One difference between the two approaches should be corrected. In 

 Equation B-2, the last term, V^ yJA, represents a buoyancy effect; the deeper 

 an object is embedded, the greater will be the force required to cause a bear- 

 ing capacity failure because of the tendency of the surrounding soil to buoy 

 the object upward. In breakout, because this buoyancy effect will cause a 

 reduction in the required uplift force, the final term should be subtracted 

 rather than added. This effect may also be incorporated by including the 

 buoyancy effect in the breakout force term. The resulting breakout force 

 equation, as patterned after Skempton's bearing capacity equation, is 



-^ = 5Sh.O + 0.2 IjM.O + 0.2 ^j (B-3) 



where Fji^ = portion of immediate breakout line force, Fe,,^, carried by soil 



= Fg,, - W, + W3 



W,^ = buoyant weight of object 



Wj = buoyant weight of soil displaced by object 



= V 7 

 s 's 



The limiting equilibrium problem could also be approached by 

 formulating the general equations of elasticity and plasticity numerically 

 and solving them on the computer. This has been done for the bearing 

 capacity of an unembedded strip footing, and the results obtained were 

 found to agree well with Prandtl's equation.'"'' The investigation was valu- 

 able in that it illustrated how plastic zones develop in soil before the bearing 

 capacity is reached. It did not, however, lead to the development of a new 

 bearing capacity equation. 



The numerical approach has also been applied to the breakout 

 problem.^ Once again, the results of the computer program written for 

 the problem yield information on how plastic zones develop during shear. 

 However, in terms of predicting the required ultimate breakout force, there 

 is little difference between this approach and that summarized by Equation B-3. 

 In the sample problem included in Reference 6, the breakout force required to 

 extract a particular submarine from a specific seafloor site was predicted to be 

 91 ,000 pounds. By use of the same input parameters. Equation B-3 predicts 

 a breakout force of 86,500 pounds. 



In comparing these results, it should be noted that the numerical 

 solution of Reference 6 required considerable computer time to process and 

 that the computer program required as input the soil elastic modulus and 



41 



