Model Test and Numerical Analysis of Landslides in Layered Ion-Type Rare Earth Ore Under Rainfall and Mineral Leaching Conditions

Abstract

The South China region is characterized by diverse landforms and significant stratification of geological materials. The rock and soil layers in this area have obvious layering characteristics. The stability of layered slopes is a critical issue in the safe mining of southern ion-type rare earth ores. This study investigates the morphological changes, pore water pressure, and moisture content variation of layered ion-type rare earth ore slopes under the combined effects of rainfall and liquid infiltration through indoor model tests. A numerical simulation was conducted to analyze the variations in pore water pressure, moisture content, slope displacement, and safety factor under different working conditions. As rainfall intensity increases, the interface between soil layers in sandy–silty clay slopes is more likely to form a saturated water retention zone, causing rapid pore water pressure buildup and a significant reduction in shear strength. For the silty–sand clay slopes, the low permeability of the upper silty clay layer limits the infiltration rate of water, resulting in significant interlayer water retention effects, which induce softening and an increased instability risk. The higher the initial moisture content, the longer the infiltration time, which reduces the matrix suction of the soil and significantly weakens the shear strength of the slope. When the initial moisture content and rainfall intensity are the same, the safety factor of the silty–sand clay slope is higher than that of the sandy–silty clay slope. When rainfall intensity increases from 10 mm/h to 30 mm/h, the safety factor of the sandy–silty clay slope decreases from 1.30 to 1.15, indicating that the slope is approaching a critical instability state.

1. Introduction

Ion-type rare earth mineral resources are primarily distributed across the seven southern provinces in China, with Jiangxi Province being a representative example. The terrain of the southern Jiangxi Province of China is predominantly mountainous and hilly, with a clear vertical stratification of the geological formations [1,2]. In the process of in situ leaching of ion-type rare earth ores, the stability of the mine slopes is a critical factor for their safe extraction [3,4]. The infiltration mechanisms and deformation characteristics of layered soils differ significantly from those of homogeneous soils, exerting a direct influence on slope stability [5]. Under the combined effects of in situ leaching and rainfall, water infiltration and the redistribution of pore water pressure lead to significant variations in soil strength [6]. Compared to slopes composed of homogeneous rock and soil materials, the stratified structure adds complexity to the analysis of slope stability. Under environmental forces and human-induced disturbances, slopes with layered characteristics are more susceptible to landslides, collapses, and other geological hazards [7]. Therefore, investigating the variations in moisture content, pore water pressure, and the evolution of the safety factor in layered slopes under rainfall conditions is of great practical significance for assessing the slope stability of ion-type rare earth mines.

Many valuable studies have been conducted by scholars both domestically and internationally on the slope stability of ion-type rare earth mines and the stability of layered geotechnical materials. Tang et al. [8] classified the landslides in ion-type rare earth in situ mining sites into surface-, middle-, and bottom-layer landslides, identifying ion exchange reactions and atmospheric rainfall as the main causes of slope failure during the leaching process. Rao et al. [9] conducted investigations on different types and occurrences of ion-type rare earth mines, finding that the majority of landslides were shallow and small-scale landslides. Deng et al. [10] used similarity theory to build a slope model under liquid infiltration and rainfall conditions, and analyzed slope stability through numerical simulations under the coupling effects of liquid infiltration and rainfall. Li et al. [11] conducted indoor simulation experiments on rainfall infiltration into slopes with dominant flow paths, using a dual permeability model for numerical simulations on the COMSOL Multiphysics^®6.2 Version, with results consistent with the experimental findings. Jia et al. [12] employed monitoring equipment to continuously track the physical parameters of the soil at slope monitoring points in ion-type rare earth mines, providing a more convenient and in-depth understanding of the slope failure mechanisms. Geng et al. [12] used the MatDEM 4.5 Version software to compare the infiltration distribution and instability evolution of ion-type rare earth tailings slopes across different seasons.

Existing studies mainly focus on the effects of soil unit weight, initial moisture content, and rainfall environment on slope stability, but research on the stability and seepage deformation characteristics of layered soil slopes remains inadequate. Dai et al. [13] selected a two-layer slope consisting of clay and silt, and proposed a study on the stability of layered soil slopes under unsaturated conditions. Jiang et al. [14,15,16] proposed a layered solution method for the infiltration rate of soil layers, and found that the slope volumetric water content distribution and stability factor calculated by the modified Green–Ampt model were more consistent with the numerical solution of the Richards equation. Zhou et al. [17] combined random field theory with horizontal integration to derive an analytical solution for the safety factor and failure probability of layered slopes, studying the stability of stratified slopes. Ren et al. [18] conducted saturated drainage experiments on layered soils with different thicknesses and textures, finding that as the layer thickness decreased, the overall water retention capacity of the soil increased, with a critical thickness value existing. Li et al. [19] observed the wetting front, cumulative infiltration, and profile moisture content, revealing that the interlayer position and soil texture significantly affected the infiltration rate of layered soils. Using finite element limit analysis [20,21], SHIAU et al. [22] and Qian et al. [23] analyzed the undrained stability of heterogeneous cohesive soil slopes. Liu et al. [24] discussed the influence mechanism of soil configuration on water movement in soil, suggesting that heterogeneity caused by heterogeneous layers in soil leads to water flow stagnation effects, which in turn affect water movement. TSAI et al. [25] used a layered infinite slope model to examine the importance of soil layer distribution on shallow landslides triggered by rainfall, and the results indicated that the pressure head caused by rainfall infiltration was closely related to the soil layer stratification. Guo et al. [26], studied the morphological destruction of ionic rare earth slumps under different rainfall - mining conditions, and revealed the variation patterns of the water distribution field. Fan et al. [27], based on the Hydrus-1D software, simulated the moisture infiltration process under different conditions of sand layer texture, burial depth, thickness, pressure head, and initial moisture content, finding that the soil profile moisture content was primarily controlled by layer thickness. Chen et al. [28] analyzed the causes of landslides in the Gan Nan region’s ion-type rare earth mining areas, concluding that differences in permeability between soil layers and ion exchange reactions played a key role. Current research mainly analyzes the stability of slopes formed by homogeneous rock and soil bodies, with insufficient systematic research on the stability of layered ion-type slopes. Given the clear vertical distribution of geological layers and the prominent stratification in the Gan Nan region, the stability analysis of layered ion-type slopes can more accurately reflect the slope stability mechanisms.

