Numerical Simulation of Rainfall-Induced Erosion on Infiltration and Slope Stability

Rainfall-induced slope failures in natural terrains are destructive natural disasters. Transport of fine particles may be induced by the rainwater seepage in a natural terrain slope comprising mixed coarse and fine particles. In this study, the interaction of internal erosion and infiltration in a soil slope is investigated. A coupled model of unsaturated flow and internal erosion is established. The effects of internal erosion on pore water pressure profiles and slope stability are studied. Parametric studies on erosion parameters and hydraulic parameters are conducted. The results of the numerical example show that internal erosion occurs mainly in the zone within the wetting front, which accelerates the advance of the wetting front and decreases the slope stability. The coefficient of erosion flux rate, βer of the erosion law, is the main factor that affects the internal erosion. The effect of erosion on the wetting front movement is more significant with large values of βer. The effects of parameters i* and αer are less significant when compared with βer. When the rainfall flux is equal or greater than the saturated coefficient of permeability, the influence of internal erosion on water infiltration and slope stability is significant. The effect of internal erosion can be neglected as long as the rainfall flux is less than the saturated coefficient of permeability. When the air-entry value of the soil is greater, the influence of internal erosion on infiltration and slope stability becomes less significant.

Rainfall infiltration is the main factor that causes slope instability. To study the effect of hydraulic parameters on the final saturation line and stability of slopes, a numerical slope model is established with a saturated–unsaturated seepage analysis method. Analysis results show the following, (1) When parameter a increases, the effective rainfall duration decreases linearly, and the ultimate safety factor increases gradually; when parameter m increases, the effective rainfall duration increases linearly, and the ultimate safety factor decreases linearly; when parameter n increases, both the effective rainfall duration and the ultimate safety factor decrease first and then remain stable. (2) When the saturated permeability coefficient decreases, the effective rainfall duration presents a crescent trend, and the ultimate safety factor decreases first and then remains the same after rainfall intensity exceeds the saturated permeability coefficient of soil. (3) When rainfall intensity is less than the saturated permeability coefficient of soil, the location of the final saturation line rises as the saturated permeability coefficient decreases and is thus independent of parameters a, m, and n.

Rainfall infiltration is an important factor that leads to slope instability. In this study, a numerical model for slope is built to investigate the effects of rainfall intensity and its pattern on slope stability. Results show that rainfall intensity and its pattern significantly influence slope stability due the following instances: (1) the safety factor of slope initially decreases and then stabilizes during rainfall, which is defined as the effective rainfall days; moreover, when the rainfall intensity is less than the saturated permeability coefficient of soil, the effective rainfall days decreases with the increase of rainfall intensity. (2) Under the same rainfall intensity, the ultimate safety factor of sand slope when the effective rainfall days is reached is greater than that of the clay slope. (3) When the rainfall intensity is less than the saturated permeability coefficient of soil with the same total precipitation, a longer rainfall duration indicates a larger slope safety factor; conversely, when the rainfall intensity is greater than the saturated permeability coefficient of soil, a longer rainfall duration indicates a smaller slope safety factor.

Based on the saturated-unsaturated seepage theory and shearing strength theory, the coupling analysis of the seepage field and the stress field was carried out by the numerical simulation using the two-phase flow configuration in FLAC. The safety factor of the slope was calculated by the shear strength reduction method. The influence of the duration and intensity of the rainfall and soil's saturated permeability coefficient to the slope stability was investigated. The results show that as the duration of the rainfall increases, the safety factor of the slope decreases; while the intensity of the rainfall keeps smaller than the saturated permeability coefficient, there is a positive correlation between the saturated permeability coefficient and the safety factor.

Rainfall is the main cause of erosion damage in loose slope deposits. During rainfall infiltration, fine particles in the soil mass will move with water infiltration, thus changing the localized particle distribution of the soil mass, which, in turn, causes changes in the pore water pressure and volumetric water content within the slope and ultimately affects slope stability. In order to develop advanced soil and water conservation programs to prevent slope damage, it is crucial to understand and accurately reproduce the particle migration and aggregation characteristics of soils under different rainfall conditions. Therefore, this paper systematically investigates the soil particle migration characteristics of the soil body under rainfall conditions by simulating the internal erosion of the lateritic soil slope body under rainfall conditions via slope internal erosion simulation experiments and experimentally analyzing the migration and aggregation of fine particles in the slope body, as well as the changed rules regarding pore water pressure and volumetric water content at different locations of the slope body with rainfall. The results of this study show that (1) with the infiltration of rainfall, the fine particles in the slope body mainly infiltrate in the vertical direction in an early stage of rainfall; in a later stage, there is vertical downward and down-slope seepage. Therefore, fine particles always gather at the toe of the slope, which leads to relatively high water content and pore water pressure at the toe of the slope, and thus, the slope is always damaged from the toe of the slope. (2) Inside the slope, the fine particles always gather at the smallest pore diameter. With the enhancement of hydrodynamic force, they will be lost again, which leads to a sudden decrease in the local volumetric water content of the slope, and the pore space increases. Then, it is filled with seepage water, which makes the pore water pressure fluctuate or increase. (3) Based on the particle distribution parameter, the present study produced a distribution map of the fine particle content of the slope body under different rainfall intensities and established a model of the dynamic change of fine particles, which improves the understanding of the effect of the change in the fine particle composition of the slope body on the water content and the pore water pressure and may be helpful for the assessment of the initiation of the mudslides.

