Fracture Fractal Dimension Research Articles

Fractal evolution characteristics of fracture meso-damage in uniaxial compression rock masses using bonded block model

With regard to deep mining in metal mines, an investigation into the failure mode of deep fractured rock masses and their corresponding acoustic emission signal characteristics is conducted via uniaxial compression tests. Subsequently, a fractal damage renormalization group mechanical model is developed to explain the behavior of those fractured rock masses. Employing the bonded block model (BBM) numerical simulation method, fracture process in synthetic rock samples is analyzed, thereby validating the efficacy of the mechanical model. The numerical simulations highlight the critical role of fractures expansion in underlying the deterioration of rock mass strength. As the peak load decreases, the fracture fractal dimension increases, leading to a significant 14.2% reduction in compressive strength accompanied by an approximate 8.7% rise in average fracture fractal dimension. A comparative analysis of tetrahedral and voronoi block synthetic rock samples reveals the tetrahedral block samples exhibit a superior ability to depict the fracture behavior of fractured rock masses. Specifically, they offer a more accurate simulation of acoustic emission characteristics and failure modes. Furthermore, variations in the fracture fractal dimension with respect to the hole defect’s position are observed, with the maximum value occurring along the vertical axis of the hole defect. This observation underscores the potential utility of visually monitoring deep rock fracture dynamics as an effective mean for quantitatively evaluating fracture damage and strength degradation in deep rock formations.

Suitability evaluation of an equivalent porous medium method for characterizing 2-D flow based on the fracture fractal dimension

ABSTRACT An equivalent porous medium (EPM) method is widely utilized in various practical applications of hydrogeological engineering with areas of a fractured geological medium (FGM). However, the suitability of the EPM model still lacks sufficient scientific evaluation when considering the distribution of water head and flow velocity. In this study, the suitability of the EPM model to the fractal dimension of a discrete fracture network (DFN) is analyzed based on numerical simulation experiments. The simulation results of the EPM model are compared with those of the DFN model. Then, a formula is proposed to evaluate the suitability of the EPM model for characterizing two-dimensional flow distribution. The results show that with the increase of the fractal dimension, the suitability of the EPM model decreases first and then increases, and the inflection point gradually appears in advance with the increase of the length–width ratio of the simulation domain. The length–width ratio of the simulation domain has a linear relationship with the suitability of the EPM model, and the slope increases with the increase of the fractal dimension. Finally, a quantitative evaluation method is proposed to calculate the suitability of the EPM model based on the fractal dimension and the length–width ratio of the simulation domain.

Characteristics and Sources of CBM Well-Produced Water in the Shouyang Block, China

The Shouyang Block was selected as the research subject. Comprehensive analysis was conducted using coalbed methane (CBM) well production data, geochemical test data on water produced from the coalbed methane well, and fundamental geological information. The findings reveal the water dynamics in the Shouyang Block are characterized by weak groundwater runoff or retention in most areas. The groundwater head height exhibits a gradual decrease from the north to south, which is closely associated with the monoclinic structure of the Shouyang Block. Overall, water production is relatively high. As the average water production increases, the average gas production gradually decreases. A concentration of high water production wells is observed in the northern part of the Shouyang Block, which gradually increases towards the southeast direction. A comprehensive analysis was conducted on the factors influencing water production, including total water content of coal seams, coal seam porosity, groundwater stability index, groundwater sealing coefficient, D value of the fracture fractal dimension, fault fractal dimension, and sand–mud ratio. The correlation degree was calculated and ranked in order of magnitude through grey correlation analysis. The order of factors that influence water production, from strongest to weakest, is as follows: sand–mud ratio > porosity > fractal dimension of fault > fracture fractal dimension D value > groundwater sealing coefficient > groundwater stability index > total water content of coal seams. The dissolution amounts of carbonate and sulfate are both small, and the water source may mainly come from the sandstone aquifer. Attention should be paid to the distribution and lithological combination of sandstone aquifers in coal-bearing strata in the future exploration and development process of the Shouyang Block. This will help to avoid the potential influence of fault structures and enable the identification of favorable areas for low water and high gas production.