This study is based on a self-designed simulation experimental apparatus to investigate the slope failure model of ion-type rare earth ores under the combined effects of rainfall and liquid infiltration. The variation in soil moisture content and pore water pressure, as well as the evolution process and instability modes of slope failure, were analyzed. Using COMSOL multiphysics numerical simulation technology, the seepage characteristics, displacement distribution, and slope stability of ion-type rare earth ore slopes under different conditions were examined. The effects of rainfall intensity, initial moisture content, and layering sequence on slope stability were revealed. The results provide a theoretical basis for understanding the instability mechanisms of layered rare earth slopes and for the development of preventive measures, as well as offering scientific support for the optimization design of slope monitoring and early-warning systems in in situ leaching mining sites. This research is of significant value for ensuring mining safety in ore extraction areas.

2. Model Test

2.1. Experimental Materials

The soil used in this experiment was sourced from an ion-type rare earth mine in the Gan Nan region. Based on the stratigraphic information provided by geological exploration data and the indoor basic physical and mechanical property tests conducted on mine soil samples, the basic physical and mechanical parameters of sandy clay and silty clay from the rare earth mining area were obtained, as shown in

Table 1. Basic physical and mechanical parameters of different soil types.

Soil	Young’s Modulus E/ MPa	Poisson Ratio μ	Moisture Content ω/ %	Porosity n	Unit Weight γ/kN/m3	Saturation Permeability Coefficient Ks/ m/s	Cohesion c/ kPa	Internal Friction Angle φ/ °
Silty clay	2.23	0.3	21.23	0.45	15.6	1.25 × 10−5	24.3	18
Sandy clay	6	0.3	12.31	0.50	17.5	4.5 × 10−5	18.2	25

. The particle gradation curves for the two types of soil are presented in Figure 1.

2.2. Experimental Apparatus

Based on the field survey of slopes in ion-type rare earth mining areas, this study designed and fabricated an indoor model test for slope failure of layered ion-type soils under the combined effects of rainfall and in situ leaching. In the establishment of a model test for accumulated soil under rainfall infiltration, the principle of geometric similarity must be adhered to. This means that the physical model of the slope must maintain proportional relationships with the prototype in terms of volume, area, and length, while the slope angle direction should remain consistent. In this study, the scale ratio between the indoor physical model and the numerical simulation model of the slope is 1:50. The experimental apparatus consists of a soil container, rainfall system, monitoring system, and water tank. The slope platform has a length of 15 cm, a height of 50 cm, and a base length of 70 cm. The soil stratification occurs at a height of 30 cm from the left side of the slope, with the upper layer of soil having a thickness of 20 cm. At the slope surface, a liquid injection hole with a diameter of 3 cm and a depth of 10 cm is excavated to simulate the actual leaching process in the mining area. Rainfall was modelled as a constant-intensity spray, overlooking natural fluctuations in drop size, intensity, pauses, and antecedent dry spells that drive preferential flow and variable infiltration. The slope model box is made of self-designed tempered glass panels with good permeability and strong explosion-proof performance, with a thickness of 1.2 cm. The box dimensions are 1.2 m × 0.6 m × 0.6 m (length × width × height), capable of withstanding the pressure from the accumulated soil model and allowing for clear observation of soil changes inside the box. A scale is attached to the surface of the box for convenient soil deposition and sensor placement. The experiment includes pore water pressure sensor measurement points (M1, M2, M3, M4) and moisture content measurement points (P1, P2, P3, P4). A schematic diagram of the landslide model test apparatus is shown in Figure 2, the physical image of the apparatus is shown in Figure 3, and the monitoring point layout of the layered slope is shown in Figure 4.

2.3. Design of Experimental Conditions for Indoor Simulation Tests

The indoor simulation experiment mainly considers two key factors: rainfall intensity and in situ leaching degree, where the leaching degree also represents different stages of liquid injection. Only two idealized layering sequences with uniform properties were tested, whereas real ion-type rare earth slopes feature complex stratigraphy, varied mineralogy, fissures, and macro-voids that influence water retention and strength. The experiment is designed by simulating different initial moisture contents:

(1)

Under different rainfall intensities, the geometric dimensions and initial soil moisture content are kept constant. The study focuses on a layered slope with an upper sandy clay layer and a lower silty clay layer, based on the rainfall intensity historical data from the Gan Nan region and references [29,30]. Three rainfall intensities are set: 30 mm/h for heavy rain, 60 mm/h for torrential rain, and 90 mm/h for extremely heavy rain. The impact of rainfall intensity on slope failure modes and infiltration patterns is explored.

(2)

Under different initial moisture contents, a medium rainfall intensity of 30 mm/h is set with a sandy–silty clay layered slope. The initial moisture content of two types of remolded soils is controlled in groups of 15% and 20%, 20% and 25%, and 25% and 30%. The slope failure modes and rainfall infiltration patterns are studied during the experiment.

(3)

Under different soil layering sequences, with the same geometric dimensions of the slope model and a slope angle of 42°, the moisture content of completely dried silty clay and sandy clay is remolded. The moisture content of the two soils is controlled at 15% and 20%. A rainfall intensity of 30 mm/h is set for heavy rain, and the failure modes and rainfall infiltration patterns of the layered slope are investigated during the experiment. The six experimental groups are represented as T1, T2, T3, T4, T5, and T6, with the corresponding model conditions shown in

Table 2. Indoor slope model test condition setting table.

Test Number	T1	T2	T3	T4	T5	T6
Slope Layering Order	Sandy–silty clay	Sandy–silty clay	Sandy–silty clay	Sandy–silty clay	Sandy–silty clay	Silty–sandy clay
Rainfall intensity (mm/h)	30	60	90	30	30	30
Initial moisture content of sandy clay (%)	15	15	15	20	25	15
Initial moisture content of silty clay (%)	20	20	20	25	30	20
Duration of rainfall (min)	480	480	480	480	480	480

.

