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Review

A Review of the Evolution Characteristics and Argillization of Clay Interbeds in Rockslides

by
Qi Song
1 and
Kun Song
2,*
1
College of Hydraulic & Environmental Engineering, China Three Gorges University, Yichang 443002, China
2
Hubei Key Laboratory of Disaster Prevention and Mitigation, Yichang 443002, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(21), 11646; https://doi.org/10.3390/app132111646
Submission received: 26 September 2023 / Revised: 20 October 2023 / Accepted: 23 October 2023 / Published: 25 October 2023
Browse Figures
Versions Notes

Abstract

:
Weak interlayers in rockslides often become the controlling factor for slope deformation due to their poor physical properties and tendency to undergo argillization. Achieving a comprehensive understanding of the weak interlayers will help us comprehend the mechanism of rock slope failure and protect people’s property. Some weak interlayers will develop into clay interbeds after tectonic activity and long-term groundwater action. A comprehensive review of clay interbeds includes a discussion on the formation conditions, the governing factors of the argillization process, and the investigation methods. The clay minerals in clay interbeds, illite, montmorillonite, and Kaolinite, play a major role in the acceleration of the argillization process. The argillization process can be observed through scanning technology and investigated using nonlinear dynamics, statistics, and numerical modeling methods, which may result in limitations for extracting parameters. It is necessary to conclude a unified evaluation standard to find the basic commonness during the argillization process of clay interbeds. It would be a future trend to establish the quantitative relationships among the mechanical strength, micro-structure and content of clay minerals of the clay interbeds during the argillization process.

1. Introduction

Weak interlayers in rock slopes tend to develop into clay interbeds and cause slope deformation due to their poor physical properties and argillization characteristics. Moreover, the presence of clay interbeds in the rock slopes was a significant factor in several major landslides, such as the Vajont landslide [1] in Italy, the Qingjiangping landslide [2] and the Shanshucao landslide [3] in the Three Gorges area of China. However, there is no unified consensus on the formation mechanism, stage division, and evolutionary characteristics of clay interbeds. In this paper, a comprehensive review of clay interbeds in rockslides is presented. The review includes a discussion on the formation conditions, the governing factors of the argillization process, and the investigation methods.

2. Definition and Formation of Clay Interbed

2.1. Definition and Formation Conditions of Clay Interbed

Clay interbeds are formed through the transformation of weak interlayers following weathering or tectonic activity, as well as long-term groundwater processes. The clay in the interbed is between the plastic and liquid limits (Figure 1). However, scholars still have many different opinions regarding the necessary conditions for the formation of argillation.
Based on the research of the Gezhouba hydropower project in China, it has been found that clay interbeds require specific rock foundation and formation conditions [4,5]. The strength gap between the rocks in the lithologic group needs to be significant. And the water–physical properties of at least one type of rock needs to be poor so that it will have enough time for weathering.
The compressive stress of the overlying strata could be the reason for the formation of a clay interbed [6]. By comparing the connection form and arrangement order of silt under different degrees of compaction, the structure of soil with a higher degree of compaction transitions from a loose, honeycomb-like structure to a more compact aggregate or condensed structure. The condensed structure exhibited slight directivity and uniformity. It indicated the effect of the load from the overlying strata [7]. However, since increased pressure could hinder the hydration process, it is necessary to examine whether pressure has a positive effect on the entire argillization process. At the same time, it should be noted that the confining pressure controls the formation of clay interbeds when discussing the formation conditions. In addition, the difference between the actual sample and the original state should also be considered [8].
The formation of clay interbeds is closely related to tectonism. Based on the actual distribution of clay interbeds, the bedding fault zone showed a high degree of consistency with the spatial distribution of the clay interbeds. It has been proven that argillization requires tectonism as a foundation [9]. In addition, tectonism would also promote the enrichment of clay minerals, which accelerated the process of agrillization [10].
To summarize, the necessary conditions for the formation of clay interbeds include suitable parent rock, tectonism, and long-term groundwater action (Figure 2). And in the order of occurrence, tectonism would first cause the formation of a fractured zone, resulting in stress concentration and the retention of groundwater in the loose structure. The loose structure is conducive to the action of groundwater. Besides that, the presence of clay minerals will significantly expedite the degradation of physical properties deterioration and structural alteration in the crushed zone, facilitating the process of argillization.

