From this plot it appears that a selection of i> equal to 25 will be 

 conservative for all_but the most plastic soils. If the other drained 

 strength parameter, c, is assumed equal to 0, a conservative design is 

 virtually guaranteed. 



In summary, the following procedures are suggested for predicting 

 the long-term holding capacities of embedment anchors in cohesive sea- 

 floor soils subjected to static loads. These are immediate use 

 suggestions and should be supplemented by additional future research. 



1. Assume (t>=25,c = 0. _ 



2. Use Equation 6 with the estimate of 't' to calculate long-term 

 holding capacity. 



3. Calculate the short-term holding capacity using Equation 4 

 (with c = short-term or undrained shear strength) . 



4. If a short-term breakout test is performed, eliminate the 

 suction effect according to Figure 3 to determine short-term holding 

 capacity. 



5. Compare short and long-term static capacities and use the 

 lower value for design. 



It is anticipated that the critical static situation for shallow anchors 

 will be long-term loading and for deep anchors, short-term loading. 



Creep . Many cohesive sediments are susceptible to shear creep 

 as well as strength reduction as a result of drainage. Shear creep 

 implies a situation whereby long-term shear straining occurs under the 

 influence of a constant state of stress, and on land a situation known 

 as "creep rupture" has been found to occur. In this situation the rate 

 of shear creep increases with time until ultimately a complete failure 

 occurs. In some soils creep rupture has occurred at stresses as low 

 as 60 percent of the measured strength (Singh and Mitchell, 1968). 



Virtually nothing is known about the creep response of seafloor 

 soils. It is anticipated, however, that their creep characteristics 

 will not be any worse than those of the worst terrestrial soils. There- 

 fore, a factor of safety of 1.7 should be adequate to prevent "creep 

 rupture" type failures. This factor of safety is probably overconserva- 

 tive for most installations and is recommended only for use in the 

 design of critical or manned structures. 



Cohesionless Soils 



Cohesionless soils are generally not susceptible to creeping and 

 the techniques for predicting short-term holding capacities given 

 earlier assume that drainage occurs instantaneously. It is reasonable 

 to assume, therefore, that long-term static holding capacity in a 

 cohesionless soil is the same as the short-term capacity. 



