Critical Reinforcement-Depth Ratio
The preceding relationships suggest that the bearing capacity increase is
realized only when the reinforcement is located within a depth of 2B for a
continuous foundation and a depth of 1.2B for a square foundation.
Critical Reinforcement-Width Ratio
According to Eq. (7.16), (b/B)cr is about 8 for a continuous foundation and about 4.5 for a square foundation. It needs to be realized that generally, with other parameters remaining constant, about 70% or more of BCRu is realized with b/B = 2. The remaining 30% of BCRu is realized when b/B increased from about 2 to (b/B)cr.
Critical Reinforcement-Length Ratio
The magnitude of (u/B)max was recommended by Binquet and Lee [2,3] to be about 0.67. However, model test results of Guido et al.  on a square foundation and Omar et al.  on a continuous foundation show that (u/B)max is about 0.9 to 1. Based on the existing studies it appears that (u/B)cr is approximately equal to 0.25 to 0.4.