2.2. Formation Stage of Clay Interbeds

The formation of clay interbeds is the combined effect of macro and micro changes. The change in macro-structure is beneficial for the change in micro-structure. The localized non-uniform stress caused by microscopic changes exacerbates the failure of the macrostructure.
Based on the assessment of structural damage, the argillization and disintegration of soft rock strata need to go through three processes: (1) failure process in the macrostructure; (2) loss of water in soft rock; and (3) the physicochemical process of clay mineralization produced via water absorption of damaged rock mass [11]. This stage division highlighted that the argillization process was a gradual transition from macroscopic disturbance to microscopic destruction, and it could be further divided into: (1) relatively calm lacustrine sediments being deposited and undergoing diagenesis; (2) undergoing intense tectonic activity; (3) physical weathering of rock masses due to changes in environmental conditions and the influence of tectonism; (4) weathering further causing water to penetrate into the rock mass, resulting in chemical weathering that generates various clay minerals; and (5) the function of clay minerals in terms of repeated flocculation and dispersion due to changes in the electric layer. It continuously breaks the particle, and the argillization process continues to occur [12].
The formation stages from the weak interlayer to the clay interbed were further divided based on relevant laboratory experiments [13]: the sedimentary stage of soft rock, tectonic shear stage (interbed of shear fracture zone), groundwater hydrochemical stage (shear fracture zone with clay), and formation stage of clay interbed. This classification only expressed the main behavior in the process of the stage, and there were some differences between laboratory and in-site experiments. The argillization rate is very slow in the laboratory tests, and it could not be completely consistent with the actual conditions of inflow and loss of water.
The evolution process could be divided into the ion exchange adsorption stage, mineral dissolution stage, and softening and disintegration stage according to the chemical elements. It was closer to the actual situation. The groundwater action was regarded to be the main influence in the whole evolution process [14]. Clay mineralization occurs not only in the final stage but also at different rates in each stage.
According to the grain size composition and thickness of the clay interbed, it can be divided into five types: (1) clay, (2) clay with breccia, (3) clay with silty sand or silty sand with clay, (4) clay membrane, and (5) breccia with clay [9].
The formation stages of clay interbeds have been discussed, including structure damage, laboratory experiments, chemical elements, and grain size. These views are mutually supportive and complementary to one another. But it still lacks a more uniform criterion for classification without considering the parent rocks, structural types, and mineral compositions of numerous clay interbeds as a basis.

3. Parameter Changes during the Argillization Process

The argillization process is a complicated process concluding disintegration and softening, structural change, the transformation of clay minerals, etc. When considering changes in the external environment and other factors, the process of argillization becomes more complex and difficult to distinguish. But, the above-mentioned changes have some degree of promoting or inhibiting effect on each other (Figure 3). In the process, the required energy should be transformed from the mechanical energy provided by tectonism to the chemical energy of a chemical reaction. Furthermore, the energy is partly displayed in the form of phase transformations, thermodynamic changes, ion transfers, and structural changes. So as to produce all kinds of reactions in Figure 4 and ultimately lead to the complete occurrence of the argillization process.

3.1. Disintegration

Groundwater action is an indispensable part of the argillization process, and the parent rocks will inevitably soften and disintegrate to a certain extent under its influence. In order to understand the parameter changes during the argillization process, the first step is to understand the change in parameters during the disintegration process.
Water acts as a medium, uniformly transmitting pore pressure to both fractures and the pore. And the pore pressure could be passively enlarged due to the swelling action. It could also be used as a lubricating medium to reduce the effective anti-slip force. At the microscopic level, the hydroxide radicals decomposed by water would enter the lattice. The stronger valence bonds would be transformed into OH-OH bonds, which greatly reduces the shear strength of the sandwich. However, it should be noted that the degree of cementation, rather than the composition of minerals, was the primary controlling factor in the process of water-immersion softening failure. Attention should be paid to differentiate them when parameters are considered in laboratory experiments [15].
Nowadays, indoor disintegration and softening tests are mainly carried out by immersing the material in still water. Although a complete simulation of immersion and water loss under natural conditions was not possible, it still had some significant reference value. Through a long-term immersion test, the immersion softening coefficient of mudstone in the reservoir water level fluctuation zone decreased by 10~20% compared with the laboratory results. The amplitude decrease between silty mudstone and mudstone was different. But the softening coefficient all decreased with increased immersion time. And the test data showed a convergent trend, which indicates the timely occurrence of water–rock interaction [14].
Based on the structural change, the disintegration of mudstone was believed to mainly be due to the change in the macroscopic fracture structure and the simultaneous occurrence of rock softening after the mudstone met water [16]. Both of them were closely related to the microstructure of mudstone itself. The disintegration process would be greatly accelerated after undergoing the cycle of immersion–dehydration–reencountering water. In the first three cycles, the degree of deterioration of the internal friction angle was significantly lower than that of the cohesive force.
Chemical changes also occur simultaneously and take on a gradual state during the disintegration process [17]. Zhou analyzed the saturated softening process of siltstone and argillaceous siltstone from the view of the change in chemical composition [18]. The aqueous solution changed from weak alkalinity to acidity in the whole saturation process. And the critical point of water–rock chemistry would be reached when saturated with water for 3 months. Before 3 months, the ion concentration would decrease due to active exchange and adsorption. After 3 months, the ion concentration recovered and began to stabilize due to the gradual weakening of the aforementioned two effects.
Longitudinal and transverse wave velocities were obtained from a saturated softening test on Gonghe tunnel shale using an acoustic wave test [19]. A saturated softening equation was deduced, which has significant implications for reducing the disintegration test pattern and duration. To a certain extent, it overcame the difficulty of the softening property being greatly changed due to the different mineral composition and expansion potential, thus avoiding a significant amount of workload.
Attention should also be paid to the selection of parameters that describe the characteristics of the argillization process and to select the most “sensitive” parameters, such as using fractal dimension to describe the collapsibility of mudstone [20]. The fractal dimension of mudstone disintegration is positively correlated with the initial fragmentation and the number of disintegration cycles. Among them, the fractal dimension varied for the same lithology in different areas, which could provide a more accurate representation of the collapsibility of different rock masses.

