Rock Mechanics and Rock Engineering

The surrounding rock of deep engineering is under high stress and often affected by dynamic disturbance loads such as excavation blasting, fault sliding, and earthquake. These loads may cause rock instability and even rockburst disaster. To study the dynamic responses of deep marble under high stress, true triaxial compression (TTC), true triaxial disturbed compression (TTDC) along with synchronous acoustic emission (AE) tests were conducted on the marble specimens. It is found that the lateral impulsive disturbance weakens the strengths of the specimens, but improves their integrity to some extent. The surfaces of the TTC specimens exhibit stronger splitting failure characteristics than those of the TTDC specimens. Under TTDC, the specimen deformation in the direction of intermediate principal stress (σ2) shows a trend of ‘expansion–contraction-expansion’ and the shear fracturing in the direction of minor principal stress (σ3) become obvious with the increase of σ2. After the lateral impulsive disturbance, the specimens have an “outbreak period” of AE events before and after their failure. More energy is released during the failure process, and severer damage occurs at higher σ2. The lateral impulsive disturbance changes the micro-fracture activities and the crack distribution of the specimens. It is easier for shear cracks to occur and develop in the disturbed specimens. After the lateral impulsive disturbance, the specimens are more sensitive to σ2. When σ2 is high, the AE b values decrease significantly several times before and after the specimen failure.

The first part of this paper outlines a simple method for assessing seismic damage to rock tunnels prior to an earthquake, based on the Seismic Damage Classification of Tunnels (SDCT), proposed by Reddy and Singh (2024) ['A Simplistic Method for Assessing Seismic Damage in Rock Tunnels Before Earthquake: Part 1-Damage Prediction and Validation Using Seismic Damage Classification of Tunnels', Rock Mechanics and Rock Engineering, pp. 1-32]. However, to implement the proposed methodology in the field, the data on seismic sources within 250 km are necessary. Seismic sources within this range should be mapped in Google Earth Pro to obtain source-to-site distances, which are then used to calculate PGA values. After gathering all inputs, the engineer needs to check critical parameter combinations to determine damage class and predict damages before an earthquake. Though the method is simple, the process is time-intensive, requiring precision at each step. This study simplifies the process and develops a software implementation of the proposed methodology. This software is an outcome of a Python-based GUI tool developed for India and adjacent countries. To develop this tool, the seismic sources are collected and mapped in QGIS to create a database of 4602 faults across India and adjacent countries. The written Python code identifies the sources within 250 km of the tunnel site and also calculates the PGA from each source through empirical attenuation relationships. This GUI tool evaluates the Seismic Vulnerability of Rock Tunnels (SVRT) using input parameters, such as latitude, longitude, Rock Mass Rating, Overburden, lining type, and tunnel shape. Using the obtained PGA values for seismic sources within 250 km and by critically combining the entered input parameters, the tool predicts the damage class and probable damages for any location within the study region. The reports are generated in .txt (notepad) format and graphs for the distribution of total faults in each damage class are provided for the user's location. The validation of this tool is done by performing SVRT of the Daliang tunnel site. The results of the software and actual damages of the Daliang tunnel due to the Menyuan earthquake (2022/Mw 6.6) in Qinghai, China are compared. The tool showed a good agreement with the proposed software. The software's user-friendly interface makes it easy to input data and quickly obtain results. Highlights • For the first time, a simple method is proposed for predicting and assessing the seismic damages to the rock tunnels before an earthquake using the Seismic Damage Classification of Tunnels. • Employing the methodology, this study proposes a software, which is a Python-based GUI tool that can perform the Seismic Vulnerability of Rock Tunnels (SVRT) before an earthquake in India and adjacent Countries. • Utilizing this tool, engineers can perform the SVRT for tunnel sites by generating reports and graphs before an earthquake. • The proposed software is simple to use and delivers quicker results through obtained reports, as it can be used by engineers for preliminary seismic investigations.

