Properties Of Fractured Rocks Research Articles

Effective elastic properties of fractured rocks considering fracture interactions

Effective elastic properties of fractured rocks considering fracture interactions

Relating Hydro‐Mechanical and Elastodynamic Properties of Dynamically Stressed Tensile‐Fractured Rock in Relation to Applied Normal Stress, Fracture Aperture, and Contact Area

AbstractWe exploit nonlinear elastodynamic properties of fractured rock to probe the micro‐scale mechanics of fractures and understand the relation between fluid transport and fracture aperture under dynamic stressing. Experiments were conducted on rough, tensile‐fractured Westerly granite subject to triaxial stresses. We measure fracture permeability for steady‐state fluid flow with deionized water. Pore pressure oscillations are applied at amplitudes ranging from 0.2 to 1 MPa at 1 Hz frequency. During dynamic stressing we transmit ultrasonic signals through the fracture using an array of piezoelectric transducers (PZTs) to monitor evolution of interface properties. We examine the influence of fracture aperture and contact area by conducting measurements at effective normal stresses of 10–20 MPa. Additionally, the evolution of contact area with stress is characterized using pressure sensitive film. These experiments are conducted separately with the same fracture and map contact area at stresses from 9 to 21 MPa. The measurements are a proxy for “true” contact area for the fracture surface and we relate them to elastic properties using the calculated PZT sensor footprints via numerical modeling of Fresnel zones. We compare the elastodynamic response of the fracture using the stress‐induced changes in ultrasonic wave velocities for transmitter‐receiver pairs to image spatial variations in contact properties. We show that nonlinear elasticity and permeability enhancement decrease with increasing normal stress. Additionally, post‐oscillation wave velocity and permeability exhibit quick recoveries toward pre‐oscillation values. Estimates of fracture contact area (global and local) demonstrate that the elastodynamic and permeability responses are dominated by fracture topology.

Fluid‐Driven Particle Migration and Its Impact on Hydraulic Transmissivity of Stressed Filled Fractures

AbstractThe accurate assessment of hydraulic transmissivity in rock fractures filled with particles is not only a scientific challenge but also a critical need for various industrial applications. However, the intricate dynamics of particle erosion and pore clogging that govern transmissivity evolution remain largely unexplored. In this study, we experimentally examine the fluid‐driven particle migration behavior in filled fractures and its consequent impact on fracture transmissivity under various hydraulic gradients, normal stresses, and fracture apertures. We find that escalating hydraulic gradients not only intensify particle erosion through amplified fluid drag forces and hydro‐mechanical coupling effects but also lead to an increase in the size of migrating particles, thereby augmenting pore clogging. The dynamics of erosion and clogging define four distinct migration phases within the filled fractures. Variations in normal stress and initial fracture aperture significantly alter the particle arrangement and the soil structure stability within the fractures, thereby modulating the progress of particle migration in response to hydraulic gradients. The pattern of particle migration in filled fractures dictates the development of the internal pore structure and normal deformation, ultimately affecting fracture transmissivity. We propose an empirical expression to encapsulate the comprehensive evolution of fracture transmissivity across different particle migration patterns. Our research advances the understanding of fluid‐driven particle migration within filled fractures and provides a practical tool for the precise determination of hydraulic properties of fractured rocks amidst complex geological settings.