3.2. Environmental Impact

Cyclic changes in different environmental conditions would have a great impact on argillization. G. Pardini [21] thought that periodic changes in temperature would cause clay interbed to undergo freeze-thaw cycles. The higher the cycle number, the greater the degree of argillization would be. And the periodic change in water level would cause the clay interbed to undergo a dry–wet cycle, accelerating the destruction of the rock mass structure. So, the argillization process was promoted.
Zhang, D. [22] carried out 24 cycles of mudstone disintegration tests under various environmental conditions, including wetting-drying (WD), saturation (ST), refrigeration-heating (RH), a combination of wetting–drying and refrigeration–heating (WDRH), and a combination of saturation and refrigeration–heating (STRH). Mudstone disintegration was divided into collapsing disintegration, exfoliation disintegration, and imperceptible disintegration. The specific disintegration is shown in Figure 5, and the main cause of this disintegration is hydrothermal interaction, which is also the main cause of the chemical effect of argillization. The disintegration rate was closely related to the number of cycles. When subjected to varying environmental conditions, the disintegration rate of the material differed during the initial stage. However, it gradually became more consistent after 16 cycles. It is significant for the prevention and treatment of reservoir landslides with clay interbeds.

3.3. Macroscopic Mechanical Properties

Most of the clay interbeds originated from weak interlayers. After argillization, the mechanical and engineering properties of weak interlayers would significantly deteriorate. During argillization, the deterioration of mechanical properties can be influenced by other physical property parameters in a mutually dependent manner. The certain relationship among the physical property parameters should be established in terms of the overall study.
There are numerous related parameters that require an index to represent them synthetically. The natural property index was adopted to include water content, compactness, storage environment, composition and content of clay minerals, consistency state, and comprehensive evaluation of strength [23]. And through the natural property index, correlation equations were established between particle size composition and shear strength parameters under various consistency conditions. The friction coefficient decreased rapidly with an increase in clay content. When the clay content exceeded 30%, the friction coefficient decreased gradually as the clay content increased, eventually reaching a stable state.
Clay interbeds in different places have different critical water content. And the critical water content would also change with different stages of clay interbed formation [24,25]. The moisture content of clay interbeds directly affects their macroscopic mechanical properties. With the increase in water content, the cohesive force increased first, then decreased after reaching the critical water content value. And when the water content exceeded 19%, the stress–shear displacement curve of the clay interbed would change from an obvious gradient to a gentle gradient. The angle of internal friction decreases with an increase in water content in red sandstone interlayers. The cohesive force increased first and then decreased with the change in water content [26,27]. If the water content suddenly increases sharply due to continuous rainstorms, the ongoing process of argillization would further decrease the shear strength [28].
When the clay content is low and the coarse particle content is high, the shear strength will still be high. This is because the coarse particles play a skeleton role when subjected to shear. Moreover, the coarse particles would be uniformly distributed in the matrix if the clay content were high. The particles were separated by the clay filling, and the main slip under shear would occur at the clay layer, which facilitated directional arrangement. As a result, a low shear strength was obtained. Therefore, the dynamic response characteristics of a bedding rock slope will be affected to a certain extent by the particle content.
The grain size distribution trend, relief degree, and thickness will also influence the shear strength of clay interbeds [29,30]. When the relief degree of the slip surface exceeded the thickness of the clay interbed, it resulted in a higher shear strength due to the slip-up effect caused by the change in contact mode.
The dynamic strength index of clay interbeds is much higher than the static strength index [31]. Based on the clay interbed found in the left bank of the Xiaolangdi hydropower project of the Yellow River, the suggested value of the total dynamic strength index was obtained.
The temperature will affect the shear strength after a critical degree, although the normal temperature did not play a strong controlling role in the formation of clay interbed. If the temperature exceeded 800 degrees, sandstone would completely change from shear failure or tensile failure to total collapse. But before 800 degrees, the shear strength of sandstone only fluctuated slightly [32]. The peak shear strength, residual shear strength and terminal normal stress all display an exponential variation with temperature, i.e., initial fluctuations or a slight increase, then a dramatic decrease, achieving a threshold temperature of 400 °C. The secant peak shear stiffness declines by 43.79–70.48% in a temperature range of 400–800 °C due to enhanced ductility and decreasing peak shear strength [33].
Clay interbeds often exist at great depths, making them difficult to evaluate. However, the relationship between the in situ stress of a rock mass and the engineering geological properties of weak interlayers could be revealed through simulation [34]. In the simulation compaction experiment, the process of dehydration, compaction, and consolidation of soft interlayer under saturated conditions was reproduced. It had great reference value for the evolution of clay interbeds, which was at a great depth.
It is more important to explore the factors that lead to the change and its change trend during the study of the physical and mechanical properties of clay interbed. The mechanical strength of clay interbed is closely related to the water content, and the clay interbeds from different locations have varying critical water content. In this paper, the shear strength is mainly taken as the mechanical strength index, and it is necessary to differentiate dynamic strength and static strength. In addition, the state of the initial stress field should be considered for different depths of clay interbed. Meanwhile, there are a few studies related to the residual strength of clay interbeds, which need to be addressed urgently.

