随着地铁建设迅猛发展，地铁运行引起的软土地基长期不均匀沉降问题，尤其是沿海地区砂黏土混合体沉降变形问题日益突出，而解决沉降变形的核心关键在于明晰砂黏土混合体在地铁运行作用下的动力响应特征。因此，基于室内试验明确砂黏土混合体的静动力学特性，揭示含砂量和含水率对砂黏土混合体静动力学特性的影响机理，以均一化理论及流动法则为出发点，较为系统地总结了砂黏土混合体地层的各向异性及循环加载特性，建立了考虑含砂量和含水率的砂黏土混合体动态本构模型，结合室内模型试验验证本构模型的正确性，最后利用数值模拟进行实际工程应用，为进一步探究控制及减少土体长期沉降提供了可靠的理论依据。论文以苏州地铁隧道建设为依托工程，采用试验、理论分析和数值分析等方法进行研究，研究成果和结论如下：

    （1）通过对砂黏土混合体物理特性宏微观试验，阐明了静载作用下砂黏土混合体的工程特性及含砂量、含水率对砂黏土混合体工程特性的影响规律。研究表明：砂黏土混合体的压缩和蠕变性能随着含水率的增加而增加，随着含砂量的增加而减小；随着含砂量的增加，混合体的微观结构由细粒控制逐渐向粗粒控制转变；存在一个特征粗颗粒质量分数f_m，当含砂量大于f_m时，砂颗粒起骨架作用，砂黏土混合体的强度以及抵抗变形特性增强；采用孔隙比及孔隙指数双指标将砂黏土混合体的压缩和蠕变特性归一化。

    （2）基于砂黏土混合体开展静动三轴试验，阐明了含水率和含砂量对砂黏土混合体的静动力特性的影响规律，揭示了含砂量及含水率对砂黏土混合体力学参数的影响机制。研究表明：当含砂量小于f_m时，随着含砂量的增加强度增长不明显；而当含砂量大于f_m时，试样的结构强度增长迅速。内摩擦角随着含砂量增大而增大，黏聚力随着含砂量的增大而减小。含水率对砂黏土混合体的内摩擦角影响规律不明显，对砂黏土的黏聚力影响较大。含水率、含砂量、振动频率和循环加载均对砂黏土混合体孔压和应变发展、应力-应变滞回曲线演化、阻尼比、模量软化及应变累积特性有着不同程度的影响，解释其原因是含水率对其黏聚力影响较大，导致其对力学特性产生影响，而砂含量在大于粗颗粒质量分数f_m砂骨架逐渐形成，从而对混合体的力学参数产生较大影响。

    （3）基于砂黏土混合体的结构特征，考虑黏土的双屈服面循环模式，借助发展的循环本构关系，构建了考虑非饱和和循环加载影响的砂黏土混合体本构模型。研究表明：砂黏土混合体本构关系主要受到所含两相成分本身的应力-应变关系影响，采用均一化方法可将两相成分中相应的材料特性联系起来。采用扩展的混合体均一化模型，引入结构变量，确定屈服面F和塑性势面G，从而确定屈服方向矢量和塑性流动方向矢量，基于流动法则建立了砂黏土混合体的柔性矩阵张量。通过对数值模拟软件进行二次开发，将所建立的砂黏土混合体本构模型嵌入，建立数值计算模型，结合室内试验证该模型的合理性。

    （4）构建了砂黏土混合土层考虑循环动载作用下的物理模型试验系统，探究了砂黏土混合土层隧道动力响应规律，并结合数值模拟二次开发验证了所提出本构模型对于工程应用的可行性。研究表明：循环荷载对隧道围岩土压力影响范围为隧道中心15m范围左右。受循环荷载作用，隧道围岩不同位置处测点加速度沿隧道中心水平方向及竖直方向均呈一定程度衰减。当加载幅值一定，随着加载频率的增加，水平方向各测点加速度幅值沿距振源距离衰减，振动边界具有明显的放大作用。隧道围岩测点加速度随加载次数增加整体呈抛物线增长，同时距离隧道中心振源越近，加载次数对加速度影响更大，收敛更为缓慢。荷载频率的减小会导致隧道沉降位移增加。

    （5）基于前面所验证的本构模型，结合数值模拟软件阐明了运营地铁在多次循环次数下围岩土体的塑性变形趋势，预测了该线某区间在未来运营期内的沉降值。研究表明：隧道底部土体塑性应变总体上随列车荷载的循环次数的增加而不断增大，随着深度的增加，土体的竖向累积沉降量逐渐减小，影响范围为隧道4倍直径。随着地铁运营周期的增长，土体的沉降幅度及范围逐渐减小，影响范围为隧道轴线两侧各3倍隧道直径距离内。累积塑性变形引起的沉降在开始阶段发展迅速，到第5年后沉降增长缓慢，趋于稳定。随着振次的增加孔压消散引起的沉降在开始阶段迅速增加，半年后基本达到稳定，并在列车荷载作用100万次后，总沉降趋于稳定。