3. Analysis of Model Test Results

3.1. Different Rainfall Intensity Conditions

(1)

Variation Law of Slope Morphology

The variation in the layered slope profile and surface morphology under different rainfall intensities is shown in Figure 5. When the rainfall intensity is 30 mm/h, with continuous rainfall, the slope surface undergoes a process of gradual development from an initially stable state to localized damage. After 60 min of rainfall, initial cracks appear on the surface of the slope, and local peak infiltration water is concentrated in the shallow region of the slope. After 240 min of rainfall, local sliding occurs in the middle of the slope, and the damaged area expands. After 480 min of rainfall, a larger area of soil sliding occurs on the slope, and the upper structure of the slope becomes significantly unstable. Under moderate rainfall intensity, the morphological change in the slope mainly involves localized sliding, with the damage gradually progressing to deeper layers. Under a rainfall intensity of 90 mm/h, slope damage is most prominent. After 60 min of rainfall, the surface layer of the slope has already cracked extensively and shows sliding, with the moisture peak developing deeper into the slope. After 240 min of rainfall, the sliding area in the middle of the slope further expands, and significant soil loss occurs at the toe of the slope. After 480 min of rainfall, the entire slope structure becomes completely unstable, with large-scale soil collapse occurring on the slope. Both the depth and extent of the damage significantly increase. High-intensity rainfall leads to rapid moisture infiltration into the deeper layers of the slope, forming a large saturated water retention zone. The pore water pressure increases sharply, causing a rapid reduction in the slope’s shear strength, ultimately leading to a large-scale slope failure.

With the increase in rainfall intensity, the extent and depth of slope damage gradually increase. The slope transitions from shallow, localized sliding to overall instability in deeper layers. Under high-intensity rainfall conditions, the slope failure occurs significantly earlier, and the surface deformation and sliding phenomena are the most severe.

(2)

Variation law of moisture content

Under the T1 condition, the moisture content inside the slope increases slowly, and the curve shows a prolonged transition phase, as shown in Figure 6. After 480 min of rainfall, the final moisture content at monitoring points P1, P2, P3, and P4 reaches 37.2%, 40.4%, 34.5%, and 42.3%, respectively. Among them, the moisture content at the slope toe monitoring point P4 is the highest, indicating that the toe area is more prone to accumulating rainwater and approaching saturation. In contrast, at P3, which is near the soil boundary interface, the moisture content increase rate is slower due to the water-retaining effect of the layered soil. As the rainfall intensity increases, the rate of moisture content increase significantly accelerates, and the moisture content curve becomes steeper, reaching saturation more quickly.

Under the same rainfall duration, the moisture content at the top of the slope (P1) increases slowly and stabilizes at a lower saturation level. In the middle of the slope, P2 and P3 are affected by the water-retaining effect of the layered soil, resulting in a slower rate of increase in moisture content, but they eventually reach higher values. At the bottom of the slope, P4, being close to the seepage convergence area, has the fastest moisture content increase rate and reaches the highest value.

(3)

Variation Law of Pore Water Pressure

As the rainfall intensity increases, the pore water pressure at each monitoring point increases more significantly, and the time required for stabilization shortens, as shown in Figure 7. Under the T1 condition, the initial pore water pressure at monitoring point M1 is −1.3 kPa. With the continuation of rainfall, the pore water pressure gradually increases and stabilizes after 420 min, eventually reaching 0 kPa. M2 increases from an initial −2.2 kPa to −1.1 kPa, M3 rises from −3.31 kPa to −1.7 kPa, and M4 increases from −2.76 kPa to −1.18 kPa. Under the T3 condition, the increase in pore water pressure is the most significant in terms of both magnitude and rate. Monitoring point M1 reaches 0 kPa and stabilizes after 300 min. M2 increases from an initial −2.15 kPa to −0.5 kPa, M3 rises from −3.27 kPa to −1.2 kPa, and M4 increases from −2.74 kPa to −0.8 kPa. Under the T3 condition, the pore water pressure increases the fastest, with each monitoring point reaching stability in a shorter time, and the final pore water pressure is significantly higher than those under T1 and T2 conditions.

Increasing rainfall intensity will boost the infiltration rate only until the soil’s maximum absorption rate is reached. Past that threshold, any additional rainfall simply runs off, and infiltration stays constant at the soil’s capacity.

3.2. Different Initial Moisture Content Conditions

• Variation law of slope morphology

Under different initial moisture content conditions, the slope profile and surface morphology changes of the layered slope are shown in Figure 8. Comparing the slope profile morphology after rainfall infiltration with different initial moisture contents, the slope with initial moisture contents of 15% and 20% for sand–silty clay, which is close to the natural moisture content, shows that after 60 min of rainfall infiltration, the upper sand clay layer was infiltrated by water up to the middle region, and surface layer sliding damage began to appear at the top of the slope. Observing the evolution of the internal wetting front, the initial moisture movement was relatively uniform, and the wetting front showed a gradual infiltration pattern. Under the influence of the self-weight of the soil particles and water transport, the interface between the two different soil types at the bottom of the slope was the first to undergo moisture migration. After 240 min of rainfall infiltration, the slope body was fully wetted, and the eroded sliding zone gradually became more apparent, with clear surface collapse and substantial changes in the slope morphology. After approximately 480 min of rainfall, the signs of landslides and collapses became more pronounced, especially in the areas at the top and bottom of the slope, where the damage was most significant.

After 30 min of rainfall infiltration, numerous small cracks formed around the top of the slope and the infiltration holes, which gradually developed into through-going cracks. As time passed, the pore water pressure in the slope body continued to increase as the soil absorbed water, leading to soil liquefaction and causing fine particles on the surface to gradually peel off and wash away under the erosion of the flowing water, forming multiple small channels. The water flowed along these small channels and continued to erode the soil, with the width and depth of the channels increasing. Inside the slope body, the infiltration of rainwater formed multiple runoff pathways, further promoting the overall sliding and slow movement of the slope. The deformation of the slope body gradually intensified, and noticeable subsidence and deformation occurred in localized areas, making the trend of soil loss and sliding more evident in the middle and lower parts of the slope. At the base of the slope, due to continuous erosion by the water flow and the deposition of sediment, a small-scale alluvial surface formed. Because the alluvial surface had a certain thickness, its surface displayed the characteristics of sediment accumulation, resulting in temporary water stagnation and localized soil loss.

As the initial moisture content increased, the softening effect of the moisture on the soil became more pronounced, causing the roughness of the slope morphology to gradually increase, and the mudification phenomenon became more significant. For the layered slope with initial moisture contents of 15% and 20% for sand–silty clay, the fine channels on the slope surface after rainfall infiltration were mainly concentrated at the base of the slope, with their profile lines appearing as continuous straight lines.

2.