3.4. Microstructure and Characteristics

Macroscopic phenomena are the external manifestations of microscopic structures and characteristics. The macroscopic physical properties must be linked to the microscopic structure and material properties in order to facilitate a mutual response. The decline in rock strength can be attributed to the initial inhomogeneous distribution of numerous microfractures and pores in the rock, which tend to be easily destroyed over time, and not due to the inherent weakness of the rock mass itself.

3.4.1. Structure

Based on the concepts of matrix clay and bonded clay, a study of soil particles was extended to include directional alignment. The concepts of “agglomerate structure” and “texture” were put forward one after another, and then the research stage transitioned from qualitative research to semi-quantitative research. Meanwhile, the research results and indicators, such as pore fractal characteristics, directional frequency, anisotropy, structural entropy, fractal dimension index, and various models, obtained during the process have greatly advanced the study of clay interbeds.
Scholars often define the different microstructures of clay interbeds to show their main characteristics. According to the microstructures of clay interbeds, various texture types have been utilized to illustrate the different stages of the argillization process, such as sheet-like lamellar structures, scale-like frames contacting sponge-like structures, heterogeneous agglomerated basement structures, massive overhead filling structures, silt-mudstone containment structures, scale-like imbricate structures, and massive mosaic structures [35].
The force of the connection force between the structures is also an important factor that affects the stability of structures. The decrease in capillary force between mineral grains was the main reason for the change in three softened slate structures. At the same time, the cohesive force was not affected greatly, and the cohesive force would decrease as a whole [36].

3.4.2. Pore

The porosity will show different phenomena at different pH values [37]. When the pH value was 1, a large number of honeycomb-like clay minerals, dissolution voids, micro cracks would appear. The voids were filled with a large number of iron and aluminum oxides, peroxides and aluminosilicate. When it was 3, local kaolinite enrichment would occur. When it was 5, sheet-like mica minerals would curl, peel off and accumulate at the entrance of dissolution holes and cavities. Sheet-like mica minerals would accumulate. When it was 7, flocculent clay minerals appeared, and micro-dissolution could be seen on the surface of albite.
Tan, Y.Z. et al. [38] studied the argillization process through the response evolution law of pore distribution characteristics, load and dry–wet cycle times. Both expansibility and disintegration were observed in the mudstone under load. The load restrained the expansion of the mudstone and promoted its disintegration. The pore volumes of compacted samples decreased significantly after undergoing dry–wet cycles. It also exhibited bimodal characteristics, primarily distributed within the pores of 0.3 and 10.
The underlying cause of the disintegration and eventual weakening was the microstructural changes in soil particles and pores. It was attributed to the change in the joining mode between particles [39]. Transition changed from surface-to-surface contact to surface-to-edge and surface-to-corner contact (Figure 6) [40]. The experiment led to an increase in the number of pores and a peak in the distribution of pore diameters. In addition, the rate of rock fracture growth would gradually decrease until reaching equilibrium as the number of dry–wet cycles increases, but the porosity disturbance would show a change from large to small [41].
There are differences between the water absorption capacity and swelling force of the remolded samples of silty mudstone and sandstone–mudstone interlayers. Furthermore, it can be explained from the pore view [42]. The pores in the remodeled silty mudstone samples were intergranularly compacted residual pores, and the presence of intergranular pores allowed for the formation of reservoir spaces and channels for fluid migration. So, the samples were easy to absorb water. However, the pores in sandstone–mudstone interlayers consisted of intergranular pores and interlayer pores. Even though they had the same capacity, they could store and transport water, and the structure was different. Water can enter the sandstone–mudstone interlayer, remolding the sample quickly and uniformly. Eventually, it would lead to the swelling of the rock sample.

