provide a basis for accounting for time effects. It is significant that the data 

 are bounded by the upper limit, Fji^/F^ equal to 1 .0, and that a decrease in 

 normalized embedment depth results in a systematic decrease in the normal- 

 ized breakout force. The trend of the data suggests an exponential form; the 

 coefficients of the appropriate exponential equation were obtained through 

 trial and error in such a manner that the number of points outside the plus- 

 or-minus-50% band was minimized. The resulting empirical equation for 

 predicting the force required for immediate breakout is 



T^ or -^ = 1.0 - 0.97e-2-^5(D/B) (5) 



This equation is plotted in Figure 3 along with the lines of plus or 

 minus 50% of the predicted breakout force. This band includes 88% of the 

 data points. 



A discussion of how to apply Equation 5 practically to predict the 

 required immediate breakout force is given in the SUGGESTED PROCEDURE 

 and SAMPLE PROBLEM sections. 



Long-Term Breakout 



Predicting the time required for extracting objects with a force less 

 than the immediate breakout force is somewhat more difficult than predicting 

 the immediate breakout force itself. Existing soil mechanics knowledge pro- 

 vides little information on the subject, and virtually no empirical data exist 

 other than the results of the NCEL tests. A further complication arises from 

 a general lack of knowledge as to the mechanism which leads to breakout under 

 long-term conditions. It is necessary, therefore, to resort to empirical data cor- 

 relation tempered with observational information on the mechanism to produce 

 a usable prediction technique. Along these lines, the long-term laboratory tests 

 of FY-71 were given first consideration because of their large number, the variety 

 of parameters considered, and the extent of control. 



The qualitative characteristics of long-term breakout were determined 

 observationally and by measuring the upward object deflection as a function 

 of time after load application and the depths of the holes left by objects which 

 were broken out. The characteristics determined were: 



1 . After the application of a line force, Fg^,, less than the estimated 

 immediate breakout line force, Fgi^. but greater than the object weight, a 

 somewhat rapid, upward object motion occurred over a period of about 5 

 minutes. This initial displacement appeared to vary directly with the force 

 applied relative to the estimated immediate force. 



19 