Variation law of moisture content

The changes in water content with rainfall duration for the two different initial moisture content soils after rainfall infiltration are shown in Figure 9. In the experimental conditions with initial moisture contents of 15% and 20% for the two soil types, the water content at all four monitoring points continuously increased with the duration of rainfall. At the early stage of rainfall, the water content in the silty clay in the lower part of the slope was significantly higher than that in the surface sand–silty clay, but the variation in water content in the surface sand–silty clay was much greater than that in the internal silty clay. The infiltration rate of rainwater was faster in the sand–silty clay than in the silty clay, and the rainfall on the slope surface was more concentrated, leading to a faster and more pronounced infiltration of moisture into the slope surface.

For the slope with initial moisture contents of 20% and 25% in sand–silty clay, after rainfall infiltration, the water content at monitoring points P1, P2, and P3 showed an increasing trend, while the water content at monitoring point P4 exhibited a trend of initially rising and then gradually stabilizing. Compared to the experimental conditions with initial moisture contents of 15% and 20% in the two soil types, after the rainfall, the water content values at monitoring points P1, P2, P3, and P4 increased by 2.3%, 1.5%, 1.2%, and 0.8%, respectively. After 420 min of rainfall infiltration, the monitoring point P4 inside the slope had reached a saturated state. Once the slope was completely infiltrated, water accumulated at the toe of the slope due to self-weight and erosion, forming small puddles, which led to a significantly higher water content at the slope toe than on the slope surface during the later stage of rainfall infiltration.

Due to the sliding force and self-weight stress of the slope, part of the rainwater at the top of the slope flowed along the surface as runoff, while another portion infiltrated into the slope surface and toe. As the wetting front reached the interface between the two layers, the volumetric water content at the slope surface and toe slightly increased.

3.

Variation Law of Pore Water Pressure

The pore water pressure variation after rainfall infiltration for soils with different initial moisture contents is shown in Figure 10. The pore water pressure curves inside the slope exhibit similar trends with respect to the duration of rainfall. Initially, the pore pressure inside the slope increases gradually, then rises and stabilizes. As the initial moisture content increases, the time for the pore pressure to rise and reach a stable state is advanced. During the continuous rainfall, due to the difference in permeability coefficients between the upper and lower soil layers, the infiltration rate is greater than the outflow rate, resulting in a reduction in the infiltration rate. At the interface between the soil layers, saturation and water retention phenomena occur, leading to a significant increase in both the volumetric water content and pore water pressure at the interface.

3.3. Different Layering Sequence Conditions

4.

Variation law of slope morphology

The slope morphology changes under different layering sequences are shown in Figure 11. As the rainfall duration increases, at 60 min, both the sand–clay and clay–sand slopes show surface cracks. At 240 min of rainfall, the sand–clay slope exhibits localized soil collapse, while the clay–sand slope shows a trend of the wetting front gradually advancing, though no large-scale collapse occurs. The sand layer has higher permeability, allowing rainwater to quickly infiltrate the soil interface, creating localized saturated zones, which reduce shear strength and trigger shallow sliding or collapse. In contrast, the clay layer, due to its lower permeability, retains water for a longer period, and the wetting front advances slowly, resulting in a slower sliding development.

At 480 min of rainfall, the sand–clay slope experienced significant collapse damage, with the sliding area clearly extending along the soil layer interface. The clay–sand slope exhibited large-scale cracking, with an uneven surface and an expansion of the sliding area. This indicates that the layering sequence significantly affects the slope failure mode: the sand–clay slope is more likely to induce localized sliding due to the formation of a saturated zone at the soil layer interface, while the failure of the clay–sand slope is mainly characterized by the expansion of surface cracks and the gradual advance of the deep wetting front.

The soil layering sequence significantly influences slope stability and failure mode under rainfall infiltration conditions. When the sand layer is positioned on top, the slope is more prone to rapid water accumulation at the soil layer interface due to its higher permeability, forming a saturated zone that triggers a landslide. Conversely, when the clay layer is on top, water retention leads to significant surface crack expansion, while the wetting front advances more slowly, resulting in a relatively slower slope failure process.

5.

Variation in water content and pore water pressure

The pore water pressure variation with rainfall duration in the sandy–silty clay layered slope (T1 condition) shows a characteristic of a faster increase in the upper layers and a smaller increase in the lower layers, as shown in Figure 12. At the shallow monitoring points at the top of the slope (M1, M2), after 480 min of rainfall, the pore water pressure increased from the initial values of −1.321 kPa and −2.23 kPa to −0.12 kPa and −0.53 kPa, respectively, with increases of 91% and 76%. In contrast, the pore water pressure at the deep monitoring points at the foot of the slope (M3, M4) showed minimal changes, increasing from the initial values of −3.327 kPa and −2.761 kPa to −2.78 kPa and −2.43 kPa, with increases of 16% and 12%, respectively.

In the silty–sandy clay layered slope, the pore water pressure at the upper silty clay monitoring points (M1, M2) increased from the initial values of −1.717 kPa and −2.788 kPa to −0.81 kPa and −1.25 kPa, with increases of 53% and 55%, respectively, after 480 min of rainfall. At the lower sandy clay monitoring points (M3, M4), the pore water pressure increased from the initial values of −2.662 kPa and −2.209 kPa to −1.75 kPa and −1.38 kPa, with increases of 34% and 38%, respectively. The water retention effect in the silty–sandy clay layered slope is more significant, and the cumulative effect of pore water pressure is mainly concentrated in the upper silty clay layer and the soil layer interface region.

The upper sandy clay (P1, P2) in the sandy–silty clay layered slope has a higher permeability, leading to rapid rainfall infiltration and a quicker increase in moisture content, eventually approaching saturation. The lower silty clay (P3, P4), with lower permeability, experiences slower moisture content increase, and the final moisture content is slightly lower than that of the upper layer. In the silty–sandy clay layered slope, the upper silty clay (P1, P2) shows significant water retention effects, with a large increase in moisture content and a higher final value. The lower sandy clay (P3, P4) exhibits a rapid initial increase in moisture content, followed by stabilization, and the final value is lower than that of the upper layer.

4. Numerical Simulation Analysis

4.1. Establishment of Numerical Model

Based on an ion-type rare earth mine in southern Jiangxi as the engineering background, the layered slope at the most unstable location in the study area was selected and a simplified COMSOL two-dimensional model was established. According to the slope morphology parameters from indoor landslide simulation experiments, a two-dimensional scale model was created using CAD software. The model has a slope height of 50 m, with the study area slope height set to 25 m. The distance between layering interfaces from the slope top differs by 10 m, and the lengths of the slope top and bottom of the study area are 8 m and 35 m, respectively. The distance between the interface and the right slope foot differs by 10 m, and the horizontal slope angle is 42°. Considering the frequent rainfall in the region during summer and the rising groundwater level, the initial groundwater level was set at 10 m. The mesh of the slope’s two-dimensional model and the study area divisions are shown in Figure 13.

