Abstract: Based on the field test results of uplift bored piles for a deep-embedded 500 kV underground transformer substation in a coastal city,the uplift behavior of under-reamed bored piles and bored piles with pileshaft post-grouting was studied.The load transfer characteristics and mechanism of uplift capacity improvement for the two different types of uplift piles were investigated.Generally,for long piles in sandy soils,the uplift piles with pileshaft post-grouting were the first option,and the construction period would be shortened.However,for short piles in clayey soils,the under-reamed uplift piles could be more economic in terms of the equivalent pile size and capacity requirements.