Crystalline bedrock generally has very low permeability, preventing fluid flow; however, fractures can provide critical flow pathways. In the context of landfills developed in crystalline bedrock, a comprehensive understanding of fracture networks is a key for environmentally safe and sustainable waste storage. This study presents the interpretation of borehole logs and Lugeon tests combined with the isotopic composition of well water surrounding a rock quarry to assess its potential as a landfill site. The Rekefjord quarry is located on the southwestern coast of Norway. Structural interpretations indicate randomly distributed fractures, but also steeply dipping fractures that are not well captured by vertical boreholes. To correct for orientation biasness the Terzaghi correction was applied. Most of the borehole fractures display apertures in the range of 2–10 mm and average degrees of connection (D) varies between 3.09 and 1.92. The hydraulic conductivity of 5 m borehole sections range from 1.2 × 10–6 to 1.6 × 10–10 m/s, showing no clear relationship with depth or fracture frequency or aperture. The ⁸⁷Sr/⁸⁶Sr ratio measured in groundwater supports the fluid connectivity within the fracture network and indicates mixing with surface water. Although this study provides site specific results, the integrated methodology combining structural analysis, hydraulic testing, and isotopic characterization and findings provide a robust framework applicable to evaluating fracture networks and fluid connectivity in similar crystalline bedrock settings.

To study the response mechanisms of slopes under seismic action after different rainfall conditions, a comprehensive analysis was conducted through large-scale shaking table model tests and wave theory. Different instability modes of slopes under seismic conditions after various rainfall events were identified. The changes in modulus and damping caused by the combined effects of rainfall and earthquakes were clarified, determining the mechanism of the acceleration response of slopes. A method for evaluating internal damage in slopes based on the difference between positive and negative energy amplification was established. These findings indicate that the reduction in modulus and damping caused by rainfall leads to a decrease in the natural frequency and an increase in the slope response. The decrease in modulus and increase in damping due to earthquakes result in a lower natural frequency and weakened slope response. The difference between positive and negative energy amplification reflects the internal damage and instability modes of the slope more accurately than the natural frequency and dynamic pore water pressure. The response mechanism of slopes is a theoretical bottleneck in the study of rainfall-induced seismic landslides. The internal damage assessment method is crucial for translating theory into application, with its accuracy and universality being essential.

Fractures are diverse geological features. Despite extensive research on their varied geometries and growth mechanisms, there has been relatively little focus on how acid–rock interactions, such as reactive transport and precipitation, influence crack growth. To understand the chemical corrosion of sandstone exclusively from chemical reaction, we have developed a chemical corrosion model that highlights that the dissolution of calcite grains within the sandstone, coupled with the diffusion of Ca²⁺ ions and precipitation of calcite, contributes to time-dependent crack formation and their subsequent filling. Based on this theory, we propose a heterogeneous grain-based phase-field method (PFM) model to analyze the failure pattern and changes in ion behavior in sandstone. Our model results are in good agreement with experimental data, validating the proposed chemical corrosion theory and the grain-based PFM model. Both numerical and experimental results reveal that the chemical corrosion of sandstone is a time-dependent deterioration process that progresses from the exterior to the interior of the sample. The cracks that form act as pathways for ion transport, leading to a gradual decrease of Ca²⁺ ions with increasing distance from the surface of calcite grains. Following the validation of our approach, we used the heterogeneous grain-based PFM model to analyze the effect of rock heterogeneity and to simulate chemical corrosion induced by calcite grains within a sandstone sample. The numerical results reveal that a lower homogeneity index leads to a larger damaged area and accelerates crack initiation. Additionally, we identify and categorize the acid–rock interactions into three distinct stages: initial surface erosion, subsequent crack propagation, and further deepening of etched features. The proposed chemical corrosion theory enhances our understanding of the degradation and fracture mechanics of sandstone in an acidic environment, and can be further extended to elucidate the sealing of hydraulic fractures and formation and dissolution of calcite veins.

It is common that the underground filling mine stopes are always encountered with disturbed stress, and the rock–backfill composite structure (RBCS) material is exposed to dynamic disturbance stress conditions. To understand the impact of cyclic disturbed frequency (CDF) on the damage and failure characteristics of RBCS, a series of multi-stage cyclic loading experiments were conducted on cylindrical RBCS specimens with different CDFs, combined with real-time acoustic emission monitoring and post-test CT scanning. The experimental results show that volumetric strain and AE events increase with increasing CDF. The AE signals characterized with high-amplitude and high-frequency increase with CDF, the wider of low-frequency band increases accordingly. In addition, the tensile cracking and shear/mixed cracking mode is well classified, it is found that the tensile cracking mode plays a dominant role for RBCS under low-frequency stress disturbance condition and shear cracking mode is dominant for RBCS under high-frequency condition. Moreover, post-test CT images reveal the differential cracking pattern, the percentage of the tensile cracks decreases and shear/mixed cracks increases with increasing cyclic disturbed frequency.