Investigation into the Failure Characteristics and Mechanism of Rock with Single Elliptical Defects under Ultrasonic Vibrations

In order to investigate the effects of elliptical defects on rock failure under ultrasonic vibrations, ultrasonic vibration tests and PFC2D numerical simulations were conducted on rocks with single elliptical defects. The research results indicated that the fracture fractal dimension, axial strain, and crack depth of specimens with elliptical defects at 45° and 90° were the smallest and largest, respectively. The corresponding strain and fractal dimension showed a positive linear and logarithmic function relationship with time. The maximum crack depth of 46.50 mm was observed on the specimens with an elliptical defect angle of 90°. Specimens with elliptical defects at 0°, 30°, 75°, and 90° exhibited more dense and frequent acoustic emission events than those with elliptical defects at 15°, 45°, and 60°. During the ultrasonic vibration process, the maximum total energy (87.86 kJ) and energy consumption coefficient (0.963) were observed on specimens with elliptical defect angles of 30° and 45°, respectively. The difference in the stress field led to varying degrees of plastic strain energy in the specimens, resulting in different forms of crack propagation and triggering differential acoustic emission events, ultimately leading to specimen failure with different crack shapes and depths. The fractal dimensions of elliptical defect specimens under ultrasonic vibration have a high degree of consistency with the changes in axial strain and failure depth, and the fractal dimension of defect specimens is positively correlated with the degree of failure of defect specimens.

Thermally-induced cracking behaviors of coal reservoirs subjected to cryogenic liquid nitrogen shock

The benefits of using cryogenic liquid nitrogen shock to enhance coal permeability have been confirmed from experimental perspectives. In this paper, we develop a fully coupled thermo-elastic model in combination with the strain-based isotropic damage theory to uncover the cooling-dominated cracking behaviors through three typical cases, i.e. coal reservoirs containing a wellbore, a primary fracture, and a natural fracture network, respectively. The progressive cracking processes, from thermal fracture initiation, propagation or cessation, deflection, bifurcation to multi-fracture interactions, can be well captured by the numerical model. It is observed that two hierarchical levels of thermal fractures are formed, in which the number of shorter thermal fractures consistently exceeds that of the longer ones. The effects of coal properties related to thermal stress levels and thermal diffusivity on the fracture morphology are quantified by the fracture fractal dimension and the statistical fracture number. The induced fracture morphology is most sensitive to changes in the elastic modulus and thermal expansion coefficient, both of which dominate the complexity of the fracture networks. Coal reservoir candidates with preferred thermal-mechanical properties are also recommended for improving the stimulation effect. Further findings are that there exists a critical injection temperature and a critical in-situ stress difference, above which no thermal fractures would be formed. Preexisting natural fractures with higher density and preferred orientations are also essential for the formation of complex fracture networks. The obtained results can provide some theoretical support for cryogenic fracturing design in coal reservoirs.

Quantifying Fracture Dynamics in Clean Energy: A Novel Fractal Perspective

Geothermal energy, as an emergent source of power, has consistently been a focal point of scholarly investigation both domestically and internationally, with a particular emphasis on the prediction and assessment of its extraction efficiency. In common geothermal extraction projects, the seepage behavior of water within the fractures for the geothermal reservoirs significantly impacts the efficiency of resource extraction. This study introduces an interdisciplinary fractal model for geothermal extraction. An enhanced fractal theory, tailored for geothermal reservoirs, is proposed, employing four innovative fractal parameters— fracture fractal dimension, fracture tortuosity fractal dimension, fracture roughness parameter, and maximum fracture length—to quantitatively characterize the fracture structure. This refined fractal theory is applied to geothermal reservoir extraction under the complex thermal-hydromechanical coupling. The findings indicate that the proposed structural parameters effectively characterize the micro-macro interactions during the geothermal extraction process. Significant evolution of these fractal parameters is observed throughout the extraction process. Furthermore, there is an inverse relationship between geothermal extraction efficiency and two key fractal parameters—fracture tortuosity fractal dimension and fracture roughness parameter—while the reservoir stress is directly proportional to these parameters.