Effect of lateral stress on frictional properties of a fracture in sandstone

The injection of large volumes of natural gas into geological formations, as is required for underground gas storage, leads to alterations in the effective stress exerted on adjacent faults. This increases the potential for their reactivation and subsequent earthquake triggering. Most measurements of the frictional properties of rock fractures have been conducted under normal and shear stresses. However, faults in gas storage facilities exist within a true three-dimensional (3D) stress state. A double-direct shear experiment on rock fractures under both lateral and normal stresses was conducted using a true triaxial loading system. It was observed that the friction coefficient increases with increasing lateral stress, but decreases with increasing normal stress. The impact of lateral and normal stresses on the response is primarily mediated through their influence on the initial friction coefficient. This allows for an empirical modification of the rate-state friction model that considers the influence of lateral and normal stresses. The impact of lateral and normal stresses on observed friction coefficients is related to the propensity for the production of wear products on the fracture surfaces. Lateral stresses enhance the shear strength of rock (e.g. Mogi criterion). This reduces asperity breakage and the generation of wear products, and consequently augments the friction coefficient of the surface. Conversely, increased normal stresses inhibit dilatancy on the fracture surface, increasing the breakage of asperities and the concomitant production of wear products that promote rolling deformation. This ultimately reduces the friction coefficient.

Detection methods for strength deterioration and structural characteristics of fractured rock based on digital drilling

Fractures ubiquitously exist in strata, and large-scale fractures can cause low bearing capacity and poor stability in rock masses and can sometimes lead to hazards such as roof falls and collapses in mines and tunnels. The foundation of the control of surrounding rocks and effective prevention of such hazards is to be able to accurately determine the structural characteristics of fractures and strength parameters of rocks. Traditionally, testing is carried out mainly through indoor compression tests and borehole inspection, which require drilling of the surrounding rock on site and core testing indoors. The effective in situ and accurate detection of the structure and strength properties of surrounding rock has become a challenge. Digital drilling testing of rock mass is a new approach to solve the above problems and future research direction. In this study, a multifunctional rock mass digital drilling test system is used to perform tests on different types of fractured rocks. The response of drilling parameters to fracture parameters is analysed, and a strength deterioration evaluation index for fractured rock based on drilling parameters is established. Then, the response mechanism of the strength deterioration rate while drilling in the fracture zone is defined, and a model for identifying fracture parameters based on drilling is established. Furthermore, an in situ detection method for fractured rock properties based on digital drilling is proposed. The results of the validation test show that the average difference ratio between the equivalent compressive strength measured by the drilling test and the laboratory compressive test is 3.97 %. The strength degradation of the fractured rock is significant, with a maximum strength degradation rate of 82.1 % obtained through drilling. The average identification accuracies of the fracture position and width are 2.56 mm and 0.85 mm, respectively. The identification accuracy of rock properties while drilling is high, which proves the effectiveness of the prediction model while drilling. This study can provide a theoretical and methodological basis for the accurate in situ prediction of rock properties in underground engineering fields, such as mine roadways, transportation tunnels, and hydraulic tunnels.

Study on the influence of thermo-pressure coupling environment on the fracture properties of shale in deep reservoirs

Deep reservoir in-situ environments enhance rock plasticity. Most current experiments that combine temperature and pressure are difficult to observe the crack tip directly, impeding the understanding of deep rock fracture properties. This paper used a digital image correlation method and the principle of fracture mechanics to establish an innovative visual experimental rock fracture system with in-situ environment. Crack tip initiation and propagation were captured dynamically in real-time temperature and pressure coupling conditions, and the influence on shale fracture properties was analyzed, providing a feasible method for examining the fracture behavior of deep reservoir rocks. The results showed that the fracture characteristics of deep shale should be reflected and characterized by jointly considering temperature and confining pressure. Shale plasticity produced crack tip passivation in combined in-situ temperature and pressure conditions, and reduced the local stress concentration effect. The post-peak softening relationship showed a nonlinear “convex” trend, indicating nonlinear fracture characteristics with increased toughness. The exponential coefficient representing the degree of nonlinearity rose as the buried depth increased. The exponential coefficient related to the buried depth was introduced to modify the constitutive model, which was used to effectively characterize the nonlinear fracture behavior of deep reservoirs and provide a basis for deep reservoir fracturing reform research.

