Characterization model for internal erosion evolution of accumulations based on coupled seepage-erosion-stress effects

Soil-rock accumulations are widely distributed in hilly and mountainous regions across China. As loose geomaterials with widely graded particles and mixed soil-rock components, they are highly prone to internal erosion induced by rainfall infiltration and subsurface seepage. During internal erosion, fine soil particles detach and migrate through intergranular pores, which simultaneously deteriorates the bearing capacity of the soil skeleton and alters the internal permeability of the accumulations. This coupled deterioration is the dominant triggering mechanism for rainfall-induced landslides, posing major threats to geotechnical facilities and geological disaster prevention. Therefore, developing an accurate method to predict the evolution and magnitude of fine particle erosion is of great theoretical significance and practical value for ensuring the safe operation of geotechnical projects and mitigating landslide risks.

Traditional internal erosion prediction models fail to consider the influence of complex in-situ stress states, limiting their application in practical engineering. To address this research gap, this study first regarded saturated soil-rock accumulations as a five-phase mixed medium and established a set of coupled governing equations for seepage, erosion, and stress fields based on mass conservation, momentum balance, and the effective stress principle. The finite element numerical model was compiled and solved on the COMSOL Multiphysics platform, and Voronoi diagrams were adopted to reconstruct geometric models of accumulations with different rock contents. A series of triaxial erosion-shear tests under three deviatoric stress conditions with a constant confining pressure of 50 kPa were carried out to verify the reliability and calculation accuracy of the proposed numerical method. On the basis of massive numerical simulation results, this study took volumetric strain, rock content, average seepage velocity, and erosion time as four input parameters, and adopted the least squares method for regression fitting. A novel evolution characterization model for internal erosion was further established, realizing the full quantitative characterization of internal erosion under complex stress conditions. Additionally, the intrinsic mechanisms of how rock content and volumetric strain affect internal erosion behaviors were systematically explored.

The combined results of physical tests and numerical simulations demonstrated that the proposed characterization model could accurately predict the evolution of eroded fine particles in soil-rock accumulations with different stress levels and rock contents, provided that no erosion-induced instability failure occurs in the soil skeleton. The increase of rock content could extend the seepage path of pore water and reduce the average seepage velocity inside the medium, thereby effectively restraining the development of internal erosion. In contrast, when the accumulation was subjected to deviatoric stress, shear dilatancy occurred, leading to a continuous rise in volumetric strain. The enlarged volumetric strain further increased porosity and permeability, which was the essential internal factor aggravating the degree of internal erosion.

This newly developed internal erosion evolution model can quantitatively describe the dynamic evolution of permeability in soil-rock accumulations under the combined action of seepage, internal erosion, and complex stress fields. The major innovations of this study lie in two aspects. First, it introduces volumetric strain to quantitatively characterize the stress effect on internal erosion, compensating for the inherent limitations of traditional models that neglect stress influence. Second, it integrates multiple key parameters to establish a multi-factor coupling model, which greatly expands the applicability of classical erosion equations. This study not only enriches the theoretical system of coupled seepage-erosion-stress effects for wide-graded soils, but also provides reliable theoretical support and technical references for stability evaluation of accumulation slopes and foundations, as well as the prevention and control of rainfall-triggered landslides in mountainous areas.