Characterization of the Migration of Soil Particles in Lateritic Soils under the Effect of Rainfall

The initiation mechanism of debris flow is regarded as the key step in understanding the debris-flow processes of occurrence, development and damage. Moreover, migration, accumulation and blocking effects of fine particles in soil will lead to soil failure and then develop into debris flow. Based on this hypothesis and considering the three factors of slope gradient, rainfall duration and rainfall intensity, 16 flume experiments were designed using the method of orthogonal design and completed in a laboratory. Particle composition changes in slope toe, volumetric water content, fine particle movement characteristics and soil failure mechanism were analyzed and understood as follows: the soil has complex, random and unstable structures, which causes remarkable pore characteristics of poor connectivity, non-uniformity and easy variation. The major factors that influence fine particle migration are rainfall intensity and slope. Rainfall intensity dominates particle movement, whereby high intensity rainfall induces a large number of mass movement and sharp fluctuation, causing more fine particles to accumulate at the steep slope toe. The slope toe plays an important role in water collection and fine particle accumulation. Both fine particle migration and coarse particle movement appears similar fluctuation. Fine particle migration is interrupted in unconnected pores, causing pore blockage and fine particle accumulation, which then leads to the formation of a weak layer and further soil failure or collapses. Fine particle movement also causes debris flow formation in two ways: movement on the soil surface and migration inside the soil. The results verify the hypothesis that the function of fine particle migration in soil failure process is conducive for further understanding the formation mechanism of soil failure and debris flow initiation.

In slopes where a mixture of coarse and fine particles is present, the infiltration of rainfall can cause the migration of fine particles. This migration alters the hydraulic properties of the soil and has implications for slope stability. In this study, the slope under investigation is a tailings dam composed of loosely consolidated soil with a wide particle size distribution. Due to rainfall infiltration, fine particles tend to migrate within the voids of the coarse particle framework, leading to changes in hydraulic properties and inducing slope instability. The classical internal erosion constitutive model, known as the Cividini and Gioda erosion criterion, is commonly used to predict the behavior and effects of fine particle erosion in geotechnical engineering. However, certain parameters in this erosion criterion equation, such as long-term density, are challenging to obtain through experiments. To investigate the coupled evolution of seepage and erosion within landfill slopes under the influence of rainfall infiltration and to understand the mechanisms of slope instability, this research assumes the erosion of fine particle suspension and adopts the Worman and Olafsdottir erosion criterion to establish a coupled model of unsaturated seepage and internal erosion. The developed model simulates the coupled response of seepage and erosion in unsaturated landfill slopes under three different rainfall intensities. It is then combined with the infinite slope model to quantitatively analyze the impact of fine particle migration on soil permeability and slope stability. The numerical simulations provide the following findings: The Worman and Olafsdottir erosion criterion, unlike the Cividini and Gioda erosion criterion, only requires the determination of the soil’s gradation curve to estimate the erosion rate. Internal erosion primarily occurs within the leading edge of moisture penetration, accelerating the advancement of the wetting front and reducing slope stability. When the rainfall intensity is lower than the saturated permeability coefficient, the influence of internal erosion can be disregarded. However, under rainfall intensities equal to or greater than the saturated permeability coefficient, considering internal erosion results in a difference in the depth of the wetting front of up to 34.2 cm after 6 h in the R2 scenario. The safety factor without considering internal erosion is 1.12, whereas considering internal erosion yields safety factors between 1.08 and 1.09. In the R3 scenario, the difference in the depth of the wetting front reaches 53.8 cm after 6 h, with a safety factor of 1.12 without considering internal erosion and safety factors between 1.06 and 1.07 when considering internal erosion.

Based on the theory of rainfall infiltration, the surface infiltration model of multilevel filled slope was established by using the SEEP/W module of GeoStudio. The changes of the volumetric water content (VWC) and pore water pressure (PWP) in the surface of the slope during the rainfall infiltration were analyzed, and the influence of the change of the rainfall conditions on the VWC and PWP was considered. The analysis showed that VWC and PWP increased when the rain fell, and the growth rate of the higher feature point was higher. The affected area was concentrated on the upper part of the surface about 0.75 m. With the increasing of rainfall intensity, the slope surface getting to transient saturation state was quick, and the time of the PWP increasing to 0 among the feature points of same elevation was shortened. Meanwhile, the PWP presented a positive value, and as the infiltration depth increased, the transient saturation region expanded. The safety coefficient of the multistage filled slope was continuously reduced; after the stop of rainfall, the VWC and the PWP decreased, and the decline rate of the higher feature points was higher. In addition, the PWP of the lower part increased, and the safety factor of the slope presented a trend of rebound.