A CLUSTERED FRACTAL DISCRETE FRACTURE NETWORK MODEL FOR FRACTURED COAL

The fracture network in fractured coal is the main channel of coal seam gas flow. Not only the geometric topology properties (such as fractal characteristics) of a single fracture but also the connection topology properties (interconnection characteristics between fractures) of the fracture network have an important impact on the fluid flow in fracture networks. In this study, the connection topology properties of the fracture network in the fractured coal are explored based on the complex network theory for the first time. The property parameters such as the fracture node degree, the clustering coefficient, and the average path length are analyzed. It shows that the average clustering coefficient of the fracture network in fractured coal is larger, and the average path length is smaller. The connection property of the fracture network in the fractured coal presents a typical “small-world” clustering model. Further, by considering the fractal characteristics of the single fracture and the clustering characteristics of the fracture network, an improved clustered fractal discrete fracture network (DFN) model is developed. Then, based on the lattice Boltzmann method, the permeability properties of the generated clustered fractal DFNs are analyzed. The results show that the permeability of DFNs is positively correlated with the average clustering coefficient of fracture network, and negatively correlated with the fractal dimension of fracture. Therefore, the topological clustering characteristics of fracture networks and the fractal characteristics of fractures cannot be ignored in describing the fluid flow in the fracture network, and our clustered fractal DFN model provides a new idea for guiding the optimization design in DFN engineering.

Analysis of Coalbed Methane Production Characteristics and Influencing Factors of No. 15 Coal Seam in the Shouyang Block

Based on the basic geological data and production data of coalbed methane wells in the Shouyang Block, the characteristics and influencing factors of coalbed methane well production were analyzed, and the primary controlling factors were identified by the grey correlation method. The results show that the average daily gas production of the coalbed methane wells in the study area for the single mining of No. 15 coal ranges from 0 to 604.34 m3/d, with an average of 116.82 m3/d. The overall average gas production is relatively low, with only 7 of the 42 wells having an average gas production greater than 200 m3/d. Gas production tends to increase as the gas content increases. There is a significant positive correlation between gas saturation and average gas production. Burial depth and coal seam thickness also show a positive correlation with average gas production. On the other hand, there is a negative exponential relationship between average gas production and critical desorption pressure. Permeability, as determined by well tests in the area, exhibits a negative correlation with the gas production of coalbed methane wells. The correlation between gas production and the mean three-dimensional stress is weak. As the fractal dimension D value of fractures increases, gas production decreases. A smaller difference in horizontal principal stress is more favorable for the formation of network fractures, facilitating reservoir fracturing and resulting in better reconstructive properties. Moreover, an increase in the sand–mud ratio leads to a decrease in average gas production. The correlation between fault fractal dimension and average gas production is weak. The grey correlation method was employed to determine the controlling factors of coalbed methane production in the study area, ranked from strong to weak, as follows: coal thickness > fracture fractal dimension D value > gas saturation > coal seam gas content > horizontal stress difference coefficient > permeability > critical desorption pressure > mean value of three-dimensional principal stress > coal seam burial depth > sand–mud ratio > fault fractal dimension.

Generation mechanism and influencing factors of fracture networks during alternate fracturing in horizontal wells

Hydraulic fracturing is an effective means to increase natural gas production, and alternating fracturing is a special fracturing method. Compared with sequential fracturing, alternating fracturing is easier to form complex fracture networks. In order to investigate the generation mechanism of complex fracture networks during alternate fracturing, we conducted several multi-stage hydraulic fracturing experiments indoors with different fracturing sequences. Based on the cohesive element method, a numerical hydraulic fracturing model was established, from which the stress interference affected by the fracture spacing and horizontal in-situ stress were analyzed. The results show that: Alternate fracturing is more effective in controlling fracture geometry than sequential fracturing, which is also advantageous in shortening fracture spacing and avoiding the penetration of adjacent fractures. The critical factors affecting the branch fractures are the fracture spacing and the number of main fractures. Alternating fracturing is easier to produce branch fractures perpendicular to the main fracture. The stress interference among main fractures will change or reverse the stress field. The branch fractures closed due to the competition mechanism at the fracture tip are reactivated and extended perpendicular to the main fracture. Alternate fracturing is beneficial in activating microfractures near the main fractures, especially under a larger stress difference. The number of micro-fractures activated by alternating fracturing was more 14% and 21% than the sequential fracturing with the stress difference of 3 MPa and 11 MPa. The increase in stress difference and fracture spacing will decrease the fracture fractal dimension, and the fractal dimension of alternating fracturing is 0.054 more than that of sequential fracturing.

