路堤荷载下加筋碎石桩复合地基变形和破坏机理研究

Compared to the conventional stone columns, geosynthetic-encased stone columns(GESCs) apprently improve the bearing capacity whilst reduce the settlement of foundation; therefore, GESCs are a novel method to improve the soft foundation and have an extremely extensive application prospect. However, the present experimental researches on GESCs basically applied down-scaled 1g physical modeling tests but lacked the field tests or modeling tests under the actual fields of stresses. Thus its deformation and failure mechanism are still far from clear. Furthermore, the researches on the bearing characteristic of GESCs subject to the axial and lateral loading have not been carried out. Therefore firstly, this research is to investigate the stress-strain relationship of GESCs under the axial loading and the bending characteristic under the lateral loading using large triaxial tests, uniaixal compression tests, and bending capacity tests. Secondly, this research is to investigate the deformation characteristic, load transfer mechanism, change law of the bearing ratio in columns and soil and the excess pore water pressure, distribution law of the additional stress in underlying stratum, and the failure mechanism and failure modes of GESCs composite foundation subject to embankment loading using full-scale field tests and centrifuge modeling tests based on PIV technology, in combination of 3D coupled mechanical and hydraulic numerical simulation. Thirdly, this research is to present the design computation methods of settlement and stability of GESCs composite foundation based on the findings. This research results are expected to provide theoretical basis for the design and construction of embankments supported by GESCs composite foundation in future.

加筋碎石桩较传统碎石桩显著提高了承载力，减少了地基沉降量，是一种应用前景极为广阔的新型软土地基处理方法。目前对其研究多采用小比例尺的1g相似物理模型试验，缺少符合实际应力场条件的现场试验和模型试验研究，其变形和破坏机理尚不明确，且缺少对桩体结构自身在轴向和侧向荷载下承载特性的研究。本项目拟通过大三轴试验、单轴压缩试验和抗弯承载力试验，研究加筋碎石桩桩体在轴向荷载下的应力应变关系以及在侧向荷载下的抗弯承载特性；通过现场试验和基于PIV图像处理技术的离心模型试验，并结合三维有限元流固耦合数值模拟，研究加筋碎石桩复合地基在路堤荷载下的变形特性、荷载传递机理、桩土荷载分担比和超孔隙水压力变化规律、下卧层附加应力分布规律，以及复合地基破坏机理和破坏模式；在此基础上提出加筋碎石桩复合地基路堤沉降和稳定性设计计算方法。研究成果为今后加筋碎石桩复合地基路堤的设计和施工技术提供理论基础。

开展了不同筋材强度、不同填料类型和压实度加筋碎石桩桩体的单轴压缩试验和大三轴试验，获得桩体在轴向荷载下的应力-应变关系及强度特性；根据莫尔-库伦理论和极限平衡原理推导出加筋碎石桩桩体强度及模量计算公式，并采用试验结果对公式进行修正。开展了加筋碎石桩桩体抗弯承载试验，结合三维数值模拟得出桩体受弯截面正应力分布规律，基于此分布规律及Euler-Bernoulli梁模型，建立了加筋碎石桩极限抗弯承载力和抗弯刚度计算公式。开展了基于人造软土的加筋碎石桩复合地基路堤相似模型试验和离心模型试验，研究其在不排水条件下的变形特性和破坏机理，路堤中心下的加筋碎石桩桩体可发生向路堤内的屈曲失稳，而边坡下的桩体在不排水的桩间土体滑移作用下产生显著的向路堤内的弯曲变形；开展了基于高岭土的加筋碎石桩复合地基路堤离心模型试验，并结合现场试验和三维有限元流固耦合数值模拟，研究了其在排水条件下的变形特性和破坏机理，桩体具有很好的排水性能，边坡下桩体发生向路堤外的弯曲变形，而有水平加筋垫层时，桩体则向路堤内弯曲变形。基于弹塑性理论，建立了路堤荷载下加筋碎石桩复合地基桩土应力比计算模型，考虑应力扩散效应和自重应力，推导得出加筋碎石桩复合地基桩土应力比计算公式。基于复合地基理论，提出了路堤荷载下加筋碎石桩复合地基承载力、沉降和稳定性计算公式，研究成果已纳入中国土木工程学会标准编制计划《土工织物散体桩处理软土地基技术规程》中。