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Fractal Analysis and Its Applications in Rock Engineering, Second Edition

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A special issue of Fractal and Fractional (ISSN 2504-3110). This special issue belongs to the section "Engineering".

Deadline for manuscript submissions: 15 December 2026 | Viewed by 12006

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Editors

Dr. Xiaohan Yang

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Guest Editor

Resources Engineering, Central Queensland University, Brisbane, QLD, Australia
Interests: fractal analysis; rock mechanics; coal/rock burst; underground mining; geomechanics
Special Issues, Collections and Topics in MDPI journals

Dr. Lihai Tan

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Guest Editor

School of Resource Environment and Safety Engineering, University of South China, Hengyang 412001, China
Interests: fractal analysis; rock mechanics; discreet numerical modelling; damage mechanics
Special Issues, Collections and Topics in MDPI journals

Dr. Gaofeng Liu

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Guest Editor

1. School of Resources and Environment, Henan Polytechnic University, Jiaozuo 454003, China
2. Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Henan Polytechnic University, Jiaozuo 454003, China
Interests: fractal characteristics; fuel; energy; geology; mining
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

In rock mechanics, fractal analysis is used to study the behavior and properties of rock fractures, including their size distribution, orientation, and connectivity. This allows researchers to better understand the mechanics of rock fractures and their impact on different aspects of rock mass behavior, including deformation, stability, and permeability. Fractal analysis has also been applied to study the fragmentation of rocks, including the study of rock blasting and rock cutting. By analyzing the fractal dimensions of rock fragments, researchers can develop models that predict the size, distribution, and shape of rock fragments after blasting, which is crucial for planning mining operations. The application of fractal analysis in rock mechanics and rock engineering has broadened our understanding of the mechanical behavior of rocks and rock masses at various scales, and has the potential to improve the design and safety of rock engineering projects.

The scope of this Special Issue includes, but is not limited to, the following topics:

• The fractal analysis of rock fractures and their properties, such as their size distribution, orientation, and connectivity.

• The fractal modeling and simulation of rock fragmentation processes, including the study of rock blasting and rock cutting.

• Applications of fractal analysis in rock engineering, including the characterization of rock mass properties and the prediction of rock mass behavior.

• The fractal analysis of geomechanical processes, such as the faulting, folding, and deformation of rocks.

• The fractal analysis of rock microstructures, including the study of grain size distribution and pore space characterization.

Dr. Xiaohan Yang
Dr. Lihai Tan
Dr. Gaofeng Liu
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 250 words) can be sent to the Editorial Office for assessment.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-anonymized peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Fractal and Fractional is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

fractal analysis and modeling
fragmentation
rock mechanics
rock fractures
rock microstructures
rock engineering
geomechanical processes
engineering application

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Fractal Analysis and Its Applications in Rock Engineering in Fractal and Fractional (21 articles)

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Research

35 pages, 8405 KB

Open AccessArticle

Fractal Acoustic Emission Characteristics and Energy Evolution of High-Water-Resistance Concrete Backfill: Roles of Water-to-Cement Ratio and Fiber Volume Fraction

by Shuaigang Liu, Zizheng Zhang, Jianxiong Yang, Kun Fang, Zilu Liu and Xiaohe Wang

Fractal Fract. 2026, 10(8), 555; https://doi.org/10.3390/fractalfract10080555 - 14 Aug 2026

Viewed by 203

Abstract

Fiber-reinforced high-water-resistance concrete backfill (FHWCB) is a rapid-setting cementitious backfill system used for underground support and backfilling, but its stability is strongly affected by mixture water content and fiber dispersion. This study investigated the fresh-state behavior, mechanical performance, acoustic emission (AE) fractal characteristics, b-value response, and energy evolution of FHWCB. Mixtures with water-to-cement ratios (w/c) of 1.0–1.8 and fiber volume fractions (Vf) of 0–0.5% were prepared and tested using fresh property measurements, unconfined compression, thermogravimetry, AE monitoring, correlation dimension analysis, b-value analysis, and strain energy partitioning. Increasing w/c improved flowability and delayed setting, but weakened the hydration skeleton and reduced early-age compressive strength by approximately 56–61%. Fiber reinforcement showed a non-monotonic effect: Vf = 0.3% increased compressive strength by approximately 16–26%, whereas excessive fiber addition reduced strength because of fiber clustering and weak local zones. AE amplitude sequences exhibited measurable fractal characteristics. A higher correlation dimension indicated distributed microdamage, while decreasing correlation dimension and b-value reflected the transition toward localized macrocrack growth. Energy analysis showed that the peak elastic strain energy density decreased from approximately 0.60 to 0.39 MJ/m³ as w/c increased. The proposed AE fractal–b-value–energy framework provides a quantitative basis for tracking damage progression and optimizing FHWCB for underground engineering. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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24 pages, 4471 KB