Study on Failure Mechanism and Numerical Simulation of Argillaceous Slate in Southeastern Guizhou.

The content of quartz and clay minerals has a crucial influence on the mechanical properties of slate, which is manifested by the differences in microscopic displacement and energy conversion during uniaxial compression. With Qiandongnan argillaceous slate in Guizhou Province as the research object, the parameters for numerical simulation were determined through indoor uniaxial tests, and argillaceous slate models with different mineral contents were established to analyze crack formation and energy conversion during uniaxial compression. The damage rate of the specimen was qualitatively analyzed by the fractal dimension of the crack. The results show that ① the number of fractures increases with the increase of quartz content, and the number of cracks in Group 4 is 33.4% higher than that in Group 1, and macroscopic fractures are dominated by cracks while microscopic fractures by tension fractures, accompanied by a small amount of shear; ② the energy change curves of the four groups of specimens are consistent with the stress-strain curves, which are divided into three stages, namely elastic deformation, yield, and strength loss. The strain energy in the strength loss stage is negatively correlated with the dissipated energy, and the dissipated energy of the four groups of specimens is positively correlated with the content of quartz; ③ the damage rates for the specimens in Groups 1 to 4 quantitatively analyzed using fracture fractal dimension are 56.8, 59.1, 65.8, and 70.7%, respectively. The research results obtained in this article have further improved the failure mechanism of argillaceous slate under uniaxial compression.

Investigation on radial fracturing around borehole under combined static stress and blasting

Blast-induced radial fracturing is generally subjected to geo-stress. This study experimentally, theoretically and numerically investigates the radial fracturing behaviours around a rock borehole under combined static stress and blast loading by combining blast testing, elastic mechanics analysis and finite element modelling. The study begins with 11 blast tests on 100 mm cubic granite samples under various uniaxial and biaxial stress states based on a biaxial hydrostatic loading system. The fracture networks are subsequently image-processed in ImageJ where the morphology and fractal characteristics of radial fracturing in the rock surface are examined. Then, the variations in fracture patterns are analyzed using elastic mechanics, and the corresponding crack extension is numerically simulated in LS-DYNA. The mechanisms of the influence of static pressure on blast-induced radial cracking are revealed. The results demonstrate that the static stress greatly suppresses the blast-induced radial fracturing, resulting in fewer and shorter radial cracks with lower propagation velocity. This leads to a reduction in the fractal dimension of fractures and rock damage, which is attributed to the initial stress offsetting the blast-formed tangential tensile stress. Meanwhile, uniaxial pressure governs the propagation direction of radial fractures, as the cracks initiate at the borehole wall where the maximum hoop tensile stress presents and propagate along a path with the smallest circumferential compression. At last, the implications of the current results for practical blasting are discussed. It is advisable to leverage the directional cracking advantage offered by blasting under anisotropic in-situ stress to achieve a smoother excavation profile.

How pore structure evolves in shale gas extraction: A new fractal model

The reservoir can be divided into four components with different properties: inorganic matrix, hydraulic fracture, natural fracture and kerogen according to the internal structure characteristics and the natural gas migration mode after hydraulic fracturing. Previous studies usually focus on the relationship between reservoir porosity and permeability, but ignore the influence of reservoir microstructure on gas production. The microstructure of different reservoir components can be described by corresponding fractal dimensions. The structure of fractures can be characterized by fractal dimension of fractures and the fractal dimension of pores can characterize the pore structure of inorganic matrix. Considering the complexity of mass transfer between components and structural deformation, in order to describe the evolution of reservoir microstructure and the influence of various factors on gas production, a fully coupled multi-scale fractal model is established. To verify that the model is reliable, the simulation results contrast with the conclusions of field tests. The effects of adsorption parameters, initial diffusion coefficient and compressibility coefficient on gas production and pore structure evolution were studied. The results show that:(1) when the adsorption constant decreases, the variation of the fractal dimension of pore structure becomes more obvious, and the shale reservoir's gas production and pore structure are most sensitive to the volume constant; (2) the deformation of pore structure increases with the increase of effective diffusion coefficient; (3) the change of compressibility of a single medium only has a major impact on the evolution of its own structure.