Physics-based time-of-failure determination of rainfall-induced instability in lateritic soil slopes

Internal erosion is a complex phenomenon that is one of the main risk factors to soil destruction. Its occurrence is mainly due to water infiltration and can cause slope instability. “Karst soil” is a type of loess with special soil and water sensitivity that makes it prone to landsliding. The processes of internal erosion include transport erosion and chemical dissolution, which strongly effect loess structure and strength. To reveal the internal processes and effects of the loess due to water infiltration, field investigations and indoor tests, including infiltration tests, undrained triaxial tests, particle analysis, chemical analysis, and scanning electron microscopy (SEM), were conducted. The results show that (1) the fine particles (clay and silt) and chemicals can move within the matrix of the macro-pores under seepage flow. The physical internal erosion is mainly due to fine particle migration out of the water and clay and silt particles, and the sample column settlement was 3.3 cm with a settlement ratio of 16.5%, which results in changes to the soil skeleton, increasing the porosity and infiltration rate of loess. (2) Chemical dissolution is also an important internal erosion process in loess, especially cations of Na, Mg, Ca, and K and anions of Cl, SO4, and CO3, which are mainly lost due to dissolution and flow out of with water and clay particles, resulting in altered physical characteristics of the soil. (3) Soil particles’ mitigation and chemical dissolution change the loess structure, leading to skeletal destruction and decreased peak strength and residual strength of the infiltrated sample to 7.75% and 8.13%, respectively. During internal erosion, physical fine particle migration and chemical dissolution are important for loess stability and loess slope susceptible to failure during water infiltration.

Reservoir water level change will change the seepage field dramatically. Saturated permeability coefficient is one of the most fundamental and important calculation parameters of slope seepage and stability analysis. This paper aims at establishing the relationship between Saturated permeability coefficient and factor of safety of bedding slope under the condition of reservoir water level fluctuation. Based on saturated-unsaturated seepage theory, seepage fields of one bedding rock slope with different Saturated permeability coefficient under the condition of water level fluctuation were calculated by finite element method, the factor of safety of the slope under the condition of water level fluctuation were calculated by Morgenstern-Price method, and then the relationship between the factor of safety and saturated permeability coefficient of rock and soil mass of the bedding slope under the fluctuation of the reservoir water level, which provides a references to stability analysis of bedding slopes in Three Gorges Reservoir Area.

Cyclic hydraulic loading frequently affects embankment dams during reservoir regulation, tidal fluctuations, and intense rainfall. It potentially worsens fine particle migration during internal erosion and increases dam failure risks. This study is the first to systematically explore the influence of cyclic hydraulic loading on the critical hydraulic gradient (icr) of gap-graded soils, providing new insights into suffusion behavior. Transparent soil experiments, which enable direct observation of soil structural evolution, are combined with coupled DEM–Darcy simulations that offer microscopic mechanical insights, marking the first integrated use of these two approaches to investigate suffusion behavior. To quantify fine particle migration, we propose a novel modified grayscale threshold segmentation (MGTS) method for analyzing cross-sectional images captured during transparent soil experiments. The results from both methods show consistency in fine particle migration, clogging formation, and failure, with differences in permeability and icr remaining within acceptable limits. Fine particle content significantly influences the post-cyclic icr of internally unstable soils. For soils with lower fine particle content (15%), icr increases after cyclic hydraulic loading and rises with the mean hydraulic gradient during cycling. Conversely, soils with higher fine particle content (20%) exhibit a decrease in post-cyclic icr. This behavior is explained by changes in the average contact force between fine particles (Fff) observed in DEM simulations.

