Seepage Behavior Research Articles

Inversion Model for Permeability Coefficient Based on Random Forest–Secretary Bird Optimization Algorithm: Case Study of Lower Reservoir of C-Pumped Storage Power Station

The geological complexity of the karst regions presents significant challenges, with the permeability coefficient being a critical parameter for accurately analyzing seepage behavior in hydraulic engineering projects. To overcome the limitations of traditional inversion methods, which often exhibit low computational efficiency, poor accuracy, and instability, this study utilizes a finite-element forward model and orthogonal experimental design to establish a sample set for permeability-coefficient inversion. A surrogate model for seepage calculation based on the Random Forest (RF) algorithm is subsequently developed. Furthermore, the Secretary Bird Optimization Algorithm (SBOA) is incorporated to propose an intelligent RF–SBOA inversion method for permeability-coefficient estimation, which is validated through a case study of the C-pumped storage power station. The results demonstrate that the RF model’s predictions for water levels at four boreholes closely align with the measured data, outperforming models such as CART, BP, and SVR. The SBOA effectively identifies the optimal geological permeability coefficient, with the borehole water-level inversion achieving a maximum relative error of only 0.128%, which meets the accuracy requirements for engineering applications. Additionally, the computed distribution of the natural seepage field is consistent with the typical distribution patterns observed in mountain seepage systems. During the normal water-storage phase, both the calculated seepage flow and gradient comply with engineering standards, while the seepage-field distribution aligns with empirical observations. This inversion model provides a rapid and accurate method for estimating the permeability coefficient of strata in the project area, with potential applicability to permeability inversion in other engineering geology contexts, thus demonstrating considerable practical value for engineering applications.

Research on the Characteristics of Seepage Failure in the Surrounding Rock (Coal) of the Goafs

During mining, the brittle fracture structure of coal makes it highly susceptible to disturbance, leading to changes in the permeability of the coal seam from non-conductive to water-conductive, which poses a significant threat to the stability and safety of coal pillars in goafs. Therefore, understanding the damage mechanisms of coal during water–rock interactions is crucial for ensuring mine safety. In this paper, based on laboratory seepage tests, the impact of hydrodynamic forces on the microstructure of fissured coal and its subsequent effect on permeability is examined. The study found that increasing confining pressure causes the “closure” of coal fissures, leading to a reduction in permeability. Additionally, during the initial stage of seepage, fine particles within the coal samples are mobilized due to seepage damage, leading to channel blockages and further reductions in permeability. However, as seepage continues, the hydraulic channels eventually open fully, resulting in a sharp increase in permeability. Furthermore, using a two-dimensional fracture seepage model, the study investigated how the scale of fractures in the water-conducting channels influences seepage behavior. A critical fracture width method was proposed to predict permeability surges, offering a new approach for analyzing the stability of coal pillars in mining areas.

Two-dimensional numerical analysis of seepage in an undersea mining area considering hydrochemical corrosion

The complex hydrogeological conditions and unique hydrochemical environment in the seabed rock often cause serious mine water inrush during undersea mining. With the exploitation of the undersea gold mining area in Sanshandao as the research background, a numerical analysis method for the seepage behavior of the surrounding rock in undersea gold mining considering the hydrochemical corrosion was put forward. A numerical simulation analysis of the undersea mining area in Sanshandao was conducted by the established method, and the results showed that mining the submarine ore body significantly changed the seepage field of the surrounding rock around the stope and water levels dropped obviously in the ore-controlling fault zone. Differences in the hydrochemical environment significantly influenced the distribution of the seepage field of the surrounding rock. The calculation results based on three fluids with different pH values show that the head variation range is the largest when pH = 2, followed by pH = 4, and pH = 7 has the smallest head variation range. The water inflow in the stope was also directly affected by the hydrochemical environment. With the change of pH, the water inflow at each boundary of the stope increases exponentially.

