由于基桩纵向截面形式的差异，竖向荷载作用下桩侧摩阻力和桩端阻力发挥存在明显的差异，尽管纵向截面异形桩在工程中得到了一定的应用，然而针对极限荷载下桩端和桩侧土体破坏形式的研究却相对较少。基于透明土材料和粒子图像测速（particle image velocimetry）技术，针对等体积的扩底楔形桩、楔形桩和等截面桩的承载特性及破坏形式进行对比模型试验，测得桩顶荷载-沉降曲线，研究了各级荷载下桩端和桩侧土体位移场的变化规律以及极限荷载下桩端和桩侧土体的破坏形式；同时分析了不同桩长情况下各类型桩的承载力特性。研究结果表明，在此试验条件下，扩底楔形桩的极限承载力约为常规楔形桩的3.5倍和等截面桩的2.5倍；极限荷载作用下各类型纵向截面异形桩桩端的破坏形式规律基本一致。

Due to the difference in their longitudinal sections, foundation piles have significantly different shaft frictions and tip resistances under the vertical loading condition. Although the piles with a variable longitudinal section have extensively used in engineering, little research effort is made to investigating the failure modes of the surrounding soil or the end bearing soil under ultimate load. Based on transparent soil material and particle image velocimetry (PIV) technique, comparative model tests on the bearing capacities and the failure modes of belled wedge pile, tapered pile and uniform-section pile with an equivalent volume are carried out. The load-settlement curves of the pile top and the displacement field of the surrounding soil under different loads are measured; the failure modes under ultimate loading and the bearing capacities influenced by pile length are analyzed in details. The results show that the ultimate bearing capacity of a belled wedge pile is about 3.5 times that of the conventional tapered pile, and is approximately about 2.5 times that of the uniform-section pile under this model test condition; and the failure modes of soils around the pile tips of the variable longitudinal section piles are similar.