Abstract: Based on the theory of soil plastic mechanics,a method to calculate the variation of soil resistance with depth during pile installation has been presented,three problems about minimal thickness from pile toe to the top of soft substratum,critical depth of pile toe penetrating into the bearing stratum and variation of minimal thickness or critical depth for different soil stratum have been discussed.The calculated results are in good agreement with the site measurements.Minimal thickness and critical depth determined by current pile code are compared with the presented results.