1. Introduction
Coal, as a traditional major source of energy, plays a critical role in the stable development of the economy, society, and environment. However, the impact of water immersion on mudstone in the weak interlayer during open-pit coal mining, leading to instability in open-pit mine slopes, cannot be underestimated in terms of its implications for sustainable resource development. The presence of weak interlayers primarily composed of mudstone increases the probability of slope instability and landslides in open-pit coal mines. These weak interlayers have a loose internal structure and weak bonding forces between particles. Due to the softening effect of water and the swelling characteristics of mudstone in water, the mechanical properties of the weak interlayers are significantly reduced under the influence of rainfall and groundwater, which is prone to rapid disintegration and failure, leading to accelerated instability of the slope rock mass. Such weak interlayers often become the dominant failure surfaces that contribute to slope sliding [
1,
2,
3]. Therefore, it is of significant engineering importance to study the mechanism of immersion degradation of mudstone in weak interlayers. Gaining a comprehensive understanding of the variations in mudstone under water immersion conditions can significantly enhance the assessment and prediction of potential geological hazards during coal mining, effectively reducing safety risks associated with mining operations. Concurrently, these research findings can offer vital insights for optimizing coal mining schemes and resource utilization, contributing to the reduction in resource wastage and environmental impact, and propelling the coal industry towards a more sustainable direction.
There has been extensive research on the interaction between water and rocks, focusing primarily on the following aspects: microstructural evolution of rocks under water–rock interactions [
4,
5,
6,
7], the static mechanical behavior of rocks under water–rock interactions [
8,
9,
10,
11], the dynamic behavior of rocks under water–rock interactions [
8,
12,
13], and the physical and chemical interaction mechanisms between water and rocks [
2,
14,
15]. It can be concluded that the presence of water to some extent disrupts the macroscopic and mesoscopic structure of rocks, weakening the mechanical properties.
The softening effect of water on mudstone is closely related to the characteristics of the internal clay mineral composition, as well as the presence of inherent defects such as pores and cracks. Scholars have conducted relevant research from different scales to investigate the relationship. At the microscopic scale, the clay minerals in mudstone exhibit strong water molecule adsorption behavior in aqueous solutions, leading to significant expansion of the crystal lattice in the stacking direction [
16]. Additionally, clay particles as a whole carry fixed negative charges, and the surfaces of clay particles in water solutions often form a diffuse double-layer structure, causing the clay mineral particles to repel each other [
17,
18]. Consequently, the presence of clay minerals serves as the material basis for the softening of mudstone. Moreover, the strength of swelling characteristics of mudstone is closely related to the types of clay minerals. Kuang and Oueslati [
18,
19] quantitatively analyzed the relationship between water content and interlayer spacing in montmorillonite and kaolinite using X-ray diffraction and neutron scattering, as well as the formation of hydration shells and the difficulty of binding water molecules in illite interlayers. Based on the analysis of the physical properties of crystal structures, Li et al. [
20] pointed out that the hydration expansion of these three clay minerals could be ranked from highest to lowest as follows: montmorillonite, illite, and kaolinite. At the mesoscopic scale, the influence of water on mudstone is primarily studied using techniques such as scanning electron microscopy (SEM), X-ray CT scanning technology, and optical microscopy. These techniques allow researchers to investigate the effects of water on mudstone at a fine-scale level. Huang et al. [
21] investigated the influence of pore structure on the adsorption and desorption behavior of mudstone using water vapor physical adsorption. He pointed out that the moisture absorption of mudstone was mainly controlled by micro fractures rather than pore structure, and the hydration fractures serve as the main pathways for capillary water absorption, controlling the capillary water absorption coefficient and height in mudstone. Huang et al. [
22] developed a three-dimensional mesoscale finite element model of concrete based on in-situ X-ray computed tomography (XCT) images and conducted simulations of three-dimensional uniaxial tensile tests on concrete. The study revealed that the internal inhomogeneity of the mesostructure, resulting from the multi-phase random spatial distribution, has a significant impact on the macroscopic behavior of concrete. Furthermore, the distribution of voids plays a crucial role in influencing the tensile strength and crack mode of the material. Feng et al. [
23] observed the microstructure and morphological changes of mudstone after water immersion. The results indicated that the nonhomogeneous strain of mudstone led to the expansion between adjacent clay mineral aggregates and particle clusters, resulting in the reduction in or disappearance of pores, which causes a decrease in the number of large-diameter pores and an increase in the number of small-diameter pores in the internal pores of mudstone. At the macroscopic scale, the effect of water on mudstone primarily involves investigating the expansiveness. Zhou and Zhong [
24,
25] conducted expansion experiments on mudstone from the Chuanzhong red beds; the results showed that the uneven expansion caused by variations in water absorption capacity and the distribution of clay minerals led to swelling and cracking in mudstone. The water absorption and expansion process of mudstone could be divided into three stages: rapid expansion in the initial short period, continuous expansion with decreasing expansion rate, and stabilization with almost no further expansion deformation.
