路堤下刚性桩复合地基桩-土应力比计算

路堤下刚性桩复合地基普遍存在的负摩阻力对桩-土相互作用特性影响显著。基于极限剪切位移受应力水平影响较小而受试样尺寸影响较大的试验结果，提出了桩-土界面剪切刚度随应力水平变化的等单位长度极限剪切位移理想弹塑性模型，针对桩-土相互作用的上部负摩阻力塑性区、中部协调变形弹性区和下部摩擦承载塑性区3区段模式，建立了塑性区非均匀、弹性区非线性的摩阻力分布图式；以均布路堤荷载下等桩长复合地基中单桩等效加固单元体为研究对象，利用单元体荷载传递微分方程，结合桩-土-垫层压缩变形协调条件，推导出了中性面位置及桩-土应力比的解析表达式，分析了路堤荷载及垫层柔度系数的影响。研究表明：中性面位置随垫层柔度系数增加而下移，随路堤荷载增大先下移后上抬；桩-土应力比随垫层柔度系数增加而减小，随路堤荷载提高先增大后降低，据此提出了采用最大桩-土应力比进行桩体布置和垫层设计的技术思路。

Calculation of stress ratio of rigid pile to composite embankment

The negative skin friction of composite foundation with rigid pile has a significant effect on the pile-soil interaction under embankment load. Based on the results of shear tests that ultimate shear displacement is less affected by stress levels but greatly influenced by specimen sizes, an ideal elastic-plastic load transfer model considering the variation of stress-dependent shear stiffness of pile-soil interface is developed. According to the three-section distribution pattern of pile-soil interaction, i.e., the upper plastic zone of negative friction, the central elastic zone of coordinate deformation and the lower plastic zone of friction bearing, a distribution model for skin friction on pile-soil interface describing the nonlinear elastic-zone and inhomogeneous plastic-zone is established. To investigate the equivalent reinforcement of single pile with equal area in rigid pile composite ground under uniformly distributed embankment load, the formulas of the depth of the neutral plane and stress concentration ratio are derived according to the force equilibriums differential equations of a simplified analytical unit cell and the compression deformation compatibility equations of pile-soil-cushion,. the effect of the embankment load and the flexibility coefficient of cushion on the depth of the neutral plane and the stress concentration ratio are analyzed. The analysis indicates that the depth of the neutral plane descends with the increasing of the flexibility coefficient of cushion, and then ascends with the increase of embankment load. The stress concentration ratio decreases with the increasing of the flexibility coefficient of cushion, and decreases afterwards with the increase of embankment load. Hereby, the technical ideas including the pile arrangement and the cushion design using the maximum stress concentration ratio are provided.