In this work, the effects of three-way confining pressure on the force distribution, fracture expansion and damage extent in the blasted rock mass were observed. With the fluid attenuation law introduced, the effect of blasting gas on crack propagation was primarily considered. Based on the ideal gas equation of state, the initial propagation pressure of the cracked zone as a function of the peak pressure on the blast hole wall was established. The method for determining the internal stress intensity following blast loading of the rock mass under confining pressure was derived utilizing the principles of elastic mechanics. A forecasting model of the extent of the cracked zone was proposed, taking into account the effect of confining pressure on the strength of the rock mass. Subsequently, the force distribution within and the extent of the cracked zone in the blasted rock mass were analyzed. Using Polymethyl Methacrylate (PMMA) specimens as an equivalent replacement for rock specimens, blasting experiments under different initial confining pressures were conducted on the in-house triaxial hydraulic loading rock blasting test jig, and the morphology, crack distribution, fractal dimension and damage of the blasted specimens were examined. Finally, the stress distribution and damage extent in the near-field, far-field blast zone in the blasted rock under the confining pressure action were analyzed by the FEA software LS-DYNA. Results showed that the initial confining pressure applied to the rock mass would reduce the fractal dimension, the average damage and the extent of the cracked zone of the rock mass. As the confining pressure increases, the ratio of such reduction gets smaller. As the confining pressure further increases, the tensile failure zone of the rock mass gets smaller, and the confining pressure was found to suppress the tensile failure of the rock mass and cause the transition of the rock mass to compression failure. The blast loading predominates in the near-field blast zone, where the initial confining pressure has little impact on the blast stress field; in the far-field blast zone, the initial confining pressure begins to impact the coupled stress field as the blast stress wave further attenuates. As a consequence, the tangential peak stress in the rock mass arrives at a time lagging behind the radial peak stress, and the tangential tensile peak stress would decrease as the confining pressure increases, which in turn restrains the crack propagation.

Rock particle size distribution (PSD) during ball milling is heavily influenced by mechanical parameters like Vickers hardness (HV) and brittleness (B). This study analyzes nine rocks with different lithologies, first using multifractal theory to examine the effects of hardness and brittleness on PSD. The results show that the range, uniformity, and concentration of the PSD follow distinct patterns with varying hardness and brittleness. The brittleness index has the greatest impact on the range and concentration of PSD, followed by hardness, while uniformity is similarly influenced by both hardness and brittleness. Furthermore, the heterogeneity of PSD increases with hardness, showing fluctuations, decline, and subsequent increase. With brittleness, it exhibits fluctuations, decline followed by increase, and final decline. As hardness and brittleness increase, the PSD variation shifts from larger to both large and small particles. Finally, the joint multifractal approach quantifies the impact of hardness and brittleness on PSD, revealing the relationship between rock properties and particle size distribution.

Fault-induced surface subsidence and roadway water inrush pose significant risks to both the surface ecological environment and the safety of underground mining operations. Over the long-term geological process, fault broken zone can lead to the formation of cemented rock strata. Typically, these strata experience dry–wet cycles due to the invasion of underground aquifers and the impact of roadway drainage systems, which affects the pore structure distribution and permeability evolution of cemented rocks. Considering the particle size distribution of specimen and the effect of wet–dry cycle, nuclear magnetic resonance (NMR) technology and transient pulse method were employed to investigate the pore structure characteristics and permeability evolution of cemented rock specimens. The test results showed that both the porosity and permeability increased with the Talbot index η and the cycle number. Based on the influence of η and the wet–dry cycle on porosity and permeability, three pore-permeability models were established: the classical estimation model, the pore absorption model, and the pore connectivity model. It was founded that the pore adsorption model was suitable for specimens after low numbers of dry–wet cycle treatment, while the pore connection model was applicable for specimens after high numbers of dry–wet cycle. The study revealed that the porosity development gradually transformed into pore connection development during the wet–dry cycle treatment. The enhancement of pore connectivity serves as an early warning indicator, suggesting the information of seepage channels and the high risk of water inrush disasters in underground roadways. To mitigate the potential menace of the increasing permeability, two prevention and control suggestions are proposed: (i) borehole grouting, to enhance the cementation strength and integrity of the fault rock strata. (ii) Prestressed anchor support, to proactively provide compressive stress for the fault cemented rock strata.