Fluorescence visualization dynamics of carbonated water injection processes in homogeneous glass micromodels

Understanding the flow behavior of oil–water phases in porous media is crucial for revealing the kinetic mechanisms underlying oil–water movement. In this study, we designed and constructed a fluorescence visualization experimental platform for carbonated water injection (CWI). We evaluated the effects of nine distinct injection rates on oil production and CO2 sequestration capabilities using three types of homogeneous porous structure micromodels. The microscale oil–water seepage behavior at the pore scale was also explored. The experimental results indicate that optimally increasing the injection rate can improve displacement efficiency, whereas excessively high rates may lead to viscous and capillary fingering. The geometry of the micromodels, particularly the hexagonal matrix, demonstrates potential advantages in enhancing oil displacement due to its uniform flow paths. The capillary number (Ca) significantly influences oil droplet capture behavior within micromodels. For instance, micromodel A captures the largest ganglion area at low Ca values, with a noticeable reduction in average ganglion area as Ca increases. Conversely, micromodels B and C exhibit similar trends in ganglion size variation at high Ca values, suggesting a more uniform impact of pore structure on oil droplet capture behavior. At elevated Ca values, capillary forces exert a more uniform and controlled effect on oil droplet capture and release, minimizing size variability. Additionally, the injection rate plays a critical role in CO2 sequestration, with higher rates promoting convective mixing between CO2 and reservoir fluids, thereby enhancing dissolution and storage efficiency. The complexity of the pore structure, characterized by the presence of micropores and macropores, affects the dissolution kinetics and mobility of CO2, influencing sequestration efficiency. These experimental findings provide valuable insights into the dynamics of oil–water motion and the potential for enhanced oil recovery (EOR) and CO2 storage, offering essential guidance for optimizing CWI strategies in the petroleum industry.

Evolution of Permeability and Sensitivity Analysis of Gas-Bearing Coal under Cyclic Dynamic Loading

It is imperative to conduct experimental studies on the seepage behavior of gas-bearing coal under cyclic dynamic loading conditions. This paper focuses on the evolution of coal permeability under the combined effects of dynamic loading, static loading, and gas adsorption. The principal conclusions are as follows: (1) As the frequency and amplitude of dynamic loading increase, the development of pore and fissure structures within the coal body becomes increasingly pronounced during dynamic loading cycles, resulting in a gradual rise in permeability. Notably, as the coal approaches its yielding stage, the permeability can increase by up to 47%. (2) The permeability curve is divided into four regions: the compaction reduction zone, the oscillation zone, the gradual recovery zone, and the abrupt failure increase zone. Ultimately, in the failure phase, the permeability surges dramatically, potentially reaching four to five times the initial permeability. (3) When the static loading stage and dynamic load are constant, the rate of change in coal permeability decreases with increasing adsorption amounts. When the adsorption amount is constant, the rate of change in permeability of the coal under dynamic loading increases with the increase in the static loading stress stage, with the maximum increase reaching 75.2%. It can be concluded from the rate of change in permeability and the dynamic loading sensitivity coefficient that the permeability of coal is highly sensitive to cyclic dynamic loading, with increased sensitivity associated with larger static loading stages and decreased sensitivity with greater adsorption amounts.

Characteristics of Overburden Damage and Rainfall-Induced Disaster Mechanisms in Shallowly Buried Coal Seam Mining: A Case Study in a Gully Region

Shallow coal mining in gully regions has resulted in significant subsidence hazards and increased the risk of surface water inflow into mining panels, compromising the sustainability of surface water management and underground resource exploitation. In this study, the chain disaster process caused by shallow coal seam mining and heavy rainfall is quantitatively analyzed. The findings reveal that shallow coal seam mining leads to the formation of caved and fractured zones in the vertical direction of the overlying rock. The fractured zone can be further classified into a compression subsidence zone and a shear subsidence zone in the horizontal direction. The shear subsidence zone is responsible for generating compression and shear deformations, intercepting rainfall runoff, and potentially triggering landslides, necessitating crack landfill treatments, which are critical for promoting sustainable mining practices. The HEC-RAS program was utilized to integrate annual maximum daily rainfall data across different frequencies, enabling the establishment of a dynamic risk assessment model for barrier lakes. Numerical simulations based on unsaturated seepage theory provide insights into the infiltration and seepage behavior of rainfall in the study area, indicating a significant increase in saturation within lower gully terrain. Precipitation infiltration was found to enhance the saturation of the shallow rock mass, reducing matric suction in unsaturated areas. Finally, the disaster chain is discussed, and recommendations for managing different stages of risk are proposed. This study offers a valuable reference for the prevention and control of surface water damage under coal mining conditions in gully regions.

