6.2.1 Strength Loss During Anchor Cyclic Loading 



Some soils, such as uniform fine sand, coarse silts, and some clean 

 oozes, are susceptible to true liquefaction failure when subjected to 

 cyclic loads. These soils are excluded from the prediction procedures 

 that follow. Recommendations for these soils are given by Herrmann 

 (1980) (to be published). Most other soils, including very plastic 

 cohesive soils, are subject to strength loss, especially from extended 

 cyclic loading. In general, denser soil, more plastic soil, and lower 

 cyclic loads lead toward lower susceptibility to strength loss. 



For all soils, the loading is characterized using the above defini- 

 tions and Figure 6-5 to determine t , from the soil's permeability. The 

 number of load cycles during a period equal to t , is found, and limit- 

 ing design bounds as a function of soil type are then established using 

 Figure 6-6. Figure 6-6 can be used to find the limiting number of 

 cycles for a given loading or the limiting loading for a given number of 

 cycles. The upper bounds apply to cases where the average quasi- 

 static load is 1/3 or less of the static holding capacity. For the unlike- 

 ly case where the average quasi- static load is greater than 1/3 of the 

 static holding capacity, the excess is added to the double amplitude 

 cyclic load prior to using Figure 6-6. 



6.2.2 Cyclic Creep During Anchor Cyclic Loading 



The mechanism leading to cyclic creep of an embedment anchor is 

 not well understood but is known to occur under loading conditions that 

 in some cases are quite safe relative to the criteria for cyclic strength 

 loss presented in the preceding section. For cyclic creep considera- 

 tions, the number and magnitude of significant loading cycles occurring 

 during the lifetime of an anchor are controlling factors. They should 

 be summarized in spectral or quasi- spectral format. The number of 

 significant loading cycles may not be as large as one would expect. If, 

 for example, a mooring system has a planned 20-year life, is continu- 

 ously in use, and is subjected to strong wave loading during ten 3- day 

 storms per year, then the total number of significant cyclic loads will 

 likely be less than 1 million. 



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