Open AccessArticle

Study on the Impact of Seepage Filtration Under Wet–Dry Cycles on the Stability of Mudstone Limestone Slopes

Rui Li

¹,

Puyi Wang

²,

Xiang Lu

^1,*

Wei Zhou

Yihan Guo

¹,

Rongbo Lei

¹,

Zixiong Zhao

¹,

Ziyu Liu

¹ and

Yu Tian

⁴

School of Mines, China University of Mining & Technology, Xuzhou 221116, China

Inner Mongolia Datang International Xilinhot Mining Co., Ltd., Xilinhot 026000, China

State Key Laboratory of Coal Exploration and Intelligent Mining, China University of Mining & Technology, Xuzhou 221116, China

⁴

School of Environmental Science and Spatial Informatics, China University of Mining & Technology, Xuzhou 221116, China

Author to whom correspondence should be addressed.

Water 2025, 17(4), 592; https://doi.org/10.3390/w17040592

Submission received: 15 January 2025 / Revised: 11 February 2025 / Accepted: 14 February 2025 / Published: 18 February 2025

(This article belongs to the Special Issue Geotechnic and Geostructure Modelling for Landslides: Prediction and Control)

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Browse Figures

Figure 1
Diagram of the pilot study programme. "> Figure 2
Flow chart of the test. "> Figure 3
Flowchart of the wet and dry cycle scheme. "> Figure 4
Slope status and model establishment. "> Figure 5
Variation curve of specimen shear strength parameters. "> Figure 6
Effect of the number of wet and dry cycles on porosity and pore diameter. "> Figure 7
X-ray diffraction physical image analysis. "> Figure 8
Changes in seepage field in argillaceous limestone slopes after different rainfall cycles. "> Figure 9
Pore water pressure distribution of argillaceous limestone slope detection line under dry and wet cycle. "> Figure 10
Patterns of change between stability coefficients and depths of infiltration lines. ">

Versions Notes

Abstract

Open-pit mining often exposes weak rock layers, the strength of which significantly affects the stability of slopes. If these rock layers are also prone to disintegration and expansion, cyclic rainfall can exacerbate instability. Rainfall-induced changes in the seepage field also indirectly threaten the stability of slopes. Therefore, investigating the characteristics of mudstone limestone and the impact of the seepage field on slope instability under different wet–dry cycles is of great significance for the safe mining of open-pit mines. This paper takes the mudstone limestone slope of a certain open-pit mine in the southwest as the starting point and conducts experiments on saturated density, water absorption rate, permeability coefficient, compressive strength, and variable angle shear strength. Combined with scanning electron microscopy and phase analysis of X-ray diffraction analysis, the macroscopic and microscopic characteristics of the samples are comprehensively analyzed. FLAC3D software is used to explore the changes in the seepage field and the mechanism of instability. Our research found that for the preparation of mudstone limestone samples, a particle size of less than 1 mm and a drying temperature of 50 °C are optimal, with specific values for initial natural and saturated density, and natural water content. As the number of wet–dry cycles increases, the saturated density of mudstone limestone increases; the water absorption rate first rises sharply and then rises slowly; the permeability coefficient first rises sharply and then stabilizes, finally dropping sharply; the compressive and shear strength decreases slowly, and the internal friction angle changes little; frequent cycles also lead to mudification and seepage filtration. At the microscopic level, pores become larger and more regular, and the distribution is more concentrated; changes in mineral content weaken the strength. Combined with numerical simulation, the changes in the seepage field at the bottom of the slope exceed those at the slope surface and top, the transient saturated area expands, and the overall and local slope stability coefficients gradually decrease. During the third cycle, the local stability is lower than the overall stability, and the landslide trend shifts. In conclusion, wet–dry cycles change the pores and mineral content, affecting the physical and mechanical properties, leading to the deterioration of the transient saturated area, a decrease in matrix suction, and an increase in surface gravity, eventually causing slope instability.