The Seepage Behavior of Rockfill Dam Due to Change of Reservoir Conditions

The Seepage Behavior of Rockfill Dam Due to Change of Reservoir Conditions

Investigate on the mechanical properties and microscopic three-dimensional morphology of rock failure surfaces under different stress states

The macro and micro morphology of rock failure surfaces play crucial roles in determining the rock mechanical and seepage properties. The morphology of unloaded deep rock failure surfaces exhibits significant variability and complexity. Surface roughness is closely linked to both shear strength and crack seepage behavior. Understanding these morphology parameters is vital for comprehending the mechanical behavior and seepage characteristics of rock masses. In this study, three-dimensional optical scanning technology was employed to analyze the micromorphological properties of limestone and sandstone failure surfaces under varying stress conditions. Line and surface roughness characteristics of different rock failure surfaces were then determined. Our findings reveal a critical confining pressure value (12 MPa) that influences the damage features of Ordovician limestone failure surfaces. With increasing confining pressure, pore depth and crack formation connecting the pores also increase. Beyond the critical confining pressure, the mesoscopic roughness of the failure surface decreases, and the range of interval-distributed pore roughness diminishes. Additionally, we conducted a detailed investigation into the water conductivity properties of rocks under different stress states using Barton's joint roughness coefficient (JRC) index and rock fractal theory. The roughness features of rock failure surfaces were classified into three categories based on mesoscopic pore and crack undulation forms: straight, wavy, and jagged. We also observed significant confining pressure effects on limestone and sandstone, which exceeding the critical confining pressure led to increased water conductivity in both rocks, albeit through different mechanisms. While sandstone exhibits fissures running across it, limestone shows shear abrasion holes. Beyond the critical confining pressure, the rock failure surface becomes smoother, leading to decreased water flow blocking capacity. The fractal dimension of Ordovician limestone increases significantly under critical confining pressure, leading to a more complex mesoscopic crack extension route.

Assessment of the Applicability of a Constant-Head Borehole Permeameter Test to River Levees

Abstract The Guelph Permeameter (GP) test, one of the constant-head borehole permeameter tests, is a potential tool for studying the heterogeneous alluvial deposits in the foundation of river levees. However, the applicability in the targeted environment and adequacy of the information obtained by the tests are still unclear. Experiments are conducted in a model ground and in the field to verify the applicability of the GP test concerning underseepage through river levees. Discussions focus on the effects of the groundwater table, seepage behavior during the test, and the representative volume of soil tested. It is noted that for sandy and silty soils, reasonable estimations of the hydraulic conductivity can be made by applying the Reynolds’ solution, but the estimation of the hydraulic conductivity in clayey soil is affected by the macropores in the soil. The GP tests performed in this study have representative volumes on the order of a few to tens of centimeters so that heterogeneity can be investigated at the meter scale. In summary, the GP test is a useful tool for evaluating the underseepage through river levees.

Analysis of seepage behavior of porous asphalt pavement based on scale test and three-dimensional simulation

This study implemented indoor scaled drainage testing and three-dimensional (3D) finite element simulations to analyze the seepage behavior of ultra-wide cross-section porous asphalt concrete (PAC) pavement with voids of 18 %, 20 % and 22 %. The seepage water level curves under the rainfall intensity of 0.5, 1, 2, 10, 50 and 100 years recurrence intervals were obtained by scaled test. The Seep3D software was used to analyze the seepage state of the ultra-wide cross-section porous pavement. Results indicated that void ratio was the paramount factor affecting the drainage capacity, which decreasing porosity significantly intensifies roadway surface water depth over 25 %. Water level curves obtained in scaled test showed similar trends to pore water pressure curves in 3D simulation. According to the theory of non-uniform gradual seepage, the pore water pressure can be used to effectively characterize the seepage state inside the porous pavement. Implementing cross drain with 10 m spacing maximally alleviated inundation by 23 %. Spacing intervals of 50 m, 30 m, and 20 m proportionately mitigated peak value by 4 %, 7 % and 12 % respectively.

Constructing a Microdiffusion-Seepage-Stress Multifield Coupling Model for Nanopore Gas.