Fracture Density Prediction of Basement Metamorphic Rocks Using Gene Expression Programming

Many methods have been developed to detect and predict the fracture properties of fractured rocks. The standard data sources for fracture evaluations are image logs and core samples. However, many wells do not have these data, especially for old wells. Furthermore, operating both methods can be costly, and, sometimes, the data gathered are of bad quality. Therefore, previous research attempted to evaluate fractures indirectly using the widely available conventional well-logs. Sedimentary rocks are widespread and have been studied in the literature. However, fractured reservoirs, like igneous and metamorphic rock bodies, may also be vital since they provide fluid migration pathways and can store some hydrocarbons. Hence, two fractured metamorphic rock bodies are studied in this study to evaluate any difference in fracture responses on well-log properties. Also, a quick and reliable prediction method is studied to predict fracture density (FD) in the case of the unavailability of image logs and core samples. Gene expression programming (GEP) was chosen for this study to predict FD, and ten conventional well-log data were used as input variables. The model produced by GEP was good, with R2 values at least above 0.84 for all studied wells, and the model was then applied to wells without image logs. Both selected metamorphic rocks showed similar results in which the significant parameters to predict FD were the spectral gamma ray, resistivity, and porosity logs. This study also proposed a validation method to ensure that the FD value predictions were consistent using discriminant function analysis. In conclusion, the GEP method is reliable and could be used for FD predictions for basement metamorphic rocks.

Analysis of Dynamic Mechanical Properties and Energy Consumption of Fractured Sandstone under Dynamic–Static Combination Conditions

Under dynamic loads, such as blasting, excavation, or quarrying, rocks with fissures are the first to sustain damage, leading to instability in the engineering rock mass. To investigate the kinetic properties of fractured rocks, fractured coal mine sandstone specimens underwent impact compression tests using a dynamic–static combination SHPB (split Hopkinson pressure bar) test device at different loading rates under combined dynamic and static conditions. The damage characteristics of the specimens were analyzed from an energy point of view. The results show that under the dynamic and static combined condition, when five impact loading air pressures are used for loading at different impact rates, the trends of the dynamic stress–strain curves of prefabricated fissured rock samples under various impact pressures were discovered to be similar and were mainly categorized into three main stages of elasticity, yield, and destruction; the specimen’s dynamic compressive strength increases according to a power function relationship; as the average strain rate increases, the dynamic strain increases linearly and the dynamic modulus of elasticity increases in a quadratic relationship, all of which show a significant strain rate effect. The incident energy is a power function of the loading rate. The reflected, transmitted, and absorbed energies by the sample increase with the incident energy. The degree of the sandstone specimen fragmentation gradually grows with increasing impact loading rate and incident energy, as evidenced by a decrease in the scale of the fragments. The absorbed energy in the sample is mainly used for the deformation damage of the rock, and the more intense the fragmentation of the specimen, the more absorbed energy is required.

Non-constant evolution of mixed mode fracture process zone in heat-treated salt rock during the whole loading

Exploring the evolution of the fracture process zone (FPZ) in mixed fracture within salt rock is vital for enhancing our comprehension of salt rock fracture properties and assessing the long-term stability of energy reservoirs. Prior research has identified the existence of constant evolution within the FPZ, a topic that has sparked debate. For this purpose, we acquired data on the evolution of the FPZ during the whole loading process of mixed fracture in salt rock at different temperatures by employing Digital Image Correlation (DIC) technology. After the experiments, we employed the computerized tomography (CT) scanning technique to analyze the pore structure within the mixed-mode salt rock fracture. This analysis aims to provide further insights into the impact of temperature on the evolution of the FPZ. The findings demonstrate that the evolution of the FPZ in mixed mode salt rock fracture exhibits non-constant behavior, involving intricate evolutionary processes. When the specimen was subjected to a temperature of 700 °C and an angle of 10°, the length of the FPZ increased from 0.4 mm to 10.7 mm, before decreasing to 9.2 mm. Moreover, during the whole loading process, the non-constant evolution of the FPZ is notably influenced by temperature. Notable variations in the evolution of the FPZ emerged during the whole loading process, especially when the temperature was below 400 °C, as opposed to when it exceeded 500 °C. When the temperature exceeded 500 °C, the FPZ emerged at an earlier stage. At a temperature of 700 °C, the FPZ emerged in the 30 % pre-peak stage of the process. Additionally, during the post-peak stage, a more complex and non-constant evolution was observed. A bilinear relationship between the length of the FPZ and temperature at the peak load stage was observed.