Time-dependent fracturing causes irreversible property degradation over time and may eventually result in the time-delayed instability. The essence of time-dependent rock failure lies in the initiation, propagation and coalescence of internal cracks, accompanied by irreversible energy dissipation. Thus, investigating the time-dependent failure process from an energy evolution perspective will enhance understanding of this complex phenomenon. In this paper, multi-stage compression tests with sustained loading were conducted on Jinping marble, and the characteristics of energy accumulation and dissipation were thoroughly analysed. Results show that the continuous input of energy is primarily driven by the increase of dissipative energy, which is used for the formation of new crack surfaces and the microstructure adjustment. Meanwhile, the elastic energy density remains relatively constant over time, indicating that the influence of sustained loading on the elastic energy evolution could be overlooked when analysing the time-dependent energy evolution. During time-dependent loading, the increase in friction energy density is primarily observed in the decelerating creep stage and the accelerating creep stage, while friction energy density remains relatively constant in the stable creep stage. Additionally, the structural integrity of hard rock also gradually deteriorates due to continuous fracturing, leading to time-dependent deterioration of long-term load-bearing capacity. The decreasing trend in long-term strength corresponds to that of fracture energy density, which is also reflected in the patterns of volumetric strain hysteresis loops.

Due to the complex mining conditions and environment in coal mines, there has been a lack of universally applicable theories for the occurrence of coalburst for a long time, which limits the assessment of coalburst risk and the safety design of prevention. In this work, a new criterion for coalburst occurrence based on the general energy principle is proposed, and rigorous theoretical derivation is conducted to prove it. Based on this, a model for roadway coalburst is established, and the formula for its occurrence is derived. The results indicate that two positive real number solutions regarding the depth of the softening zone and two critical stress values for the occurrence of coalburst can be obtained by solving the formula. Only when the depth of the softening zone is between these two positive real number solutions will roadway coalburst occur. Compared with results of elastic mechanics method, the theoretical formula for coalburst occurrence has considered the post-peak mechanical behavior of the coal rock. The burst proneness index is one of the most important parameters in determining the difficulty of coalburst occurrence. To verify the critical stress formula, a biaxial loading experiment is conducted on a coal specimen featuring a prefabricated circular hole. The results indicate that the theoretical critical stress value is 16.48 MPa, while the experimental value is 16.37 MPa, with an error of approximately 0.67%.

Fracture propagation due to fluid pressurization is frequently encountered in the oil and gas industry, enhanced geothermal systems (EGS) as well as carbon capture, utilization and storage (CCUS). In the operation of hydraulic fracturing, acidizing treatment is often incorporated for low-permeability, tight, unconventional reservoirs, to soften the rock and promote crack connectivity, which has been proven effective for carbonate-rich reservoirs. However, the complex interplay between the evolution of the stress field, deformation, hydraulic properties and chemical processes (e.g., mineral dissolution) during the stimulation and maintenance phases demands a sophisticated understanding. How a fluid-driven crack propagates in a stressed frictional rock undergoing mineral mass removal as surrounded by a chemical environment, remains elusive. Here we investigate the acid-assisted fracking problem for pressure-sensitive rocks by adopting a coupled reactive-chemo-mechanical model, considering a combined effect of micro-cracking enhancement on the chemically driven shrinking of the yield surface, a dissolution induced ductile transition post-yield, as well as a chemically affected elastic modulus. Our numerical results show that the subcritical propagation of a single blunt-tip crack can be chemically driven and the yielding concentrates at the crack tip which expands penetrating into the material in front of the tip point. A typical three-region development of Mode I crack propagation is identified by the plotting of crack propagation velocity versus stress intensity factor. By adopting the Drucker–Prager yield criterion, more pronounced material yielding and chemical mass removal arise in the near-tip region compared to the frictionless reference case. Microstructural heterogeneity in the form of inhomogeneous distribution of the initial porosity leads to a substantial acceleration of the crack propagation, which is attributed to the interaction between the microstructure and the chemo-mechanical process during evolution, as well as a distinct self-organized pattern of micro-bands formed in front of the crack tip.

