桩底土的成层性对桩体纵向刚度的影响

 为了确定桩底土(桩底与基岩间的土体)的厚度和分层情况对桩体刚度和阻尼的影响，使用锥形虚土桩模型对单桩纵向振动进行求解。桩底应力以一定扩散角度传递至基岩表面，将扩散锥面内部土体看作虚土桩。由虚土桩底部的位移限制条件和阻抗传递法得到桩底和桩顶的复阻抗。将桩底土层的参数转换为虚土桩的参数，通过分析这些参数的变化，进而判断桩底土层对桩体刚度和阻尼的影响。分析结果表明：桩底土层厚度小于5倍的桩体半径时，桩体刚度明显提高，阻尼减小；桩底土层存在软夹层，则桩体刚度会相应降低，阻尼增大，若存在硬夹层，结论相反。桩底土存在不同土层时，若只使用第一层土体的参数估算桩底刚度，误差较大，尤其对处于准静力状态的桩体。

INFLUENCE OF LAYERING OF STRATUM UNDER PILE TIP ON PILE LONGITUDINAL STIFFNESS

In order to identify the influence of the thickness and layering of stratum between the pile tip and bedrock on the pile stiffness and damping，a longitudinal vibration response of single pile was obtained by a taper fictitious soil pile model. The soil in the tapered area of influence，which is extending to the surface of the bedrock，is treated as a fictitious soil pile. The pile complex impedance was obtained by impedance transfer method combining the boundary condition at the fictitious pile tip. A parameter analysis of the stratum under the pile was carried out to determine the effect of the soil under the pile on the complex impedance at both pile tip and head. The results show that，the stiffness of pile has a significant improvement，and the damping decrease，when the stratum between the pile tip and bedrock is less than 5 times the radius of the pile. The soft interlayer in the stratum under the pile will result in the decrease of the pile stiffness and increase of the pile damping；and the hard interlayer has an inverse effect. In addition，for multilayer stratum under the pile tip，a major error of pile complex impedance may be created only considering the first layer，particularly for the pile subjected to quasi-static load.