基于天津于家堡南地下车库工程扩底桩抗拔极限承载力试验，结合数值模拟手段对扩底桩抗拔承载特性、破坏模式和受力机制开展了分析研究。计算分析表明，扩底桩（有效桩长19 m）相比等截面桩抗拔极限承载力提高约50%，材料增加仅8.5%，扩底桩扩大头周边土体提供的抗力显著提高了抗拔承载力；荷载较小时抗拔力主要由等截面段侧摩阻力提供，扩头段抗拔力占桩顶加载的比值随加载近似呈线性增加；扩底桩等截面段沿桩土界面先发生剪切破坏，扩头段周边土体后发生受压破坏，抗拔承载力达到极限；扩头段位于同一土层时，不同桩长扩头段提供的极限抗拔力相差不大，桩长越长，扩头段抗拔力贡献率越低；扩头段抗拔力主要由自重、扩头段法向力竖向分力和侧摩阻力组成，其中法向力竖向分量提供了扩头段的主要抗拔力，占扩头段总抗拔力约70%。

Based on the ultimate uplift bearing capacity tests of the enlarge base piles in Tianjin Yujiabao South Underground Garage Project, the uplift bearing behavior, the uplift failure mode and the bearing mechanism of the enlarged base piles is analyzed by numerical simulation. The results show that the ultimate uplift bearing capacity of the enlarge base pile with the increase of 8.5% of concrete material, increases about 50% more than the uniform section pile; the ultimate bearing capacity is significantly improved by the soil around the enlarged base. When the loading is small, the uplift resistance is mainly provided by the shaft fiction of the uniform section part of the enlarged base pile; the ratio of the uplift resistance of the enlarged base to the loading on the pile top increases approximately linearly with the loading increase. The shear failure firstly occurs in the pile-soil interface of the uniform section part with the loading increase; then the compression failure of the soil around the enlarged base occurs, and the uplift resistance reaches the ultimate bearing capacity. The ultimate uplift resistance of the enlarge base of the different length piles is almost the same when the enlarged bases are located in the same layer of soil; and the uplift bearing efficiency reduces as the pile length increases. The uplift resistance of the enlarge base is composed of the dead weight, the vertical component of the normal force and the shaft friction; the vertical component of the normal force provides the major uplift resistance of the enlarged base, the percentage of the total uplift resistance of the enlarged base is about 70%.