Drilled Shaft Tip depth beyond fixity depth

[minorchord2000]

After determining the depth to fixity for a concrete drilled shaft, what is the rule of thumb (if such a thing exists) for the increase in depth beyond fixity to the tip elevation?

 

[BridgeSmith]

Same question posed in the foundation engineering forum. Please do not double post.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Settingsun]

If you've used the fixity method where the depth to fixity = 1.4*R (clay) or 1.8*T (sand), the method is valid for embedded length > 4*R or 4*T.