Existing research is difficult to fully capture the correlation between gas molecules and pore wall interactions, multiphase flow, and stress distribution in nanopores. Taking gas as an example, a microscopic model was constructed. At the same time, diffusion, seepage, and stress were considered to accurately predict and manage gas transport in nanopores. First, molecular dynamics (MD) simulation methods were adopted to simulate the motion trajectories and interactions of gas molecules in nanopores. Second, a multiscale model was established based on continuum mechanics to consider the interaction between pore walls and gas molecules, and a diffusion equation was established to describe the diffusion process of gas molecules in pores. Then, finite element analysis and porous media models were used to simulate the seepage behavior of gas in the nanopores. Finally, the stress distribution in the pores was analyzed, and the influence of the interaction between the pore wall and gas molecules on stress was considered. The multifield coupling model was experimentally evaluated from three aspects: diffusion coefficient, seepage behavior, and stress distribution. The root-mean-square error (RMSE) and mean absolute error (MAE) of the model in different testing directions were calculated using different simulation tools, such as COMSOL, ANSYS, OpenFOAM, and CFX. The mean values of RMSE and MAE were lower than 0.20 and 0.17, respectively. The constructed model can comprehensively describe gas transmission within nanopores, improving the management accuracy and efficiency.

Study on the pore blocking characteristics and cleaning methods of permeable asphalt mixtures based on accelerated clogging simulation experiments

Clogging in permeable asphalt pavements compromises their functional properties. Understanding clogging characteristics and developing appropriate maintenance methods is crucial. This study integrates indoor accelerated clogging simulations with Grey Relational Analysis to investigate the factors influencing clogging behavior. Computer tomography scanning and seepage model simulations are utilized to examine the distribution of clogging materials, seepage properties, and changes in pore parameters in permeable asphalt mixtures before and after clogging. Additionally, variable frequency vibration tests are conducted to evaluate the impact of vibration frequency on clog removal efficiency. Findings show that the cementation hardening effect of clay progressively seals pores, with sand particles of 0.15 mm diameter exerting the most significant influence on clogging. Wet-dry cycles lead to an accumulation of clay cementation, resulting in a 26.1 % and 72.4 % decrease in pore channel length and volume, respectively, at depths of 0–4.2 cm. The area within this depth range is most prone to clogging, increasing pore tortuosity. Seepage behavior is primarily determined by several main interconnected pores. Vibration test outcomes indicate that frequencies between 100 and 400 Hz are optimal for disrupting cementation blockages.

A fluid–solid coupling model for hydraulic fracture of deep coal seam based on finite element method

The fluid–solid coupling effect is more pronounced in the process of deep coal seam development compared to shallow coalbed methane, exerting a greater influence on production, and cannot be disregarded. Throughout the extraction process, the interaction between effective stress and gas desorption triggers deformation within the coal seam, leading to dynamic changes in both porosity and permeability. This paper has developed a fully coupled gas flow and deformation model that contains the coal matrix and discrete fractures to describe the dynamic gas seepage behavior and deformation of deep coal seams within a coupled wellbore–hydraulic fractures–matrix system. The model's validity is corroborated through the examination of fracture aperture, employing the finite element numerical simulation capabilities of COMSOL Multiphysics. Subsequent to the model's validation, an in-depth investigation into the permeability and production variations under diverse parametric conditions is conducted. This analysis also encompasses the assessment of hydraulic fracture geometry's impact. The simulation outcomes reveal that the permeability alterations during coal seam development are subject to the counteracting influences of gas desorption and effective stress. Moreover, it is observed that an increase in the Langmuir volume strain constant and initial porosity correlates with enhanced production, whereas a diminution in the hydraulic fracture compression coefficient leads to increased cumulative production. Notably, the optimal production is attained when hydraulic fractures are oriented vertically yet asymmetrically relative to the horizontal well.

Effect of particle size distribution variability on the permeability of hydrate-bearing sediments: A CFD study

Natural gas hydrates (NGH) are considered as a future clean energy in the era of carbon neutrality. The seepage behaviour of hydrate-bearing sediment (HBS) and its controlling factors are significant for developing effective production strategies. In this study, we aim to elucidate how the heterogeneous spatial distribution of clayey-silty sediment particles, particularly the non-uniform particle size distribution (PSD) affects the permeability of HBS. An algorithm was developed to generate the heterogeneous spatial distribution of sediment particles based on the PSD of hydrate-bearing cores extracted from Japan Nankai Trough (D50 = 122 μm). A series simulation of fluid flow in porous media based on CFD was conducted for the HBS with varying hydrate saturations (SH). The watershed segmentation algorithm was employed for the identification of pore structure, including pore space and pore throat with a detailed analysis on the pore structure evolution with SH. Host sediments with higher uniformity result in initial high permeability but the permeability reduction with increasing SH is more significant compared with that of lower uniformity. Pore throat width reduced to less than 5 μm with SH above 50% compared with its original width of ∼30 μm. Tortuosity exhibits a linear increase with SH, while permeability decreases exponentially with increasing tortuosity. We propose an improved permeability model (KCpro) based on the classical Kozeny-Carman model accounting for the change in specific surface area for non-uniform PSD in HBS with good agreement. This study confirmed the applicability of CFD modelling as a promising tool to study the seepage behaviour of HBS complementary to classical permeability experimental tests. Results of this study provide fundamental understanding on how permeability evolves in HBS with significant PSD variations in natural gas hydrate reservoirs in nature.

