LONG-TERM REPEATED LOAD HOLDING CAPACITY 



Embedment anchor systems which are used to moor surface vessels or 

 buoys will be subjected to a combination of sustained and repeated loads 

 which will vary with the tautness of the system and the nature of wave 

 or tidal action. Experience with land soils indicates that soil-struc- 

 ture systems do not react in the same way to sustained-repeated load 

 combinations as they do to strictly sustained loads of the same magnitude. 

 In almost all cases failure occurs at a lower force level if a portion 

 of the load is repeated. In designing anchor systems for long term use, 

 therefore, it is necessary to consider the amount by which repeated 

 loading will reduce the holding capacities . 



There has been a good deal of research on the response of soils to 

 repeated loading. Most of this has consisted of applying repeated loads 

 to cylindrical soil samples in triaxial cells and determining the amount 

 of strength reduction produced by different numbers of load repetitions. 

 The purposes of this research were to determine how natural soils respond 

 to earthquake loadings and how compacted soils respond to vehicular 

 traffic. No research has been conducted to determine how natural soils 

 respond to repeated loads extending for long periods of time. Since this 

 may be the critical case for anchor loading, it is necessary to extra- 

 polate the results of the existing research. 



Virtually all soils respond adversely to repeated loading. However, 

 some soils are affected more strongly than others. Lee and Eitton (1969) 

 provide an indication of the influence that particle grain size has on 

 the strength under repeated load conditions. Results show that soils in 

 the fine-sand to silt range (median grain size between 0.2 and 0.02 mm) 

 are the most susceptible to repeated loading with clays, sands, and 

 gravels being less susceptible. Data provided in this reference cannot 

 be used quantitatively; however, it is of value in indicating which soils 

 are most troublesome. 



Cohesive Soil 



There are several reports available which provide specific informa- 

 tion about the repeated load response of particular soil types . An 

 extensive study of the behavior of San Francisco Bay mud, a cohesive 

 marine soil, is described by Seed and Chan (1966). Figure 5 summarizes 

 many of the test results obtained during this study in which pulsating 

 stresses were applied to samples of bay mud. The plot indicates the 

 stress state (in terms of pulsating stresses normalized by the "normal" 

 or static undrained strength) at which failure will occur following a 

 specified number of transient stress pulses. Values exceeding 100 can 

 be attributed to the short duration of the pulses compared to the static 

 strength test. As may be seen the worst situation investigated is that 

 in which the applied stress is repeated 900 times. The resulting strength 

 is about 60 percent of the static strength test. 



Since these tests were performed to investigate earthquake response. 