Open AccessArticle

Multiscale Fractal-Dimension-Constrained Coherent Phase Processing of Seismic-While-Tunneling Signals for Fault Prediction

by Qi Guan, Qianzong Bao, Xuefei Wu, Chao Chen and Huicong Xu

Fractal Fract. 2026, 10(7), 464; https://doi.org/10.3390/fractalfract10070464 - 10 Jul 2026

Viewed by 286

Abstract

Seismic-while-tunneling signals acquired during coal-mine excavation are typically characterized by strong nonstationarity, intense mechanical noise, weak reflection responses, unstable inter-trace phases, and complex waveform fluctuations. These characteristics make conventional energy- or amplitude-based picking methods susceptible to false triggers and missed detections. To reveal the local complexity mutation of mine seismic signals under strong-noise backgrounds, this study proposes a multiscale fractal-dimension-constrained coherent phase processing method for signal enhancement, first-arrival picking, and fault prediction. First, the raw seismic-while-tunneling records are reorganized into shot gathers, windowed, and downsampled to preserve the effective early-arrival information. A damped multichannel singular spectrum analysis method is then used to extract coherent low-rank components and suppress incoherent random noise. Second, short-window and long-window box-counting fractal dimensions are calculated to characterize local and background waveform complexity, and a fractal-dimension mutation index is constructed to identify abrupt complexity transitions associated with effective seismic arrivals. On this basis, the fractal mutation index is incorporated into a coherent phase picking function that combines multichannel phase consistency and stacked amplitude, forming a fractal-dimension-constrained CCPP detection criterion. This criterion enhances true coherent arrivals while suppressing isolated noise spikes and unstable local amplitude disturbances. Finally, phase-weighted stacking is applied to further strengthen phase-consistent reflection responses and improve the interpretability of seismic-while-tunneling imaging profiles. Field application at the WII02040503 working face of Tunbao Coal Mine demonstrates that the proposed method can effectively improve the continuity of coherent events, stabilize automatic picking results, and enhance anomalous reflection bands under complex underground noise conditions. During the engineering trial, a total of 2558 m of ahead prospecting was completed, and 29 faults were predicted. The field-confirmation rates of the predicted faults with throws greater than 3 m, between 1 and 3 m, and less than 1 m were 100%, 87.50%, and 81.25%, respectively. Overall, 25 of the 29 predicted faults were confirmed by field exposure, corresponding to an overall field-confirmation rate of 86.21%. After velocity-synchronization time-difference correction, the average planar positioning deviation of the confirmed fault predictions decreased from 7.86 m to 5.08 m, corresponding to a 35.37% reduction in positioning error. These results indicate that the proposed fractal-dimension-constrained coherent processing framework provides an effective approach for complexity-aware signal enhancement and robust fault prediction in seismic-while-tunneling monitoring. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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18 pages, 13951 KB

Open AccessArticle

Fractal Characterization of Temporal and Spatial Evolution of Microseismic Events Before Rock Instability

by Lin Tian, Shijia Liu, Yunxing Cao, Menglong Wang and Shengliang Lu

Fractal Fract. 2026, 10(6), 389; https://doi.org/10.3390/fractalfract10060389 - 5 Jun 2026