To investigate the crushing behavior and energy dissipation patterns of rocks subjected to impact loads, impact experiments were performed on limestone samples with various aspect ratios using a split Hopkinson pressure bar testing apparatus. The effects of aspect ratio and impact velocity on the failure modes and energy consumption during the fragmentation process of the specimen were examined. High-speed photography techniques and scanning electron microscopy technology were utilized to investigate the morphological features of the fracture surfaces in rock fragments. The results indicated that at a constant aspect ratio, an increase in the impact velocity resulted in a decrease in the equivalent particle size and an increase in the fractal dimension. At the same impact speed, the equivalent particle size increased with an increase in the aspect ratio, whereas the fractal dimension decreased as the aspect ratio increased. The failure modes of the limestone specimens gradually shifted with higher aspect ratios, transitioning from axial splitting to compressive shear as the dominant failure mechanism. The fractal dimension of the fragments increased with a higher energy dissipation density. As the aspect ratio of the specimen increases, the length-to-thickness ratio of the fragments increases linearly. The proposed prediction function model for the variation of the equivalent particle size of fragments with dissipated energy agrees well with the experimental results. Numerical simulations employing the finite element method confirmed the dynamic crushing characteristics of limestone. These simulations illustrated the variations in crack propagation paths in limestone specimens with various aspect ratios under impact conditions.

The combined finite-discrete element method (FDEM) has been widely used to simulate the rock fracturing process. However, the penalty-based contact interaction algorithm commonly utilized in FDEM is element size-dependent, which may yield artificial non-smoothness and abrupt jump of contact force. Specifically, the amplitude and direction of the obtained contact force between two contacting blocks can vary even when their overlap area is unchanged. To circumvent the limitations, we establish a unified distance potential field using the boundary node lists to calculate contact forces and consider the update of the local distance potential field when new fractures are generated. The proposed algorithm not only ensures momentum and energy conservation but also avoids the dependence of contact force on element size, which is thus suitable for any complex cases. The effectiveness and robustness of the proposed method for contact interaction between discrete bodies are verified, and its advantages are demonstrated. Finally, we present two application examples to demonstrate the capability of the proposed approach for evaluating rock slope stabilities when multi-block contact is involved. The work provides a new effective solution to analyze discontinuous computation models associated with complex rock mass systems.

Hydraulic test is one of the most common methods to determine the in situ stress conditions in the rock mass, especially at great depth, where the test locations are accessible by drillholes only. This paper discusses the hydraulic fracturing of intact rock (HF) and hydraulic test on pre-existing fractures (HTPF). The paper critically reviews the current understanding of HF and HTPF test methods and suggests corrections and improvements. The associated calculation methods using HF and HTPF data are also re-introduced with detailed discussions. To demonstrate the calculation process, SINTEF uses ISDM for the calculation of in situ stress with Monte Carlo simulation to get a priori estimates, which is also done by other researchers. SINTEF runs the stress calculations for different a priori estimates based on repeated Monte Carlo simulations to reduce the sensitivity of the solution to the a priori estimates. This calculation process is applied to data obtained from hydraulic tests carried out in a group of short drillholes (30 m) at Røldal hydropower project in Norway. Two calculation alternatives were performed, which are (a) Calculation using ISDM for HF tests, (b) Calculation using ISDM for HF and HTPF tests. A comparison is also made for the obtained results with the result from classical HF tests. The paper discusses the applicability of the ISDM method, impact factors on the accuracy of the ISDM, practical challenges and uncertainties during measurement and calculation processes, and possible improvements.

Blasting excavation and mechanical vibration in underground engineering often induce “three-dimension (3D) stress + dynamic disturbance” coupled failure of rockmass structural planes, leading to engineering disasters. However, the disturbance effects on rockmass structural planes under 3D stress are unclear. Therefore, true triaxial shear disturbance tests with different amplitude (A) and frequency (f) were carried on limestone structural planes with acoustic emission (AE) monitoring. The effects of A and f on mechanical properties and failure characteristics were systematically studied. The disturbance damage evolution and energy release characteristics, shear fracture evolution process, microscopic fracture mechanisms, and failure precursor were further revealed. As the A and f increase, the shear failure stress and critical strength of limestone structural planes rapidly decrease and then converge. As the A increases, the disturbance strain energy (U) at failure significantly increases, the proportion of dissipated energy (Ud) changes little, while as the f increases, the U at failure gradually decreases, the proportion of Ud increases. Furthermore, through the analysis of AE signals, the failure mechanism of limestone structural planes is primarily the shear–tensile mixed failure mode dominated by tensile cracks. As the shear stress increases, the proportion of shear cracks gradually increases. As the A increases, the proportion of shear cracks first decreases slightly and then increases significantly. As failure approaches, the number of AE events increases rapidly, with a large number of high-energy events occur mainly focusing on shear failure surfaces, especially as the A increases, the trendy become more pronounced. Near the failure, the b-value shows a continuous and significant downward trend, reaching the minimum at the time of failure, the high A signal increases significantly, and the fractal dimension (Dt) decreases rapidly to the minimum. The variation in lgN/b is opposite to that of the b-value, but remains relatively stable during the early stages of failure, with more pronounced changes at failure, which makes it a more suitable indicator for shear failure precursors.