Keywords:

cyclic rainfall; open-pit mine; seepage filtration; permeability coefficient; slope stability

1. Introduction

Safety has always been a key research direction in coal resource mining. In recent years, with the increasing national energy demand and the adjustment of national policies, the proportion of open-pit coal resource mining in the total coal volume has been increasing year by year, and the safe mining of open-pit coal mines has also attracted more attention [1,2,3]. In open-pit mining, safety requirements primarily stem from the stability of slopes. The stability of open-pit mine slopes is not only affected by intrinsic geological conditions of the strata but also by external factors such as rainfall and mining disturbances [4,5,6], with rainfall having an especially significant impact on slope stability [7,8,9]. According to statistics, landslides caused by rainfall in China account for about 90% of the total number. Rainwater infiltration changes the seepage field, and wet–dry cycles also cause significant changes in the mechanical field of the soil, leading to a decrease in slope stability [10,11,12]. In the wet–dry cycles, the interaction between water and rock is more likely to exacerbate the deformation and fracturing of weak rock layers, leading to a decrease in strength compared to a single wet or dry state. Mudstone limestone, as a typical weak rock body, is greatly affected by the rainfall cycle and often triggers disasters such as landslides. The exposed mudstone limestone slopes in open-pit mines pose a serious threat to the safety of construction personnel. In its natural state, mudstone limestone has good mechanical properties and is hard in texture, but it is prone to disintegration when it encounters water, and its resistance to weathering is weak, turning into mudstone limestone soil upon contact with water [13]. Mudstone limestone is widely distributed in the open-pit mines of Southwest China. Its physical and mechanical parameters are influenced by the climate. Rainfall and evaporation not only directly affect the water content of the slope but also indirectly change the groundwater level line [14,15,16]. Long-term exposure to the rainy season cycle leads to large-scale instability of mudstone limestone slopes, which greatly affects the safety of open-pit mining [17,18,19]. To ensure the stability of easily disintegrating rock slopes in open-pit mines, it is of great significance to study the impact of seepage filtration [20] under wet–dry cycles on slope stability.

Nan Xuan [21] used experimental and numerical methods to study the movement patterns of tailings particles in waste rock systems under different flow velocities. A computational relationship between pressure drops, porosity, and waste rock thickness was established, revealing the self-similarity of internal blockage and accumulation in waste rock. This provides theoretical guidance for preventing particle penetration. Li Hongwei [22] conducted research on the Meijiatai landslide, using finite element analysis and seepage field inversion to obtain unsaturated hydraulic parameters, among others. The results showed that drainage led to a decrease in the groundwater level, and the saturated area range decreased under heavy rainfall. The landslide has three modes of instability, and after treatment, the stability coefficient exceeded the safety value, stabilizing the landslide. The findings can serve as a reference for similar projects. Li Zian [23] discovered through physical simulation experiments that the amount of spring water replenishment affects the failure forms of debris layer slopes. Low flow rates lead to the formation of traction landslides, medium flow rates evolve into translational landslides, and high flow rates cause soil flow phenomena. Landslide disturbances cause the sliding mass to liquefy, and after deformation, the pore water pressure dissipates, leading the landslide into a new stage. By comparing the on-site data of the Nafeng landslide, monitoring and prevention suggestions are provided. Huang Liqun [24] addressed the construction of impermeable bodies on landslide debris and complex geological structures. The upstream consists of lacustrine soft foundations, with landslides and unstable mountains on both sides, and the reservoir area has an average annual rainfall of 1640 mm. Different working conditions were considered, and seepage in typical dam sections was calculated and evaluated. During the reservoir flood season and under adverse conditions such as rising groundwater levels, design optimization and construction process improvements were implemented to enhance construction quality and reduce risks, providing valuable references for projects under similar geological and construction conditions. Qihang Li et al. [25] used MatDEM v2.02 software to analyze slope failure at 45°, 55°, and 65° angles under rainfall-mining coupling. Results show that a 55° slope angle minimizes damage and reduces instability risks. Energy decreases significantly during mining, with stress concentrating in goaves and failure zones. In addition, Qihang Li et al. [26] used engineering methods and UDEC simulation methods, and the results showed that the strata damage increased non-linearly during the mining process, especially near the top of the coal seam. The subsidence curve of the lower strata is inverted trapezoidal, and the subsidence curve of the upper overburden is funnel-shaped. The critical strata reduced upward displacement, resulting in separation from the lower strata. The critical stratum breaks at 120 metres and stops surface subsidence. Convex landforms have a higher rate of subsidence than concave landforms, leading to geological hazards.

However, in current research, there is a deep qualitative study mainly targeting the causes and influencing factors of seepage filtration, but the research on the impact of the occurrence of seepage filtration on the hydrological properties of soil and rock, as well as its impact in engineering applications, is relatively weak. Therefore, taking the mudstone limestone slope of an open-pit mine in the southwestern region as an example, this paper explores the variation law of the permeability coefficient of mudstone limestone and the mechanism of slope stability under the phenomenon of seepage filtration.

2. Experimental

This section introduces the entire process of experimental materials and methods, as shown in the Figure 1.

