<p>&nbsp; &nbsp; &nbsp;&ldquo;海绵城市&rdquo;将是城市功能发展的必然方向，相应的渗水路面和给排水管网建设规模不断扩大；与此同时城市化进程的推进、城市交通量的日益增长，使得黄土地区城市路基面临着动载渗透及力学性能劣化的灾变风险。针对黄土工程性质及力学效应研究已取得了长足发展，但随着城市道路功能的改变，动载渗透效应下黄土路基工程病害问题趋于严重。本文以城市黄土路基为研究对象，从路基动载渗透、渗透促劣角度入手，通过现场调研、室内试验、理论分析和数值模拟等手段，研究城市黄土路基动载渗透效应和渗透作用下黄土动力特性劣化效应，建立黄土动载渗透控制方程组和考虑渗透作用的土体动力本构方程，提出基于动载渗透效应的黄土多场耦合模型，实现车辆动载与管道渗漏作用下城市黄土路基变形破坏数值模拟，揭示考虑动载渗透效应的城市黄土路基变形破坏机理。主要研究内容和成果如下：</p> <p>&nbsp; &nbsp; &nbsp;（1）基于自主研制的动载渗透试验装置和微细观试验手段，研究了车辆动载对路基黄土渗透特性的影响规律，揭示了城市路基黄土动载促渗效应：①车辆动载引起的振动作用具有加快水分渗透，推进湿润锋运移，加速水气驱替的促渗效应，土体饱和渗透系数为无振动下的3～14倍，且在土体自振频率附近（约20～25Hz）增幅最大，土体体积含水率提升时间提前，增长进程加快，基质吸力衰减提前，气体逃逸加剧；②车辆动载具有激发渗水超静孔压和加剧渗透侵蚀的促渗效应，土体孔隙水压力增长时间提前，水分渗透速率和侵蚀速率提升显著；③动应力和振动联合作用于渗水和土体结构，产生渗透驱动力提升，土粒黏结键错断，封闭气泡打开，孔隙迂曲度降低，孔隙（微裂隙）贯通，结合水弱结合化等土水-固液气三相多重效应，造成土中水快速渗透的促渗效应。</p> <p>&nbsp; &nbsp; &nbsp;（2）通过考虑渗透作用的路基黄土动力特性多尺度试验研究，揭示了渗透压力具有弱化土体动力学参数，加剧土体累积塑性变形的动力特性劣化效应：①骨干曲线硬化幅度降低，渗透压力增至30kPa时，曲线由应变强硬化型转化为弱硬化型，动剪切模量显著降低、阻尼比增加、动强度衰减；②累积塑性应变曲线由稳定型向发展型过渡，土体累积变形为无渗透压力下的1.5～3.5倍，且渗透压力越大、动应力幅值越高、加载频率越低，累积塑性变形越大；③动应力和渗透压力联合作用于土骨架，带来土体孔隙粗化、胶结溶解、裂隙滋生的结构损伤效应，造成土体动黏聚力骤减，动强度损失，动力特性衰减的劣化效应。</p> <p>&nbsp; &nbsp; &nbsp;（3）基于孔隙率的概念，考虑动载作用产生的物理损伤，推导了黄土动载损伤孔隙率和渗透性方程，结合颗粒侵蚀运移和土体水力特性方程，提出了黄土动载渗透控制方程组；基于土的弹塑性理论、损伤理论和边界面理论，考虑渗流损伤及循环加载效应，提出了考虑渗透作用的黄土动力本构模型；通过孔隙率和渗透系数演化架起了黄土动载渗透控制方程组和土体动力本构模型之间的桥梁，建立了基于动载渗透效应的黄土多场耦合模型，通过对物理模型试验开展数值模拟，验证了多场耦合模型的合理性。</p> <p>&nbsp; &nbsp; &nbsp;（4）以西安市咸宁东路路基塌陷为例，应用黄土动载渗透多场耦合模型，考虑车辆动载和管道渗漏作用，数值分析了路基多物理力学场的演化特征，结合物理模型试验结果，阐明了该路基变形破坏的&ldquo;三阶段、五步骤&rdquo;演化过程：动载渗透损伤累积阶段（渗水饱和软化、振动促渗致裂）、动载渗透侵蚀流失阶段（振散侵蚀贯通、振溃流失成洞）和动渗促劣剪冲致陷阶段；确定了该路基塌陷的原因：复杂的地层结构，特殊的土层岩性以及多变的多物理场环境是导致塌陷破坏的先决条件；车辆动载和管道渗漏产生的动载促渗效应是发生塌陷破坏的直接诱因；渗透作用下，土体动力特性的劣化效应是形成塌陷破坏的主要原因。</p> <p>&nbsp; &nbsp; &nbsp; &nbsp;研究成果将有助于揭示动载作用下黄土体渗流演化与动力响应机制，深化对城市黄土路基管道渗漏诱发塌陷等灾害发生机理的认识，对创新城市黄土路基灾变机制理论，提升城市防灾减灾能力具有重要意义。</p>

<p>&ldquo;Sponge city&rdquo; is the essential development direction of urban functions, which brings a continuous expansion of corresponding water seepage pavement, water supply pipeline network and underground utility tunnel construction scale. Meanwhile, urban pavements in loess areas are facing with dynamic infiltration and structural deterioration-induced disaster risks due to the progresses in urbanization and daily increase of urban traffic volume. Researches on loess engineering properties and mechanical effects have achieved significant progress. With functional changes of urban roads, loess pavement diseases under dynamic infiltration effect are intensifying gradually. From the perspective of dynamic load infiltration and infiltration promoting deterioration of subgrade, this paper took urban loess subgrade as the research object, field investigation, laboratory test, theoretical analysis and numerical simulation were conducted and the pro-infiltration effect of urban loess subgrade under dynamic load as well as the degradation effect of loess dynamic characteristics under infiltration were explored. Then, the dynamic infiltration control equations of loess and the dynamic constitutive equation of soil considering infiltration were established, and a multi-field coupling model of loess based on dynamic infiltration effect was proposed. While realizing the numerical simulation of deformation and failure of urban loess subgrade under vehicle dynamic load and pipeline leakage, this study revealed the deformation and failure mechanism of urban loess subgrade considering dynamic infiltration effect. The main research content and achievements were as follows:</p> <p>&nbsp; &nbsp; &nbsp;(1) Based on the self-developed dynamic load permeability test device and microscopic test means, this paper studied the influence of vehicle dynamic load on loess permeability characteristics of subgrade, and demonstrated the effect of dynamic load on promoting permeability of urban loess subgrade: 1) The vibration caused by vehicle dynamic load could accelerate water infiltration, advance wet front migration and speed up water vapor displacement. The saturated permeability coefficient of soil was 3 - 14 times of that without vibration, and it was most significant near the natural vibration frequency of soil (approximately 20-25 Hz). The increase time of soil volume moisture content was advanced, the growth process was accelerated, the matrix suction attenuation was accelerated, and gas escape was intensified. 2) Vehicle dynamic load could stimulate excess pore pressure of seepage and intensify infiltration erosion. The growth time of pore water pressure in soil was advanced, and the water infiltration rate and soil erosion rate were significantly improved. 