Viewed by 326

Abstract

Disaster warning is crucial for coal mine safety. However, capturing the precursor information of disaster occurrence remains a challenging task, especially the spatiotemporal fractal characteristics of microseismic events before large-scale fractures in rock masses and their indicative significance for disasters. In this study, we proposed a method for precursor identification and early warning indication of high-energy microseismic events by a combination of temporal and spatial fractal of low-energy events based on a set of 78 days of on-site microseismic monitoring data. The rationality of this method was validated by numerical simulation, which focuses on the fractal characteristics of rock fracture morphology under compression loading. The results show: (1) Before the occurrence of high-energy microseismic events, the fractal dimension of the microseismic events in the time series was relatively low, and the variance and autocorrelation coefficient of the time fractal dimension showed an increasing trend, exhibiting a typical slowing down phenomenon. (2) The spatial fractal dimension of the microseismic events rapidly decreased to 1.3–1.5 before the occurrence of high-energy events, with an average decrease of over 0.48. (3) Microseismic events are associated products of coal rock mass fracture processes, and their uniqueness in time series and spatiotemporal evolution characteristics have important indicative significance for the incubation and early warning of coal rock mass disasters. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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25 pages, 6665 KB

Open AccessArticle

Evolution of Mechanical Properties and Fractal Characteristics of Acoustic Emission of Sandstone–Concrete Composites Under Acidic Sulfate Attack

by Zhijun Zhang, Zheng Yang, Min Wang, Lingling Wu and Yakun Tian

Fractal Fract. 2026, 10(5), 308; https://doi.org/10.3390/fractalfract10050308 - 1 May 2026

Viewed by 416

Abstract

The long-term stability of rock–concrete composites largely depends on the mechanical properties and durability of the rock–concrete interface. This study investigated the coupling effect of interfacial roughness and acid sulfate corrosion on sandstone–concrete composites by using uniaxial compression tests combined with acoustic emission (AE) monitoring. The results showed that corrosion continuously reduces the mechanical properties of the specimens with peak strength and elastic modulus, exhibiting a two-stage evolution: rapid degradation in the early stage followed by a slow decline in the later stage. After 60 days of corrosion, the peak strength for composites with JRC = 5, JRC = 10, and JRC = 15 interfaces decreased by 46.59%, 44.34%, and 50.43%, respectively. The elastic modulus exhibited the same pattern of variation, and the decreasing rate was 68.90%, 66.96%, and 76.46% for the JRC = 5, JRC = 10, and JRC = 15 groups. Acoustic emission activities appeared earlier and were more significant after corrosion. With the effect of corrosion, the fracture mode evolved from tensile-dominated cracks to mixed tensile–shear cracks with a stronger shear component. Fractal analysis of AE energy revealed that the Hurst exponent decreased from 0.842–0.864 in the natural state to 0.503–0.567 after 60 days of immersion, whereas the fractal dimension increased from 1.136–1.182 to 1.433–1.497, indicating a decrease in the persistence and increase in complexity of the acoustic emission energy release process. Overall, the moderately rough interface (JRC = 10) achieved a better balance between initial strengthening and long-term corrosion resistance. These findings provide experimental support for evaluating the durability of sandstone–concrete composites in acidic sulfate environments. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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20 pages, 4107 KB

Open AccessArticle

Surface Fractal Characterization of Granite Cut by Diamond Wire Saw

by Yihe Liu, Yufei Gao and Jiahao Xu

Fractal Fract. 2026, 10(5), 276; https://doi.org/10.3390/fractalfract10050276 - 22 Apr 2026