2.1. Materials and Specimen Preparation

2.1.1. Material

The mudstone limestone samples used in this experiment were taken from an exposed mudstone limestone stratum in an open-pit mine in the southwestern region. The samples were obtained according to “Standard for test methods of engineering rock mass” [27] specifications and were tightly wrapped after extraction to ensure their original structural integrity. The basic physical and mechanical properties are shown in the following Table 1.

2.1.2. Specimen Preparation

Firstly, the particle size was selected. Through experiments, it was found that when the selected particle size was greater than or equal to 2 mm, the prepared samples had poor adhesion between particles, obvious stratification, and extremely poor physical and mechanical properties, with significant differences in hydrological properties compared to the original rock. Therefore, particles with a size of less than 1 mm were finally selected for the experiment.

To determine the hydrological properties of mudstone limestone, the ring knife method is employed, utilizing a Ø61.8 × 40 ring knife for sample preparation. Initially, a filter paper with a diameter of 70 mm is cut and placed on the surface of a Ø70 × 5 mm permeable stone. Subsequently, the ring knife with a side is positioned on the permeable stone side with the filter paper. Mudstone limestone raw material with a particle size of 1 mm is added to the ring knife in three portions. After each addition, a compactor is used to consolidate the material. Immediately following compaction, a scraper is utilized to roughen the compacted surface, eliminating stratification and ensuring uniform density of the sample. Once the ring knife is filled, a filter paper with a diameter of 70 mm and a Ø70 × 5 mm permeable stone are added on top, forming a single closed ring knife sample with a side. The masses of the three ring knife samples are recorded. These three samples are then stacked together and placed on a saturation rack with a diameter of 61.8 mm to prevent swelling during saturation. The set of ring knife samples is soaked in a bucket of water for 24 h. Afterwards, they are removed and placed in an oven to dry, resulting in the prepared samples.

2.2. Experimental Methods

2.2.1. Laboratory Experiments

This section introduces the indoor experimental scheme and methods. The experimental process is shown in the Figure 2 below.

(1) Wet–Dry Cycle Test

The wet–dry cycle consists of two steps: drying and wetting. First, the mudstone limestone is ground into rock soil with a particle size of less than 1 mm. The assembled sample group is placed in a bucket of water, ensuring that the samples are completely submerged for a soaking duration of 24 h, allowing water to fully penetrate the pores of the mudstone limestone. After the wetting process, the samples are removed and placed in a drying oven set at a temperature of 50 °C for a drying period of 24 h. The mass of the samples returns to the initially recorded values of m1, m2, and m3, promoting dehydration and shrinkage of the mudstone limestone. The aforementioned steps of wetting and drying are repeated for a total of 5 wet–dry cycles. After each cycle, the dried mass and the mass of the saturated samples are measured to obtain the saturated water absorption rate.

(2) Saturation Permeability Coefficient Test

There are mainly two experimental methods for measuring the permeability coefficient: the falling-head and constant-head permeability tests, which apply to fine-grained and coarse-grained soils, respectively. The subject of this study is mudstone limestone, which belongs to fine-grained soil. Therefore, the falling-head permeability test method is used to determine the permeability coefficient of mudstone limestone. The experimental instrument used is the TST-55 soil permeameter. (Nanjing Huade soil Instrument Manufacturing Co., Ltd., Nanjing, China). The experiment is conducted in accordance with the “Test Methods of Soils for Highway Engineering” (JTG3430-2020) [28]. The height of the piezometer is recorded every 2 min, along with the temperature of the outflowing water. The test ends after 10 min, and the permeability test for the next sample begins.

(3) Variable Angle Shear Test

In the study of the physical and mechanical properties of rocks, cohesion and the internal friction angle are extremely important parameters for judging their mechanical properties. The testing equipment used is the WAW-1000 electro-hydraulic servo rock mechanics testing system (Jinan Zhongluchang Testing Machine Manufacturing Co., Ltd., Jinan, China). This test adopts the variable angle shear method to test the shear strength parameters of mud tuff specimens. Due to the influence of the wet expansion and dry contraction characteristics of mud tuff specimens in the process of dry and wet cycling, it is not possible to obtain a standard specimen, but only a cylindrical specimen with a diameter of 48 mm and a height of 97 mm. Therefore, when performing the variable angle shear test, it is necessary to pad a layer of steel plate with an inner diameter of 48 mm inside the variable angle plate fixture, the outer diameter of which coincides completely with the fixture, because the angle of the variable angle plate fixture can not be lower than 30°, nor can it be higher than 70°. Shear angles of 40°, 50°, and 60° are selected for two sets of parallel tests during each wet–dry cycle, with continuous loading at a speed of 0.2 mm/s until the system automatically stops. The final results are averaged from three sets of parallel tests.

