随着我国西北黄土地区重载铁路建设的快速发展，列车传递给以黄土填筑路基的动载呈现重载化趋势，这将对长期服役条件下重载铁路路基的稳定性产生威胁。与此同时区域性极端降雨频发，环境因素所引起的增湿作用加剧了黄土路基服役性能劣化，需要对黄土进行改良，而粗颗粒掺量对改良黄土的性质影响明显，黄土路基的粗级配调控问题亟需解决。因此，从粗颗粒调控后形成的改良黄土路基填料物理力学特性为出发点，探究粗颗粒含量、压实度和含水率等因素对其强度的影响规律，并得到动载作用下改良黄土路基填料动变形的变化规律，据此构建改良黄土路基填料的动变形计算模型；考虑湿化程度对路基动力响应的影响，借助模型试验和数值模拟，阐明增减湿作用下路基内部水分迁移规律，明确增减湿作用下路基动力响应特征，进一步探讨增湿作用下其内部结构演化机制，并预测其累积沉降发展趋势，得到黄土最佳粗级配调控方式。主要研究内容和成果如下：

（1）开展改良黄土物理力学特性试验，明确了粗颗粒含量、压实度和含水率对改良黄土力学特性的影响规律，并在细观层面揭示了粗级配调控方法增强土体力学性能的内在机制，探讨了改良方法降低土体水分敏感性的作用效果。研究表明：①当改良黄土粗颗粒占比R_g低于20%时，细颗粒主控阶段土体受含水率增大导致的静强度劣化更显著；②当R_g为20%至60%时，土体逐渐转变为粗颗粒骨架主控模式，抑制了含水率增加对改良黄土静力性能的不利影响；③改良黄土渗透系数随着R_g的增大呈指数增长，表明改良方法能够降低水分在路基的滞留效应。

（2）开展动三轴试验，得到了动载作用下粗颗粒含量、压实度、含水率和加载频率对改良黄土累积塑性应变、回弹模量与阻尼比的影响规律，构建了改良黄土路基填料多参数动变形计算模型。研究表明：①当R_g低于20%时，含水率对动变形的影响更显著，压实度和加载频率对动变形影响程度相对较低；②当R_g为20%至60%时，粗颗粒含量对动变形的影响占主导地位；③根据土体物性参数对改良黄土动变形影响程度的差异性，构建黄土改良前、后关于各自影响因素的临界动应力模型、累积塑性应变模型和动回弹模量模型，揭示了动载作用下改良黄土路基的动变形规律。

（3）开展路基缩尺物理模型试验，得到了增减湿作用下改良黄土路基水分迁移规律，以及不同湿化程度下路基的动力响应特征。研究表明，当R_g为40%时，不同填筑土体间的渗透系数差异抑制了水分竖向迁移，增湿作用下基床表层与底层、底层与路堤交界面存在水分聚集现象。路基增湿后，随深度增加，土体的土压力和竖向位移逐渐降低；排水减湿后，随着湿化程度降低，路基土压力与竖向位移随深度增加而明显衰减。重现了粗颗粒含量不足导致的动力失稳演化过程，当R_g为10%时，交界面阻水能力减弱，渗透路径扩展至路基深部，增加了路基整体失稳风险。

（4）建立路基离散元计算模型，揭示了增湿前、后动载作用下改良黄土路基填土颗粒和孔隙结构的内部结构演化机制。研究表明，路基未增湿时，动载作用易引起路基内部土颗粒松动，从而引起路基强度劣化；增湿作用易引起路基底部浸水软化，底部支撑力不足将导致粗颗粒自重下移。当R_g为40%时，动载作用下路基应力传递路径完整，减少了路基局部应力集中；增湿作用下部分细颗粒流失并未改变路基整体应力分布形态；即使路基处于高湿化程度，动载作用下粗颗粒骨架形式限制了细颗粒失稳重组，路基整体的抗侵蚀能力得到增强。

（5）在重载铁路提速增运背景下，建立改良黄土路基三维数值计算模型，开展了重载铁路改良黄土路基的长期服役动力稳定性研究。研究表明，路基增湿后，随着路基深度的增加，动应力衰减速率降低，轴重越大，这一现象越明显；相比于列车轴重对路基竖向位移的影响，提速主要影响动应力的频率和幅值，列车提速所引起的瞬态动载对路基竖向位移影响较小。路基长期服役条件下，当R_g为40%时，随着振动次数的增加，路基的累积变形快速增长并趋于稳定；增大轴重和湿化程度提高引起路基出现了变形累积效应，但随着振动次数增加，两种因素引起的沉降量呈收敛趋势。