Viewed by 628

Abstract

The surface quality of granite cut by diamond wire saw significantly impacts the cost of subsequent processes such as grinding and polishing. Traditional evaluation parameters like surface roughness (Ra) or peak-to-valley value (PV) face challenges in characterizing the surface morphology. This study introduces fractal dimension (FD) as a potential auxiliary parameter for evaluating the surface quality of sawn granite. Cutting experiments were conducted on Shanxi Black granite using varying wire speeds, feed speeds, and workpiece sizes. The box-counting method was employed to extract the three-dimensional fractal dimension (3D FD) of the granite surface, which characterizes the overall surface complexity, as well as the distribution of two-dimensional fractal dimensions (2D FD) for granite surface cross-sectional profiles at different angles. The results indicate that the granite-sawn surface exhibits complex micro-morphology featuring brittle micro-pits and wavelike saw marks along the feed direction. A strong negative correlation exists between the 3D FD and both surface roughness Ra and PV value, suggesting that 3D FD can serve as an indicator of granite surface quality, with higher FD values corresponding to better surface quality. Moreover, compared to the PV value constrained by material heterogeneity, 3D FD more effectively represents the true surface quality of the granite. Additionally, the distribution characteristics of 2D FD at different angles effectively reveal surface anisotropy and damage. The results suggest that a more symmetrical 2D FD distribution is associated with consistent surface integrity in the evaluated samples. This suggests that FD has the potential to serve as a meaningful auxiliary parameter for characterizing granite surface quality. The findings hold significant importance for the accurate evaluation of diamond wire-saw-cut granite surfaces and provide a basis for the formulation of subsequent grinding process. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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19 pages, 5513 KB

Open AccessArticle

Laboratory Measurement and Analysis of Permeability of Sandstone Reservoir Microstructure Based on Fractal Geometry Theory for Porous Media

by Zhen Zhang, Gaofeng Liu, Yongliang He, Huan Liu, Xiaoming Wang, George Barakos and Ping Chang

Fractal Fract. 2025, 9(12), 817; https://doi.org/10.3390/fractalfract9120817 - 15 Dec 2025

Cited by 1 | Viewed by 721

Abstract

The pore complexity and heterogeneity in porous media display obvious fractal characteristics, which can be characterized by the fractal dimension for the pore tortuosity (D_T) and the fractal dimension for the pore size (D_f). Correspondingly, a three-dimensional (3D) fractal permeability model for porous media is proposed based on the D_T and D_f. The accuracy of the proposed model is verified by the classical theoretical relation of the permeability versus porosity, the measured permeability, and the previous study. The sensitivity analysis of model parameters (D_f, D_T, λ_min and λ_max) based on elasticity coefficient indicates that the proposed model is much more sensitive to D_f and D_T than λ_min and λ_max, and more sensitive to D_f than D_T. The proposed model is much more sensitive to λ_min than λ_max. Furthermore, the proposed model is compared with the modified Kozeny–Carman equation. The root mean square error (RMSE) analysis shows that the RMSE of the proposed model and the modified Kozeny–Carman equation in predicting permeability are 8.9857 × 10⁻⁴ and 0.5082, exhibiting high prediction accuracy of the proposed model. The proposed fractal permeability model achieves a more accurate characterization of the fluid transport by more comprehensively describing the complexity and tortuosity of pore structure, which can also provide the prospective theoretical significance and method reference for predicting the permeability of 3D porous media. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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23 pages, 3312 KB

Open AccessEditor’s ChoiceArticle

Automatic Picking Method for the First Arrival Time of Microseismic Signals Based on Fractal Theory and Feature Fusion

by Huicong Xu, Kai Li, Pengfei Shan, Xuefei Wu, Shuai Zhang, Zeyang Wang, Chenguang Liu, Zhongming Yan, Liang Wu and Huachuan Wang

Fractal Fract. 2025, 9(11), 679; https://doi.org/10.3390/fractalfract9110679 - 23 Oct 2025

Cited by 36 | Viewed by 1715

Abstract

Microseismic signals induced by mining activities often have low signal-to-noise ratios, and traditional picking methods are easily affected by noise, making accurate identification of P-wave arrivals difficult. To address this problem, this study proposes an adaptive denoising algorithm based on wavelet-threshold-enhanced Complete Ensemble Empirical Mode Decomposition with Adaptive Noise (CEEMDAN) and develops an automatic P-wave arrival picking method incorporating fractal box dimension features, along with a corresponding accuracy evaluation framework. The raw microseismic signals are decomposed using the improved CEEMDAN method, with high-frequency intrinsic mode functions (IMFs) processed by wavelet-threshold denoising and low- and mid-frequency IMFs retained for reconstruction, effectively suppressing background noise and enhancing signal clarity. Fractal box dimension is applied to characterize waveform complexity over short and long-time windows, and by introducing fractal derivatives and short-long window differences, abrupt changes in local-to-global complexity at P-wave arrivals are revealed. Energy mutation features are extracted using the short-term/long-term average (STA/LTA) energy ratio, and noise segments are standardized via Z-score processing. A multi-feature weighted fusion scoring function is constructed to achieve robust identification of P-wave arrivals. Evaluation metrics, including picking error, mean absolute error, and success rate, are used to comprehensively assess the method’s performance in terms of temporal deviation, statistical consistency, and robustness. Case studies using microseismic data from a mining site show that the proposed method can accurately identify P-wave arrivals under different signal-to-noise conditions, with automatic picking results highly consistent with manual labels, mean errors within the sampling interval (2–4 ms), and a picking success rate exceeding 95%. The method provides a reliable tool for seismic source localization and dynamic hazard prediction in mining microseismic monitoring. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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13 pages, 6555 KB

