Figure B-3 represents a simple breakout problem involving the 

 following mechanism: an upward load per unit length, P, is applied to a 

 long, narrow footing. Because loading is rapid (immediate breakout), the 

 soil permeability is low, and there are no open passageways, water is pre- 

 vented from moving into the zone beneath the footing. Instead the soil 

 itself is forced to move into this zone and the flow pattern of the soil is 

 given by the Prandtl failure surfaces. 



The situation involved in the NCEL breakout tests was somewhat 

 different from that given by Figure B-3. The objects were not long, narrow 

 footings but rather were cubes, spheres, and prisms; and they were partially 

 embedded in the subsoil. The downward short-term bearing capacity for 

 objects in this situation has been investigated by Skempton,^° who pre- 

 sented the equation 



-^ = 5SM.0 + 0.2 |)(l-0 + 0.2 ^j + -^ {B-2) 



where P = bearing capacity force 



A = horizontal cross-sectional area of object 



S = undrained shear strength of soil 



D = depth of embedment 



B = width of object 



L = length of object 



Vj = volume of object embedded in soil 



7j. = buoyant unit weight of soil 



This equation is somewhat empirical; but it does reduce approxi- 

 mately to the solution of Prandtl's equation for a long, narrow footing with 

 zero embedment depth, and it has been shown to be applicable to actual field 

 situations. In view of the similarity between breakout and bearing capacity 

 as noted in the simpler Prandtl example, it is reasonable to assume that this 

 equation will also yield an approximate solution to the breakout problem for 

 these more complex circumstances. The comparison between the two is pro- 

 bably not exact for this case because of some distortion of the symmetric 

 failure surfaces and differences in elastic deformation patterns. However, an 

 equation of this sort can serve as a rough first approximation in estimating 

 the immediate breakout force. 



40 



