吸湿与上拔荷载共同作用下膨胀土桩基极限破坏分析与性能优化

Wetting unsaturated expansive clay behaves as swell, which usually causes a considerable reduction of critical uplift capacity of pile. Using water content path instead of complex suction-path, this project aims to advocate an applicable analysis framework based on water content-swelling pressure-swell strain and shear strength approach, and then to assess critical uplifting performance of pile in wetting expansive clay. By means of laboratory tests and initial-strain finite element method (FEM), it is intended to estimate wetting-induced heave of expansive clay such that active zone of wetting expansive around pile can be determined. Subsequently, a new pile-soil interface strength criterion along wetting path is also formed. In addition, lower bound method is applied to explore uplift failure mode of pile in wetting expansive clay, and then effects of pile size on uplift performance is investigated to achieve an optimum analysis on critical capacity . Eventually, classical limit equilibrium method is employed to build a practical method for estimation of critical uplift capacity of pile. This work is intended to provide a basic guideline for the optimum design of beamed pile. Scientifically, this research is also attempted to provide a theoretical support for building design code of uplift pile foundation and preventing engineering disaster in expansive clay.

非饱和膨胀土吸湿体胀行为将严重劣化桩基抗拔承载性能。本项目基于含水率路径代替复杂的吸力路径，建立含水率-膨胀压力-应变及强度的非饱和土应用型分析框架，以评估吸湿环境下膨胀土桩基的极限抗拔承载性能。综合室内单元及模型试验，结合初应变有限元方法，定量分析吸湿与上拔荷载共同作用下桩周土的隆起变形以界定主动区；同时构建桩-土界面强度准则，基于下限分析理论探讨吸湿路径下抗拔桩极限破坏机制，进而探讨桩型尺寸对抗拔承载力的影响规律，以实现承载性能的优化分析，最后应用极限平衡法建立湿胀环境下抗拔桩极限承载力的实用计算方法。本课题研究成果旨在为膨胀土桩基选型与优化设计提供依据，同时也为膨胀土桩基设计标准的建立与灾变防治提供理论支撑。

膨胀土作为一种特殊非饱和土，因含大量强亲水性黏土矿物，具有高塑性、干缩湿胀性及增湿强（刚）度劣化等物理力学特性，增湿（如降水）环境的桩基承载性能，尤其是抗拔承载性能往往产生显著的劣化效应，目前膨胀土抗拔承载设计主要依赖于经验方法，缺乏科学理论予以支撑。纵观膨胀土桩基理论及应用相关课题的研究现状，在全吸力范围非饱和膨胀土持水特性、强度准则及变形特性在机理上尚未完全厘清，增湿环境下膨胀土桩基抗拔承载劣化机制尚不明确，桩基增湿抗拔性能定量评价缺乏现场试验印证，现行规范对膨胀土桩基抗拔设计方法尚无明确性指导意见。.本项目综合运用室内及现场等多尺度试验观测技术、非饱和土力学理论及数值模拟手段，建立了膨胀土的持水曲线、强度准则、本构方程及桩-土接触模型，提出了增湿条件下膨胀土桩基抗拔极限承载的分析方法，再现了增湿条件下桩基抗拔承载劣化规律，最终实现了桩基抗拔承载性能的优化分析。首先，针对膨胀土持水特性，综合运用轴平移技术、滤纸法和盐溶液蒸汽平衡法，开展了广吸力范围非饱和膨胀土持水行为的室内试验观测，通过对毛细饱和度与吸附饱和度的解耦，建立了依赖于孔隙比的统一持水曲面模型；其次，针对膨胀土强度及变形特性，对常规非饱和三轴仪体变量测系统辅以差压传感器（DPT）的升级改造，开展广吸范围非饱和膨胀土三轴剪切观测，建立了广吸力范围非饱和土的统一双折线强度准则，并基于下负荷塑性理论框架构建了广吸力范围膨胀土的本构模型；此外，针对增湿膨胀土桩基抗拔承载的评价方法，基于大型直剪仪对桩-土接触面的试验观测，建立了膨胀土-桩三维临界状态接触模型，在ABAQUS有限元中实现了接触模型子程序UINTER嵌入，结合含水率与膨胀力及毛细吸力的关联方程，建立了含水率变量的桩-土相互作用数值评价方法；最后，针对桩基抗拔承载性能的优化分析，开展了增湿下桩基现场足尺试验观测，基于膨胀土含水率、吸湿变形、桩身轴力及摩阻力的时空演化规律，再现增湿膨胀土现场桩基承载劣化行为，基于抗拔承载性能对桩基尺寸及埋深等关键设计参数进行优化分析，提出了膨胀土抗拔桩设计的选型原则。