2.2.2. Numerical Simulation

The suction force in unsaturated soil is closely related to its structure and mechanical properties. In unsaturated seepage, the changes in the seepage field, while considering the saturation degree of this basic parameter, cannot fully reveal the solid–gas–liquid interactions in unsaturated soil pores. Additionally, they fail to adequately reflect and simulate the change patterns of the seepage field in slopes. Consequently, the changes in the flow field do not provide a sufficient theoretical basis, and the mechanical effects of the force field on the deformation and strength of the unsaturated soil skeleton cannot be accurately represented. The influence of the force field cannot be reflected. To understand the seepage field regarding the hydrophysical properties and mechanical strength changes in slope stability studies, we must first introduce the seepage field in relation to the changes in the permeability coefficient. This enables a simulation of the seepage field’s actual conditions. Next, we will analyze the water’s characteristics within the slope and the slope itself, applying the mechanical properties to determine the stability coefficient of the slope. Thus, we also need to incorporate matrix suction and saturation through the function of negative correlation. Therefore, the matrix suction force is introduced and linked to the saturation degree through a negative correlation function.

Currently, there are various numerical simulation software available for calculating slope stability. FLAC3D 6.0 software, using finite difference methods, has unique advantages in solving problems such as large deformations and discontinuous deformations. From the above presentation of unsaturated soil theory, the command flow chart for the wet–dry cycle scheme in this numerical simulation is shown in Figure 3 below.

(1) Model Establishment

The regional slope model selected in this paper is 150 m in length, with the lowest slope bottom height at 20 m and the highest slope top height at 60 m. The mudstone limestone fragmentation area has a minimal impact on the selected region and is thus ignored in this study. The thickness of the slope model is set at 10 m, composed of 5 terraces. The two main terraces, namely the highest and lowest horizontal terraces, are both 7 m in height. The lowest terrace is at the 1040 level, and the highest terrace is at the 1070 level. The areas of the slope model that change with each rainfall cycle are grouped as shown in the Figure 4 below.

For the setting of boundary conditions, the actual situation must be considered. The boundary conditions for this cross-sectional model are such that no displacement restrictions are applied at the top and the sloping edges, allowing for a free mode, while the rest of the boundary parts restrict vertical displacement.

(2) Parameter Settings

Open-pit mine landslides are influenced by a variety of rainfall conditions, especially the intensity and duration of rainfall, which have different impacts on slope stability. The rainfall intensity is set at 24 mm/d, and the duration of rainfall is 3 days. The internal friction angle remains almost constant and is taken as a fixed value of 32° in this simulation. The main parameters of the slope model are shown in the Table 2 below.

2.2.3. Microscopic Analysis

(1) Phase Analysis of X-ray Diffraction Analysis

An X-ray Diffractometer is used to analyze the diffraction patterns of materials by X-ray diffraction, obtaining the composition, internal atomic and molecular structure, and morphology of the materials. The testing instrument parameters are as follows: the model used is the D8 ADVANCE from Bruker (Karlsruhe, Germany), with Cu Kα radiation, voltage and current set at 40 KV and 30 mA, respectively; the testing target material is copper–palladium, and the scanning speed is 0.07–0.2 s/step.

(2) Scanning Electron Microscopy

Scanning Electron Microscopy utilizes the principle of focused electron beams to observe the surface morphology and microstructure of samples. The instrument parameters are as follows: The model used is the JSM-7800F from Bruker Germany. A trace amount of the sample is directly adhered to a conductive adhesive and then gold-sputtered for 45 s using a sputtering coater. Subsequently, the sample morphology is captured using the scanning electron microscope. During morphology capture, the accelerating voltage ranges from 0.02 to 30 kV, using a thermal field emission electron gun with stability better than 0.2%/h, and the magnification ranges from 50 to 2,000,000.

3. Results and Analysis

3.1. Experimental Results and Analysis

3.1.1. Saturation Permeability Test

To ensure the accuracy of the data and reduce the occurrence of randomness, five samples were used for this test, and each underwent five wet–dry cycles. The average permeability coefficients under standard water temperature after five wet–dry cycles of mudstone limestone are shown in the Table 3 below.