Effect of grouting on damage and fracture characteristics of fractured rocks under mode I loading

Grouting is widely used to improve the mechanical properties of fractured rocks, but its impact on the fracture behavior of fractured rocks remains incompletely understood. This research aims to investigate the effects of grouting on the damage and fracture characteristics of fractured rocks under mode I loading. We used epoxy resin and ultrafine cement to grout granite and sandstone centre-cracked Brazilian disc specimens. We conducted Brazilian splitting tests on intact specimens, as well as specimens with and without grouting under mode I loading. Acoustic emission (AE) and digital image correlation (DIC) techniques were utilized to study the effect of grouting on rock fracture behavior. Furthermore, we employed an improved fracture toughness calculation method to evaluate the anti-fracture capacity of grouted specimens. The results show that grouting positively reinforces fractured rocks by increasing the shear crack ratio, mitigating the stress concentration effect at the crack tip, and fostering a more complete and stable development of the fracture process zone (FPZ). Materials with superior strength and bonding properties can effectively improve the mechanical performance of fractured rocks. Notably, epoxy resin significantly boosts the strength, deformation adaptability, and anti-fracture capacity of the specimens, outperforming ultrafine cement in terms of grouting effect. The entropy change of AE energy during the loading process indicates that the rock system undergoes three stages of order-to-disorder-to-order. The second stage and the abrupt changes in entropy value can serve as precursors to rock damage and fracture. This research enriches our understanding of how grouting influences the damage and fracture behavior of fractured rocks, providing valuable insights for risk assessment and disaster prediction in geotechnical grouting engineering.

Research on the evaluation method of rock brittleness characteristics based on stress-strain curve

Abstract In this study, not only the variability of the internal texture of rocks is considered in the in-depth investigation of the brittle fracture properties of rocks, but also the effects of different stress states and temperature factors on the fragility of rocks are focused on. Through experimental studies and data analysis of various types of rocks under different stress conditions and temperature environments, this study found that the plasticity of marble is positively related to stress. Under other stress states of 5, 15, 25, and 35 MPa, the stress-strain curves become flatter with increasing stress. Under uniaxial stress condition, the fragile strengths of different rocks are sandstone, red sandstone, granite, marble, chert, and cement mortar in that order. Whereas, under triaxial stress condition, the fragility of the rock samples was in the order of fine-grained granite, coarse-grained granite, sandstone, tuff, and marble. It was further found that the rock specimens after different high-temperature water-cooling treatments exhibited four different stages of brittle fracture, and the peak of the first stage of the rock was higher as the temperature decreased. The inclination of the peak was inversely proportional to the temperature. This study provides an essential scientific basis for the in-depth understanding of the brittle fracture characteristics of rocks and their engineering applications.

Experimental study on effects of cyclic loading paths on cracking behavior and fracture characteristics of granite