Experimental Research on Supercritical Carbon Dioxide Fracturing of Sedimentary Rock: A Critical Review

AbstractSupercritical carbon dioxide (ScCO2) fracturing has great advantages and prospects in both shale gas exploitation and CO2 storage. This paper reviews current laboratory experimental methods and results for sedimentary rocks fractured by ScCO2. The breakdown pressure, fracture parameters, mineral composition, bedding plane angle and permeability are discussed. We also compare the differences between sedimentary rock and granite fractured by ScCO2, ultimately noting problems and suggesting solutions and strategies for the future. The analysis found that the breakdown pressure of ScCO2 was reduced 6.52%–52.31% compared with that of using water. ScCO2 tends to produce a complex fracture morphology with significantly higher permeability. When compared with water, the fracture aperture of ScCO2 was decreased by 4.10%–72.33%, the tortuosity of ScCO2 was increased by 5.41%–70.98% and the fractal dimension of ScCO2 was increased by 4.55%–8.41%. The breakdown pressure of sandstone is more sensitive to the nature of the fracturing fluid, but fracture aperture is less sensitive to fracturing fluid than for shale and coal. Compared with granite, the tortuosity of sedimentary rock is more sensitive to the fracturing fluid and the fracture fractal dimension is less sensitive to the fracturing fluid. Existing research shows that ScCO2 has the advantages of low breakdown pressure, good fracture creation and environmental protection. It is recommended that research be conducted in terms of sample terms, experimental conditions, effectiveness evaluation and theoretical derivation in order to promote the application of ScCO2 reformed reservoirs in the future.

An equivalent analysis of non-linear flow along rough-walled fractures based on power spectrum and wavelet transform

The classical cubic law deviates from the estimation of the flow rate because of the surface roughness distributed along the fracture surface, which can be decomposed as the primary and the secondary roughness. A piecewise equivalent model is developed to characterise the relationship between primary and secondary roughness and flow rate inside the fracture, setting the fractal dimension 2.0 as the piecewise point. The rough-walled fracture influenced by the primary and secondary roughness is quantitatively evaluated based on the wavelet transform and the power spectrum method. A classification criterion is then proposed to accurately capture the secondary roughness by investigating the non-linear flow behavior along rough-walled fractures using the lattice Boltzmann method. The results found that secondary roughness greatly enhances the flow non-linearity when the fracture fractal dimension exceeds 2.0, while secondary roughness has little effect on non-linear flow behavior with the fracture fractal dimension less than 2.0. The developed piecewise equivalent model shows that the non-linear flow pattern is affected by both fracture primary and secondary roughness. The flow variation along fractures with identical fracture apertures and primary roughness is greatly related to the influence of the secondary roughness. The increase of secondary roughness greatly decreases the hydraulic aperture and the permeability. This work provides an improved equivalent model which can be potentially used in field practices to precisely predict the non-linear flow property by emphasising the great impacts of the secondary roughness.

A new approach to evaluate the interactions between the surrounding rock microstructure and water inrush for tunnel excavation

Risk assessments during tunnel excavation are a constant concern for researchers and practitioners. During excavation in water-rich and karst regions, an assessment of the risk of water inrush mediated by coupled hydrological–mechanical effects is crucial. However, as the major channel for water transport, few published studies have analysed the effects of the fracture microstructure on the risk assessment under the combined effects of in situ stress, matrix deformation and water pressure. Based on fractal theory, this study proposes a highly coupled fractal analysis model for tunnel excavation that is capable of simultaneously and quantitatively investigating the interaction between the distribution and fracture evolution of the surrounding rock, the risk of water inrush, and the tunnel stress. We adopt the maximum fracture length (lmax) and fracture fractal dimension (Df) to quantitatively characterize the fracture density and fracture size of the surrounding rock. A comparison with field data and analytical solutions is conducted to verify the correctness. The rock stress, water pressure, deformation and microstructural evolution of rock under different excavation processes of the tunnel have been comprehensively analysed. The contributions of the evolution of various microstructural parameters during tunnel excavation to the risk of water inrush are assessed. This provides a new micro-macro solution for the risk assessment of tunnel water inrush at the engineering scale and provides technical guidance for underground engineering practitioners