To ensure the accuracy of the numerical simulation calculation, soil areas are set at the lower part and sides of the model and are connected as a whole with the slope. To ensure the integrity of the boundary conditions of the model and the solution domain during the calculation process, COMSOL Multiphysics^®6.2 Version software was used to repair the geometric model. The repair tolerance was set to 1 × 10⁻⁵ to ensure that there were no geometric errors or discontinuities during the numerical calculation process, thus guaranteeing the accuracy and reliability of the simulation. Meanwhile, in order to improve the efficiency and accuracy of the model mesh partitioning, the user-controlled “free triangle” mesh partitioning method was adopted. Extremely fine meshes were set up throughout the two-dimensional model to ensure high-precision calculation results. Finally, the number of grid domain elements of the model was set to 11,778, the number of boundary elements to 508, the maximum and minimum sizes of the elements in the grid division were 0.9 m and 0.0018 m, respectively, the maximum growth rate of the network elements was 1.1, the curvature factor was 0.2, and the network resolution in the narrow area was 1.

The numerical simulation results are verified by establishing a physical simulation experiment platform [31,32]. Numerical models used simplified soil–water curves, linear elastic–plastic shear-strength laws, and basic boundary conditions. When solving the rainfall infiltration–flow–solid coupling problem, it is necessary to accurately simulate and analyze the unsaturated seepage process of the soil and rock mass. The Richards equation is the core fundamental equation of unsaturated seepage theory, used to describe the movement of water in unsaturated soils during the rainfall infiltration process. As the basic equation for describing water movement in unsaturated soils, the expression of the Richards equation in a two-dimensional coordinate system can be represented as the following formula [33]:

$$n{\displaystyle \frac{\partial S_r}{\partial h}}{\displaystyle \frac{\partial h}{\partial t}}-{\displaystyle \frac{\partial }{\partial x}}\left(K{\displaystyle \frac{\partial h}{\partial x}}\right)-{\displaystyle \frac{\partial }{\partial z}}\left(K{\displaystyle \frac{\partial h}{\partial z}}\right)-{\displaystyle \frac{\partial K}{\partial z}}=0$$

(1)

In the equation, h is the pore water pressure head, in meters (m); it is defined as $h=p/{\gamma }_w$. p is the pore water pressure in kPa, ${\gamma }_w$ is the unit weight of water in N/m³, n is the porosity of the porous medium, S_r is the saturation degree, in percentage (%), K is the permeability coefficient, in meters per second (m/s), x is the horizontal coordinate, z is the vertical coordinate, and t is time, in seconds (s).

Let the spatial volume of the soil body be $\Omega$, and the boundary be $\Gamma$, where n is the unit normal vector on the surface of the porous medium. At the initial time t = 0, the pore water pressure head of the soil body is h₀, and the initial condition is [34,35]:

$$h\left(x,z,t\right){\mid }_{t=0}=h_0\left(x,z\right)$$

(2)

The pressure head applied on the boundary $\Gamma$ of the soil body is a known value h_b:

$$h\left(x,z,t\right){\mid }_{{\Gamma }_h}=h_b\left(x,z,t\right)$$

(3)

Darcy’s Law describes the relationship between the flow rate of a fluid in a porous medium and the pressure gradient. For water flow in soil, the expression of Darcy’s law is:

$$q=-K\nabla h$$

(4)

Here, $q$ is the seepage flux per unit area, $K$ is the permeability coefficient, and $h$ is the gradient of the pore water pressure head. This equation indicates that the seepage flux $q$ is inversely proportional to the gradient of the pore water pressure head, and its direction is opposite to the pressure gradient.

If the boundary is a seepage flux q:

$$-n\cdot K\nabla h\left|{\Gamma }_q\right.=q\left(x,z,t\right)$$

(5)

In the equation, $h\left(x,z,t\right)$ represents the pore water pressure head at a certain point in the soil body at time ttt; $h_0\left(x,z\right)$ is the initial distribution of the pore water pressure head; $h_b\left(x,z,t\right)$ is the pore water pressure head at the boundary of the soil body; and $q\left(x,z,t\right)$ is the seepage flux per unit area.

In solid mechanics analysis, the governing equation for solid deformation is typically composed of the stress equilibrium equation, the geometric deformation equation, and the constitutive equation, as shown in the following formula:

$$\nabla \cdot \sigma +F=0$$

(6)

$$\epsilon ={\displaystyle \frac{1}{2}}\left(\nabla u+\nabla u^T\right)$$

(7)

$$\sigma =D:\epsilon$$

(8)

In the equation, $\sigma$ represents the stress tensor (Pa); F is the body force (N/m); $\epsilon$ is the strain tensor; $u$ is the displacement vector (m); D is the elasticity modulus tensor (Pa); and $\nabla$ is the gradient operator.

During the infiltration process, the soil matrix suction, permeability, and other properties change with the variation in the soil’s saturation degree. To estimate the relationship between the volumetric water content θ_w, permeability coefficient k_w, and matrix suction ψ, the Van Genuchten model [36] was used for the analysis:

$$\theta ={\theta }_r+{\displaystyle \frac{{\theta }_s-{\theta }_r}{{\left[1+{\left(ah\right)}^n\right]}^m}}$$

(9)

In the equation, θ represents the volumetric water content; h represents the negative pressure, which is taken as a positive value; θ_s and θ_r represent the saturated volumetric water content and residual volumetric water content, respectively; and $a$, m, n are model parameters.

This study utilizes COMSOL Multiphysics software, employing the theoretical framework of flow–solid coupling and the solution of Richards’ equation, to conduct an in-depth analysis of the water flow movement and solid stress deformation within the slope under rainfall infiltration conditions.

4.2. Numerical Simulation Condition Design

This study primarily considers the impact of different soil types’ initial moisture content, rainfall intensity, and layering sequence on the stability of layered ion-type rare earth mineral slopes. To examine the influence of rainfall on the stability of layered slopes, historical rainfall intensity data from the Gan Nan region are referenced. Three rainfall intensities are set: heavy rain (U₀ = 10 mm/h), torrential rain (U₀ = 20 mm/h), and extremely heavy rain (U₀ = 30 mm/h). The initial moisture content of sandy–silty clay is set to 15% and 20%; 20% and 25%; and 25% and 30%. Layered slopes of sandy clay–silty clay and silty clay–sandy clay are studied, with a uniform rainfall type and a rainfall duration of 10 days, as shown in

Table 3. Table for setting experimental conditions for numerical simulation.