To investigate the impact of cyclic loading paths on the Mode I fracture characteristics of granite, a series of fracture tests were conducted on semi-circular bend (SCB) specimens using acoustic emission (AE), digital image correlation (DIC) and 3D scanning. The tests included monotonic loading, variable amplitude (VA) cyclic loading, and tiered constant amplitude (TCA) cyclic loading. The results demonstrated that VA cyclic loading leads to an average enhancement of 5.6% in fracture toughness, whereas TCA cyclic loading results in an average reduction of 3.7% in fracture toughness. During cyclic loading and unloading, the TCA loading path exhibits a higher rate of cumulative plastic deformation growth and greater magnitude cumulative plastic deformation compared to the VA loading path. Additionally, detailed observations from 3D scanning revealed that under cyclic loading, the fracture trajectories and surface morphology exhibit more tortuosity and roughness compared to monotonic loading, particularly in the case of TCA cyclic loading. Furthermore, according to the AE and DIC analysis, the proportion of microcrack initiation and stable propagation stage, cumulative AE counts and fracture process zone (FPZ) size generated under VA cyclic loading are greater than those generated under TCA cyclic loading, displaying a consistent variation pattern with fracture toughness. The changes of fracture properties can be attributed to a competition between the compaction-induced hardening effect and the cracking-induced damage effect. A higher cyclic upper limit and incomplete unloading are more likely to accelerate the growth of the irreversible crack damage and rapid crack extension, resulting in weakened rock fracture properties. These findings provide important references for rock instability control in underground rock engineering.

Quantitative assessments on fluid flow through fractures embedded in permeable host rocks: Experiments and simulations

An accurate understanding on hydraulic behaviors of fluid flow through fractured rocks is of central importance for both underground resources exploitation and CO2 geological sequestration. Despite its importance, fluid flow through fractures surrounded by permeable matrix is rarely investigated. Here, this study aims to explore fluid flow through fractures surrounded by permeable host rocks using a combined experimental and numerical approach. In particular, core-flooding experiments were sequentially conducted on both intact and fractured cores subjected to different rock types, under varying confining stress and fracture roughness. According to the experimental results, fracture permeability decreases rapidly with increasing confining stress and fracture roughness, while matrix permeability is less affected by confining pressure, resulting in an increment in contributions of matrix flow to the overall flow with increased confining pressure. In addition, numerical simulations were performed on digital fractured rocks with a broadened range of roughness from 2.58 to 18.2, and under confining pressure ranging from 2 MPa to 53 MPa. The fluid flows within matrix and fracture were assumed to follow linear Darcy's law and nonlinear Forchheimer's law, respectively. The simulated results also show that Km/Kf increases with the increase of confining pressure, reflecting the enhancement in contributions of matrix permeability to the overall transportation. If Km/Kf = 0.1 is defined as a threshold over which matrix permeability should not be ignored, then the critical confining pressure of 18 MPa, 20 MPa, and 46 MPa is determined for mudstone, sandstone and limestone, respectively. In addition, the fluid flow through rough-walled fractures is observed to be susceptible to normal loading and fracture roughness, which directly alters flow paths within the fractures. With an increment in confining pressure and fracture roughness, the contact area between rough fracture surfaces grows, promoting flow tortuosity and generating eddies inside fracture, which consequently increases flow resistance and hence weakens fracture permeability. Ultimately, by fitting the simulated data, an empirical equation linking fracture permeability with confining stress, fracture roughness and hydraulic gradient is provided. This study provides quantitative characterizations on fluid flow through fractures accounting for permeable matrix, and thus can potentially improve the accuracy in predicting hydraulic properties of fractured rocks.

Influence of injection methods on bio-mediated precipitation of carbonates in fracture-mimicking microfluidic chip

Enzymatically induced carbonate precipitation (EICP) is a potential method to alter the hydraulic properties of fractured rocks. The pore-scale in-situ formation of bio-mediated precipitated carbonates in fracture-mimicking channels was investigated experimentally through a visualization platform and microfluidic technology. Image processing was employed to analyze the precipitation behavior. Three representative biogrouting methods (one-phase continuous injection, one-phase staged injection, and two-phase staged injection) and the role of nucleation agents were examined in this study. Results revealed that more amorphous substances formed during one-phase injection process, and the one-phase staged injection method exhibited superior precipitation efficiency, while two-phase staged injection method demonstrated better effectiveness in channel clogging. The addition of nucleation agents resulted in the channel-like precipitation pattern, compared to formation of more crystals in the absence of additives. The hydrodynamic coupling between precipitation and flow velocity was used to explain the mechanisms of fracture sealing. Precipitation and aggregation led to the reduction in microchannel aperture, thereby influencing the flow field. Under flow-induced shearing, the amorphous substances could be transported and settled on the channel surface, leading to a self-enhancing process in channeling. This study provides insights into the microscopic mechanisms of EICP and advances the application of EICP in fracture repairing.