Numerical study of the seepage behavior of droplets in porous materials

This article analyzes the spread and infiltration dynamic process of liquid droplets on the surface of porous materials and establishes a mathematical model of dynamic changes in droplets. To accurately describe the dynamic effects of droplet flow, a mathematical model of droplet dynamics was developed by applying the level-set method, and the seepage process of the droplets was also analyzed numerically. The effects of a series of control parameters on the droplet percolation process are analyzed, and the pore degree of porous materials performed a numerical study of the droplet deformation process. We observe the spread and infiltration process of liquid droplets in porous materials models. It was found that there is a competitive relationship between diffusion and penetration of droplets. The depth of penetration of droplets decreases with increasing viscosity and increases with increasing porosity. The results of the study help to understand the seepage behavior of the droplet on the surface of the porous material.

Fluorescence-Based Image Analysis of Seepage Behavior in Drip Irrigation: Exploring Varied Fractal Grading in Media Permeability

In the recycling of low-value metallic elements, heap leaching is commonly employed. The particle size distribution is a crucial parameter in heap leaching implementation, and the percolation behavior of a heap has always been a focal point in heap leaching technology. This paper utilizes a novel form of ultraviolet fluorescence image acquisition and fluorescence image analysis to investigate the percolation process with different fractal dimension particle size distributions, where the maximum particle diameter is 10 mm. The ore used was low-grade copper ore. The results indicate that the new fluorescence image analysis method can reveal different percolation regions during the heap leaching process, aiding in a better understanding of heap leaching behavior. The combined study found that under irrigation conditions of 10 mL/min, seepage was more uniform under the heap structure formed by a particle gradation with a fractal dimension of 2.2.

Effect of pore structure characteristics on gas-water seepage behaviour in deep carbonate gas reservoirs

The study of gas-water two-phase flow behavior has always been a focus and a difficult problem in the field of oil and gas seepage, and it is related to the formulation of reasonable development plans for oil and gas reservoirs. Deep carbonate gas reservoirs (depth>4500m) develop cross-scale porous media, which have extremely strong heterogeneity and very complex seepage characteristics. To the best of our knowledge, there has been no systematic study on the two-phase flow patterns of gas and water in deep carbonate gas reservoirs. This paper uses cast thin sections, scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) to study the influence of carbonate rock pore structure characteristics on gas storage and seepage capabilities. The results show that the existence of cavities enhances the gas storage capacity of the reservoir and improves the seepage capacity of pore-throat space near the cavity. Fractures can effectively communicate with isolated cavities, enhance reservoir connectivity, and thereby significantly improve the overall seepage capacity of the reservoir. The centrifugal technique was combined with NMR to study the movable fluid characteristics of carbonate reservoirs. The critical flow throat radius and movable fluid saturation of the reservoir reveal that the degree of development of cavities and fractures determines the mobility of the fluid in the reservoir. The fluid mobility in the fracture-cavity-type reservoir is the strongest, while the fluid mobility in the pore-type reservoir is the weakest. Analysis of the normalized curve of gas-water relative permeability under high temperature and pressure reveals that formation water preferentially enters connected pores with low seepage resistance, and the strong storage capacity of cavities can effectively delay the non-uniform intrusion of formation water. The results of water invasion simulation experiments show that the water invasion phenomenon can improve the gas production rate and recovery rate of pore-type reservoirs. However, the formation water flow and channeling phenomena caused by reservoir heterogeneity will cause a large amount of gas to be sealed in the pore space of cavity-type and fracture-cavity-type reservoirs, reducing the recovery rate and economic benefits of these reservoirs. These findings further provide solid fundamentals for efficient and effective exploitation of deep carbonate gas reservoirs.

