抗拔桩极限平衡方程及其应用

Limit equilibrium equation for uplift piles and its application

将抗拔桩侧阻力分解为与桩侧正压力不相关的桩-土黏结强度pc、与桩侧有效正压力成正比的摩擦力sF两部分,采用摩擦定律计算摩擦力sF。基于轴对称条件,假定土体为半无限弹性体,以Mindlin公式积分计算分析极限平衡状态下桩-土共同作用,依据平面应变条件下柱状孔扩张的弹性力学解建立桩-土界面位移协调方程,推导出抗拔桩极限平衡方程,给出了求解方法及计算参数确定方法。该方程能反映桩与土的材料特性、桩体尺寸、桩顶埋深、群桩效应、卸荷效应等多因素对抗拔桩极限承载力的影响。结合海上风电大直径超长抗拔钢管桩足尺试验进行验证。对比分析结果表明,该方法计算的抗拔极限承载力与实测值接近,计算精度远高于现行规范推荐方法,其结果可为工程应用及抗拔桩承载力机制研究提供参考。

The ultimate shaft resistance of uplift pile is divided into two parts, one is the setting strength of pile and soil cp, which is irrelevant to the earth pressure of pile, and the other is the force of friction F, which is proportional to the earth pressure of pile and can be calculated using the friction law. With assumption of elastic soil, this paper studies the pile-soil interaction through the integral calculation using Mindlin formulation. Pile-soil interface displacement coordinate equation is derived based on the solution of cavity expansion in infinite soil. Then a limit state equation of uplift piles and its solution are presented. The methods for determining the parameters of soil and pile are also given. The equation can be used to calculate ultimate bearing capacity of uplift piles with considering the influence of such factors as pile and soil materials, pile length, pile diameter, depth of pile top, group piles effects, unloading effect due to excavation, and so on. As an example, a full-scale test was performed on a large steel pipe pile with a length of 81.3m, a diameter of 1.7 m and a wall thickness of 25mm. The calculated and measured ultimate uplift bearing capacities are compared, showing that the proposed method for calculating the bearing capacity of uplift piles is satisfactory and applicable and has an accuracy much better than the methods recommended in the current codes of China.