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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.