With the rapid development of heavy haul railway construction in the loess area of Northwest China, the dynamic load transmitted by trains to the loess filled subgrade shows a trend of heavy haul, which will threaten the stability of heavy haul railway subgrade under long-term service conditions. At the same time, regional extreme rainfall occurs frequently, and the humidification effect caused by environmental factors aggravates the deterioration of the service performance of the loess subgrade. It is necessary to improve the loess, and the content of coarse particles has obvious influence on the properties of the improved loess. The problem of coarse gradation regulation of loess subgrade needs to be solved urgently. Therefore, starting from the physical and mechanical properties of the improved loess subgrade filler formed after the regulation of coarse particles, the influence of factors such as coarse particle content, compaction degree and moisture content on its strength is explored, and the dynamic deformation of the improved loess subgrade filler under dynamic load is obtained. The change law is used to construct the dynamic deformation calculation model of the improved loess subgrade filler ; considering the influence of wetting degree on the dynamic response of subgrade, with the help of model test and numerical simulation, the law of moisture migration in subgrade under the action of humidification and dehumidification is clarified, and the dynamic response characteristics of subgrade under the action of humidification and dehumidification are clarified. The evolution mechanism of its internal structure under humidification is further discussed, and the development trend of its cumulative settlement is predicted, and the optimal coarse gradation control mode of loess is obtained. The main research contents and results are as follows:

(1) The physical and mechanical properties of the improved loess were tested, and the influence of coarse particle content, compaction degree and water content on the mechanical properties of the improved loess was clarified. At the meso level, the internal mechanism of the coarse gradation control method to enhance the mechanical properties of the soil was revealed, and the effect of the improved method on reducing the water sensitivity of the soil was discussed. Research shows that: ① when the proportion of coarse particles of improved loess R_g is less than 20%, the static strength degradation of the soil in the fine particle-dominated stage is more significant due to the increase in water content; ② when R_g is between 20% and 60%, the coarse particle skeleton effect is greater, the fine particles only play a filling role, and the soil gradually changes to the coarse particle skeleton dominant mode, which inhibits the adverse effect of the increase in water content on the static properties of the coarse-graded loess; and ③ the permeability coefficient of coarse-graded loess increases exponentially with increasing R_g, indicating that the improved method can reduce the retention effect of water in the subgrade.

(2) The dynamic triaxial test was carried out to obtain the influence of coarse particle content, compaction degree, water content and loading frequency on the cumulative plastic strain, resilient modulus and damping ratio of improved loess under dynamic load, and the multi-parameter dynamic deformation calculation model of improved loess subgrade filler was constructed. The research shows that: ① When R_g is lower than 20%, the effect of the moisture content on dynamic deformation is more significant, and the influence of the compaction degree and loading frequency on dynamic deformation is relatively low; ② when R_g is between 20% and 60%, the influence of the coarse particle content on dynamic deformation is dominant; and ③ according to the difference of the influence degree of soil physical parameters on the dynamic deformation of improved loess, the critical dynamic stress model, cumulative plastic strain model and dynamic rebound modulus model of loess before and after improvement are constructed, and the dynamic deformation law of improved loess subgrade under dynamic load is revealed.

(3) The scale physical model test of subgrade was carried out, and the water migration law of improved loess subgrade under the action of humidification and dehumidification and the dynamic response characteristics of subgrade under different humidification degrees were obtained. The study revealed that when R_g is 40%, the difference in the permeability coefficients among the different filling soils inhibits the longitudinal migration of water, and there is a significant phenomenon of water accumulation at the interface between the surface and bottom layers and between the bottom layer and the subgrade under the humidification effect. After the subgrade is humidified, the soil pressure and vertical displacement of the soil gradually decrease with the increase of depth. After dewatering and dehumidification, with the decrease of wetting degree, the soil pressure and vertical displacement of subgrade decrease obviously with the increase of depth. The dynamic instability evolution process caused by insufficient coarse particle content was reproduced, and it was found that when R_g is 10%, the water blocking capacity of the interface is weakened, and the penetration path extends to the deep part of the subgrade, which increases the overall instability risk of the subgrade.

(4) The discrete element calculation model of subgrade is established to reveal the internal structure evolution mechanism of improved loess subgrade filling particles and pore structure under the action of dynamic load before and after humidification. The research shows that when the roadbed is not humidified, the dynamic load is easy to cause the loosening of the soil particles inside the roadbed, thus causing the deterioration of the roadbed strength. Humidification is easy to cause water softening at the bottom of the subgrade, and insufficient bottom support force will lead to the downward movement of coarse particles. When R_g is 40%, the stress transfer path of subgrade under dynamic load is complete, which reduces the local stress concentration of subgrade. The loss of some fine particles under the action of humidification does not change the overall stress distribution of the subgrade. Even if the subgrade is in a high degree of humidification, the coarse particle skeleton form under dynamic load limits the instability and reorganization of fine particles, and the overall erosion resistance of the subgrade is enhanced.

(5) Under the background of speeding up and increasing operation of heavy haul railway, a three-dimensional numerical calculation model of improved loess subgrade is established, and the long-term service dynamic stability of improved loess subgrade of heavy haul railway is studied. The research shows that after the subgrade is humidified, the dynamic stress attenuation rate decreases with the increase of subgrade depth, and the larger the axle load, the more obvious this phenomenon. Compared with the influence of train axle load on the vertical displacement of subgrade, the speed increase mainly affects the frequency and amplitude of dynamic stress, and the transient dynamic load caused by train speed increase has little effect on the vertical displacement of subgrade. Under the condition of long-term service of the subgrade, when R_g is 40%, the cumulative deformation of subgrade increases rapidly and tends to be stable with the increase of vibration times. The increase of axle load and wetting degree caused the deformation accumulation effect of subgrade, but with the increase of vibration times, the settlement caused by the two factors showed a convergence trend.