Open AccessArticle

Effect of Layer Spacing on Fracture Development and Seepage Evolution of Surrounding Rocks During Repeated Mining Under Insufficiently Collapsed Gob

by Dingyi Hao, Guozhong Liu, Shihao Tu and Wenlong Li

Fractal Fract. 2025, 9(6), 376; https://doi.org/10.3390/fractalfract9060376 - 12 Jun 2025

Cited by 3 | Viewed by 1124

Abstract

Repeated mining under insufficiently collapsed gobs is a complex process in underground mining and is associated with safety hazards such as ground collapse and subsidence. The effect of layer spacing on the fracture network evolution and fluid transport mechanisms in rock strata during this process has not been systematically studied. In this work, the discrete element method was employed to analyze the fracture development and seepage evolution of surrounding rocks in the Nanliang coal mine across varying layer spacings (5, 20, 35, 50, and 65 m). A systematic evaluation of the rock mass integrity was conducted through damage coefficient quantification. The key findings revealed that an increase in the layer spacing progressively reduced the damage coefficients in both the overburden strata above the goaf and in the interlayer formations ahead of the working face, accompanied by reduced fracture propagation intensity. Shear failure mechanisms dominated throughout the mining process. Fractal characteristics of the fractures intensified with the advance of the working face, while the hydraulic conductivity and interstitial pressure in the interlayer strata exhibited declining trends with reduced attenuation rates. Our findings provide critical insights for ensuring the safety and improving the efficiency of repeated mining under insufficiently collapsed gobs. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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23 pages, 5181 KB

Open AccessArticle

Fractal Characterization and NMR Analysis of Curing-Dependent Pore Structures in Cemented Tailings Waste RockBackfill

by Jianhui Qiu, Xin Xiong and Keping Zhou

Fractal Fract. 2025, 9(6), 367; https://doi.org/10.3390/fractalfract9060367 - 4 Jun 2025

Cited by 5 | Viewed by 1764

Abstract

This study investigates the coupled effects of waste rock-to-tailings ratio (WTR) and curing temperature on the pore structure and mechanical performance of cemented tailings waste rock backfill (CTRB). Four WTRs (6:4, 7:3, 8:2, 9:1) and curing temperatures (20–50 °C) were tested. Low-field nuclear [...] Read more.

D_{a}

D_{b}

D_{c}

) at micropore, mesopore, and macropore scales. Uniaxial compressive strength (UCS) and elastic modulus (E) were also measured. The results reveal that (1) the micropore structure complexity was found to be a key indicator of structural refinement, while excessive temperature led to pore coarsening and strength reduction.

D_{a}

= 2.01 reaches its peak at WTR = 7:3 and curing temperature = 40 °C; (2) at this condition, the UCS and E achieved 20.5 MPa and 1260 MPa, increasing by 45% and 38% over the baseline (WTR = 6:4, 20 °C); (3) when the temperature exceeded 40 °C,

D_{a}

dropped significantly (e.g., to 1.51 at 50 °C for WTR = 7:3), indicating thermal over-curing and micropore coarsening; (4) correlation analysis showed strong negative relationships between total pore volume and mechanical strength (

R

= −0.87 for

δ_{a} v s . U C S),

and a positive correlation between

D_{a}

and UCS (R = 0.43). (5) multivariate regression models incorporating pore volume fractions,