Study on the Relationship between Early Shrinkage Cracking and Mechanical Properties of Nano-Clay Cement Mortar Based on Fractal Theory

In order to study the influence of nano-clay on the crack resistance of cement-based materials, two kinds of nano-metakaolin (NMK) and two kinds of nano-attapulgite clay (NMA) were considered. The early cracking process and mechanical properties of nano-clay cement mortar (NCM) was studied by using a plate knife-edge constraint test. Based on fractal theory, the distribution characteristics of NCM surface cracks were revealed, and the calculation method forNCM maximum crack width was given. The results show that the cracking time of the NMK-3 specimen is 2 and 6 h later than that of NMK-1 and NMA-2, respectively; the smaller the particle size of nano-clay, the earlier the cracking time of the specimen. However, nano-clay effectively inhibited the expansion of mortar cracks, and the cracks on the surface of NCM were thin and sparse. At 28 days, the maximum crack width of NMK-3 was 46.7% and 33.3% lower than that of NMK-1 and NMA-2, respectively. NMK hadthe best improvement effect on the mechanical properties cement mortar. The smaller the particle size, the more pronounced the improvement effect.The flexural strength ratio and compressive strength ratio at 7 and 28 days are 76.7%, 67.4%, and 61.2%, respectively.The distribution of surface cracks on NCM has fractal characteristics, and the fractal dimension of surface cracks is smaller than that of ordinary cement mortar. The larger the particle size of nano-clay, the smaller the fractal dimension of cracks. The quantitative relationship between fracture fractal dimension and NCM elastic modulus and shrinkage tensile stress is established.

Experimental study on the impact disturbance damage of weakly cemented rock based on fractal characteristics and energy dissipation regulation

In geotechnical engineering construction, the surrounding rock is often disturbed by dynamic loads such as blasting and earthquakes, resulting in varying degrees of disturbance damage. To study the fracture fractal characteristics and energy dissipation law of different dynamic disturbance-damaged rock masses were subjected to the same failure impact. A series of laboratory tests were carried out by means of a split Hopkinson pressure bar system. Firstly, the disturbance impact test was carried out and the test variables were impact velocity (including high-velocity impact in the velocity range of 1.65 m/s and low-velocity impact in the velocity range of 1.27 m/s) and cycle impact times. Based on the disturbance impact test, the failure impact test with the same velocity (with an average velocity of 3.2 m/s) was carried out to study the relationship between the fractal characteristics of fragmentation and energy dissipation. The results show that the macro damage (crack propagation, edge fracture, and surface particle detachment) increases with the increase of disturbance times. At the same time, after the impact of low-velocity disturbance, the fragments of disturbed specimens have larger fractal dimensions than those of undisturbed and high-velocity disturbed specimens. This indicates that low-velocity impact may produce internal damage that cannot be easily observed directly. The energy dissipation law shows that the high-velocity disturbance impact specimens have higher reflection energy and transmission energy than the low-velocity impact specimens. The regulation and rates of dissipated energy and energy dissipation density are, on the contrary, indicating that the energy utilization rate of the specimen is higher under the impact of low-velocity disturbance. Under the dynamic stress disturbance, the final fracture fractal dimension of the specimen is negatively correlated with the reflected energy during the disturbance impact and positively correlated with the dissipation energy and dissipation energy density. Finally, the meso failure mechanism of weakly cemented red sandstone, subject to disturbance, was analysed based on SEM photos of debris.

