In the process of using transportation infrastructure, contact erosion between different particle sizes soil layers can easily occur under complex hydro-mechanical coupling, leading to deformation and damage of structures. To investigate indirect erosion between soil layers under cyclical load effects from a microscopic perspective, a volume of fluid-discrete element method (VOF-DEM) coupled method was adopted in this study. The influence of different water table levels and particle size ratios (PSR) was considered. The study found that: (1) The compressive effect of coarse particles during loading and the stress relaxation effect during unloading can both cause migration of fine particles within one loading-unloading cycle; (2) Immersion of the contact surface between coarse and fine particles is a key factor in inducing particle migration, with the interaction between particles being the most intense at the contact surface; (3) Fully saturated soil experiences the most severe particle erosion and macroscopic deformation; (4) Reducing PSR can effectively improve the integrity of soil structure and suppress erosion of fine particles; (5) Particle migration inevitably leads to axial deformation of the soil, resulting in reduced stiffness and increased energy dissipation during loading-unloading cycles. This study provides new insights into contact erosion under complex hydraulic coupling from a microscopic perspective.

The underestimated risk of contact erosion failure in railway substructures poses a significant threat to railway safety, particularly at the interface between the ballast/subballast and subgrade. The larger constriction size at this interface exacerbates the potential for long-term erosion, necessitating attention to safeguard railway integrity. This study introduces a novel laboratory erosion testing apparatus to evaluate contact erosion at the subballast-subgrade interface under cyclic loading. Subgrade soils with varying fines contents are tested, and the effect of pressure head on erosion is investigated in detail. The results indicate that sandy soil with higher internal stability exhibits a higher critical pressure head for contact erosion. Cyclic loading induces oscillations in pore water pressure within the subballast layer, with higher pressure heads leading to larger amplitudes. Excess pore water pressure is generated in the sandy soil layer during cyclic loading and gradually dissipates over time. Fine eroded particles migrate into the subballast layer, forming mud, while coarse eroded particles accumulate at the base, creating low-permeability interlayers. Notably, the geometric conditions alone may not guarantee effective prevention of contact erosion in railway substructures. The hydraulic conditions for contact erosion are more easily achieved under cyclic loading compared to static loading. These distinctive features of contact erosion in railway substructures, different from those observed in hydraulic structures, provide some insights for the development of remediation strategies and improvements in railway substructure design.

Transportation infrastructure, being exposed to natural environments for a prolonged period, is susceptible to contact erosion between different particle size soil layers due to complex water-force interactions such as cyclical loading and water infiltration. The significant loss of particles leads to uneven deformation and decreased stability of the soil mass, and in severe cases, it can even result in the overall collapse of structures. To reveal the mechanism of contact erosion, a coupled solid-liquid-gas contact erosion model based on the VOF-DEM method was established. The study investigates the particle migration process, macroscopic deformation response, and evolution of contact forces under different particle size ratios (PSR) and seepage path influences. The following conclusions were drawn: (1) Significant particle migration between coarse and fine particles occurs only after being subjected to the effects of seepage. The particle erosion rate reaches its maximum after the soil mass becomes saturated. Cyclic loading intensifies the severity of particle erosion under seepage conditions. (2) Particle erosion mainly occurs at the contact surface, and the squeezing action of coarse particles and stress relaxation during unloading contribute to particle migration and loss. (3) Particle migration induces significant axial deformation in the soil mass and increases energy dissipation during loading and unloading processes. (4) Reducing the PSR effectively suppresses particle loss in the contact erosion process. (5) Seepage perpendicular to the contact surface results in more severe particle loss and soil deformation.

Levees and the beneath foundation ground sometimes cause consolidation settlements. When the consolidation estimation is underestimated in advance, the structures and residents are damaged by consolidation. In addition, levees are subjected to piping and boiling with heavy rainfalls, which often have coarse permeable layers. The seepage is concentrated through coarse permeable layers. Owing to the repetition of regular rainfall and the changes in water levels, internal erosion occurs between a permeable and a finer soil layer. This study aims to reveal the effect of internal erosion on consolidation. Clay, including granular soil, is used to simulate both internal erosion and consolidation. First, long-term seepage is imposed on a soil specimen with a coarse layer. Then, clayey soil samples are obtained from the specimen subjected to seepage, and a consolidation test is conducted. The soil properties related to consolidation are revealed in several experimental cases. A numerical analysis is conducted using the experimentally obtained parameters to simulate the consolidation for embankment construction. Finally, the influence of internal erosion on consolidation is discussed. The soil consolidation behavior is similar to that of the clayey material when the specimen is loosely compacted. However, the behavior of the dense material appears to be sandy; consolidation is rapidly completed. The loosening caused by internal erosion is dissipated, and the consolidation properties approach those of the soil sample with a similar initial compaction degree. Finally, a practical application method for the internal erosion effect is proposed.

Bei dem Beitrag handelt es sich um die erweiterte Fassung der gleichnamigen Keynote Lecture auf der 4. Bodenmechanik Tagung im Rahmen der Fachsektionstage Geotechnik, die auf Anregung der Fachsektionsleitung auch in der Zeitschrift geotechnik veroffentlicht werden soll. Bei den Phanomenen der inneren Erosion in durchstromtem Boden und in Erdbauwerken geht es um das Losen, den Transport und die Ablagerung bevorzugter Fraktionen mit der Folge einer anderung der Bodeneigenschaften. Die Phanomene der inneren Erosion werden als Kontakterosion, Suffosion, Kolmation und ruckschreitende Erosion charakterisiert. Die Kinematik dieser physikalischen Prozesse ergibt sich mit der Energie einer Sickerstromung aus der Bewegung des Einzelkorns im Porenraum, den moglichen Freiheitsgraden beim Transport. Der Artikel gibt einen uberblick uber die Art und Bedingungen der verschiedenen Phanomene sowie uber deren spezifische Kinematik innerhalb der Bodenstruktur. Die relevanten international verwendeten Nachweismethoden und Kriterien werden aufgefuhrt und in ihrer Aussagekraft bewertet. Die kennzeichnenden Einflussparameter werden aufgezeigt. Fur die einzelnen Phanomene der inneren Erosion werden Strategien zur Bewertung und Beherrschung des Erosionsrisikos diskutiert. Phenomena, kinematics and risk assessment strategies of internal erosionInternal soil erosion due to seeping water in natural sediments as well as in earthworks can lead to a significant change in soil properties and could even destroy the structural integrity. The physical process of erosion always is induced by loosening, migration, and deposition of predominant fine particles within the soil structure. Depending on the kinematics, the phenomena are divided into contact erosion, suffusion, colmation and backward erosion piping. The kinematics is controlled by the energy of a seepage on the one hand, on the other by the degrees of freedom and the boundary conditions of an individual grain movement within the pore space. This article provides an overview of the characteristic, specifics, and conditions of the different phenomena considering their kinematics within the soil structure. Internationally used approaches and methods of assessment are listed, their significance and their limitations will be evaluated. The impact parameters that control the different processes are shown. Strategies for assessing and controlling the risk of structural damage are discussed for the different phenomena of internal soil erosion.