Internal erosion of debris-flow deposits triggered by seepage

Rainfall infiltration exerts great effect on the stability of soil slopes. Based on the actual rainfall data, saturated-unsaturated seepage theory and the mechanical theory of unsaturated porous media, numerical simulations were conducted to examine the seepage field of saturated-unsaturated slope during rainfall infiltration in this paper. The influence of the coefficient with rainfall duration, rainfall patterns, rainfall intensity and soil saturated permeability on seepage field are studied, and the relationship between the factor of safety and the position of sliding surface is obtained. The simulated results show that the saturated hydraulic conductivity has a great effect on slope stability. If the permeability of soil is relatively large, the changing range and the velocity of the factor of safety increase with the rainfall intensity; and if it is relatively in a low degree, the influence of the rainfall intensity on the slope stability is also low

Loose wide-grading soils are commonly found in the source areas of debris flows, and in landslides after an earthquake. During rainfall events, fine particles (fines) in the soils gradually migrate downward, and eventually the loss of fines results in an increase in the pore volume of the soil and a reduction in the stability of the soil skeleton, which can lead to subsequent slope failure. To gain more understanding of the fine migration process at the microscopic scale, a 3D discrete element-fluid flow sequentially coupled model is developed, based on Darcy’s Law, to simulate fluid flow through a porous medium and calculate the transportation of soil solids. The erosion model is verified using experimental data. Parametric studies are carried out to investigate the effects of coarse particle size. The results reveal that changes in pore structure caused by fine particle migration can change the local permeability of the material. For the case of the average pore throat diameter to fine particle ratio ( $$J$$ ) of 2.41, changes in local porosity with time from internal erosion in the sample can be divided into four stages: (1) a rapid increase with some variations in porosity, (2) a slow increase in porosity, (3) a rapid increase in porosity, and (4) a steady state with no change in porosity. Not all stages are present for all value of $$J$$ . Stages (1) (2) (4) are present for 2.48 ≤ $$J\le$$ 2.58 and stages (1) (4) are present for $$J$$ ≤ 2.24 and $$J\hspace{0.17em}$$ ≥ 2.74. A sharp increase in the fine’s erosion possibility occurs for a $$J$$ value lies between 2.58 and 2.74. The erosion possibility sensibility shows an exponential relationship with $$J$$ . The model provides an effective and efficient way to investigate the process of pore blockage and internal soil erosion.

Urban groundwater infiltration resulting from tropical storm rainfall can lead to internal soil erosion and ground subsidence. The long-term effects of these phenomena may result in uneven settlement of the subway system, posing risks to its safety and operational integrity. Suffusion poses a significant hazard to long-term subway operation. It is a type of soil internal erosion characterized by the migration of fine particles from the pore channels between coarse particles. This study focused on the performance degradation of different gap-graded sands caused by internal erosion, observed through triaxial compression tests. Using the cap plasticity model and considering mechanical parameters before and after soil erosion, a refined two-dimensional finite element model of a soil subway station structure based on ABAQUS was developed to analyze the deformation and damage pattern of the subway structure resulting from soil internal erosion and earthquake. The results of this study demonstrated that localized internal soil erosion significantly contributes to uneven settlement in subway stations, potentially compromising their seismic performance during oblique seismic excitations.

Rainfall infiltration is a key factor affecting the stability of the slope. To study the impact of rainfall on the instability mechanism and stability of slopes, this paper employs numerical simulation to establish a rainfall infiltration slope model and conducts a saturated–unsaturated slope flow and solid coupling numerical analysis. By combining the strength reduction method with the calculation of slope stability under rainfall infiltration, the safety factor of the slope is obtained. A comprehensive analysis is conducted from the perspectives of the seepage field, displacement field and other factors to examine the impact of heavy rainfall patterns and rainfall intensities on the instability mechanism and stability of the slope. The results indicate that heavy rainfall causes the transient saturation zone within the landslide body to continuously move upward, forming a continuous sliding surface inside the slope, which may lead to instability and sliding of the soil in the upper part of the slope toe. The heavy rainfall patterns significantly affect the temporal and spatial evolution of pore water pressure, displacement and safety factors of the slope. Pore water pressure and displacement show a positive correlation with the rainfall intensity at various times during heavy rainfall events. The pre-peak rainfall pattern causes the largest decrease in the safety factor of the slope, and the slope failure occurs earlier, which is the most detrimental to the stability of the slope. The rainfall intensity is inversely proportional to the safety factor. As the rainfall intensity increases, the decrease in the slope’s safety factor becomes more significant, and the time required for slope instability is also shortened. The results of this study provide a scientific basis for analyzing rainfall-induced slope instability and failure.

This study presents a procedure for calculating the change of the safety factor for unsaturated slopes of homogenous, residual soils suffering from rainfall infiltration within Khanh Vinh district, Khanh Hòa province. Rainfall is supposed as a main trigger caused failure of the potential sliding slopes. Rainwater into the slope due to infiltration caused an increase in moisture content and negative pore water pressure; a decrease in matric suction and in shear strength on the failure surface. Thus, slopes are reduced stability and can be failed. Soil permeability and rainfall intensity were found to be the primary factors controlling the instability of slopes due to rainfall, while the initial water table location and slope geometry only played a secondary role. A numerical model of analysis coupled seepage-stability used to simulate the seepage and slope stability under conditions of specific environment such as soil permeability, rainfall intensity, water table location and slope geometry in the study area. The relationships between safety factor and rainfall intensity, soil permeability, angle slope, high slope were identified to provide a good indication for the management of landslide hazards under the effects of rainfall.