Abstract：Four groups of vertical loading tests were performed on belled piles and equal-diameter piles at four sites to make comparative study. The results are shown as follows. At site 1，the shaft diameter and length of the two piles are the same. With a 30 mm settlement，the peak load of the belled pile is 5.5 times that of the equal-diameter pile. When the peak load is 2 700 kN，the settlement of the belled pile is only 1/22 of that of the equal-diameter pile. When the concrete quantity of the belled pile is increased by 1 m3，the peak load of the equal-diameter pile can be increased by 1 746 kN. At site 2，the length of the shorter equal-diameter pile is almost the same as that of the belled pile，and the longer equal-diameter pile is 1.65 m longer than the belled pile. With a 10 mm settlement，the peak load of the belled pile is 3.58 and 1.70 times as that of the shorter and longer equal-diameter piles，respectively. This indicates that the effect of belled pile in shallow and weak soil layer is better than that of the equal-diameter pile in deeper and stronger soil layer. At site 3，the lengths of all piles are almost the same，and the shaft diameter of the equal-diameter pile is 1.0 m larger than that of others. The volume of equal-diameter pile is 2.09 times as that of the belled pile，and the bearing capacity of the equal-diameter pile is larger than that of the belled pile when the settlement is less or equal to 5 mm. However，the ultimate bearing capacity of the belled pile is larger than that of the equal-diameter pile. At site 4，the shaft diameters and lengths of two piles are the same，respectively，and there is 0.2 m thick residue at the bottom of the belled pile. The load-settlement curves of two piles are very close，which indicates that the bearing capacity of two piles are almost equal. This demonstrates that the residue at the bottom of the belled pile should be removed in order to fully mobilize large bearing capacity of the belled pile.
