桩的存在使桩周土的竖向有效应力不等于上覆土自重应力与上部外加荷载的简单叠加，原因是桩-土相对刚度导致桩-土接触面上产生竖向剪切。通过平衡分析法，得到不同荷载形式下桩周土竖向有效应力、桩侧正（负）摩阻力、总侧摩阻力和轴力（下拽力）的计算方法，根据已开展的饱和黏土中单桩的负摩阻力试验，验证了公式的合理性，证明了考虑竖向剪切效应的必要性。通过计算分析可知：桩周土竖向均布荷载作用下，桩-土竖向剪切效应削弱了桩周土竖向有效应力、桩侧负摩阻力和桩身下拽力，剪切效应的控制参数?分别取-0.01、-0.02和-0.04时，靠近桩端的桩侧负摩阻力分别为? = 0时的91%、83%和71%，靠近桩端的桩身下拽力分别为? = 0时的93%、87%和76%。由于下拽力受截面几何参数影响，其折减程度略小于桩侧负摩阻力。因此，桩周土表面均布荷载作用时，忽略桩-土竖向剪切效应可能高估由下拽力引起的桩顶沉降。

As is well-known, the effective vertical stress of surrounding soil element of pile foundation is not identical to the dead weight of overburden soil plus the applied surcharge load. This is because the vertical shearing is developed on pile-soil interface due to the large pile-soil relative stiffness. The analytical solutions for the effective vertical stress, the unit positive (negative) shaft resistance, the total shaft resistance and the axial force (dragload), subject to axial and surcharge loads, are derived by means of equilibrium analyses. Those solutions are calibrated by the reported test result of a single pile in saturated clay. It is shown that those solutions are reasonable and it is necessary to consider the vertical shearing of pile-soil interface. Under the surcharge load, the vertical shearing weakens the effective vertical stress, the unit negative shaft resistance and the dragload. The negative shaft resistances at the pile tip of the cases ? = -0.01, -0.02 and -0.04 are reduced to 91%, 83% and 71% that of the case ? = 0, respectively. Meanwhile, the induced dragload at the pile tip of the cases ? = -0.01, -0.02 and -0.04 are reduced to 93%, 87% and 76% that of the case ? = 0, respectively. The dragload is reduced slight smaller than the negative shaft resistance since it is also controlled by the cross-sectional geometries. This indicates that pile settlement is always overestimated independent of the vertical shearing.