The change of failure mode into local is due to soil compressibility. To introduce this effect, soil compressibility factors were added to the general equation as follows:

F_cc, F_qc, & F_gc  are the soil compressibility factors .

In order to calculate these factors the following steps should be followed:

• Calculate the rigidity index, I_r, of the soil at depth approximately B/2 below the bottom of the foundation
  
  

where G = shear modulus of the soil and q' = effective overburden pressure at a depth of D_f+ B/2
 

• Calculate the critical rigidity index, I_r(cr), which can be expressed as:

  

   

• If  I_r > I_r(cr) then F_cc = F_qc = & F_gc = 1

  However if I_r < I_r(cr)

  

  For f = 0:

  
   

  For f > 0: