Abstract
Effects of drying-wetting cycles and loads on the mechanical and disintegration characteristics of carbonaceous rock were investigated by triaxial compression tests and disintegration tests in this study. The disintegration phenomena of carbonaceous rock exposed to drying-wetting cycles were analyzed, and the corresponding mechanisms were discussed. The test results showed that drying-wetting cycles lead to the deterioration of particle contact conditions in carbonaceous rock, with a more pronounced impact on the elastic modulus than on the peak strength. After 20 drying-wetting cycles with a range of 5%, the elastic modulus and the peak compressive strength were reduced by 56.86% and 23.71%, respectively, as compared to those at the initial state. Confining pressure was found to mitigate this deterioration in mechanical properties. The disintegration process of carbonaceous rock was elucidated, and its underlying mechanism was revealed from an energy perspective. The mass percentages of each particle size fraction gradually stabilized after five cycles. As the number of cycles further increased, solid particles larger than 5 mm continued to disintegrate, while particles smaller than 5 mm would become increasingly difficult to break down. Particles in the 2–5 mm size range accounted for the highest mass fraction after the complete disintegration of carbonaceous rock. The dissolution of carbonate minerals and cation exchange on the surface of clay mineral particles were the main reasons for the rapid disintegration of carbonaceous rock exposed to drying-wetting cycles. It is believed that the outcomes of this study would shed light on the deterioration mechanism of mechanical properties for carbonaceous rock under adverse natural conditions.
1 Introduction
Large areas of carbonaceous rock, characterized by a saturated uniaxial compressive strength of less than 25 MPa, are widely distributed in Southern China. Such rock typically exhibits strong hydrophilicity, susceptibility to softening and disintegration, low strength, and significant deformability (; ; ; ). Under the drying-wetting cycles induced by seasonal rainfall and evaporation, carbonaceous rock slopes and embankments are highly prone to geological hazards, such as surface cracking and shallow landslides, posing serious threats to the safety of roadway engineering (; ; ; ; Yang et al., 2022; Zen et al., 2021a; Zhang et al., 2024; Zhang and Gao, 2020). The deterioration of engineering properties of carbonaceous rock involves not only the reduction of strength parameters but also severe structural disintegration upon contact with water (). This has become a critical geotechnical engineering problem requiring urgent resolution in the infrastructure construction and maintenance of Southern China (; ; ; ).
Earlier researchers have achieved a series of useful outcomes regarding the properties of varied soft rocks. It is reported that drying-wetting cycles lead to a significant decline in the elastic modulus and peak strength of soft rocks, and also change their failure modes (; ; ; ; ; ; Wu et al., 2024; Yu et al., 2024; Zhang et al., 2021). The slake durability test has been extensively employed to assess the disintegration and water stability of varying rocks. After the durability test, the particle size distribution of disintegrated rocks could be obtained by the sieve analysis method (; Yang et al., 2023). In recent years, some advanced microscopic analysis techniques, such as X-ray computed tomography (CT), scanning electron microscopy (SEM), and so forth, have been introduced to quantitatively describe the spatial configuration and the evolution of microstructure. These techniques provided great help in obtaining a profound understanding of the disintegration mechanisms of varied rocks (; ; ; ). Some novel monitoring technologies for rockburst and soft rock disasters have also been proposed, such as charge induction monitoring (Wang et al., 2026a; Wang et al. 2026b; Wang et al. 2026c). Charge induction monitoring realized on-site dynamic early warning by capturing micro-charge signals generated during rock fracture. This kind of technology would be one of the ideal candidates for combining a static disintegration model with dynamic early warning.
The disintegration phenomenon of carbonaceous rock has garnered widespread attention, but the intrinsic correlation of macro-disintegration with microstructure has not yet been fully understood. Previous studies usually focus on the evolution of mechanical properties (e.g., unconfined compressive strength) with increasing drying-wetting cycles. The combined effects of drying-wetting cycles and external loads on disintegration behaviors of carbonaceous rock have been noticed to be limited. There are also very few studies that examine the disintegration characteristics of carbonaceous rocks from an energy perspective. Therefore, conducting in-depth research on the evolution of mechanical properties and the disintegration mechanisms of carbonaceous rock under the combined action of drying-wetting cycles and loads is of significant theoretical and engineering necessity.
This research plans to conduct triaxial compression tests and disintegration tests on carbonaceous rock. It aims to evaluate the effects of drying-wetting cycles and applied loads on mechanical properties and disintegration of carbonaceous rock. The variation of surface energy of carbonaceous rock with cycles was assessed, and the correlation of surface energy with disintegration was also analyzed. Based on the obtained results, the disintegration mechanism of carbonaceous rock under adverse conditions were discussed. It is believed that the findings from this study would provide a theoretical basis and parameter support for engineering design and long-term stability assessment of carbonaceous rock in Southern China.
2 Experimental investigation
2.1 Materials
The carbonaceous rock used in this study was collected from a highway slope in Guangxi Province, China. This slope experienced a landslide after about 1 week of rainfall in summer, and serious disintegration of the carbonaceous rock was observed on the slope surface. Therefore, this type of carbonaceous rock was selected for wetting-drying cycles and disintegration tests in this study. X-ray Diffraction (XRD) analysis was conducted on one carbonaceous rock sample, and the obtained diffractogram was presented in Figure 1. It can be observed that the tested carbonaceous rock was mainly composed of quartz, illite, dolomite, and calcite. The mass fractions of each mineral are summarized in Table 1. The total organic carbon (TOC) content of the carbonaceous rock was measured as 2.43%, and its specific gravity Gs was determined to be 2.58. Figure 2 presents the particle size distribution curve of the disintegrated carbonaceous rock. The calculated uniformity coefficient Cc and coefficient of curvature Cu values were 31.8 and 2.0, respectively, demonstrating a well-graded gravel (GW). Quintuplicate samples were tested for natural moisture content w and density ρ, and the results showed that the w and ρ values of the investigated carbonaceous rock range from 1.52% to 1.86% and from 2.65 to 2.69 g/cm3, respectively. Deionized water (pH = 6.80 and electrical conductivity <1 μS/m) was used for soaking.
FIGURE 1
TABLE 1
| Mineral composition | Value |
|---|---|
| Quartz (SiO2) | 43.86% |
| Calcite (CaCO3) | 11.70% |
| Dolomite (CaMg(CO3)2) | 13.30% |
| Illite (2K2O·3MgO·Al2O3·24SiO2·12H2O) | 22.75% |
| Pyrite (FeS2) | 3.32% |
| Others | 5.07% |
Chemical elements of tested carbonaceous rock.
All the value presented in the table is referenced to mass.
FIGURE 2
2.2 Triaxial compression test under drying-wetting cycles
Five selected carbonaceous rock specimens were first subjected to saturation in a vacuum chamber, and then their water content values were measured. The average value of the saturated water content was determined as 15.1%. Therefore, the upper limit water content for the drying-wetting cycles was set at 15% here. To simulate the influences of varying rainfall intensities and frequencies in Guangxi Province, three different lower water content values (i.e., 10%, 5%, and 0%) were designed, resulting in three different amplitudes (i.e., 5%, 10%, and 15%) of drying-wetting cycles. For each amplitude, tests were performed after 0, 5, 10, and 20 drying-wetting cycles. Three parallel rock specimens were prepared for each cycle count. Taking five cycles as an example, the drying-wetting cycle path is illustrated in Figure 3.
FIGURE 3
Each drying-wetting cycle consisted of two steps: saturation and drying. For specimen saturation, filter papers and porous stones were placed at both ends of the rock specimen. The assembly was fixed using a triaxial saturator and then placed in a vacuum saturation chamber filled with deionized water for vacuum saturation until the saturation degree reached 96%–98%. For the drying process, the saturated rock specimen was placed in an oven at 105 °C. Drying was stopped when the specimen mass reached the target water content (i.e., the lower limit water content of the drying-wetting cycle). To control the specimen’s water content, the target mass was calculated based on the known target water content (the dry density of carbonaceous rock was tested to be 2.73 g/cm3). By real-time monitoring of the mass during both the saturation and drying processes, the amplitude of the drying-wetting cycles could be accurately controlled.
Triaxial compression tests were conducted with a loading rate set at 0.1 mm/min (see Figure 4). The selection of this loading rate was based on a previous study by Xu et al. (2026), in which triaxial compression tests were conducted on cylindrical carbonaceous mudstone samples (height of 80 mm and diameter of 39.8 mm) with a loading rate of 0.1%/min. As the instability of soft rock slopes primarily manifests as rainfall-induced shallow landslides characterized by a slide mass thickness typically less than 5 m and effective stresses at the slip surface usually below 100 kPa. The confining pressures for this experiment were set at 10 kPa, 30 kPa, and 90 kPa. Taking the unit weight and Poisson’s ratio of carbonaceous rock as 22 kN/m3 and 0.30, respectively, as an example, the thicknesses of overlying rock corresponding to confining pressures of 10 kPa and 90 kPa were approximately 1 m and 10 m, respectively. This designed thickness range fully covered the actual slide mass thicknesses. Research has demonstrated that employing rubber membranes to confine rock specimens is an effective method for simulating low confining pressures, offering advantages of operational simplicity and high testing efficiency. Therefore, rubber membranes with a thickness of 0.5 mm and an inner diameter of 50 mm were adopted to simulate the in-situ confining pressure conditions. The number of rubber membrane layers required to achieve the target confining pressures of 10 kPa, 30 kPa, and 90 kPa was determined through calculation to be 4, 6, and 10 layers, respectively.
FIGURE 4
2.3 Disintegration test
To investigate the disintegration characteristics of carbonaceous rock under different loads and varying numbers of drying-wetting cycles, the modified testing apparatus was employed, as shown in Figure 5. This apparatus is capable of simultaneously applying both load and drying-wetting cycles. The embankment depths generally do not exceed 10 m, resulting in geostatic stresses below 200 kPa. Thus, the applied load levels were set at 0 kPa, 50 kPa, 100 kPa, and 200 kPa here. To prevent experimental errors arising from variations in specimen shape and size, the average particle size of the rock specimens was controlled within the range of 40 mm–60 mm, and their mass was maintained between 1.9 kg and 2.1 kg.
FIGURE 5
Representative intact rock specimens were first selected, photographed, weighed, and their masses recorded. After that, the specimen was slowly placed into the inner cylinder, with porous stones and filter papers positioned at both the top and bottom. The inner cylinder containing the specimen was then placed into the outer cylinder. The designed weights were placed to apply the required load onto the rock mass. Subsequently, the deionized water was introduced into the outer cylinder through the water inlet until the water level reached the height of the inner cylinder’s upper surface. The specimen was maintained under these specified loading conditions while immersed in water for 24 h. During the initial loading and immersion phase, some disintegrated particles generated within the first 30 min were collected. They were used for scanning electron microscopy (SEM) observation to analyse their microstructure. After 24 h of immersion, the specimen was transferred to an oven and dried at 105 °C for 24 h. The specimen was cooled to room temperature and weighed. The disintegrated specimen was subsequently sieved, and the mass of disintegrated particles within different size fractions was recorded. This sequence constituted one complete cycle. The test was concluded after 10 such drying-wetting cycles.
2.4 Surface energy
Many studies have demonstrated that the wedge cracking pressure generated by water absorption on the surface of dry carbonaceous rock is the primary cause of its disintegration (Yang et al., 2023; Zeng et al., 2024a). In a dry state, carbonaceous rock possesses high surface energy. Upon contact with water, the rock absorbs a substantial amount of water, consuming surface energy to perform external work. The disintegration of carbonaceous rock is essentially a process of external work and energy conversion. Changes in surface energy can intuitively reflect its disintegration characteristics. Assuming the disintegrated particles of carbonaceous rock are ideal spheres with radius R, the newly increased surface area ΔA due to disintegration can be expressed as following Equation 1:where Pi is the mass percentage of particles within a specific size range, and is the corresponding average particle size. There is a fact that the slaked carbonaceous rock particles are not spherical and have unique shapes that are hard to be accurately described. Although this simplification method for spheres introduces errors in surface energy calculation, it does not affect the evolution trend of surface energy with disintegration cycles. Using advanced microscopic analysis techniques, such as X-ray CT scanning technology, to accurately capture the three-dimensional morphology of slaked particles would be one of the available answers to settle this limitation.
The formula for calculating the newly increased surface energy ΔW for the first disintegration is presented as Equation 2 (; Zhang and Ouchterlony, 2022):where t is the stress state factor; E is the elastic modulus; σR is the tensile strength of the carbonaceous rock particles; r is the particle size after mudstone disintegration; f is the damage variable of the mudstone. The formula for calculating the newly increased surface energy for each disintegration event after the second (≥2) disintegration ΔWj is expressed as Equation 3:
The cumulative increase in surface energy W after n (n ≥ 2) disintegration events is calculated as Equation 4: ():
During a single disintegration event of carbonaceous rock, smaller particle sizes and higher mass percentages within specific size ranges correspond to greater newly increased surface energy. As the number of disintegration cycles increases, the overall particle size continuously decreases. It will eventually be smaller than 0.25 mm in theory.
3 Results and discussion
3.1 Mechanical properties of carbonaceous rock
The stress-strain curves of carbonaceous rock under different drying-wetting cycle paths are shown in
Figure 6. They can be roughly divided into four different stages:
Crack compression stage (OA section): The stress-strain curve exhibits an upward concave shape. With increasing load, initial cracks or micro-defects within the specimen gradually close. Under the same confining pressure, the slope of the curve in the compression stage decreases significantly with an increase in cycles. The curve for 0 cycles shows the steepest slope, indicating the most rapid pore and crack closure. In contrast, the compression stage for the curve after 20 cycles is noticeably prolonged. This suggests that drying-wetting cycles exacerbate the propagation of internal cracks within the rock mass, and the closure of these micro-cracks during initial loading generates significant non-linear deformation.
Elastic deformation stage (AB section): Cracks and micro-defects within the specimen continue to compact, and internal micro-cracks gradually propagate, resulting in elastic deformation. The slope of the curve in this stage (i.e., the elastic modulus) shows a decreasing trend as the number of drying-wetting cycles increases, indicating a reduction in the rock’s resistance to deformation.
Plastic yielding stage (BC section): With an increase in cycles, the plastic yielding stage becomes markedly prolonged. The peak stress (Point C) decreases, while the corresponding strain increases, signifying a transition in the specimen’s failure mode from brittle to plastic.
Post-peak failure stage (CD section): At this stage, microcracks inside the specimen rapidly form, accumulate extensively, propagate, and merge. It ultimately forms a macroscopic fracture surface. In addition, the stress exhibits a rapid drop after the peak.
FIGURE 6
An increase in both the number and amplitude of drying-wetting cycles leads to the deterioration of particle contact conditions. Internal pores and micro-cracks within the specimen continuously propagate until they interconnect, forming a network. This manifests on the stress-strain curve as a prolongation of the crack compression and plastic yielding stages, a weakened growth rate of strain energy in the elastic stage, a reduction in peak stress, and an increase in peak strain. Under the same drying-wetting cycle path, an increase in confining pressure improves interparticle constraint and friction. This hinders the coalescence of pores and the propagation of cracks, thereby mitigating the reduction in peak strength and elastic modulus. Consequently, the deteriorating effect of drying-wetting cycles on the mechanical properties becomes less pronounced.
Figure 7 presents the variation of peak strength and elastic modulus for carbonaceous rock under a confining pressure of 10 kPa with increasing cycles. After 10 drying-wetting cycles, specimens subjected to cycle amplitude of 5%, 10%, and 15% exhibited reductions in elastic modulus of 39.72%, 40.66%, and 44.36%, respectively, and reductions in peak strength of 11.34%, 13.13%, and 22.55%, respectively. For specimens under a cycle amplitude of 5%, after three different cycles, the elastic modulus decreased by 21.10%, 39.72%, and 56.86%, respectively, while the peak strength decreased by 3.09%, 11.34%, and 23.71%, respectively. These reduction comparisons indicate that the number and amplitude of drying-wetting cycles have a greater influence on the elastic modulus than on the peak strength.
FIGURE 7
3.2 Evolution of particle size distribution and disintegration ratio
Changes in particle size distribution represent a significant manifestation of rock mass disintegration (Zeng et al., 2024b; Zhang et al., 2024). To reveal the disintegration behaviour of carbonaceous rock, the variation of particle size distribution for carbonaceous rock under various load conditions and drying-wetting cycles was analysed. Figure 8 shows the variation in mass percentage of different particle size fractions with drying-wetting cycles under varying vertical stresses. The mass percentage of particles in each size range changed most significantly during the first cycle. After five cycles, the mass percentages of various size fractions gradually stabilised. In addition, the mass fraction of the 2–5 mm coarse fraction initially increased rapidly with increasing cycles. After the disintegration of larger-sized particles was complete, the content of this fraction began to decrease gradually. The mass fraction of the fraction larger than 40 mm decreased drastically during the first cycle, even dropping to zero under the 100 kPa load. The mass percentage of the fraction smaller than 2 mm generally exhibited a negative correlation with an increase in cycles. In contrast, the mass fractions of the three particle groups between 2 mm and 20 mm showed a trend of first increasing, then decreasing, and finally stabilising. As the number of cycles increased, particles larger than 5 mm continued to disintegrate, while it became progressively more difficult for particles smaller than 5 mm to disintegrate further. The primary reason is that internal cracks in coarse particles are prone to propagate and connect, whereas as particles become finer, the connectable internal cracks gradually decrease, making further disintegration of small particles difficult. It is noteworthy that after complete disintegration, the 2–5 mm fraction accounted for the highest mass fraction in carbonaceous rock. This fraction also exhibited the most significant changes during the disintegration process. Except for the fractions larger than 40 mm and smaller than 0.075 mm, the changes in mass fractions are mainly influenced by two factors: i) the influx of particles from the disintegration of larger-sized fractions into the target fraction, increasing its mass fraction; ii) the further disintegration of particles within the target fraction into finer fractions, decreasing its mass fraction. The combined influence of these two factors determines the dynamic characteristics of each fraction during the disintegration process.
FIGURE 8
The disintegration ratio was usually employed to quantitatively assess the d disintegration degree. The mineral composition of carbonaceous rock includes substances resistant to disintegration, such as quartz and mica (Yang et al., 2022). Consequently, a small amount of large-sized material remains even after sufficient disintegration. The disintegration ratio dr of carbonaceous rock is defined as following Equation 5:where S1 is the mass of particles smaller than 2 mm, and S2 is the total mass of particles. Figure 9 illustrates the variation of the disintegration ratio dr with the number of drying-wetting cycles under different stress levels. Under the same load condition, the disintegration ratio continuously increased with increasing cycles. For a given drying-wetting cycle, the disintegration ratio exhibited an increasing trend with increasing stress. The disintegration ratio values after 10cycles exceeded 30%, reaching 73.06% and 68.31% for 100 kPa and 200 kPa stresses, respectively. The disintegration ratio of carbonaceous rock is about 6–7 times higher than that of other common rocks (dr is approximately 10%). A positive correlation was observed between disintegration ratio and number of cycles. During the drying-wetting cycles, water-rock interaction continuously extends into the rock mass. Clay minerals are gradually eroded by water, and cementation continuously weakens until it is lost. Consequently, the mass fraction of particles smaller than 2 mm continuously increases, leading to a corresponding increase in the disintegration ratio.
FIGURE 9
3.3 Variation of surface energy
To further investigate the disintegration characteristics of carbonaceous rock, the disintegration test results were analysed based on the aforementioned surface energy. The parameters for the mudstone were assigned the following values: t = 1.0, E = 12 MPa, S = 10−7, σR = 1.5 MPa, f = 12.5%. It should be indicated that these parameter values were adopted from the previous studies (; Zeng et al., 2021b; ), in which the soft rocks were selected as the research targets. Carbonaceous rock is a kind of soft rock with the characteristic of easy disintegration, and its engineering properties are similar to those of carbonaceous mudstone that was investigated by Zeng et al. (2021b). Thus, it is reasonable to employ these parameters to explore the relationship between surface energy of carbonaceous rock and drying-wetting cycles. It should be acknowledged that if these calculation parameters are derived from experimental tests of investigated carbonaceous rock, the calculation results of surface energy will be more accurate. Additional experimental tests, such as tensile strength test, slake durability test, and so forth were suggested to be conducted on carbonaceous rock for measuring these parameter values. For particle sizes greater than 40 mm or less than 0.075 mm, the values of were taken as 40 mm and 0.075 mm, respectively; for particles within other size ranges, the average particle size was used for calculation. This yielded the relationship between the cumulative surface energy increase W, the single-increment ΔWj, and the number of cycles n during the disintegration process of carbonaceous rock, as shown in Figure 10.
FIGURE 10
The cumulative surface energy W of carbonaceous rock exhibits a trend of initial significant increase followed by gradual stabilisation with an increasing number of cycles. In contrast, the single-increment of surface energy ΔW shows a characteristic pattern of a sharp initial increase followed by decay, eventually approaching zero. Under the four load conditions of 0, 50, 100, and 200 kPa, the increase in surface energy was most pronounced during the first three disintegration cycles, with cumulative energy increases reaching 27.20 J, 38.41 J, 55.24 J, and 80.19 J, respectively. Although the curves for the single-increment of surface energy under different load conditions show some intersections, their overall trends remain consistent. In summary, during the disintegration of carbonaceous rock, a higher vertical load leads to a greater cumulative increase in surface energy and a more substantial energy surge during the first three cycles. As the number of disintegration cycles continues to increase, the cumulative surface energy gradually stabilises. This is primarily because the application of load elevates the internal stress level within the specimen, accelerating the propagation of pores and cracks, thereby enhancing the degree of disintegration. Consequently, a higher load results in more intense disintegration of carbonaceous rock and the release of more energy. Once disintegration is essentially complete, the surface energy increment approaches zero.
3.4 Analysis of disintegration phenomena
It is observed that carbonaceous rock disintegrated rapidly from the surface inward, accompanied by the release of a significant number of bubbles. This demonstrates its strong hydrophilicity and susceptibility to water-induced disintegration. The disintegrated solid particles after drying-wetting cycles showed that the proportion of large particles gradually decreased, while the content of small-sized particles continuously increased. The particle size distribution stabilised after disintegration was essentially complete, with the relative proportions of different particle sizes remaining constant. By observing the disintegration morphology of carbonaceous rock specimens under cycle numbers, it was found that the particle size of carbonaceous rock continuously decreased with increasing cycles. After the first cycle of loading and immersion, intact rock masses disintegrated intensely, forming a large quantity of elongated, needle-like particles. After five cycles, the particle size of most particles was already below 10 mm, except for a few hard rock masses. After 10 cycles, the particles further refined, and disintegration approached completion.
Based on the aforementioned macroscopic disintegration phenomena, the most significant disintegration occurred during the first immersion and loading stage. To gain deeper insight into the disintegration mechanism of carbonaceous rock under the combined action of load and drying-wetting cycles, a microstructural analysis was conducted on particles disintegrated within the first 30 min of immersion under a 100 kPa load (see Figure 11). In its initial state, carbonaceous rock primarily exhibited a flaky structure, with particles bonded mainly by cementitious materials and containing primary pores. After 5 min of immersion, a substantial amount of clay particles on the surface of the flaky structure detached and were lost. Pores gradually expanded under the combined influence of load and water, resulting in clearer particle outlines, and needle-shaped particles began to appear on the specimen surface. After 15 min of immersion, the surface clay particles were mostly absent, and the structure was dominated by needle-shaped particles. Pores further enlarged and showed a tendency to connect. By 30 min of immersion, the disintegrated particles had largely transformed into needle-shaped particles, with clear boundaries between them. Pores had noticeably enlarged and had formed interconnected fissures. In summary, under the coupled effects of load and drying-wetting cycles, the structure of carbonaceous rock particles gradually degraded from flaky to elongated, needle-shaped shapes, while primary pores continuously expanded and ultimately interconnected.
FIGURE 11
Based on the disintegration model for mudstone proposed by Zhang et al. (2024), the disintegration process of carbonaceous rock under the combined influence of load and drying-wetting cycles can be summarized as follows: The load promotes the expansion of primary pores within the carbonaceous rock. Upon water immersion, moisture infiltrates these pores, leading to the softening and swelling of clay minerals (). As the pores further enlarge, the water-rock contact area increases, intensifying their interaction. Pores progressively connect, causing the intact rock mass to disintegrate into elongated, needle-shaped particles. Under the continued action of moisture and load, the rock mass further breaks down into finer fragments and granules, eventually disintegrating into a powdered state, marking the completion of disintegration (; ; ; ; Yang and Zhang, 2022; ).
3.5 Disintegration mechanism
Feldspar within the mineral composition of carbonaceous rock is susceptible to weathering and disintegration, while minerals such as calcite and dolomite are water soluble (Yang and Zhang, 2024). Under the combined action of load and drying-wetting cycles, these soluble mineral components undergo physico-chemical interactions with water, leading to softening or dissolution. On one hand, the cementing material between particles in the rock mass is diluted and weakened by water, resulting in the loss of cohesion and triggering disintegration. On the other hand, minerals represented by calcite (CaCO3) react with water under drying-wetting cycle conditions, forming water-soluble Ca(HCO3)2. This dissolution process leads to the formation of secondary structures such as dissolution pores and seams within the carbonaceous rock.