The weakening effect of water on the mechanical properties of mudstone is primarily studied through experimental means. Liu et al. [
26] conducted uniaxial compression experiments, Brazilian splitting experiments, and triaxial compression experiments on mudstone samples with different water contents. They found that, as the water content increased, the elastic modulus, uniaxial compressive strength, tensile strength, triaxial compressive strength, and cohesion of the mudstone samples all decreased to varying degrees, while the peak strain initially decreased and then increased, and the internal friction angle remained almost unchanged. Yang et al. [
27] investigated the influence of water content on the mode I fracture toughness of mudstone; the results showed that with the increasing immersion time, the fracture toughness of the mudstone samples exhibited a decreasing trend, accompanied by a significant increase in the degree of anisotropy, and the failure mechanism of the mudstone gradually shifted from brittle failure to ductile failure. Liu and Zhang [
28,
29] studied the mechanical properties and energy evolution mechanism of mudstone under different water immersion. It was found that the influence on the mechanical properties of saturated mudstone was characterized by the decrease in characteristic strength, the weakening of brittle deformation, the enhancement of plastic deformation, and the obvious difference of each deformation section. The influence on the energy evolution of mudstone was characterized by the weakening of energy absorption and release properties of rock after saturation, the enhancement of energy dissipation properties, and the different energy distribution rules of each stage. In summary, scholars have conducted research on the impact of soaking conditions on the mechanical properties of mudstone and have gained a better understanding of the macro-scale mechanism of mudstone weakening in water. However, there are relatively few studies that comprehensively investigate the weakening mechanism of mudstone under soaking conditions, taking into account the changes in macro- and meso-structure, mechanical properties, and energy evolution of mudstone.
After being exposed to water, fissures in mudstone significantly proliferate and expand, while internal microstructures and micropores exhibit pronounced development [
30]. When the external force is subsequently applied to the rock, the combination of water-induced fissures and cracks initiated by loading gradually expands, leading to the formation of macroscopic cracks, which ultimately causes the structural failure of the rock, resulting in the loss of its load-bearing capacity [
31,
32,
33]. This process influences the mechanical properties of the rock and represents an accumulation of damage on the macroscopic scale. Cao et al. [
34] introduced the concept of a damage threshold, suggesting that rock damage does not occur immediately upon loading but rather when the stress or deformation reaches a certain level. Yang et al. [
35] used CT scanning technology to study the damage characteristics of rocks; the results indicated that under low stress levels, the degree of compaction of the holes in the rock was greater than the degree of compaction of most initial defects such as micro cracks and pores and the degree of the expansion of main cracks. The introduction of the concept of rock damage threshold can provide a better explanation for the phenomenon of elastic deformation occurring in rocks at low stress levels. For the description of the damage process in rocks under loading, scholars generally believe that rock damage follows a Weibull distribution, so statistical damage constitutive models have been widely recognized as effective tools for describing damage [
36,
37,
38,
39,
40]. The internal structure of mudstone is loose, the internal cracks increase after immersion, and the bearing capacity is further reduced, so there is a possibility of failure and instability under low stress levels. Therefore, it is necessary to establish a statistical damage constitutive model of mudstone considering immersion damage and low-stress loading.