The development of fractures in loess slopes often has a great influence on the stability of slopes. In this paper, taking the homogeneous slope model as an example, fractures with different distances and depths from the slope top angle are provided at the top of the slope. Firstly, the influence of single fracture location and depth on slope stability is analyzed, and after the most unfavorable location and depth of the fractures are determined, the influence of the slope top fractures on the slope seepage field and slope stability under the conditions with and without rainfall is studied secondly. The results show that with the increase of the distance from the fracture to the top angle of the slope, the safety coefficient of the slope shows the change law of decreasing first and then increasing. The critical fracture depth corresponding to the fractures at different locations is different, and when the critical fracture depth is exceeded, the slope safety factor tends to be stable. At the end of the rainfall moment through 1-1 section to analyze, it is found that at this time the pore water pressure and volume water content of the soil at the bottom of the fracture at different depths are greater than those of the upper soil, and the shallow fracture has less influence on the seepage field of the slope, and through 2-2 section it is found that the pore water pressure and volume water content in the soil far from the fracture are greater than those of the soil near the location of the fracture. When 288 h after the end of rainfall, the pore water pressure and volumetric water content of the soil around the fracture in section 2-2 were greater than those far from the fracture location. With the passage of time, the pore water pressure at the location of the fracture gradually dissipated and the corresponding volumetric water content gradually decreased, indicating that the fracture had an obvious influence on the characteristics of the seepage field in the soil at different times of rainfall. Under the condition of no rainfall, there exists a critical depth of fractures affecting slope safety stability, while under the condition of rainfall, the slope safety coefficient continuously decreases with the increase of fracture depth, and there is no critical fracture depth. The research results promote the understanding of the safety and stability of loess fracture slopes and provide a reference for slope management.

Investigating the mud pumping and interlayer creation phenomena in railway sub-structure

Centrifugal modeling test on failure characteristics of soil-rock mixture slope under rainfall

To study the interlayer creation and the mud-pumping phenomena in the conventional French railway substructure, a physical model was developed with a 160-mm-thick ballast layer overlaying a 220-mm-thick artificial silt layer (mixture of crushed sand and kaolin), both layers being compacted in a transparent cylinder of a 550-mm-inner diameter. One positive pore water pressure sensor, three tensiometers, and three time domain reflectometer (TDR) sensors were installed around the ballast/silt interface allowing the evolution of pore water pressure (negative or positive) and volumetric water content to be monitored, respectively. A digital camera was installed allowing direct monitoring of different movements (ballast, ballast/subsoil interface, etc.). The effects of loading (monotonic and cyclic loadings) and degree of saturation of subsoil (w = 16 %, Sr = 55 %, and near-saturation state) were investigated. It was found that the development of pore water pressure in the subsoil is the key factor causing the migration of fine particles, hence, resulting in the creation of interlayer, as well as mud pumping. In particular, the camera exposed the pumping up level of fine particles during the test, showing that the migration of fine particles was not only a result of the interpenetration of ballast particles and subsoil, but also of the pore water pressure that pushed the fine particle upward. The quality of the recorded data showed that the physical model developed worked well and the test protocol adopted for studying the mechanisms of intermixing of ballast and subsoil and mud pumping was appropriate.

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.

The evolution of seepage field in large loess-mudstone landslides is significantly influenced by rainfall infiltration, which is a critical factor affecting their stability. This paper presented a case study of the Hongya Village landslide in Huzhu County, Qinghai Province, to better understand the impact of rainfall infiltration on the triggering mechanism of loess-mudstone landslides. A combination of field investigations, high-density electrical resistivity surveys, and numerical simulations was employed to systematically analyze the temporal and spatial distribution of pore water pressure, volumetric water content, and shear strain within the landslide mass under rainfall infiltration conditions. The results indicated that the rainfall infiltration markedly alters the seepage characteristics of the landslide mass, leading to a sharp increase in pore water pressure and a significant reduction in shear strength, which consequently reduced the overall stability of the landslide. The stability analysis identified a stepwise instability process in the landslide, characterized by “shear displacement-tensile failure-pulling effect” triggered by rainfall infiltration. The findings provide a comprehensive understanding of the instability mechanisms of loess-mudstone landslides under rainfall conditions, thereby offering valuable scientific guidance and technical support for the prevention and mitigation of similar landslide hazards.