T_{2}

relaxation times, and fractal dimensions predicted

U C S

and E with R² > 0.98; (6) the hierarchical sensitivity of fractal dimensions follows the order micro-, meso-, macropores. This study provides new insights into the microstructure–mechanical performance relationship in CTRB and offers a theoretical and practical basis for the design of high-performance backfill materials in deep mining environments. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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21 pages, 7083 KB

Open AccessArticle

Pore Structure Evolution Characteristics and Damage Mechanism of Sandstone Subjected to Freeze–Thaw Cycle Treatment: Insights from Low-Field Nuclear Magnetic Resonance Testing and Fractal Theory

by Xin Xiong, Feng Gao, Jielin Li, Keping Zhou and Chengye Yang

Fractal Fract. 2025, 9(5), 293; https://doi.org/10.3390/fractalfract9050293 - 1 May 2025

Cited by 9 | Viewed by 2081

Abstract

To investigate the pore structure evolution characteristics and damage mechanism of sandstone subjected to treatment with freeze–thaw cycles, quantitative analyses were conducted on the longitudinal wave velocity (LWV) and T₂ spectrum of sandstone before and after 10, 20, 30, and 40 freeze–thaw cycles, using longitudinal wave velocity testing, low-field nuclear magnetic resonance (NMR) testing, and fractal theory. The results show that, with the increase in the number of freeze–thaw cycles, the LWV of sandstone gradually decreases, the amplitude of the saturated T₂ spectrum gradually increases, the amplitude of the centrifugal T₂ spectrum gradually decreases, the total porosity and effective porosity increase, and the residual porosity decreases. After undergoing freeze–thaw cycles, sandstone exhibits obvious fractal characteristics in both the total porosity NMR fractal dimension and the effective porosity NMR fractal dimension, and the growth rates of both decrease exponentially with the increase in the number of freeze–thaw cycles. The magnitude of the fractal dimensions reflects the complexity of the pore structure, with smaller fractal dimensions indicating better pore connectivity. In summary, the damage evolution mechanism of sandstone under freeze–thaw cycles is characterized by the gradual expansion and interconnection of internal closed micro-pores (cracks), along with increased total porosity and effective porosity, leading to enhanced freeze–thaw damage. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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18 pages, 20166 KB

Open AccessArticle

Degradation Characteristics of Coal Samples Under the Dry–Wet Cycle Action of Acidic, High-Salinity Solutions: Experimental Study and Fractal Analysis

by Leiming Zhang, Min Wang, Bin Zhang, Xun Xi, Ying Zhang and Jiliang Pan

Fractal Fract. 2025, 9(4), 221; https://doi.org/10.3390/fractalfract9040221 - 1 Apr 2025

Cited by 5 | Viewed by 1548

Abstract

Uniaxial compression tests were conducted on coal samples subjected to different dry–wet cycling treatments to investigate the damage and degradation mechanisms of coal samples under the dry–wet cyclic action of acidic, high-salinity solutions. The damage process of the coal samples was monitored in situ using acoustic emission (AE). The degradation evolution of the mechanical parameters and macroscopic failure modes with the number of cycles was analyzed. Based on the AE ringing parameters, the RA-AF distribution and the AE fractal dimension’s variation characteristics were studied. Additionally, scanning electron microscopy (SEM) was used to observe the microstructure of the coal samples. The results showed that with the increase in the number of dry–wet cycles, both the peak strength and elastic modulus of the coal samples exhibited varying degrees of degradation, and the failure mode gradually shifted from tensile failure to shear failure. AE ringing counts decreased progressively, while the proportion of shear cracks based on the RA-AF classification increased. At the same time, the mean AE fractal dimension of the coal samples increased, and the fractal dimension decreased with an increase in AE ringing counts. The sharp drop in fractal dimensions could serve as an early warning signal for a major failure in the coal samples. Furthermore, under the influence of dry–wet cycling in acidic, high-salinity solutions, defects such as pores and cracks in the microstructure of the coal samples became more pronounced, and the degradation effect continuously intensified. Full article

(This article belongs to the Special Issue Fractal Analysis and Its Applications in Rock Engineering, Second Edition)

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