Test Number	Type of Precipitation	Rainfall Intensity/(mm·h−1)	Types of Layered Slope Models	Initial Moisture Content of Two Types of Soil. (%)	Duration of Rainfall/d
Q1	Heavy rain	10	Sandy–silty clay	15, 20	10
Q2	Torrential rain	20	Sandy–silty clay	15, 20	10
Q3	Extremely heavy rain	30	Sandy–silty clay	15, 20	10
Q4	Heavy rain	10	Sandy–silty clay	20, 25	10
Q5	Heavy rain	10	Sandy–silty clay	25, 30	10
Q6	Heavy rain	10	Silty–sandy clay	15, 20	10

.

4.3. Different Rainfall Intensities

As shown in Figure 14, with the continuous rainfall, a sudden increase in and diffusion area of volumetric water content appears at the interface of the two layers of the layered slope and at the slope toe, where the volumetric water content is higher than that of the homogenous soil layers above and below. As the rainfall intensity increases, the area of the sudden increase in and diffusion of volumetric water content gradually expands, and the region with the fastest expansion rate shifts from the lower silty clay layer to the upper sandy clay layer.

As shown in Figure 15, with the increase in rainfall intensity, the expansion of the high-water-content region is accompanied by a sharp rise in pore water pressure. During the early stages of rainfall, the pore water pressure increases rapidly, mainly concentrated in the shallow region of the sandy clay layer. As the rainfall continues, the pore water pressure gradually accumulates in the silty clay layer, forming a larger pressure gradient at the layer interface. This exacerbates the mechanical differences between the upper and lower soil layers, making the slope failure surface more likely to form along the layer interface.

The changes in pore water pressure and water content at different monitoring points of the sandy–silty clay slope before and after rainfall show a significant increase with the rising rainfall intensity, as shown in Figure 16. Under the rainfall intensity of U₀ = 30 mm/h, the pore water pressure at monitoring point M1 increased from an initial value of −58.2 kPa to nearly 0, with an increase of 99.9%. The other monitoring points also exhibited similar trends, with the slope soil body gradually approaching a saturated state. The water content at each monitoring point increased significantly with the increase in rainfall intensity. Under the heavy rainfall conditions, the water content at points P1 and P2 increased noticeably, with an increase of approximately 120%. Under the extreme rainfall conditions, the water content almost reached saturation, with point P4 showing the highest water content of 44.5%. This indicates that under high rainfall intensity conditions, the area of increased soil water content gradually expands and stabilizes.

The changes in pore water pressure and water content of the sandy–silty clay slope under different rainfall intensities are shown in Figure 17. At a rainfall intensity of 30 mm/h, the infiltration rate of rainwater accelerates further, leading to a rapid accumulation of pore water pressure. Monitoring points M1 and M2 reached nearly saturated states after about 3 days, with final values of approximately 0 kPa and 2 kPa, respectively. Pore water pressures at M3 and M4 increased rapidly around the fourth day, reaching final values of approximately −20 kPa and −15 kPa, respectively. The shallow-layer water content (P1, P2) quickly approached saturation around the third day, reaching approximately 48% and 50%, respectively. The deep-layer water content (P3, P4) also increased significantly around the fourth day, with final values of approximately 44% and 51%. In both the shallow surface layer and the soil layer interface, high-intensity rainfall accelerated the rate of water content change, with both the shallow and deep layers approaching saturation. This significantly enhanced the accumulation of pore water pressure.

With the increase in rainfall intensity, both the pore water pressure and water content of the sandy–silty clay slope show a significant upward trend. The response of the pore water pressure and water content in the shallow surface layer is more sensitive to rainfall, as the upper sandy clay has higher permeability and allows for faster infiltration of rainwater. However, in the deep slope foot area, the increase in pore water pressure and water content is relatively slow due to the low permeability and water retention effect of the underlying silty clay. Nonetheless, under high-intensity rainfall conditions, a significant accumulation effect is also observed.

4.4. Different Initial Moisture Content

The variation in initial moisture content directly affects the distribution of pore water pressure and the accumulation characteristics of moisture after rainfall, with a distinct saturated water retention zone forming at the soil interface, which significantly alters the stability of the slope. As shown in Figure 18, with the increase in initial moisture content, the pore water pressure in the slope body rises significantly after the rainfall, with high-pressure areas primarily concentrated at the soil interface. When the initial moisture content reaches 30%, the maximum pore water pressure at the soil interface reaches −5×10³ Pa. The significant increase in pore pressure substantially reduces the effective stress of the soil, thereby weakening its shear strength. Additionally, the distribution range of pore water pressure increases with the rise in initial moisture content, indicating that the slope is more susceptible to rainfall-induced landslide failure.

Figure 19 shows the distribution characteristics of moisture content after rainfall for different initial moisture contents. As the initial moisture content increases, the area of the saturated water retention zone at the soil interface significantly expands, and the degree of saturation is higher. The maximum moisture content at the surface sandy clay and the soil interface near the slope toe approaches saturation. This indicates that due to the difference in permeability coefficients at the soil interface, a moisture accumulation zone is formed, reducing the drainage capacity and leading to further accumulation of moisture content. The presence of the water retention zone not only intensifies the increase in pore water pressure but also significantly reduces the slope’s shear strength.

As shown in Figure 20, under different initial moisture content conditions, there are significant differences in the variations in pore water pressure and moisture content of the sandy–silty clay slope before and after rainfall, and the change characteristics vary at different monitoring points. Before the rainfall, as the initial moisture content increases, the pore water pressure at each monitoring point gradually decreases. At the M1 monitoring point, the pore water pressure decreases from −58.2 kPa (initial moisture content of 15%) to −10.1 kPa (initial moisture content of 30%). After the rainfall, the variation in pore water pressure is relatively small under different initial moisture content conditions, but generally, the higher the initial moisture content, the higher the pore water pressure after the rainfall. At the M1 monitoring point, the pore water pressure reaches approximately 0 kPa when the initial moisture content is 30%, indicating near-saturation, while the pore water pressure is relatively low when the initial moisture content is 15%. Under low initial moisture content conditions, the soil has a stronger water absorption capacity, leading to a larger variation in pore water pressure after rainfall, but the time to reach saturation is shorter. Under high initial moisture content conditions, the soil is more likely to approach saturation during the rainfall, causing the pore water pressure to rise rapidly, with a more extensive saturated area.

