The critical depth of piles are normally assumed as 10-20 pile diameter deep and is the depth beyond which the resistance is constant and is equal to respective value at critical depth.
The critical depth is a fallacy which comes from the failure to interpret the results of full and model-scale pile tests. In full-scale test, the neglect of presence of residual loads renders a measured load distribution to be linear below the so called “critical depth”. Residual loads refer to loads that are induced in piles during and after installation of piles.
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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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my experience its not true for compression piles but might work in case of tension piles. In compression load bearing piles, the pile has got to reach to a competent rock layer in order to safely transfer the load. this is nothing to do with pile dia. Skin friction is not relied upon solely for heavy structures like bridges over 100 years life span.