In this paper, a comprehensive study was conducted to investigate the weakening mechanism of mudstone under different water immersion times. The research started by quantitatively analyzing the mineral composition of the mudstone. Subsequently, non-destructive water immersion experiments were carried out on the mudstone, and the macroscopic and mesoscopic structural changes were monitored at different water immersion times. Following this, uniaxial compression strength (UCS) experiments were performed on the mudstone samples subjected to different immersion times to determine their mechanical failure characteristics and energy evolution patterns. Through these experimental observations and analyses, the weakening mechanism of mudstone under different water immersion times was revealed by considering the changes in macro- and mesostructure, mechanical properties, and energy evolution patterns. Additionally, based on the concepts of damage mechanics and damage threshold, a statistical damage constitutive model for mudstone was developed, which takes into account the coupling damage of immersion and low-stress loading, and the accuracy and effectiveness were validated through further experiments and analysis.
5. Mechanical Damage Behavior of Water-Immersed Mudstone
5.1. Geometric Damage Model
Based on the assumption of strain equivalence, a geometric damage constitutive model for the interaction of water immersion and loading was established. The model divides the infinitesimal element into four parts: the undamaged region, the immersed damage region, the loaded damage region, and the immersion–load damaged region, as shown in
Figure 13. The nominal stress and total area of the infinitesimal element are denoted as
and
S, respectively. The effective stress and area of the undamaged region are denoted as
and
S1, respectively. The areas of the immersed damage region, loaded damage region, and immersion–load damaged region are denoted as
S2,
S3, and
S4, respectively.
According to the force balance of the micro unit, it can be seen that:
where
.
The immersion damage variable of mudstone
D1:
The mudstone after immersion is further damaged under load, and the load damage variable
D2 can be expressed as
The damage variable
D of the combined action of immersion and load of mudstone can be expressed as
Substitute Equation (10) into Equation (7) to obtain
The damage variable D can be expressed as
According to the generalized Hooker theorem,
where
and
are the elastic modulus and Poisson’s Ratio after immersion, respectively;
,
, and
are the nominal strain and confining pressure of rock, respectively.
Substituting Equation (14) into Equation (11) to obtain
5.2. Damage Constitutive Model
Defining the immersion damage variable
D1 in terms of the degree of degradation of the elastic modulus
where
and
represent the elastic modulus of the mudstone after 0 h and
n hours of water immersion, respectively.
In the initial stage of loading, the pores and initial micro cracks within the rock tend to close under the influence of pressure, and no internal damage occurs, exhibiting elastic deformation. As the stress or deformation further increases, the micro cracks inside the rock begin to grow, accompanied by the generation of new cracks, leading to continuous damage in the rock. Assuming that the load damage variable D2 in the rock is a function of axial strain , and the damage threshold is denoted as , where represents the strain corresponding to the elastic limit, when the axial strain is less than , the rock undergoes elastic deformation, and the damage D2 is equal to 0. It is considered that the failure of rock micro units is random, and Weibull distribution is used to describe the damage evolution law of rock.
When
, rock damage begins to occur, and the density function of the load damage variable
D2 is given by:
where
is the axial strain;
m and
are the shape parameter and scale parameter of the three-parameter Weibull distribution, respectively.
The load damage
D2 can be expressed as an integral of the density function, which is
Substituting Equation (17) into Equation (18), the evolution law of the load damage
D2 with respect to the axial strain
can be obtained. The expression is
Substitute Equations (16) and (19) into Equation (12) to obtain
Under different confining pressures, the stress–strain curve of rock satisfies the following conditions:
where
is the peak strength;
is the peak strain.
The model parameters are obtained by solving
5.3. Damage Evolution Law of Immersed Mudstone
By using Equations (21) and (22), the model parameters are obtained and substituted into Equation (15) to obtain the damage constitutive model of rock samples, and the damage constitutive model parameters of mudstone under different immersion times are obtained, as shown in
Table 3.
The evolution of damage variables of water-immersed mudstone under uniaxial loading is illustrated in
Figure 14. It is evident that the initial value of the immersion damage variable,
D1, gradually increases with the increasing immersion time. Furthermore, the overall total damage,
D, exhibits a characteristic “S”-shaped trend.
Figure 15 presents the damage evolution process of mudstone in the natural state under uniaxial loading, demonstrating the model’s ability to effectively reflect the mechanical response of rock under uniaxial compression. Upon specific analysis of the damage evolution under uniaxial loading, the process can be divided into three stages, which are described as follows:
(1) Initial damage retention stage: the generation of damage is very slow. Apart from the immersion damage, there is almost no new load damage, resulting in a nearly horizontal evolution curve of damage. It is believed that micro cracks gradually close without extending, and no new damage is generated. This stage corresponds to the elastic stage in the stress–strain curve.
