The geological conditions under Yichun City Audit Bureau Compartment Building are very complicated. Determination of proper load bearing layer of formation is key to foundation design. After evaluation and analyses on the formation conditions and existing local experiences as well as new pile foundation technology, the load bearing layer of piles were courageously put on the detritus soil layer which is unconsolidated, character various greatly and bearing capacity low. Bottom enlargement of bored piles was adopted to enhance the bearing capacity. Post grouting was used to reinforce the bottom of piles. The practice verified that the alternative was technically feasible and economical.