The significant features of rapid disintegration rate and high disintegration degree exhibited by carbonaceous rock under the coupled effects of load and drying-wetting cycles are closely related to its swelling and shrinkage mechanism. Mineral composition analysis indicates that carbonaceous rock contains a substantial amount of clay minerals, such as illite and kaolinite, which possess large specific surface areas and strong hydrophilicity (; ). The structural units of kaolinite are primarily connected via oxygen-oxygen bonds; however, these bonds exhibit poor stability. Upon immersion, water molecules rapidly intrude into the rock mass along pre-existing fractures, causing the breakage of these oxygen-oxygen bonds. During the water immersion process, high-valence cations such as Al3+ and Fe3+ in the rock mass can be replaced by lower-valence cations like Mg2+, resulting in an excess of negative charge on particle surfaces. These negative charges further adsorb H+ from the water, causing a significant increase in both the interlayer spacing and the thickness of the hydration film, ultimately leading to particle volume expansion. Due to differences in the duration and intensity of water interaction between the surface and the interior of carbonaceous rock particles, differential expansion occurs within the rock mass, subsequently inducing overall disintegration or surface spalling (Huang et al., 2022; Zhu et al., 2020). Particularly under the combined influence of load and drying-wetting cycles, carbonaceous rock particles undergo repeated alternating processes of drying-shrinkage and wetting-swelling, continuously deepening the degree of disintegration (; Yu et al., 2023; Zhao et al., 2018). Additional microchemical analyses, for example, ionic concentration determination, quantitative analysis of mineral composition, pore size and its distribution characteristics, and so forth, are recommended to be conducted in future for establishing a quantitative correlation between microchemical indexes and the disintegration progression. This quantitative basis for the rapid disintegration process is to contribute to further understand the disintegration mechanisms of carbonaceous rock.
4 Conclusions
This study systematically investigated the deterioration laws of mechanical properties and disintegration characteristics of carbonaceous rock under the combined effects of drying-wetting cycles and load through triaxial compression tests and disintegration tests. The intrinsic mechanism governing the disintegration of carbonaceous rock was clarified from multiple perspectives. The main conclusions derived are as follows:
The stress-strain curves of carbonaceous rock could be divided into four characteristic stages. An increase in the number and range of drying-wetting cycles led to the deterioration of particle contact conditions. This manifested on the stress-strain curves as a prolongation of the crack compression and plastic yielding stages, and a reduction in peak stress. An increase in confining pressure mitigated the reduction in peak strength and elastic modulus. The influence of drying-wetting cycles on the elastic modulus of the rock specimens was greater than their influence on the peak strength.
The mass percentages of investigated particle size fraction gradually stabilized after five cycles. It was found that during a single disintegration event of carbonaceous rock, smaller particle sizes and higher mass percentages within specific size ranges corresponded to greater newly increased surface energy. As the number of disintegration cycles increased, the overall particle size continuously decreased, theoretically becoming smaller than 0.25 mm eventually.
Loading promoted the expansion of primary pores within the carbonaceous rock, and then, water infiltrated into these pores, leading to the softening and swelling of clay minerals. As the pores further enlarged, pores progressively connected, causing the intact rock mass to disintegrate into broken particles. These processes would continue until the carbonaceous rock disintegrates into powder form. Dissolution of carbonate minerals and cation exchange of clay mineral particles were the two key points for the rapid disintegration of carbonaceous rock under drying-wetting cycles.
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
KM: Data curation, Writing – original draft. CM: Conceptualization, Supervision, Writing – review and editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This research was funded by the National Natural Science Foundation of China (Grant Nos. 42272296 and U25A20349).
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
carbonaceous rock, disintegration characteristics, mechanical properties, predictive model, surface energy prediction model
Citation
Ma K and Ma C (2026) Mechanical and disintegration behaviors of carbonaceous rock under drying-wetting cycles. Front. Mater. 13:1953993. doi: 10.3389/fmats.2026.1953993
Received
31 July 2026
Revised
05 September 2026
Accepted
08 September 2026
Published
30 September 2026
Volume
13 - 2026
Edited by
Praveenkumara Jagadeesh, Vidyavardhaka College of Engineering, India
Reviewed by
Di Wu, Jilin University, China
Chuanlong Dong, Shanxi Datong University, China
Wei Wang, Liaoning University, China
Caijin Xie, Guizhou Open University, China
Updates
Copyright
© 2026 Ma and Ma.
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*Correspondence: Chong Ma, machong@cug.edu.cn
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