筋箍碎石桩复合地基动力承载变形机理及其分析方法研究

Geosynthetic encased stone columns (GESC) is a new ground improvement technique for soft soils, which is formed by encasing stone columns with geosynthetic encasement. The capacity and stiffness of the columns can be increased and the settlement of the soil strata can be further reduced due to the confinement provided by the geosynthetic encasement. GESC has been widely used to support a wide variety of structures including embankments for highways, railways, and dams constructed on soft soils. However, the analytical studies of GESC were not well built. The research on the bearing capacity and deformations of GESC under cyclic loading has not been studied. This project would focus on the dynamic behavior of GESC based on its macroscopic and microscopic performances. The model tests, discrete element method (DEM) numerical analysis, and theoretical method would be used to understand GESC’s load transfer mechanism, particle stresses and deformations of the stone, encasement, and surrounding soil, and dynamic consolidation of the composite foundation. A series of model tests was designed to consider the effect of the encasement length, the load magnitude and the loading frequency. Three-dimensional DEM models would be proposed to simulate the aggregate, the geogrid encasement, the surrounding soil, and the column. Measurements from laboratory tests would be used to compare the DEM results and to validate the proposed models. Based on the results from model tests and DEM analysis, a theoretical method could be proposed to calculate the deformation of GESC under cyclic loading conditions. This method should be useful for understanding this new technique and giving references for the project design and construction.

筋箍碎石桩复合地基因其良好的工作性能获得了工程界的青睐，但目前对该处治方法的研究多集中于静力学领域，对动力荷载作用下筋箍碎石桩复合地基的承载机理及其变形特性研究尚缺乏系统深入的研究。本项目以高等级公路软基处理为研究背景，从筋箍碎石桩复合地基动力特性研究入手，设计多组室内模型试验，利用分布式光纤测量技术，深入探讨长期循环荷载下筋箍碎石桩复合地基的荷载传递规律、碎石-筋材-土体动力相互作用机制、复合地基动力固结特性等内容；在此基础上，基于三维颗粒流数值模拟技术，提出碎石、筋材、土体和桩体的离散元建模途径和参数标定方法，建立筋箍碎石桩复合地基动力响应的离散元分析模型，深入揭示筋箍碎石桩复合地基动力承载变形机理；最后，引入损伤时变分析方法，结合圆孔扩张理论和荷载传递法，提出长期循环荷载作用下筋箍碎石桩复合地基的沉降变形计算方法，以期完善该处治技术并为工程设计提供参考。

筋箍碎石桩复合地基处治技术针对普通碎石桩刚度小、易鼓胀、承载力低的问题，采用土工合成材料包裹碎石桩体形成环向加筋套筒，限制桩体的侧向鼓胀变形，从而有效提高桩体刚度和整体性，减少地基沉降量。但目前的研究成果主要集中于静力领域，交通荷载下的动力承载变形特性仍有待深入研究。利用室内模型试验和三维数值模拟等手段，对交通荷载作用下筋箍碎石桩复合地基的承载变形机理、荷载传递规律、排水固结效应以及微观受力变形等内容进行深入研究。首先通过级配碎石循环动三轴试验对级配碎石的动力学特性展开研究，深入探讨了不同围压、动应力以及加载频率下级配碎石的动力承载变形规律。同时，通过6组复合地基模型动静载荷试验，分析了承载形式、土工格栅套筒和加载频率对承载变形特性的影响，深入研究了竖向压缩变形特性、筋材拉伸变形特性、荷载传递机理以及软土孔隙水压力的发展和消散规律等内容，进而揭示筋箍碎石桩复合地基的动力承载变形机理。其次，针对软土和筋材的连续性以及碎石的离散性，分别提出了模拟筋箍碎石桩复合地基中软土、筋材的连续介质模型及碎石的离散元模型；进而建立了软土固结试验模型、碎石静三轴和动三轴试验模型以及土工格栅拉伸试验模型，实现了软土弹性模量，碎石回弹模量、內摩擦角及筋材拉伸刚度的标定。建立了筋箍碎石桩复合地基离散元-有限差分法耦合的三维数值模型，并同室内试验结果进行了对比验证。根据数值模型结果，重点分析了交通荷载作用下桩体竖向应力和径向应力大小、颗粒接触力分布、碎石颗粒位移规律、桩体破坏模式以及桩体径向变形等内容。最后，进一步对筋箍碎石桩复合地基的动力承载特性展开参数分析，重点探讨了加筋长度、筋材拉伸刚度、桩端持力层弹性模量、动应力幅值对桩顶沉降的影响，并提出影响因素与桩顶沉降的关系曲线和关系表达式以及影响因素的建议值为筋箍碎石桩的设计和施工提供重要参考。