Under different initial moisture content conditions, the moisture content at each monitoring point before the rainfall is almost consistent with the initial value. After the rainfall, the moisture content increases significantly, and the higher the initial moisture content, the smaller the increase in moisture content. At the P1 monitoring point, when the initial moisture content is 15%, the moisture content increases to 38.7% after the rainfall, whereas when the initial moisture content is 30%, it only increases to 42.8%, indicating that soil with a high initial moisture content has lower water absorption capacity during the rainfall, while soil with a low initial moisture content absorbs water more easily.

The changes in the pore water pressure and moisture content of the sandy–silty clay layered slope under different initial moisture content conditions before and after rainfall are shown in Figure 21. The results indicate that, in terms of pore water pressure changes, the higher permeability of the upper sandy clay causes the pore water pressure under rainfall to decrease rapidly. The pore water pressure at monitoring points M1 and M2 drops from −10 kPa to approximately −50 kPa, with a large variation. In contrast, the lower permeability of the underlying silty clay causes its pore water pressure to decrease more slowly. Monitoring points M3 and M4 show smaller changes during the early stages of rainfall but gradually stabilize between −40 kPa and −60 kPa as the rainfall continues. When the initial moisture content increases further to 25% or 30%, the changes in pore water pressure are significantly reduced for both the upper sandy clay and the lower silty clay, indicating that higher initial moisture content conditions weaken the impact of rainfall on pore water pressure.

The upper sandy clay (initial moisture content of 15%) quickly absorbs water under rainfall, with the moisture content at monitoring points M1 and M2 rising rapidly and approaching saturation, stabilizing at around 40%. In contrast, the moisture content changes in the lower silty clay (initial moisture content of 20%) are relatively slow, but due to the cumulative effect, it gradually increases in the later stages of rainfall, eventually stabilizing between 35% and 40%. Although the water absorption capacity of the silty clay is weaker, it still exhibits some moisture accumulation under rainfall. Under higher initial moisture content conditions (30%), the changes in moisture content for both the upper and lower soil layers are significantly reduced, with an increase of only 5–10%, indicating that rainfall has limited impact on soils that are already near saturation.

4.5. Different Layering Sequences

From Figure 22 and Figure 23, it can be seen that in the sand–silty clay layering, the high permeability of the upper sand clay allows rainwater to quickly penetrate to the top of the lower silty clay. However, due to the lower permeability of the lower silty clay, water accumulates at the soil interface, forming a distinct saturated water retention zone. This phenomenon is reflected in the pore water pressure distribution, where there is a significant increase in pore water pressure at the interface, and in the water content distribution, where the water content near the interface rises rapidly and approaches saturation. At this point, the high permeability of the sand clay causes rapid water movement due to rainfall, while the silty clay obstructs downward water infiltration, creating a notable water transfer lag effect between the upper and lower soil layers.

In the silty clay–sand clay layering, due to the lower permeability of the upper silty clay, rainwater is unable to quickly penetrate the lower sand clay, resulting in water accumulation in the upper soil layer, significantly increasing the pore water pressure and water content of the upper layer. When rainwater finally infiltrates the lower sand clay, its high permeability allows the water to spread rapidly within the lower layer, causing the saturated retention zone to move deeper into the slope. Compared to the sand–silty clay layering, the water retention zone in the silty clay–sand clay layering is smaller in area but more widespread.

Figure 24 compares the changes in pore water pressure and water content at monitoring points on the lower main slope before and after rainfall under different layering sequences. From the perspective of pore water pressure changes, at the initial stage, the pore water pressures at different locations (M1–M4) are all at relatively low levels. The pore water pressure change in the lower silty clay layer is more significant, with the maximum value reaching −5.24 kPa after rainfall, while the pore water pressure fluctuations in the sand clay layer are smaller, with the maximum value being −10.38 kPa. This indicates that the silty clay layer has a stronger water accumulation capacity. In terms of water content changes, the initial water contents of the sand clay and silty clay are 15% and 20%, respectively. After the rainfall, the water content of both layers significantly increases, especially in the surface layer (P1, P2) of the sand clay, where the water content rises from 14.9% to over 40.9%.

As shown in Figure 25, the variation trends of pore water pressure and water content at the monitoring points over time indicate that during rainfall, the pore water pressure in the sand clay layer rises rapidly and then stabilizes, while the pore water pressure in the silty clay layer continues to increase after the rainfall ends. The change in water content shows a rapid increase during the rainfall, gradually approaching saturation after the rainfall ends. The lower water content in the silty clay layer of the sand–silty clay slope increases the most, reaching 41.5%, indicating more significant water retention in the silty clay layer, which in turn delays the saturation time of the overlying sand clay layer.

When the sand clay overlies the silty clay, the water retention phenomenon within the slope is more pronounced, with the saturated retention zone mainly concentrated at the interlayer boundary. However, when the silty clay overlies the sand clay, the water retention effect is alleviated, but the accumulation of pore water pressure in the lower part of the slope is more obvious.

5. Comparison and Analysis of the Stability of Layered Slopes

5.1. Strength Theories and Criteria

During the rainfall process, at the interface of soil layers with different permeability coefficients, a transient water retention phenomenon will form due to the difference in permeability coefficients. Based on this concept, Fredlund et al. [37] proposed a method for evaluating the shear strength of saturated–unsaturated soils using a double stress state variable.

$${\tau }_{\mathrm{f}}=c+({\sigma }_n-u_a)\tan \phi +(u_a-u_w)\tan \phi$$

(10)

In the equation, ${\tau }_{\mathrm{f}}$ is the shear strength of the soil, ${\sigma }_n$ is the total stress, $u_a$ is the pore air pressure, $u_w$ is the pore water pressure, and c and $\phi$ and f are the cohesion and internal friction angle of the soil, respectively.

The basic principle of the finite element strength reduction method to calculate the slope safety factor is to gradually reduce the cohesion and internal friction angle of the soil by defining the strength reduction factor F_s, so that the shear strength of the soil gradually decreases. The safety factor F_s at the point where the soil reaches the limit failure is the safety factor of the soil. The expression for the reduced shear strength parameters is [38]:

$$c_m=c/F_s$$

(11)

$${\phi }_{\mathrm{m}}=\arctan (\tan \phi /F_s)$$

(12)

In the equations, c and c_r represent the cohesion of the soil before and after reduction, respectively, while $\phi$ and ${\phi }_{\mathrm{m}}$ represent the internal friction angles of the soil before and after reduction. F_s is the strength reduction factor, which is the safety factor.