3.5. Clay Minerals in the Process of Argillization

Because of the inhomogeneous stress resulting from the hydrophilic swelling of clay minerals, scholars believed that clay minerals played a controlling role in the process of argillization. It is more accurate to use mineral composition or chemical composition directly to describe the state of clay interbed.
Clay interbeds mainly consist of an illite, montmorillonite, kaolinite or illite/montmorillonite mixed layer. It should be noted that clay minerals of different types and origins exhibit variations in layer charge magnitude and distribution, ion exchange capacity, and expansibility. Clay minerals can also be transformed into each other. The study of clay minerals should be synchronized with the characteristics of structural change at each stage. Then, a more comprehensive understanding of each of them could be obtained (Figure 7) [43].
The type and content of clay minerals should be determined first. The standard methods used to measure clay content are the hydrometer method and the pipette method, and they are complex and difficult to replicate. Emmanuel Arthur [44] provided alternatives by using the clay estimation model based on vapor adsorption. By comparing the clay content estimated via the modified model, it is evident that the estimation of the measured clay content has significantly improved. And thus, it had great practicality.

3.5.1. Expansibility of Clay Minerals

The expansiveness of clay minerals is not only related to their properties but also restricted by the orientation of the bedding plane in landslides. With the increase in the angle between the bedding surface and the end surface, the expansion angle decreased gradually. When the angle between the bedding surface and the end surface was 0 degrees, the maximum expansion rate was 3.54%. This explains the high probability of clay interbeds existing in gentle dip landslides [20]. The expansiveness of clay minerals under different cementing methods was also found to vary [45]. Calcareous cement would produce a greater expansion force than siliceous cement.
The hydrophilic swelling of clay minerals mainly causes the hydration of groundwater. The mineral’s crystal form changed after absorbing water. It further alters the bonding of clay minerals, causing the entire structure to become loose and destroyed. Among them, montmorillonite is the clay mineral with the strongest expansiveness. So, it plays a controlling and promoting role in the overall hydration process [46].

3.5.2. Changes in Clay Minerals during the Argillization Process

Clay minerals are essential components in the argillization process. However, scholars are still studying whether there are necessary requirements for the types and quantities of minerals that are needed.
Based on the alteration zones at different depths, G. Riedmiiller [47] suggested that the argillization process consists of three successive clay transformation processes. They are illitization, montmorillonitization, and kaolinitization (Figure 8).
By comparing the clay mineral compositions of the mud–sandstone in the sliding body to those in the argillization zone, it was observed that the content of chlorite and illite increased gradually. The montmorillonite content remained unchanged, and the main reactions vary slightly depending on the pH values of the external aqueous solutions. When the acidity was high, it would mainly result in dissolution. Otherwise, it would mainly involve hydration, hydrolysis, and ion exchange. Overall, the higher the acidity level, the more intense the reaction will be. But in all cases, the reaction will reach a state of dynamic equilibrium after a certain period of time [13].
The reason that montmorillonite played a controlling role in argillization was first due to the fact that it disintegrated rapidly. Then, it would result in positive ion exchange and enrichment, which would influence the physical and chemical transformation of clay minerals [48], and the swelling of the montmorillonite crystal could be described through the thermodynamic relationship between the swelling pressure and suction [49]. The diffusion bilayer model was used to explain the osmotic swelling mechanism. It should be noted that the properties of liquids differ significantly between the surface of clay and the bulk phase. Moreover, it was determined that the macroscopic theories, such as DLVO, were not applicable to these unique microscopic layer-by-layer swelling phenomena.
Tan, L.R. [50] argued that argillization only required a specific quantity of clay minerals rather than specific types. Based on sampling from various locations, the investigation of clay interbeds and adjacent soft rocks revealed the presence of clay minerals in both. At the same time, even if the sandstone or siltstone were broken by tectonism. The argillization phenomenon has not yet occurred due to a lack of sufficient clay minerals.
In fact, the presence or absence of montmorillonite was not the decisive factor in mudstone argillization. Sufficient amounts of illite and kaolinite could also induce argillization [51].
Kaolinite can promote the argillization process. The electrostatic stress on the surface of kaolinite clay particles could generate a gravitational field. This attracts water molecules near to the surface of kaolinite clay particles and arranges them into a tight and orderly manner, producing a “water absorption” effect. Moreover, it could also enhance water–rock interaction and facilitate the formation of clay interbeds in fractured rock masses [52].
Illite can promote the formation of clay interbeds [53]. Clay minerals in the red strata of Sichuan Province are primarily non-swelling illite, which is distinct from the interlayer expansion mechanism observed in montmorillonite (Figure 9). It belongs to the intra-molecular swelling mechanism because the cohesive force between the cells was weakened after encountering water. Then, the water molecules were able to enter the cells, increasing the spacing between the cells and resulting in the expansion of the particles. The particles would then form the stress field due to electrostatic attraction. The stress field would lead to the macroscopic phenomenon of the “water film effect on the surface of clay minerals in mudstone”, further promoting the argillization process.
Different compositions and fabrics of clay minerals can result in varying degrees of argillization. The higher the ratio of kaolinite to montmorillonite, the muddier the degree of gangue would be [54].
The variation in specific ion concentrations could indicate the level of hydration, reflecting changes in the degree of argillization [55]. Based on the siltstones found in the Gezhouba hydropower project, it is evident that the clay-enriched substrate junction contained a higher concentration of montmorillonite and adsorptive Na+. It was prone to hydration–dispersion softening and disintegration. This process and the disintegration caused by the micro-fissure adsorption effect in siltstone could occur simultaneously. Furthermore, they would promote each other, accelerating the softening, disintegration and argillization of the soft rock.
The immersion of brine on the clay interbed also had effects on clay minerals [56]. The main mechanism for softening under brine immersion is the dissolution of particles contained in the intercalation in water. The particles would cause the clay mineral to swell and soften when exposed to water. The lower the concentration of brine (referring mainly to the salt content, specifically the content of Na+), the more pronounced the impact on the clay interbed would be. The higher the concentration of brine, the smaller and slower its effect would be.