From the table, it is known that the saturated permeability coefficients of the five samples all show a trend of first increasing, then stabilizing, and finally decreasing after five wet–dry cycles. This can be divided into three stages. The first stage is the rapid increase stage. This is because during seepage, soaking can turn the originally small connected fractures in the samples into large connected fractures and can also connect some originally unconnected fractures. Additionally, in areas with poor crystalline connections, new small fractures are formed to provide conditions for the next connection. This phenomenon is more evident in the first two cycles, hence the significant increase in permeability coefficients. As the wet–dry cycle tests proceed, they gradually enter a stable stage. During this stage, the mudstone limestone samples are clearly divided into multiple regions, and water flows quickly through these large fractures with little impact on the samples. Therefore, small fractures are formed in this stage, but most are not connected. Moreover, during this stage, the wet–dry cycles cause some of the fine particles to become mud-like. Since the seepage velocity through the large connected fractures is fast, these mud-like fine particles and small rock particles move with the seepage, and when they move to the vicinity of the large fractures, they block some of the large fractures, thus causing seepage filtration. This phenomenon will only worsen with the continuation of the wet–dry cycles, leading to the third stage of decrease, where the obvious seepage filtration phenomenon blocks the fracture pores.

3.1.2. Variable Angle Shear Test

This test employs the variable angle shear method to evaluate the shear strength parameters of mudstone limestone samples. Through the experiment, the maximum test force for each angle can be obtained, and then the shear stress τ for each angle is calculated. The calculated τ and σ values are plotted as coordinate points (σ, τ) on the coordinate axis to form a τ-σ curve. A fitting curve is drawn for each set of 6 points after each wet–dry cycle. The intercept and slope of the fitted line are determined, where the slope corresponds to the internal friction angle, and the intercept represents the cohesion. The cohesion and internal friction angle are key parameters in this analysis. As shown in Figure 5 below.

It can be seen that the cohesion shows a gradual downward trend under wet–dry cycles, while the internal friction angle remains around 32°, essentially unaffected by the wet–dry cycles. This is mainly due to two reasons: First, the dissolution of soluble minerals and matrix with wet–dry cycles leads to the softening of mudstone limestone; second, frequent wet–dry cycles cause the mineral particles to wear into a uniform and regular spherical shape, reducing the friction between particles during loading. The combined effect of these two factors ultimately manifests as a decrease in cohesion.

3.2. Microscopic Analysis

3.2.1. Scanning Electron Microscope Analysis

To more accurately describe the changes in characteristic parameters such as porosity and average pore diameter of the sample surface after wet–dry cycles, the Scanning Electron Microscope images after different numbers of cycles were processed using PCAS software. Through this step, digital images can be transformed into binary images, which only have black and white, corresponding to solid soil particles and pores. For the statistics of isolated pores, the software uses a seed filling algorithm or an improved scan line seed filling algorithm for identification and calculation. These three statistical parameters correspond to the three basic geometric characteristics of pores, namely orientation, area, and shape factor, and describe the variations in these geometric features among different pores. PCAS provides a reliable basis for the quantitative characterization of microporosity using image processing methods. The analysis of the effects of the number of wet–dry cycles on porosity and pore diameter is shown in the Figure 6 below.

It can be observed that wet–dry cycles cause the pores in mudstone limestone samples to continuously enlarge, with both small and large pores increasing in size. The changes are more significant in the early stages of the cycles, while they remain relatively stable in the later stages. Due to the influence of seepage filtration in the later stages, some of the larger cavities become blocked, leading to a slight decrease in the maximum pore diameter. This reduces the disparity in pore sizes within the mudstone limestone samples, making the pore sizes more uniform. As the wet–dry cycles progress, the number of maximum and minimum pores gradually decreases while the proportion of medium-sized pores increases. These medium-sized pores gradually fill in or erode the irregularities of the pores, causing the pore boundaries to become more regular and approach a circular shape.

3.2.2. Phase Analysis of X-Ray Diffraction Analysis

Phase analysis of X-ray diffraction analysis is a widely used technique for phase analysis and determination of crystallinity. It is extensively applied in the analysis of material phases and crystallinity. Different minerals contain different crystals, which are composed of different unit cells. These unit cells are made up of regularly arranged atoms, and the differences in unit cells can be distinguished by the varying interplanar spacings of atoms.

To enhance the visibility of the experimental results, the preparation of samples for this test involves the use of a permeameter. The prepared samples must be placed in the permeameter for a seepage process lasting 2 h. To ensure that the experimental conditions of the obtained samples are as similar as possible, samples after each cycle should be taken from the same large mudstone limestone sample. During sampling, the entire sample is divided into six parts, and a portion is taken after each wet–dry cycle until the upper half of the sample is completely taken after five cycles. The samples obtained after each cycle are thoroughly dried and ground with a mortar to a fineness of less than 300 mesh. Finally, the samples are wrapped with plastic wrap to prevent moisture absorption. As shown in Figure 7 below.

