动刚度是桩基动测分析中一个重要指标，研究动刚度的影响因素有着现实的工程意义。为了分析完整桩和缺陷桩单桩动刚度的变化规律和影响因素，推导并求解了竖向简谐荷载作用下考虑两个变截面桩的导纳响应解析解。利用测试数据验证了该模型能有效计算导纳共振频率值和动刚度值。同时，对513根相同型式桥桩进行机械阻抗法测试，并对其动刚度进行统计分析。研究表明：（1）桩端支撑刚度和桩长对动刚度影响较大，端承桩动刚度随桩长增加而降低，纯摩擦桩情况与之相反。摩擦桩在桩长较长时其动刚度可能大于端承桩。（2）桩身缺陷体积改变量与动刚度变化成正相关。（3）动刚度对桩身缩颈缺陷敏感度较高，在一定程度上可以用于判断桩身完整性和承载能力。

Dynamic stiffness is an important index in pile dynamic measurement and analysis. In practice it is important to analyze and identify the factors that influence the dynamic stiffness of pile foundation. To analyze the dynamic stiffness of the intact and defected piles, an analytical model of admittance response under vertical harmonic loads is developed and solved. In this model, two variable cross sections are considered. The measured results show that the model can be effectively used to calculate the values of resonance frequency and dynamic stiffness. Moreover, 513 bridge piles of the same type are measured using mechanical impedance method. The dynamic stiffness for all piles is analyzed statistically. It is shown as follows. (1) the supporting stiffness of pile tip and the pile length significantly effect dynamic stiffness; the dynamic stiffness of end-bearing pile decreases with its length, whereas that of pure friction piles increases with the pile length. If friction pile has a longer length, its dynamic stiffness may exceed that of end-bearing pile. (2) For defected piles, the volumetric change has the positive relation with the variation of dynamic stiffness. (3) Dynamic stiffness is very sensitive to necking defection, which can be used to estimate the pile integrality and bearing capacity to some extent.