(2) Subsequent damage generation stage: the damage evolution curve exhibits a concave upward trend, and the damage variable increases rapidly, gradually reaching a value of 1. It is considered that weak interfaces and elements within the rock start to fracture, leading to the formation of new cracks that stably propagate and penetrate, resulting in macroscopic fractures. Irreversible plastic deformation occurs, and the material progressively yields and rapidly fails. In the stress–strain curve, this stage is characterized by the rock yielding as strain increases until reaching peak strength, followed by a sudden drop in stress and rapid failure.
(3) Residual damage stage: the damage variable reaches a plateau and stabilizes at 1. During this stage, macroscopic cracks have formed, and the internal structure of the rock is completely disrupted. The rock is unable to maintain its original shape under external loading.
The comparison between the theoretical and experimental curves of uniaxial compression for different immersion times of the mudstone is shown in
Figure 16. It can be observed that the proposed uniaxial compression model in this study effectively describes the stress–strain relationship of immersed mudstone, which validates the correctness and rationality of the model proposed in this study. However, it should be noted that the theoretical model cannot reproduce the compaction stage and the residual stage during the loading process.
5.4. Immersion Deterioration Mechanism of Mudstone
Based on the X-ray mineral composition analysis and macroscopic and mesoscopic structural observations, it can be observed that the mudstone exhibits significant softening characteristics after immersion. The softening process of the mudstone after immersion can be attributed to four stages: the clay mineral expansion stage, mineral fracture and disintegration stage, pore filling stage, and structural damage stage.
(1) Clay mineral expansion stage: The internal defects (fissures, pores, etc.) within the mudstone provide pathways for water molecules to infiltrate, as observed in the disintegration experiment depicted in
Figure 2. It can be seen from
Figure 4 that there is a high content of hydrophilic clay minerals in the mudstone. The invasion of water molecules will quickly interact with clay minerals and cause the volume expansion of the rock. The main component of clay minerals in mudstone studied in this paper is illite. The illite crystal structure unit in mudstone is a three-layer structure cell composed of two layers of silicon sheets and a layer of aluminum sheets. There are counterions between the cells. In the process of interaction between illite clay minerals and water, counterions continue to escape, and the binding force between the cells gradually decreases. Water molecules fill the crystal, and the lattice expands, which will cause the original volume of the rock to increase by 50~60% [
43,
50]. The chemical reaction can be described as follows:
(2) Mineral fracture and disintegration stage: On the one hand, there are different clay minerals and non-hydrophilic minerals in mudstone, and the expansion capacity of different mineral types varies [
20]. Additionally, the distribution and structure of minerals in mudstone are not uniform, leading to uneven volume expansion of the mudstone and the generation of uneven stress within it. This uneven stress causes non-coordinated deformation around mineral particles, resulting in the generation of numerous micro cracks. With the gradual expansion and interconnection of these micro cracks, macroscopic fractures are further formed, as shown in
Figure 7, which is also one of the reasons for the fracture and disintegration of mineral components in the mudstone. On the other hand, based on scanning electron microscopy (SEM) analysis, it is observed that coarse mineral particles such as quartz, albite, and other coarse-grained minerals are embedded in the clay mineral matrix, forming a cementitious connection. When water molecules infiltrate, the clay minerals adsorb water molecules and form a hydration film, causing the clay cementation to transform into water cementation. This deterioration of the bond strength between clay minerals and coarse mineral particles promotes the fracture and disintegration of rock minerals.
(3) Pore filling stage: On the one hand, when the volume of clay minerals expands to a certain extent, the initial cracks and pores present in the mudstone are filled by the expanded clay minerals, leading to a reduction in pore size. On the other hand, due to the fluidity of pore water, larger pores and fractures within the rock gradually get filled with fragmented particles, resulting in an increased proportion of smaller voids, which leads to an improvement in the surface smoothness of the rock and a gradual loosening of its structure.
(4) Structural damage stage: With the continuous infiltration of water molecules, the mechanical effects resulting from the expansion deformation and fracture of minerals within the mudstone accumulate, leading to the progressive accumulation of structural damage, and ultimately leading to the overall destruction of the mudstone’s structure.