It can be observed that the primary mineral constituents of mudstone limestone are quartz and calcite. The diffraction patterns obtained from the X-ray diffraction analysis show no new peaks, indicating that no new substances are generated during the wet–dry cycles, thus confirming that no chemical reactions occur during these cycles. According to the data, the proportion of quartz mineral content decreases overall while the proportion of calcite mineral content increases. Since the mineral content of rocks significantly influences their mechanical properties, it can be inferred that the reduction in quartz content is one of the reasons for the decreased strength of the mudstone limestone samples, with quartz making a substantial contribution to the strength of mudstone limestone.

4. Slope Stability Analysis

4.1. Seepage Field Changes and Analysis

Since the first change caused by rainfall is in the seepage field within the slope body, and only then is the changed flow field coupled into the mechanical field, these are two distinctly separate aspects. Here, we first analyze the flow field of the slope. By studying the seepage field within the slope, we understand the distribution of water pressure within the slope body and conduct qualitative and quantitative analyses of the flow field during rainfall. The main focus is on the depth of the wetting front and the changes in its depth before and after seepage filtration. The changes in the seepage field within the slope body after rainfall under different cycles are shown in the Figure 8 below.

The maximum positive pore water pressure at the ground surface and the maximum negative pore water pressure within the slope body both increase continuously with wet–dry cycles while the matrix suction gradually decreases. This indicates that as wet–dry cycles occur, more and more water penetrates into the slope body, not just the surface layer but the entire slope. Therefore, the continuous infiltration of rainwater into the slope body not only causes the surface to become saturated from the top down but also leads to an overall increase in the water content within the slope, causing the wetting front to move downward rapidly. It can be inferred that at the beginning of rainfall, the slope surface reaches saturation first. However, rainfall not only causes the slope surface to become saturated but also continuously infiltrates, increasing the overall water content within the slope body, leading to a decrease in matrix suction and a reduction in the slope’s adsorption of rainwater. This makes the flow of rainwater more fluent, accelerating the expansion of the upper transient saturated area until the seepage field reaches equilibrium.

In addition, by comparing the distribution of pore water pressure at the bottom of the local slope with that at the top, slope face, and ground surface, it can be clearly seen that the influence of rainfall on the pore water pressure at the bottom of the slope is greater than that in other areas, and this influence increases with the number of cycles until it merges with the groundwater level line. This indicates that the rainfall intensity is greater than the saturated permeability coefficient, causing rainwater to not only accumulate along the surface layer of the slope at the bottom but also form runoff above the surface, flowing to the bottom of the slope. At this time, there is a significant water pressure on both the inside and outside of the slope bottom, causing the rainwater to infiltrate deeper at the bottom of the slope than at the slope face and the top surface.

4.2. Changes and Analysis of the Wetting Front Depth

The description of the seepage field can qualitatively understand the approximate distribution range of pore pressure and the upper and lower limits of pore water pressure values within the slope after different rainfall cycles. However, the specific change curve of pore pressure with depth and the exact influence of depth of rainfall on the seepage field is not clear enough. Therefore, to better illustrate the distribution and changes in pore water pressure within the mudstone limestone slope, a fixed vertical line segment depth is chosen as the detection line to establish a curve of pore pressure versus depth. After each rainfall cycle, not only does the wetting front deepen, meaning the boundary of the temporarily saturated surface area caused by rainfall will lower, but also the affected surface area will increase after each rainfall. At this time, there will be a boundary between the affected and unaffected areas, and this boundary will also be continuously lower, as shown in the figure below.

From Figure 9, it can be seen that as the number of wet–dry cycles increases, the wetting front continues to descend: after the first rainfall, it drops to 1.91 m, the second to 2.85 m, the third to 3.71 m, the fourth to 4.38 m, and the fifth to 4.8 m. However, the rate of descent of the wetting front gradually slows down. This is because the permeability coefficient of the transient saturated zone changes with the rainfall cycle, affecting the time it takes for rainwater to infiltrate to the same depth during subsequent rainfall. In the first few cycles, the saturated permeability coefficient increases several times, allowing rainwater to infiltrate to the same depth more quickly, expanding the transient saturated zone and lowering the wetting front. The coefficient changes are similar in the first two cycles, and so is the depth of the wetting front’s descent. Between the second and fourth cycles, the saturated permeability coefficient changes little, and the wetting front descends more slowly during the third to fifth rainfall events. After the fifth rainfall, the coefficient drops significantly, making infiltration more difficult.