Multiphysics phase-field modeling for thermal cracking and permeability evolution in oil shale matrix during in-situ conversion process

In-situ conversion process (ICP) is a promising recovery technology for economically effective exploitation of unmatured oil shale. High temperature treatment during ICP facilitates the chemical conversion of kerogen content in oil shale, while markedly improving the accessibility of pyrolyzed fluid through thermally-induced microcracks. Although extensively studied experimentally, the microstructure development in shale is not fully understood due to the lack of consideration for multiphysics effects during ICP at the mineral scale. This paper presents a numerical study on thermal cracking and permeability evolution in oil shale matrix during in-situ conversion process. To this end, a multiphysics phase-field model for investigating thermo-mechanical response, chemical reaction, and seepage behavior is developed, while arbitrary crack growth in heterogeneous shale matrix is naturally predicted using the phase-field method for fracture. Implementing the proposed numerical model, thermal cracking of heterogeneous granodiorite is first simulated, from which the numerical outcome regarding crack morphology is reasonably consistent with experimental data. Permeability evolution of oil shale matrix is found to be attributed to early-stage thermal crack propagation and late-stage kerogen decomposition by correlating multiple variables. Anisotropic permeability is also observed and investigated by examining crack morphology, pore-space interconnectivity and fluid permeation. Further analysis reveals that the shale matrix microstructure, in-situ stress and heating temperatures play vital roles in influencing thermal cracking and permeation behaviors of the shale matrix. The results provide a unique perspective to understanding thermal cracking and permeability augmentation of heterogeneous rocks.

Monitoring model group of seepage behavior of earth-rock dam based on the mutual information and support vector machine algorithms

The establishment of a high-precision piezometric water level monitoring model ensures the safe operation of earth-rock dams. The hysteresis effect of the upstream water level and rainfall should be considered during modeling. In the traditional method, the average factors are used to express this effect, and linear regression modeling is adopted. These factors reduce the accuracy of the model. In this paper, the mutual information (MI) and support vector machine (SVM) algorithms are proposed. MI has a powerful correlation analysis capability, and it is innovatively used to address hysteresis effects. SVM has a strong nonlinear modeling ability, and it is used as a modeling algorithm. During this study, it was found that the lag time of rainfall varied. In view of this characteristic, the concept of an innovative model group, which is an important extension of the traditional single model, is proposed. In the example, the mean square error (MSE) is used as the precision index. Compared with the traditional single model established by linear regression, the MSE of the MI–SVM model group can be reduced by approximately 60.98%–68.75%. Compared with the model group established by linear regression, the MSE of the MI–SVM model group can be reduced by approximately 41.28%–45.45%. The new method effectively improves the accuracy of the model and can precisely monitor the seepage state of the dam. Moreover, it is beneficial for improving the level of dam safety management and can be extended to other fields involving hysteresis effects and nonlinear modeling.

Effects of Clay Content on Non-Linear Seepage Behaviors in the Sand–Clay Porous Media Based on Low-Field Nuclear Magnetic Resonance

Clay is widely encountered in nature and directly influences seepage behaviors, exerting a crucial impact on engineering applications. Under low hydraulic gradients, seepage behaviors have been observed to deviate from Darcy’s law, displaying a non-linear trend. However, the impacts of clay content on non-linear seepage behavior and its pore-scale mechanisms to date remain unclear. In this study, constant-head seepage experiments were conducted in sand–clay porous media under various hydraulic gradients. Low-field nuclear magnetic resonance (LF-NMR) technology was utilized to monitor the bound-water and free-water contents of sand–clay porous media under different seepage states. The results show a threshold hydraulic gradient (i0) below which there is no flow, and a critical hydraulic gradient (icr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. Both hydraulic gradients increased with clay content. Moreover, the transformation between bound water and free water was observed during the seepage-state evolution (no flow to pre-Darcy or pre-Darcy to Darcy). As the hydraulic gradient reached the i0, the pore water pressure gradually overcame the adsorption force of the bound-water film, reducing the thickness of the bound-water film, and causing non-linear seepage behavior. When i0 < i < icr, the enlarging hydraulic gradient triggers the thinning of bound water and enhances the fluidity of pore water. Moreover, the increasing clay content augments the bound-water content required for the seepage state’s change.