Model of shear strength of ultra-deep fractured sandstone considering fracture morphology

Ultra-Deep fractured tight sandstone gas reservoir, the hotpot unconventional petroleum resources, has been the subject of investigation recently. Due to the presence of natural fractures, drilling in ultra-deep fractured tight sandstone may cause complicated wellbore instabilities, which resulted in a substantial challenge to the efficient exploitation and production of tight gas resources. In order to understand the rock mechanical properties of the ultra-deep fractured tight sandstone under shear stress, tight sandstone core samples were artificially shear-fractured, the 3D characteristics of the fractures were reconstructed by CT scanning, and the fracture aperture and roughness characteristics were systematically evaluated. Simultaneously, triaxial shear strength tests were conducted to evaluate the mechanical properties of fractured rock subjected to varying confining pressures. Considering the contact relationship of the fracture surface during shearing, two indexes of average effective shear angle and contact area ratio were proposed to correct the fracture roughness, and a new fracture shear strength model was constructed, which served as the failure criterion for the fractured formation. The results show that the intact rock has a high compressive strength and can only collapse under conditions of substantial differential stress. Consequently, shear failure of fracture ought to be the primary source of wellbore instability. The fracture aperture's spatial distribution is irregular and conforms to a Gaussian distribution. A clear anisotropy in the fracture roughness is present, and there is less roughness along the initial shear failure path and more roughness perpendicularly. With an increase in normal stress, the fracture's peak shear strength rises, while the friction angle steadily falls. The innovative model's accuracy in predicting shear strength is higher than that of the weak-surface model and the JRC-JCS model.

Numerical Simulation Analysis of Fracture Propagation in Rock Based on Smooth Particle Hydrodynamics.

The mechanical properties of fractured rock have always been a focal point in the rock mechanics field. Based on previous research, this paper proposes improvements to the SPH method and applies it to the study of crack propagation in fractured rocks. By conducting uniaxial compression tests and simulating crack propagation on various specimens with different crack shapes, the characteristics of crack propagation were obtained. The comparison between the simulated results in this study and existing experimental and numerical simulation results confirms the validity of the SPH method employed in this paper. The present study utilizes the proposed methodology to analyze the influence of the crack angle, width, and orientation on crack propagation. The SPH method employed in this study effectively demonstrates the expansion process of fractured rock under uniaxial compression, providing valuable insights for the engineering applications of SPH.

Heating-path dependency of fracture behavior of sandstone subjected to thermal treatment

The thermal effect has a negative impact on the strength and fracture properties of rocks, and much related research has been carried out around heat treatment methods such as temperature levels, heating rates and cyclic thermal shocks. In fact, the temperature history to which rocks are subjected is often complex, and the effects of this accumulation of thermal damage on the rock cannot be ignored. In this work, we have carried out three-point bending tests on sandstone subjected to thermal treatments under conventional and complex heating paths based on digital image correlation (DIC) method. The results show that the P-wave velocity, peak load and fracture toughness all tend to decrease with increasing temperature under conventional heating paths, and that 600 °C is the sandstone threshold temperature for strong and weak transformation. Under complex heating paths, a constant temperature of 200 °C for two hours and further heating to the target temperature does not weaken the mechanical properties of the sandstone, whereas a constant temperature of 400 °C for two hours and further heating results in a significant reduction. The variation trend of crack opening displacement (COD) with axial load shows that the cracks transition from slow opening in the early loading stage to rapid propagation near the peak load and develop into decelerated extension in the later stage. Finally, the crack propagation process was refined by determining the jump points of the maximum principal strain at different positions on the crack propagation path. At room temperature, the sandstone cracks at approximately 0.8Pm and propagates at a higher rate, exhibiting clear brittle characteristics. Both the crack initiation load and propagation rate show a significant decrease with increasing temperature after 200 °C, with ductile characteristics gradually becoming apparent.