Furthermore, the groundwater level in this area is basically unaffected by the rainfall cycle and remains stable at about 26.2 m underground. The intersection of the pore water pressure–depth curve and the groundwater level curve shows that after each rainfall, the range of the rainwater influence zone is larger than that of the transient saturated zone and expands with the increase in the number of cycles. After five cycles, the influence is deepest at 9.5 m, and the negative pore water pressure in the influence zone decreases, with the matrix suction dropping from 122 KPa to 78 KPa.

In terms of vertical depth, the intersection of the zero-pressure water level line and the curve can be used to determine the range of the unsaturated zone. Since the zero-pressure water level line near the ground surface follows the ground and slope surfaces, and the groundwater level line is almost fixed, the unsaturated zone range here can roughly represent the overall slope condition. Clearly, with the rainfall cycle, the relevant intersections gradually approach each other, indicating that the unsaturated zone is continuously shrinking, and the rate of narrowing is becoming slower and slower.

4.3. Analysis of Slope Stability Coefficient

The slope stability coefficient is a critical metric that most directly reflects the stability of a slope. Under ideal conditions, this coefficient can be monitored to predict not only the transition time from overall slope failure to localized slope failure but also the final slope failure time. Such predictions are invaluable for conducting a comprehensive analysis of the slope’s overall stability. The relationship between the stability coefficient and the depth of the wetting front is a key factor in this analysis, as illustrated in the Figure 10 below.

As the number of wet–dry cycles increases, the stability of the slope is observed to decrease. This decrease is not linear; rather, the rate of decrease gradually slows down as more cycles are completed. This pattern suggests that while the initial cycles have a more pronounced impact on slope stability, subsequent cycles have a diminishing effect. The depth of the wetting front, which is the boundary between the saturated and unsaturated zones within the slope, plays a crucial role in this process. There is a clear negative correlation between the depth of the wetting front and the stability of the slope. With each rainfall cycle, several changes occur within the slope that contributes to its decreasing stability. The range of the transient saturated zone expands, and both the positive and negative pore water pressures increase. These changes are significant because they affect the internal forces within the slope. As the wetting front moves deeper into the slope, it indicates that more water is infiltrating and saturating the soil or rock. This infiltration leads to an increase in pore water pressure, which in turn reduces the effective stress and the shear strength of the slope materials. Consequently, the slope’s ability to resist failure is compromised, and its stability coefficient decreases.

5. Conclusions

The saturated density increases with the number of wet–dry cycles. The saturated water absorption rate of mudstone limestone samples first increases slowly, then rapidly, and finally slows down again as the number of cycles increases. The saturated permeability coefficient initially rises sharply, stabilizes in the middle period, and then decreases slightly. The compressive strength and cohesion of the samples both decrease gradually and stabilize with the increase in the number of cycles, while the internal friction angle remains relatively unchanged.
From a microscopic perspective, the micro-pores within mudstone limestone expand overall as the number of wet–dry cycles increases and the pores become increasingly regular in shape, with their sizes gradually becoming more uniform. As the wet–dry cycles frequently occur, by the fifth cycle, a significant amount of mineral particles aggregate and move to block fractures and large pores, leading to seepage filtration. Quartz minerals, which have a higher strength, experience loss and displacement during the wet–dry cycles, resulting in a decrease in their content. This leads to a reduction in the strength and cohesion of mudstone limestone, and the changes in mineral content provide evidence for the changes in the strength of mudstone limestone.
Through the experiments and simulations mentioned above, the mechanism of landslides in mudstone limestone slopes under rainfall cycles is derived: As the rainfall cycles proceed, changes occur in the pores and the morphology, size, and distribution of mineral particles within the rock. These changes lead to the softening of mudstone limestone and a decrease in its mechanical strength, thereby reducing the stability of the slope.
With the rainfall cycles, changes occur in the pores and the morphology, size, and distribution of mineral particles within the rock. These changes lead to the softening of mudstone limestone and a decrease in its mechanical strength. The alteration of the permeability coefficient causes the wetting front to continually penetrate deeper, enlarging the transient saturated area of the slope while reducing the unsaturated area. The increase in water content also leads to a decrease in matrix suction. Ultimately, under the dual influence of the mechanical and seepage fields, the stability of the slope is reduced.
In this paper, the changes in the seepage field and the stability of slopes under different numbers of rainfall cycles are investigated through indoor experiments combined with numerical simulation. However, in reality, due to the complexity and variability of various conditions, not only man-made disturbances but also natural physicochemical influences, the impact on slope stability is the result of the combination of multiple conditions. Due to the time rush and the author’s level limitations, the research work in this paper is only a preliminary research stage; the consideration is not very comprehensive, and there should still be further in-depth future research to strengthen the following aspects of the work: mud tuff for the disturbance of the rock and soil, not representative of the whole open-pit mine of the rock layer condition, and due to the lack of experimental programme and the experimental data of the accidental, the experimental data are not precise enough. Therefore, the field test can be added when possible, and the indoor test can be compared and adjusted so as to make the physical and mechanical parameters of mudstone grey rock more accurate.