Soil–water characteristic curve (SWCC) is one of the input components required for conducting the transient seepage analysis in unsaturated soil for estimating pore water pressure (PWP). SWCC is usually defined by saturated volumetric water content (θs), residual water content (RWC) and air entry value (AEV). Mathematical model of PWP could be useful to unearth the important SWCC components and the physics behind it. Based on authors’ knowledge, rarely any mathematical models describing the relationship between PWP and SWCC components are found. In the present work, an evolutionary approach, namely, multi-gene genetic programming (MGGP) has been applied to formulate the relationship between the PWP profile along soil depth and input variables for SWCC (θs, RWC and AEV) for a given duration of ponding. The PWP predicted using the MGGP model has been compared with those generated using finite element simulations. The results indicate that the MGGP model is able to extrapolate the PWP satisfactory along the soil depth for a given set of boundary conditions. Based on the given AEV and saturated water content, the PWP along the depth can be determined from the newly developed MGGP model, which will be useful for design and analysis of slopes and landfill covers.

Assessment of the size selectivity of eroded sediment in a partially saturated sandy loam soil using scouring experiments

In this study, a one‐dimensional seepage test apparatus was used to investigate the effect of clay on the critical hydraulic gradient, hydraulic conductivity, migration of fine particles in soil, and percentage of fine particle loss during the internal erosion of clay‐sand‐gravel mixture, compared with clean gravel. The critical hydraulic gradient and fine sand loss percentage of the clay‐sand‐gravel mixture decreased, and critical flow velocity and the hydraulic conductivity increased. Six clay‐sand‐gravel mixture samples with different clay contents were used to evaluate the effect of different clay contents on internal erosion. As the percentage of clay mass to fine particle mass increases from 0% to 25%, the critical hydraulic gradient of soil samples decreases by nearly half and the fine sand loss percentage decreases from 13.73% to 3.48%. Overall, clay has a significant effect on the development of internal erosion of clay‐sand‐gravel mixture. And attention should be paid in engineering project; clay‐sand‐gravel mixture with a small amount of clay is more likely to be damaged than clean gravel.

According to the available geological data and monitoring data, the completely weathered phyllite slope dilates and softens under the condition of continuous rainfall, which is then prone to instability failure. The indoor artificial rainfall test was carried out through the construction of the slope model, and the soil moisture sensor, pore water pressure sensor and matric suction sensor were used to study the variation laws of moisture content, pore water pressure and infiltration line at the back edge, slope body and the foot of the slope under continuous heavy rainfall. According to the sensor data and recorded information, with the influence of heavy rainfall over a long period of time, the water content and pore water pressure increased firstly, then decreased, and finally stabilized. The infiltration line moved from the top, the surface and the foot of the slope to the slope body, and shallow slip failure occurred in the shallow layer of the slope body.

Slope failure is a one of major process that causes severe landform variation and environment variation, and slope failure has become a major hidden danger to human settlement and urban construction in this vast loess region. The physical model of slope failure as induced by artificial rainfall was constructed in the field, and monitored the pore water pressure (PWP), earth stress (ES), volumetric water content (VWC), electrical conductivity (EC), and temperature (T) of the soil using this physical simulation. The surface morphology of slope started to occur in the slope as a result of erosion caused by rainfall and rainwater infiltration at the beginning of the experiment; concurrently, PWP, ES, VWC, and EC were increased gradually. Meanwhile, the saturated weight of the soil rose. In the middle of the experiment, PWP, ES, VWC, and EC were increased rapidly as the artificial rainfall continued, and the ratio of soil pore the soil fell. The slope landform was obviously occurred during the experiment, when it was noted that PWP, ES, VWC, and EC of the soil rapidly decreased. Afterwards, slope failure evolved into a debris flow; eventually, the landform was entirely changed in the slope. The soil became more compact toward the end of the experiment, and PWP, ES, VWC, and EC were slowly increased; these factors indicated that the loess slope was temporarily stable. This study could potentially be used to provide the relevant parameters for numerical simulations of landform variation in loess regions, and provide reference for regional land use planning and environmental development.

Internal erosion and soil piping are caused by the migration of fine soil particles through a skeleton of coarse grains or by progressive mass movement of soil grains under the tractive forces produced by concentrated subsurface flow, which may lead to the failure of dams and levees. The ability of the Vision Cone Penetrometer (VisCPT) to record internal erosion and soil piping in the laboratory and in-situ has previously been reported. The present paper focuses on developing a video-processing algorithm to quantify the soil particle motions. An optical flow technique is adopted to compute particle migration velocity fields. This makes it feasible to detect, track and quantify the movement of soil particles by the VisCPT. The research defines the eroded area percentage (EAP) and the sum of the absolute velocities of mobilized fine soil particles (SAV) which may eventually be used to quantify soil erosion and piping susceptibility.