5.2. Displacement Analysis of Layered Slopes

Under heavy rainfall conditions, the displacement of the slope is mainly concentrated in the toe region. The high permeability of the upper sandy clay allows rainwater to infiltrate quickly, while the lower silty clay has a lower permeability, which causes the seepage to be hindered at the sand–silt interface, leading to small displacements in the toe region with limited deformation. The overall slope remains stable. Under torrential rain conditions, the infiltration rate of rainwater increases significantly, causing the sandy clay to approach saturation locally. The lower silty clay swells due to water absorption, reducing its shear strength. This results in an expanded displacement range and increased displacement values, with noticeable deformation occurring in both the slope crest and toe areas. Under extreme rainfall conditions, the interface between sandy and silty clay experiences concentrated displacement due to high pore water pressure, with the maximum displacement reaching approximately 0.6 m. Deformation at the slope crest and toe significantly intensifies, and the slope approaches a critical state of instability. Displacement contour maps of the layered slope after rainfall for each scenario are shown in Figure 26, Figure 27 and Figure 28.

Under different initial moisture content conditions, the relatively low initial moisture content of the sandy clay maintains a higher shear strength, and the silty clay does not reach saturation, resulting in smaller displacement values for the slope, with a maximum displacement of approximately 0.15 m, indicating good stability. When the moisture content increases to 20% and 25%, the saturation trend of the sandy clay accelerates, and the shear strength of the lower silty clay decreases further. This leads to a significant increase in displacement in the toe region, reaching approximately 0.2 m, resulting in reduced slope stability. When the moisture content further increases to 25% and 30%, the upper sandy clay rapidly reaches saturation, and pore water pressure accumulates in the lower silty clay. A clear displacement concentration zone forms at the sand–silt interface, with the maximum displacement values at the toe and crest of the slope reaching up to 0.4 m.

In the sand–silty layered soil condition, the high permeability of the upper sandy clay allows rainwater to quickly infiltrate to the sand–silt interface, while the low permeability of the silty clay causes pore water pressure to accumulate. This results in a displacement concentration zone centered at the toe of the slope, with the maximum displacement around 0.12 m and a small deformation range. The overall slope remains relatively stable. In the silty–sand layered soil condition, the low permeability of the silty clay slows down the infiltration of rainwater, and the high permeability of the lower sandy clay helps release the pore water pressure. The maximum displacement is around 0.1 m, primarily concentrated in the crest area, demonstrating better slope stability.

5.3. Analysis of Safety Factor of Layered Slope

From Figure 29, it can be observed that the soil layer structure of the sand clay layered slope has a weak control ability over moisture migration. Due to the high permeability of the upper sand clay layer, rainwater quickly infiltrates into the lower silty clay layer, forming a saturated water retention zone at the sand–clay interface. The formation of this retention zone significantly increases the pore water pressure at the soil layer interface. Under conditions of high rainfall intensity, the pore water pressure increases sharply, leading to a significant reduction in the shear strength at the sand–clay interface, which in turn significantly decreases the safety factor of the slope. The simulation results show that under extreme rainfall conditions, the safety factor of the sand–silty clay layered slope decreases to 1.15, indicating a significant increase in the risk of instability under extreme rainfall.

Under different initial water content conditions in the upper and lower layers, the safety factor of the sand–silty clay layered slope gradually decreases, indicating that as the initial water content increases and the degree of saturation of the soil increases, the moisture retention effect and the cumulative effect of pore water pressure become more pronounced. On the one hand, a high initial water content increases the soil’s rate of saturation; on the other hand, it reduces the effective stress and shear strength of the soil, exacerbating the instability trend.

When the rainfall intensity increases from 10 mm/h to 30 mm/h, the safety factor of the slope decreases from 1.30 to 1.15, and the slope gradually transitions into a critical instability state. The higher the rainfall intensity, the faster the infiltration rate, and the more noticeable the accumulation of pore water pressure, which further reduces the shear strength of the soil, leading to a significant decrease in slope stability.

Under the same initial conditions, the safety factor of the silty–sand clay layered slope is higher than that of the sand–silty clay layered slope. This is because the upper silty clay layer of the silty–sand clay slope has a lower permeability, which limits the rate of rainfall infiltration and delays the accumulation of pore water pressure, thus resulting in higher slope stability. In contrast, the upper sand clay layer of the sand–silty clay layered slope has higher permeability, which allows for rapid rainfall infiltration and leads to the rapid accumulation of pore water pressure, making the slope more prone to instability.

6. Conclusions

(1)

Injection leaching and rainfall are key driving factors influencing ion-type rare earth mining landslides. The initial moisture contents and rainfall intensities significantly alter the slope’s hydraulic response and failure characteristics. The layering sequence changes the rainfall infiltration path and the distribution pattern of hydraulic coupling, thereby affecting the slope failure mode.

(2)

As rainfall intensity increases, the accumulation of pore water pressure and the rise in moisture content significantly intensify, leading to more pronounced water stagnation in the slope. The higher the rainfall intensity, the earlier the slope destabilizes, and the landslide failure range expands significantly. Under heavy rainfall and extreme rainfall conditions, the rate of pore water pressure increase is significantly faster than moderate rainfall conditions.

(3)

The initial moisture content reflects the duration and stage of leaching mining. Under conditions of lower initial moisture content, the slope has a stronger ability to absorb water. Rainfall infiltration is rapid, causing significant increases in pore water pressure and moisture content, while the stability is weakened to a lesser extent. In conditions with higher initial moisture content, the soil is close to saturation, resulting in smaller increases in pore water pressure triggered by rainfall. However, the overall reduction in shear strength is more pronounced, making deep slip failure more likely.

(4)

In sand–silty clay layered slopes, the high permeability of the upper sandy clay allows rainwater to quickly infiltrate into the silty clay layer, causing the formation of a saturated stagnation zone at the interface, where pore water pressure rises sharply. Landslide failure is mainly concentrated near the interface.

(5)

In silty–sand clay layered slopes, the low permeability of the upper silty clay leads to a longer retention time of rainwater at the surface, forming a distinct wet stagnation zone. The slope failure mode is dominated by surface crack propagation and local slippage at the slope toe.