Experimental study on seepage-stress coupling failure characteristics and fracture fractal characteristics of cemented gangue-fly ash backfill with defects

In order to study the seepage-stress coupling failure characteristics and fracture fractal characteristics of cemented gangue-fly ash backfill (CGFB) with defects, the samples of single crack CGFB with different angles were prepared and the triaxial compression test under seepage conditions was carried out. The effects of seepage pressure and crack defects on the seepage-stress coupling failure characteristics of CGFB were analyzed. In addition, the box dimension method was introduced to calculate the fractal dimension, so as to quantitatively characterize the correlation between fracture morphology and mechanical properties. The results show that: 1) the permeability and acoustic emission change periodically with the stress-time curve during the deformation and failure of CGFB. With the increase of seepage pressure, the characteristic permeability of CGFB increases, and the characteristic permeability of CGFB with defects is higher than that of intact CGFB. 2) Under the seepage-stress coupling, there is no obvious corresponding relationship between the fractal dimension of fracture appearance and the compressive strength of CGFB: under the same seepage pressure, the higher the strength of CGFB with different angles, the greater the fractal dimension of fracture appearance; under the same crack angle, the greater the seepage pressure, the lower the strength and the larger the fracture fractal dimension. 3) Under the seepage-stress coupling, the fractal dimension of the failure fracture appearance has a good correlation with the dissipated energy, and the two increase linearly. This relative relationship is only related to the damage and fracture, and has nothing to do with the external conditions such as seepage pressure. The research results provide a theoretical basis for the stability analysis and instability prediction of artificial pillar containing joints, cracks and other geological defects under water-rich conditions in China.

Experimental Investigation on Water Seepage through Transparent Synthetic Rough-Walled Fractures

One of the impacts of climate changes nowadays is the increase in the frequency of high-intensity rainfall events alternating with extreme dry periods, which affect the components of the hydrologic cycle, such as runoff, infiltration, and aquifer recharge. Several experimental investigations and theoretical studies have demonstrated that infiltration flow in fractured media can develop along preferential pathways. However, the prediction of infiltration phenomena in fractured media still remains an open issue. This, together with erratic rainfall patterns due to climate changes, affects the prediction of aquifer recharge and contaminant transport in fractured aquifers. The present work contributes to reducing this research gap by means of experimental investigation and forecast analysis, with a focus on the geometrical properties of single fractures and their influence on flow patterns. Several fracture surfaces based on different fractal dimensions, standard deviations, and mismatch lengths were designed using the SynFrac model and were generated by 3D printing technology. The results revealed that the fracture’s fractal dimension has a significant impact on the number of flow paths, while the fracture inclination only increases the number of intermediate preferential channels, and, hence, modifies the flow rate distribution over the fracture outlet. Additionally, the change in the inclination angle of the dry fracture from 55 to 65 degrees resulted in an 8% reduction in the mean width of first flow path. A sensitivity analysis using an M5 tree indicates that there is a linear relationship between flow rate and the exponential form of the fractal dimension. The location of flow channels is a function of fracture fractal dimension, and the influence of mismatch length on their location is negligible. Finally, an accurate prediction algorithm with a Nash value of 0.81 was developed using Wavelet transform in order to estimate the time series of periodic flow rates over the fracture outlet.

Dynamic Mechanical Behaviors and Failure Mechanism of Lignite under SHPB Compression Test

There is an obvious impact effect of on-site blasting on the slope coal mass of open-pit mines, so it is of great significance to study the dynamic mechanical response characteristics of coal rock for slope stability control. In this paper, first, the mineral composition and microstructure of lignite from open-pit mine are analyzed, and it is found that the content of non-organic minerals in lignite such as clay accounts for more than 24.40%; meanwhile, the rock sample has obvious horizontal bedding characteristics and mainly micro pores and transition pores inside; further, there are obvious banded areas with high water content in the rock, which has the same extending direction as the beddings. Based on the SHPB test system, the dynamic compression tests of lignite with different impact velocities are carried out. The results show that there is a significant hardening effect caused by the increase of strain rate on the dynamic mechanical parameters of rock samples, and the stress–strain curve has obvious “double peak” characteristics; meanwhile, the macroscopic crack of the rock appears at the first stress peak and disappears after further compression until the interlayer fracture occurs; further, the fracture fractal dimension of lignite increases linearly with the impact velocity, revealing that the fragmentation of rock samples increases gradually. In addition, with the increase of impact velocity, the input energy and dissipated energy of rock samples increase linearly, while the elastic property increases slowly and at a low level. The bedding characteristics of lignite and the wave impedance difference between the layers cause the high-reflection phenomenon in the process of stress-wave propagation, and then produce the obvious tensile stress wave in the rock sample, which finally results in the interlayer fracture failure of the rock.