4. Observation Technique and Analysis Method of Clay Interbed

4.1. Observation Technique of Clay Interbed

Engineering geology is closely related to measurement and observation technology. In the present research, it is not possible to fully observe the formation of clay interbeds. It is necessary to utilize a range of new observational techniques combined with clay interlayer image processing technology to extract parameters for different perspectives. This will enhance both qualitative and quantitative research overall.

4.1.1. Digital Speckle Imaging Technology

Digital speckle imaging technology is non-contact scanning, which does not damage the sample. Moreover, it is suitable for analyzing the three-dimensional deformation characteristics, stress, and strain analysis of samples under static conditions, and it can automatically visualize the output results for easy observation and recording.
Through digital speckle imaging technology, the position and rupture angle of the broken surface could be determined in the unconfined compression test, and surface deformation could be observed under the unidirectional stress of clay interbeds. It has been proven that digital scatter imaging technology was of great help in the evolution of the entire process [57].

4.1.2. Scanning Electron Microscope (SEM)

Scanning electron microscopes (SEMs) are currently the most popular observation technique. Due to their simple operation, adjustable magnification, large depth of field, and convenient sample preparation, they have become a widely used observation method. The image result is convenient for analysis and operation, offering greater flexibility.
The SEM (scanning electron microscope) could be used to scan the clay interbed and analyze it using MATLAB image processing and gray histogram [58], and then the parameter formula for the pore ratio of the clay interbed plane was further preliminarily fitted.
The SEM image could also be analyzed by using the weighted mean nonlinear median filtering method (Figure 10). Based on watershed segmentation of labeled areas, we were able to accurately extract the micro-fabric parameters of clay interbeds. The obtained data showed a clear trend of change along the longitudinal section. It provided support for the ‘page like’ (stratification characteristics) clay interbeds [59].

4.1.3. Transmission Electron Microscope (TEM)

Transmission electron microscopy (TEM) can be used for observations in the order of 0.1~0.2 nm, with magnification up to millions of times, and it is far beyond the limits of an optical microscope. However, it is not popular in geotechnical research due to its high requirements in terms of sample preparation. In addition, its results will be greatly influenced by the sample preparation effect.
The disintegration process of soft rocks could be studied using a high-resolution transmission electron microscope (TEM). By comparing the distribution of the atomic lattice on the basis of the microstructure, mineral water absorption expansion was proven to be not the main cause of disintegration [60]. Based on the theories of an energy dissipation cycle in thermodynamics, fracture mechanics, and the fractal strength of rock, a calculation method for new surface energy was derived, and then the law of new surface energy and energy utilization changing with time during disintegration was revealed.