    With the rapid development of subway construction, the long-term uneven settlement of soft soil foundation caused by subway operation, especially the settlement deformation of sand-clay mixture in coastal areas, is increasingly prominent, and the key to solve the settlement deformation is to clarify the dynamic response characteristics of sand-clay mixture under the action of subway operation. Therefore, based on laboratory tests, the static and dynamic characteristics of sand-clay mixture are clarified, and the influence mechanism of sand content and water content on the static and dynamic characteristics of sand-clay mixture is revealed. Based on homogenization theory and flow law, the anisotropy and cyclic loading characteristics of sand-clay mixture stratum are systematically summarized, and a dynamic constitutive model of sand-clay mixture considering sand content and water content is established. The correctness of the constitutive model is verified by laboratory model tests. Finally, numerical simulation is used for practical engineering application, which provides a reliable basis for further exploring and controlling long-term settlement of soil. Based on the construction of Suzhou subway tunnel, this paper adopts the methods of experiment, theoretical analysis and numerical analysis, and the research results and conclusions are as follows:

    (1) The engineering characteristics of sand-clay mixture under static load and the influence law of sand content and water content on the engineering characteristics of sand-clay mixture are expounded through macro-and micro-experiments on the physical characteristics of sand-clay mixture. The results show that the compression and creep properties of sand-clay mixture increase with the increase of water content, decrease with the increase of sand content. With the increase of sand content, the microstructure of the mixture gradually changes from fine particle control to coarse particle control; There is a characteristic coarse particle mass fraction f_m. When the sand content is more than f_m, the sand particles play a skeleton role, and the strength and deformation resistance of sand-clay mixture are enhanced. The compression and creep characteristics of sand-clay mixture are normalized by porosity ratio and porosity index.

    (2) Based on the static and dynamic triaxial test of sand-clay mixture, the influence law of water content and sand content on the static and dynamic characteristics of sand-clay mixture is expounded, and the influence mechanism of sand content and water content on the mechanical parameters of sand-clay mixture is revealed. The results show that when the sand content is less than f_m, the strength does not increase obviously with the increase of sand content; However, when the sand content is greater than f_m, the structural strength of soil samples increases rapidly. The internal friction angle increases with the increase of sand content, and the cohesion decreases with the increase of sand content. The influence of water content on the internal friction angle of sand-clay mixture is not obvious, but has a great influence on the cohesion of sand-clay mixture. Water content, sand content, vibration frequency and cyclic loading all have different effects on the development of pore pressure and strain, the evolution of stress-strain hysteretic curve, damping ratio, modulus softening and strain accumulation characteristics of sand-clay mixture. The reason is that the water content has a great influence on its cohesion, which leads to its influence on mechanical properties, while the sand skeleton gradually forms when the sand content is greater than the mass fraction of coarse particles, which has a great influence on the mechanical parameters of the mixture.

    (3) Based on the structural characteristics of sand-clay mixture, considering the double yield surface cyclic mode of clay, and with the help of the developed cyclic constitutive relation, a constitutive model of sand-clay mixture considering the effects of unsaturated and cyclic loading is established. The results show that the constitutive relation of sand-clay mixture is mainly influenced by the stress-strain relationship of the two-phase components, and the corresponding material characteristics of the two-phase components can be related by homogenization method. The extended homogenization model of mixture is adopted, and structural variables  are introduced to determine the yield surface F and plastic potential surface G, so as to determine the yield direction vector n_kl and plastic flow direction vector m_ij. Based on the flow law, the flexible matrix tensor of sand-clay mixture is established. Through the secondary development of numerical simulation software, the established constitutive model of sand-clay mixture is embedded, and a numerical calculation model is established. The rationality of the model is proved by laboratory tests.

    (4) The physical model test system of sand-clay mixed soil layer under cyclic dynamic load is constructed, and the dynamic response law of sand-clay mixed soil layer tunnel is explored, and the feasibility of the proposed constitutive model for engineering application is verified by the secondary development of numerical simulation. The research shows that the influence range of cyclic load on the rock and soil pressure around the tunnel is about 15m in the center of the tunnel. Under cyclic load, the acceleration of measuring points at different positions of tunnel surrounding rock decreases to some extent along the horizontal and vertical directions of tunnel center. When the loading amplitude is constant, with the increase of loading frequency, the acceleration amplitude of each measuring point in the horizontal direction decreases along the distance from the vibration source, and the vibration boundary has obvious amplification effect. The acceleration of the measuring point of the surrounding rock of the tunnel increases parabolically with the increase of the loading times, and the closer it is to the vibration source of the tunnel center, the greater the influence of the loading times on the acceleration and the slower the convergence. The decrease of load frequency will lead to the increase of tunnel settlement displacement.

    (5) Based on the previously verified constitutive model, combined with numerical simulation software, the plastic deformation trend of surrounding rock and soil under multiple cycles of operating subway is expounded, and the settlement value of a section of the line in the future operation period is predicted. The research shows that the plastic strain of the soil at the bottom of the tunnel generally increases with the increase of the number of cycles of train load, and the vertical cumulative settlement of the soil gradually decreases with the increase of depth, and the influence range is 4 times the diameter of the tunnel. With the increase of subway operation cycle, the settlement amplitude and range of soil gradually decrease, and the influence range is within the distance of 3 times the tunnel diameter on both sides of the tunnel axis. The settlement caused by cumulative plastic deformation developed rapidly in the initial stage, and it grew slowly and tended to be stable after the fifth year. With the increase of vibration times, the settlement caused by the dissipation of pore pressure increases rapidly at the beginning, and basically reaches stability after half a year, and the total settlement tends to stability after the train load acts for 1 million times.