3) Dynamic stress and vibration acted on seepage and soil structure, which results in multiple effects of soil-water-solid-liquid-gas, such as increase of seepage driving force, broken bond of soil particles, opening of closed bubbles, decrease of pore tortuosity, penetration of pores (micro-fissures), and weak combination of bound water, thereby leading to rapid infiltration of water in soil.</p> <p>&nbsp; &nbsp; &nbsp;(2) Multi-scale test was performed to investigate the dynamic characteristics of subgrade loess considering seepage. It is revealed that osmotic pressure has the effect of weakening the dynamic parameters of soil and aggravating the dynamic characteristics deterioration of cumulative plastic deformation of soil: 1) The hardening degree of soil skeleton curve was weakened. When the seepage pressure increased to 30kPa, the curve changed from strain hardening type to weak hardening type. Dynamic shear modulus decreased significantly, damping ratio increased and dynamic strength decreased. 2) The cumulative plastic strain curve of soil changed from stable to developing state, and the cumulative deformation of soil was 1.5 - 3.5 times of that without infiltration pressure. Moreover, the greater the infiltration pressure, the higher the dynamic stress amplitude and the lower the loading frequency, the greater the cumulative plastic deformation. 3) Dynamic stress and infiltration pressure jointly acted on soil skeleton, bringing the structural damage effect of soil pore coarsening, cementation dissolution and crack breeding. This could trigger the deterioration of dynamic cohesion, loss of dynamic strength, and attenuation of dynamic property of soil.</p> <p>&nbsp; &nbsp; &nbsp;(3) Based on the concept of porosity of geotechnical medium, in view of the physical damage caused by dynamic load, the porosity and permeability models of loess dynamic load damage were derived. Combining with the particle erosion migration equation and the soil hydraulic characteristic equation, this paper proposed the dynamic infiltration control equations of loess. Based on the elastic-plastic theory, damage theory and boundary surface theory of soil, a dynamic constitutive model of loess considering seepage damage and cyclic loading effect was put forward. Through the evolution of porosity and permeability coefficient, the bridge between the dynamic load permeability control equations of loess and the dynamic constitutive model of soil was established. Furthermore, a multi-field coupling model of loess based on dynamic infiltration seepage effect was set up. Numerical simulation of the physical model test was carried out to verify the rationality of the multi-field coupling model.</p> <p>&nbsp; &nbsp; &nbsp;(4) Based on the subgrade collapse of Xianning East Road in Xi&rsquo;an, the multi-field coupling model of loess dynamic infiltration was embedded into the numerical simulation software, and the evolution of multi-physical and mechanical fields of subgrade was numerically simulated and analyzed considering the dynamic load of vehicles and pipeline leakage. Combining with the physical model test results, this study expounded the &ldquo;three-stage, five-step&rdquo; evolution process of subgrade deformation and failure, namely the cumulative stage of dynamic infiltration damage (saturated softening of seepage, vibration promoting infiltration cracking), the stage of dynamic load infiltration erosion loss (vibration scattering-erosion into penetration, vibration-collapse loss into holes) and the stage of dynamic-infiltration promotes deterioration-shear-impact depression. In addition, complex stratum structure, special soil lithology and changeable multi-physical field environment were prerequisites for the collapse and failure. The dynamic pro-infiltration effect caused by vehicle dynamic load and pipeline leakage was the direct cause of collapse damage. Under the action of infiltration, the deterioration effect of soil dynamic characteristics was the main reason for the collapse failure.</p> <p>&nbsp; &nbsp; &nbsp;The research result is conductive to revealing the seepage evolution and dynamic response mechanism of loess under dynamic load, and deepening the understanding of mechanism of disasters such as pipeline leakage-induced collapse of urban loess subgrade. It is also of great significance for innovating the theory of disaster mechanism of urban loess subgrade and improving the urban disaster prevention and reduction ability.</p>