4.1.4. X-ray Diffraction (XRD)

X-ray diffraction (XRD) is mainly used to study the crystal structure of metals, and it is widely used to obtain the composition and content of various substances in clay interbeds. However, this is only a preliminary application of qualitative and quantitative analysis of XRD. The final data can only be used for statistical analysis and comparative analysis. Further development is needed for dynamic changes, analysis of sample structure defects and other applications under special circumstances, such as low temperature, high pressure, instantaneous, etc.

4.1.5. Automatic Fabric Goniometer

The function of an automatic fabric goniometer is similar to that of XRD, but it is not limited by the particle size of clay minerals. Through the use of an automatic fabric goniometer, the azimuth inclination angle of any crystal plane in any mineral can be statistically determined, and the composition of the mineral crystal axis can be obtained through further analysis. If combined with the Videolab image analysis system, it could describe the orientation and anisotropy of the microstructure of clay soil through the use of interrelated indexes [61].

4.1.6. Computed Tomography (CT)

Computed tomography (CT) can scan the structural changes of the sample at a specific depth. It is characterized by a short scanning time, layered images, and a flexible angle. However, the CT image is presented in grayscale, which is not as clear as the X-ray image. However, it has obvious advantages in terms of detecting the development process of microscopic damage in the internal structure of samples [62,63].
Li X.N. [37] used CT technology to scan the entire process of rock fracture failure at the microscopic level, and scanning images were obtained (Figure 11) that were directly related to the stress–strain curve. Based on the gray histogram, the characteristics of damage evolution were further studied. The uneven distribution of fractures and defects in the rock under the unified stress state was reflected, and it showed the phenomenon of the layer having different damage evolution characteristics.

4.2. Analysis Method of Clay Interbed

Nowadays, the research methods used for clay interbeds are based on concepts from mathematics, statistics, energy, and other disciplines, and the various parameters of the entire argillization process are simulated and explained through them.

4.2.1. Nonlinear Dynamics Analysis Method

Changes in the argillization process can be considered internal changes in the energy system. The concept of ‘equilibrium’ can be further studied. Based on the phenomena of seepage, chemical reactions, ion migration, and structural damage and destruction that occur during the process, research can be conducted from three different perspectives: thermal balance, mechanical balance, and chemical balance. Most of the models currently established assume continuous media, starting with I. Biot’s porous media theory and progressing to mixture continuum theory. Nowadays, the non-continuum correlation model is only applied to macroscopic rock fractures, and it is rarely applied to micro-fine and argillization processes. After analyzing from different perspectives, it should ultimately be connected with objective changes, such as structural changes, generating theory and practice, and partially deducing the evolution.
The nonlinear dynamic analysis method, which integrates the above three aspects, is applicable to studying the argillization process. It exhibits a high level of agreement with the energy and chemical changes associated with water–rock interaction. Based on the clay mineral microstructure model proposed by Tuller, M. and Or, M. [64], Liu, Z., et al. [65] introduced structural elements to describe the characteristics of silty grains. Two types of typical soft rock microstructural units, namely granular structure and dense strip structure, were established. Through the principle of renormalization, the critical criterion for microstructure evolution in the softening process was analyzed. The quantitative relationship between microstructure and mechanical properties of soft rock was determined. By incorporating the coupling effect into the analysis of nonlinear dynamics, the softening behavior of soft rock can be divided into three distinct stages: near the equilibrium state, the principle of equilibrium self-organization state, and critical non-equilibrium phase transition. Therefore, Zhou [66,67,68] established the bifurcation evolution model for the formation of dissipative structures in soft rock. The critical point of structural change would be evident at 3 and 6 months, although there was no self-organizing critical point at 6 months. It was completely consistent with the laboratory test.
Based on chemical dynamics and the liquid–solid phase transformation of water–rock interaction, it has been demonstrated that elements with different chemical properties exhibit varying processes of spread, release, and combination. However, it would ultimately result in the destruction of the rock structure and mineral lattice. Moreover, the process of the recombination of elements in the rock would greatly disturb the original structure; therefore, it could cause rocks to undergo argillization. A significant chemical composition marker of the argillization process is the mole ratio of Si/Al.

4.2.2. Statistic Methods

There are many parameters involved in the argillization process, and the reactions occur simultaneously. However, statistics are well-suited for dealing with multiple variables, and internal correlation can be analyzed using a large amount of data, which play a significant role in mechanism research.
Marchuka, S. [69] conducted a statistical analysis using Pearson’s correlation coefficient method and an r-type cluster tree. The ion concentration, which could reflect the degree of reaction of specific clay minerals, was obtained. Three groups of ions were classified as follows: Na+, Mg2+ and Cl−; K+ and Ca2+; F−. This classification in statistical analysis showed a close correlation between Mg2+ and Na+ ions. Montmorillonite, calcite, and fluorite were the sources of Na+, Ca2+, and F−, respectively. It was concluded that K+ could be used as a standard to evaluate the hydrolysis and ion exchange between illite and montmorillonite under the influence of a strong acid because only a small fraction of K+ ions could be separated by dissolution. Furthermore, by analyzing the comprehensive data, it was found that clay minerals exhibited a strong fixation and adsorption effect on K+.