Author Contributions

Conceptualization, W.Z., R.L. (Rui Li) and X.L; Methodology, R.L. (Rui Li) and Z.L.; Software, Y.G. and Z.Z.; Formal analysis, X.L. and R.L. (Rui Li); Resources, X.L; Data curation, Z.L. and R.L. (Rui Li); Writing—original draft preparation, R.L. (Rui Li); Writing—review and editing, P.W. and W.Z.; Visualization, R.L. (Rongbo Lei) and Y.G.; Supervision, X.L and P.W.; Project administration, W.Z. and P.W.; Funding acquisition, W.Z., X.L. and Y.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Key R&D Program of China (2023YFF1306001); the National Natural Science Foundation of China (52204159); the Natural Science Foundation of Jiangsu Province, China (BK20221125); the National Nature Science Foundation of China (52304157 and 52204182); and the Jiangsu Funding Program for Excellent Postdoctoral Talent (2023ZB112).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Puyi Wangs was employed by the company Inner Mongolia Datang International Xilinhot Mining Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Diagram of the pilot study programme.

Figure 2. Flow chart of the test.

Figure 3. Flowchart of the wet and dry cycle scheme.

Figure 4. Slope status and model establishment.

Figure 5. Variation curve of specimen shear strength parameters.

Figure 6. Effect of the number of wet and dry cycles on porosity and pore diameter.

Figure 7. X-ray diffraction physical image analysis.

Figure 8. Changes in seepage field in argillaceous limestone slopes after different rainfall cycles.

Figure 9. Pore water pressure distribution of argillaceous limestone slope detection line under dry and wet cycle.

Figure 10. Patterns of change between stability coefficients and depths of infiltration lines.

Table 1. Basic physical and mechanical properties.

Items	Water Content	Cohesion	The Angle of Internal Friction	Dry Density
Parameters	5.62%	425.8 kPa	31.1°	1.654 g/cm³

Table 2. Main physico-mechanical parameters of the slope model.

Rock Stratum	Saturation	Density	Porosity	Initial Pore-Saturated Permeability Coefficient	Cohesion	Angle of Internal Friction	Poisson’s Ratio
Unsaturated zone	0.15	1594 kg/m³	0.22	5.44 × 10⁻⁶ cm/s	529 kPa	32°	0.28
Saturated zone	1	1747 kg/m³	0.22	5.44 × 10⁻⁶ cm/s	1111 kPa	32°	0.28

Table 3. Average saturation permeability coefficient record table.

Number of wet and dry cycles	0	1	2	3	4	5
Average saturated permeability coefficient/10⁻⁶ cm/s	5.44	10.74	21.57	23.10	22.68	10.00

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MDPI and ACS Style

Li, R.; Wang, P.; Lu, X.; Zhou, W.; Guo, Y.; Lei, R.; Zhao, Z.; Liu, Z.; Tian, Y. Study on the Impact of Seepage Filtration Under Wet–Dry Cycles on the Stability of Mudstone Limestone Slopes. Water 2025, 17, 592. https://doi.org/10.3390/w17040592

AMA Style

Li R, Wang P, Lu X, Zhou W, Guo Y, Lei R, Zhao Z, Liu Z, Tian Y. Study on the Impact of Seepage Filtration Under Wet–Dry Cycles on the Stability of Mudstone Limestone Slopes. Water. 2025; 17(4):592. https://doi.org/10.3390/w17040592

Chicago/Turabian Style

Li, Rui, Puyi Wang, Xiang Lu, Wei Zhou, Yihan Guo, Rongbo Lei, Zixiong Zhao, Ziyu Liu, and Yu Tian. 2025. "Study on the Impact of Seepage Filtration Under Wet–Dry Cycles on the Stability of Mudstone Limestone Slopes" Water 17, no. 4: 592. https://doi.org/10.3390/w17040592

APA Style

Li, R., Wang, P., Lu, X., Zhou, W., Guo, Y., Lei, R., Zhao, Z., Liu, Z., & Tian, Y. (2025). Study on the Impact of Seepage Filtration Under Wet–Dry Cycles on the Stability of Mudstone Limestone Slopes. Water, 17(4), 592. https://doi.org/10.3390/w17040592

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