非稳定流条件下深厚覆盖层内管涌流固耦合机理

Hydro-mechanical mechanism of suffusion under unsteady seepage in deep alluvium foundation is a critical scientific problem affecting dam safety. Suffusion, which is significantly different from backward erosion piping, occurs in foundation, and it is more concealed and harmful than backward erosion piping. It is significantly important to study the evolution mechanism of suffusion and predict the harmfulness of suffusion. First, a list of suffusion triaxial experiments under unsteady seepage was carried out to reveal the hydro-mechanical coupling effect of suffusion, and then a new constitutive relation was established, which describes the initiation conditions of suffusion, suffusion failure, and the variation of porosity in the evolution of suffusion. Second, the effective stress transfer chain in alluvium soil was investigated based on granular media theory, and then the internal relationship between the effective stress transfer chain and the initiation of suffusion was revealed from the mesoscopic point. Third, a list of flume-scale suffusion hydro-mechanical experiments was performed to research the influence of the worst geological conditions (such as open-framework gravel and the double-layer strata structure containing a coarse and fine particle layer) on the evolution of suffusion. The migration path of fine particles and the failure patterns of suffusion were also revealed. Finally, a new hydro-mechanical coupling mathematical model for suffusion was developed based on the continuum theory of solid and liquid mixtures. The new model can consider the influences of the stratum structure and overlying load, and predict the spatial and temporal progression of suffusion during reservoir filling in deep overburden foundation. The results will provide scientific evaluation and guide for the future design and safety operation of the deep overburden foundation projects.

非稳定流条件下深厚覆盖层内管涌流固耦合机理是影响大坝安全的关键科学问题。与管涌不同，内管涌的发生发展完全位于地基内部，隐蔽性强，危害性大。探讨其发生机理，对其危害性进行预报预警，具有重要科学意义。本项目首先开展一系列非稳定流条件下内管涌三轴试验，揭示内管涌流固耦合机制，建立本构关系，量化覆盖层内管涌发生条件、发展过程中孔隙率演变规律及破坏条件。其次，基于散体介质理论，研究覆盖层土体应力传递路径网络及其与内管涌发生的内在联系，从细观角度解释内管涌机理。再次，开展一系列内管涌水槽模型试验，探讨架空层、粗细颗粒层相间地层结构等最不利地质条件对内管涌的影响，明确内管涌破坏形式及细颗粒运移规律。最后，基于固液混合物连续介质理论，建立内管涌数学模型，该模型能够考虑覆盖层上覆荷载及复杂地质条件的影响，能够实时预报大坝蓄水等非稳定流条件下内管涌的时空发展过程。研究成果将为覆盖层工程设计和运行提供科学指导。

深厚覆盖层潜蚀流固耦合机理是影响大坝安全的关键科学问题。与管涌不同，潜蚀的发生发展完全位于地基内部，隐蔽性强，危害性大。探讨其发生机理，对其危害性进行预报预警，具有重要科学意义。项目开展了同一围压，不同偏压条件下的渗流-侵蚀-应力耦合潜蚀试验，探讨了剪应力比对潜蚀发生临界坡降的影响规律。研究表明，应力状态对潜蚀的发生条件具有显著影响，且存在一个临界剪应力比。当小于临界剪应力比时，潜蚀发生临界坡降随着剪应力比增大而线性增大；当等于临界剪应力比时，潜蚀发生临界坡降达到最大，此后，骤然减小；当大于临界剪应力比时，潜蚀发生临界坡降缓慢线性增大。基于上述特征，建立了潜蚀发生临界坡降与土体所处应力状态之间的本构关系。基于散体介质理论，建立了DEM-CFD耦合的潜蚀细观数值模型，探讨了内部不稳定土体中粗细料应力分担比例、细颗粒流失引发的应力传递路径演变、细颗粒流失可能诱发的土体沉降等细观机制。开展了一系列潜蚀水槽模型试验，探讨了单层、架空层、“上层反滤层+下层内部不稳定土体”的双层地基等不利地质条件对潜蚀发生发展过程的影响，明确了不同地质条件下潜蚀破坏形式及细颗粒运移规律，建立了地层结构对潜蚀发生和破坏条件的经验关系。单层地基中，潜蚀首先发生在防渗墙端部下游侧，然后逐渐向上游侧发展；架空层能显著降低潜蚀发生和破坏条件，潜蚀首先发生在架空层上游侧，然后逐渐向上游侧发展，潜蚀过程中大量细颗粒能够进入架空层；双层地基中的潜蚀发生机理完全不同于单层地基，潜蚀首先发生在防渗墙的端部或者端部的上游侧，然后逐渐向下游侧扩展。反滤层对潜蚀发生条件和破坏条件均有显著影响，潜蚀发生和破坏条件随着层间系数的增大而呈指数关系降低。项目研究成果将加深对潜蚀宏细观机理的认识，丰富渗透破坏相关理论框架体系，并为深厚覆盖层工程渗控方案设计和保障工程长效安全运行提供重要理论依据。