in strength. Ideally a prediction of long-term anchor response would 

 involve a prediction of the stress distribution, a laboratory analysis 

 of the soil to determine how these stresses change the soil strength, 

 and finally a stability analysis to determine how the holding capacity 

 will vary with time. This type of prediction is currently impossible, 

 and additional research is needed to determine under which conditions 

 negative pore pressures (which lead to a decrease in holding capacity 

 with time) are set up, the magnitude of these pressures, and the amount 

 by which the soil strength changes with time under the influence of 

 these pressures. 



Before research of this type is accomplished, it is possible to 

 apply a limited amount of previous work to yield approximate values for 

 design. Several researchers (Meyerhof and Adams 1968; Kupferman, 1971) 

 have noted the development of tension cracks on the soil surface above 

 loaded embedment anchors. These cracks probably indicate the existence 

 of negative normal stresses, caused by negative pore pressures, which, 

 if allowed enough time to dissipate would lead to reduced strength in 

 the overlying soil. The existence of these stresses is further documented 

 by the research described in Adams and Hayes, 1967. These authors also 

 conclude that for deeply embedded anchors positive rather than negative 

 pressures would develop. No data is presented to substantiate this, 

 however . 



An approximate means for analyzing anchors embedded in cohesive 

 soil under long-term drained conditions is presented in Meyerhof and 

 Adams (1968). The technique suggeste^d is t_o use the standard holding 

 capacity equation (Eq_uation_2) with N and N obtained using the drained 

 strength parameters (c and tf) ) of the cohesive soil. This is done in 

 recognition of the fact that cohesive materials behave as primarily 

 frictional materials under long-term conditions. Two problems exist, 

 however; one theoretical and the other practical. Theoretically there 

 2^s reason to doubt that the somewhat empirical relations for obtaining 

 N which were developed for sands will apply to clay. The failure modes 

 involved may differ considerably by virtue of the time element. There 

 is reason to believe that the anchors in clay will fail progressively, 

 thereby generating failure surfaces quite different from those produced 

 in rapid, drained loading of sand. Even with this theoretical complica- 

 tion, however, the use of the standard equation with drained parameters 

 should lead to approximately correct estimates. 



The major problem is the practical one of estimating the drained 

 strength properties of the soil. Good quality samples and careful tri- 

 axial testing would be required to yield the required parameters, and 

 this would be expensive, time consuming, and probably not justified for 

 most problems. Ongoing research at NCEL should provide means for approx- 

 imately estimating drained properties of cohesive seafloor soils. However, 

 for immediate use, it is necessary to resort to empirical correlations 

 developed for soils on land in lieu of performing triaxial tests. One 

 applicable relation is given by Bjerrum and Simons (1960) in which the 

 drained friction angle, cf), is plotted versus the plasticity index. 



