cohesive soils. Equation 2 reduces to 



F^ = A (cN + Y^D) (0.84 + .16 B/L) (4) 



i CD 



The only remaining problem in estimating the^ short-term holding 

 capacity in cohesive soil is that of estimating N . This quantity has 

 been found to increase almost l_inearly as a function of D/B, reaching a 

 constant final value of around N = 9 (Ali, 1969; Kupferman, 1971; and 

 Adams and Hayes, 1967), at certain D/B values_ which appear to be functions 

 of soil shear strength. The point at which N becomes independent of 

 D/B is usually indentified as the point of separation between "shallow" 

 and "deep" anchor behavior. These terms will be discussed in greater 

 detail later. 



Simplified results of short-term, small-scale pullout tests in 

 clay are plotted in Figure 1. The test data are somewhat limited and 

 scattered (scatter not shown_to simplify diagram) ; however, the plot does 

 illustrate the variation of N with the average shear strength. Con- 

 servative approximations to the data are presented in Figure 2 and can 

 be represented by the relations : 



N = 3.8 (D/B) (— + 0.3) (5) 



c c 



or N =9, whichever is smaller, for 0.75 psi <c <_4 psi. 



If c is less than 0.75 psi or greater than 4 psi, engineering judg- 

 ment indicates that it should be assumed e^ual to 0.75 psi or 4 psi, 

 respectively, for purposes of calculating N . 



The suggested technique for estimating the short-term holding capa- 

 c^ity of an embedment anchor in cohesive soil is to use Equation 4 with 

 N obtained from Equation 5. 



This procedure appears to be valid for predicting short-term 

 holding capacities and in designing anchors. However, when the results 

 of field anchor tests are to be evaluated, it is necessary to consider 

 another factor which is usually identified as suction. 



When a load is first applied to an anchor embedded in soil, it may 

 be carried either by shearing stresses in the soil over the anchor or 

 by negative gage pressures (suction) in the water contained by the soil 

 beneath the anchor. In time the suction pressures will dissipate and 

 thereby decrease the holding capacity. This is almost strictly a problem 