4.2.3. Numerical Modeling Analysis Methods

Numerical modeling analysis methods are useful in attempts to unify the meso-scale and macro-scale in terms of argillization. The Finite Element Method (FEM) and Discrete Element Method (DEM) can be used to connect the meso- and macro-scale in order to obtain a more accurate numerical model. Through FEM, a mesoscopic numerical model of a clay interbed could be established on a scale of 100 microns. Peak strength can be analyzed through numerical shear tests [70]. However, because the model is usually simplified from the actual clay interbeds, it is often considered a special case in terms of analysis. The universality of it needed to be improved.
At present, DEM is mainly used to simulate the movement and deposition process [71,72] because its simulation range occurs during the early stage of argillization evolution. The early stage does not show an obvious control effect. It is rarely used in the entire process of clay interbed evolution.

5. Conclusions

Clay interbeds play a crucial role in the stability of landslides, and their formation requires long-term groundwater action following tectonic activity within suitable strata. The strata have to contain a certain amount of clay minerals rather than a specific kind.
The change of clay minerals in clay interbeds will greatly accelerate the entire process. The three clay minerals, illite, montmorillonite, and kaolinite, play a major role in the acceleration of the argillization process. Changing levels of Na+ and K+ can indicate the characteristic change degrees of argillization.
Observation techniques and analysis methods with different perspectives can result in limitations when extracting parameters. It is necessary to conclude a unified evaluation standard to find the basic commonness of argillization by comparing the different parent rocks, structural types, and mineral compositions of clay interbeds.

Author Contributions

Conceptualization, K.S.; methodology, Q.S. and K.S.; validation, Q.S. and K.S.; formal analysis, Q.S. and K.S.; investigation, Q.S.; data curation, Q.S. and K.S.; writing—original draft preparation, Q.S. and K.S.; writing—review and editing, K.S.; supervision, K.S.; project administration, K.S.; funding acquisition, K.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 42077239 and 41702378.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Typical clay interbed in a rock slope.
Figure 1. Typical clay interbed in a rock slope.
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Figure 2. Formation of clay interbed.
Figure 2. Formation of clay interbed.
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Figure 3. The relationships among the transformation of clay minerals, softening disintegration, and structural changes in the argillization process.
Figure 3. The relationships among the transformation of clay minerals, softening disintegration, and structural changes in the argillization process.
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Figure 4. The promotion relationship of chemical reaction during argillization.
Figure 4. The promotion relationship of chemical reaction during argillization.
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Figure 5. The relationship between the number of cycles and disintegration rate of soil under different conditions (modified from Zhang, D. [22]).
Figure 5. The relationship between the number of cycles and disintegration rate of soil under different conditions (modified from Zhang, D. [22]).
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Figure 6. Contact mode of clay particles during disintegration (modified from Zhang S. [40]).
Figure 6. Contact mode of clay particles during disintegration (modified from Zhang S. [40]).
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Figure 7. Transformation of clay minerals (modified from Ko’da, B.A., 1981 [43]).
Figure 7. Transformation of clay minerals (modified from Ko’da, B.A., 1981 [43]).
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Figure 8. The sequence of shear zone alteration (modified from Riedmüller, G. [47]).
Figure 8. The sequence of shear zone alteration (modified from Riedmüller, G. [47]).
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Figure 9. The expansion of montmorillonite under the effect of water–rock interaction.
Figure 9. The expansion of montmorillonite under the effect of water–rock interaction.
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Figure 10. Preprocessing of SEM scanning image. (a). Original image; (b) histogram equalization; (c) binarization process.
Figure 10. Preprocessing of SEM scanning image. (a). Original image; (b) histogram equalization; (c) binarization process.
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Figure 11. The CT scanning equipment.
Figure 11. The CT scanning equipment.
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Song, Q.; Song, K. A Review of the Evolution Characteristics and Argillization of Clay Interbeds in Rockslides. Appl. Sci. 2023, 13, 11646. https://doi.org/10.3390/app132111646

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Song Q, Song K. A Review of the Evolution Characteristics and Argillization of Clay Interbeds in Rockslides. Applied Sciences. 2023; 13(21):11646. https://doi.org/10.3390/app132111646

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Song, Qi, and Kun Song. 2023. "A Review of the Evolution Characteristics and Argillization of Clay Interbeds in Rockslides" Applied Sciences 13, no. 21: 11646. https://doi.org/10.3390/app132111646

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