The deterioration of the microstructural features of mudstone during water–rock interaction weakens the bond strength between mineral particles, reduces frictional forces, and increases pore water pressure. The stress concentration at the tips of fractures promotes the initiation and propagation of the fracture, ultimately resulting in the disruption of the internal structural system of mudstone. This phenomenon is macroscopically manifested as a softening of the physical and mechanical properties, which explains the significant decrease in elastic modulus and UCS of the mudstone after 48 h of immersion (reduced by 88.84% and 79.83% respectively), as shown in
Table 1.
From the energy perspective, the absorption and release of energy are the driving forces behind the initiation, development, and propagation of micro cracks in rocks. As mentioned in
Section 4.2, compared to the samples in the natural state, the energy storage and dissipation capacities of water-immersed mudstone decrease, while the energy dissipation capacity increases. Based on the scanning electron microscopy (SEM) results of water-immersed mudstone shown in
Figure 8, it is evident that the easy disintegration characteristic of mudstone when exposed to water leads to a looser structure. According to the results in
Table 1, the mechanical properties of the mudstone are significantly weakened. Water–rock interaction is believed to reduce the fracture toughness of the original micro pore tip in the mudstone, and the presence of pore water pressure causes stress concentration near primary defects and new cracks. These effects contribute to the initiation of new cracks in water-immersed mudstone, resulting in an increase in energy dissipation at the macroscopic level [
28]. In addition, the increase in axial load promotes the development, connectivity, and cumulative damage of micro cracks, weakening the supporting capacity of the rock framework. This is manifested macroscopically by a decrease in elastic strain energy and an increase in the surface energy required for crack propagation and extension. The physical softening and chemical damage caused by water weaken the bond strength between mineral particles and disrupt the mechanical structure of the mudstone, driving the rock towards a state of minimum energy.
In conclusion, the water-softening characteristic of mudstone can be attributed to the propensity of clay minerals to expand and disintegrate upon water contact, changes in pore structure, variations in mineral types and distributions, and the presence of pore water pressure.
To comprehensively understand the mechanism of mudstone weakening in water, a combination of laboratory testing and numerical simulation methods can be employed. In addition to laboratory testing, the finite element method and discrete element method can be utilized to simulate the evolution of internal particles and pore structure of the immersed mudstone, which allows for a more accurate depiction of microscopic structural changes within the mudstone. Considering the inherent complexity and randomness of the mudstone’s internal structure, the use of random field modeling [
51] offers distinct advantages in simulating the spatial randomness of the mudstone, enabling a more authentic representation of its structural evolution when encountering water.
By integrating experimental data and numerical simulation results, the mechanism of mudstone weakening in water can be comprehensively explored from multiple perspectives, which provides valuable theoretical support and practical guidance for the field of geological engineering, contributing to enhanced safety and reliability of geological engineering practices.
6. Conclusions
In order to investigate the macroscopic and mesoscopic structural changes, mechanical failure behavior, and damage evolution mechanism of mudstone under different water immersion times, the degradation characteristics of mudstone were observed under different water immersion times, and the degradation mechanism of water–rock interaction was explored. UCS experiments were conducted on mudstone samples immersed for different times to obtain the mechanical and energy evolution characteristics under uniaxial compression. Based on the results, a constitutive model considering the coupling damage of water immersion and low-stress loading was established and validated in mudstone.
(1) Based on the analysis of X-ray mineral composition and macro-microstructural observations, the main cause of macroscopic failure in water-immersed mudstone is the initiation, propagation, and interconnection of micro cracks. The microstructural characteristics of water-immersed mudstone include improved surface smoothness, reduced large-scale pore–crack networks, the appearance of dense crack networks on the surface, and fusion of unit boundaries.
(2) With the increase in immersion time, the mass and relative water content of the mudstone increase and the uniaxial mechanical parameters and energy parameters decrease, and the deterioration of the physical and mechanical properties of the mudstone is more severe during the early stage of immersion. Lastly, the proposed statistical damage constitutive model for mudstone considering the coupling damage of water-immersion and low-stress loading can effectively describe the stress–strain evolution during uniaxial compression of immersed mudstone.
(3) The water-softening characteristic of mudstone can be attributed to the propensity of clay minerals to expand and disintegrate upon water contact, changes in pore structure, variations in mineral types and distributions, and the presence of pore water pressure.