Stress field analysis: Construction of site descriptive model of underground research site in KAERI

This study aims to integrate geological investigation data and construct a rock mechanical site descriptive model relative to site selection research for deep geological disposal of spent nuclear fuel. As preliminary research for the model construction process, a sub-divided numerical domain was introduced by adopting existing fracture zone in the site region of an underground research facility in KAERI. The spatial properties of the domain were decided based on the rock classification information of core specimens obtained from several boreholes. Stress field estimation modeling was performed using the constructed domain and the results were compared with the in-situ state of the field test. When the depth-dependent stress ratio was assigned to the model condition, it was confirmed that an appropriate stress state was ensured with the similarity of the in-situ stress along the depths. Additionally, uncertainties of the input properties in the stress estimation model were also assessed. The effect of uncertainties could be considered through a sensitivity analysis of rock mass and fracture zone properties to identify stress dependence on mechanical properties. Significant stress distribution was not indicated with a variation in the rock mass properties. However, weak fracture zone modeling induced a slightly disturbed stress in a shallow depth range. These results indicated that the uncertainty of the rock mechanical site descriptive model could be increased due to the fracture zone properties. This study is a case of prior research in Korea on the site evaluation model that is already applied in leading foreign countries related to site selection research for nuclear fuel disposal. The applicability based on the achievement of this study can be improved through consideration of derived equivalent properties of fractured rock. And more reliable results can be succeeded by assigning properly defined boundary conditions for the simulation of various geological scenarios.

Hybridized random forest with population-based optimization for predicting shear properties of rock fractures

Rock mass fractures play an essential role in controlling the shear behavior of natural rocks. The clear understanding of the fracture's shear parameters is of great significance in maintaining underground structure stability. A novel approach combining the random forest (RF) model and two population-based optimization algorithms respectively (the Sine Cosine Algorithm (SCA) and the whale optimization algorithm (WOA)) was proposed to predict the shear strength, peak shear displacement, and dilation angles of rock fractures. 84 direct shear data was divided into training set and test set for training and testing the proposed models. Coefficient of determination (R2), root mean square error (RMSE), and mean absolute error (MAE), three indexes were utilized to evaluate the model prediction accuracy. The prediction performance indicated that SCA-RF and WOA-RF models achieved high accuracy in predicting the shear strength and dilation angles of rock fractures. These two models also perform satisfactorily in predicting the peak shear displacement by optimizing the dataset division and fitness function. Comparing with Barton-Bandis and Asadollahi-Tonon empirical models, the proposed models have significant advantages in estimating the shear properties of rock fractures. It has important implications for the development of accurate and reliable models for predicting the shear behavior of natural rocks, that some data processing techniques were used based on the characteristics of the direct shear dataset.

A new approach for predicting direct shear tests on rock fractures

The shear process can significantly affect the hydro-mechanical properties of rock fractures. In the present study, a new approach is developed to predict the direct shear process of rock fractures under the constant normal load condition. This approach is validated by three sets of laboratory direct shear test data and it is then used to study the impact of rock mechanical properties on the shear process. The results show that the presented shear modelling approach can reliably predict the entire shear process of direct shear tests on rough rock fractures under the constant normal load condition. The shear strengths and the normal displacement are sensitive to different physical parameters, which are demonstrated through a comprehensive sensitivity analysis of the physical parameters (i.e., the mechanical properties). The presented shear modelling approach and the sensitivity analysis results are useful for further hydro-mechanical investigations of fractured rocks.