长期循环荷载作用下泥炭土累积变形特性与微观演化机理研究

Under the long-term cyclic loading, the foundation constructed on peat soil emerges a considerable additional post-construction settlement due to the loose structure of peat. However, the study on deformation characteristics of peat soil is still not systematically developed and the deformation mechanism is also still indeterminacy. Considering that peat soil is rich in organic matter and its effect on mechanical properties is significantly, this study will conduct undrained cyclic triaxial tests against peat soil samples with different organic contents, where the combination of different control conditions are designed. And the development law of accumulative deformation and the effect of organic matter will be determined, and a normalized calculation model of cumulative plastic strain will be proposed. Then, from the micro perspective, the evolution process of microstructure of peat soil will be understand combining with the macro mechanical tests, and evolution equation of microstructure will be established, also the accumulative deformation mechanism will be essentially revealed. On this basis, taking into account of the effect of organic matter and evolution characteristics of microstructure, elasto-plastic boundary surface model for peat soil will be built up relying on the critical state theory and boundary surface model theory. In addition, the constitutive model will be compiled and an efficient numerical algorithm will also be provide. Finally, deformation characteristics of foundations constructed on peat soils under will be analyzed based on the finite element model. The productions of this study will be capable of predicting the post-construction settlement for those foundations built on peat soil. Consequently effective measures will be suggested based on the results of this study to control and reduce the long-term settlement for the foundations under construction as well as the structures on schedule.

长期循环荷载作用下泥炭土地基由于疏松的结构形态会产生显著的工后附加沉降，而目前对其变形特性的研究尚未系统展开，累积变形发展机理尚不明确。本项目针对泥炭土富含有机质且有机质对力学性能影响显著这一特征，以不同有机质含量泥炭土为研究对象，开展考虑不同试验控制条件组合的不排水循环三轴试验，明确累积变形发展规律及有机质对其影响效应，建立归一化的累积塑性应变计算模型；从微观角度出发，查明泥炭土微观结构演变规律，耦合分析宏观试验结果，构建微观结构演化方程，揭示累积变形发展机理；在临界状态理论和边界面模型理论框架内，考虑有机质影响以及微观结构演化特征，建立泥炭土弹塑性边界面动力本构模型，对模型程序化以及开发高效的数值算法，结合有限元分析泥炭土地基在不同循环荷载作用下产生的累积变形发展规律。研究成果可为泥炭土地基上已建工程的工后沉降合理预测、在建和拟建工程采取有效措施控制和减小长期沉降提供科学依据。

从岩土工程视角，泥炭土因其富含有机质的物质组成特征而常常被视为“问题土”，其在交通荷载等长期循环荷载作用下的变形特性尚未引起足够重视。项目采用微宏观试验与理论分析结合的手段，对泥炭土在长期循环荷载下的累积变形特性与微观演化机理展开研究。首先，系统开展了泥炭土的静力力学特性试验，揭示了有机质含量与分解度对泥炭土力学特性的影响效应。其次，基于大数目循环三轴压缩试验，明确了循环荷载幅值、频率、循环周次、静偏应力、固结围压、排水条件等因素对泥炭土累积塑性应变、累积超孔压以及割线模量的影响规律，并建立了可归一化考虑这些因素的累积塑性应变模型。结合刚度衰减特征和微观电镜扫描照片，研究了高分解度泥炭土的结构性特征，指出由于有机质的作用土颗粒间易形成散碎的团粒结构，导致泥炭土中未形成明显的结构性。通过与典型软粘土循环变形特性的对比，发现在相同应力状态下，初始一定循环周次内泥炭土累积变形相较软粘土要小，然而由于泥炭土具有较大的累积变形速率，随着加载的继续，泥炭土累积变形逐渐大于软土。此外，对比分析了不同循环次数与试验条件下泥炭土的电镜扫描照片，指出荷载应力水平越高，振动次数越多，振动频率越低，泥炭土受压越明显，微观结构的变化程度越大，表现为土体内部孔隙越少，单元体排列越紧凑密实。最后，将泥炭土累积变形等效为静力蠕变变形，结合临界状态理论，建立了长期循环荷载下泥炭土累积变形计算模型，采用回退应力积分算法对模型进行了程序化，利用室内试验数据和具体的边界值问题，对模型进行了验证。这些研究为泥炭土地基上已建工程的工后沉降合理预测、在建和拟建工程地基处理与设计提供了理论依据与技术支持。