强膨胀土—桩基相互作用及隔胀效果数值模拟

黑棉土是一种强膨胀土,其受湿度变化影响呈现很强的胀缩特性,对桩基的承载力和变形不利,易导致断桩等多种问题。本文建立了强膨胀土-桩基有限元模型,分析了胀缩条件下桩基内力和位移的变化规律,与桩-土共同作用解析解对比,验证了数值模型的有效性并解释离心机试验现象。利用该模型进行参数分析,指出黑棉土胀缩对桩的不利影响及工程中采用隔胀措施的必要性,进而模拟局部换填和界面隔胀措施。结果表明:浅表层黑棉土适合采用局部换填隔胀,换填宽度宜不小于膨胀土厚度;深厚层黑棉土适合采用双层分离式隔胀,隔胀套管应保证界面折减程度达到土体参数的30%且布置至黑棉土和普通土分界面附近;桩基承台底部宜增加柔性垫层,预留胀缩空间。

Numerical Simulation of Strong Expansive Soil-pile Foundation Interaction and the Effect of Swelling Isolation Measures

Black cotton soil is a kind of strong expansive soil. It presents obvious swelling-shrinkage characteristics and has negative effects on bearing capacity and deformation of piles which would cause serious problems such as broken piles. This paper built a finite element analysis model which contains black cotton soil and pile foundation and analyzed the force and displacement in swell-shrink process. The results were verified by the analytical solutions of pile-soil interaction and the findings in the centrifuge test were explained. Parameter study showed the negative effects and the necessity of adopting expansion isolation measures. Furthermore, part-replacing and filling measure and interface isolation measure were simulated. The calculation results indicated that, part-replacing and filling should be used in shallow surface black cotton soil layer and the width of replacing and filling earth should be not less than the thickness of expansive soil. As for thick black cotton soil layer, interface isolation measure should be applied and the isolation pipe should be extended to the interface between the black cotton soil and ordinary soil with more than 30% shear stiffness reduction. For pile foundation with cap, flexible cushions should be installed under the caps, leaving space for soil expansion.