Double-prevention boreholes (DPB), which simultaneously incorporate gas drainage and pressure relief boreholes within the same coal seam, effectively mitigate the compound disaster risks of rock bursts and coal–gas outbursts. However, improper spacing between the boreholes often leads to crack propagation and synergistic deformation, resulting in gas leakage or inadequate pressure relief. Therefore, this study, based on a coal–rock deformation and damage testing platform using digital image correlation (DIC), conducted surface deformation experiments on specimens with different borehole spacings during the progressive damage process. Diameter contraction was used as an indicator, and the least squares method for elliptical fitting and principal axis analysis was applied to derive the diameter variation patterns of the double boreholes along the Y-axis. Pearson’s correlation analysis was then performed to reveal the deformation behavior characteristics of boreholes with different spacings. The results show that: (1) the specimen exhibits an optimal borehole spacing, and deviating from this optimal value significantly reduces the mechanical parameters. As the borehole spacing increases, the peak stress and elastic modulus increase by 6.84% and 17.07%, respectively. However, when the spacing reaches 50 mm, these parameters decrease by 17.61% and 42.54%, respectively. (2) Borehole spacing influences the evolution of the strain field. The root and tip of cracks around boreholes with different spacings experience varying degrees of shear and tensile forces, resulting in significant differences in crack propagation modes. When the borehole spacing is less than 50 mm, cracks concentrate in the effective load-bearing strain region between the two boreholes. At a spacing of 50 mm, cracks propagate from the local spalling zone along the axis towards the edges. (3) A significant correlation exists between the diameter contraction of the double boreholes (|R|> 0.5), indicating mutual deformation and damage between the boreholes. Before peak stress, the boreholes tend to recover, leading to non-synergistic deformation. After peak stress, this behavior is suppressed. (4) The peak value Rm in the double-borehole specimen is related to the specimen’s strength, with the specimen at 50 mm spacing showing the highest Rm of 0.85 (strong correlation), indicating synergistic deformation behavior. The valley value correlation parameter Rd for different borehole spacings appears at different loading stages. At the initial stage of loading, Rd reaches its minimum value (Rd, 25 mm = − 0.56), indicating a tendency toward non-synergistic deformation. The specimen with a medium spacing of 37.5 mm exhibits a R̅ closer to 0, demonstrating superior performance in terms of arrangement.

Carbon dioxide phase transition fracturing (CDPTF) represents an environmentally sustainable method for rock fragmentation, with applications in coal seam permeability enhancement, tunnel excavation, and open-pit mining operations. However, current CDPTF research primarily focuses on rock specimens without inherent flaws, leaving uncertainties regarding internal flaw pressure and damage characteristics in flawed rocks. To investigate these aspects, this study conducted CDPTF experiments on specimens containing pre-cracks and pre-holes, monitoring flaw pressure and lateral strain measurements. The research methodology incorporated scanning electron microscopy to examine fracture characteristics in the erosion zone and rock fracture surfaces, while mercury injection tests assessed rock damage at various positions. The results demonstrate that under CDPTF conditions, pre-crack specimens fracture along the crack orientation; the breakdown pressure of pre-crack specimens increases when the crack width is increased or the crack length is reduced. Conversely, larger pre-hole diameters correlate with decreased breakdown pressure in pre-hole specimens. As release pressure intensifies, the microscopic characteristics of the pre-crack sandstone’s erosion zone and fracture surface transition from intergranular to transgranular fracture patterns. The high-pressure gas impact generates an erosion zone at the sandstone specimen’s top. Additionally, stress waves induce damage to the sandstone, reducing its pore structure complexity, with damage severity increasing in proximity to the nozzle.

The extensive application of hydraulic fracturing in unconventional reservoirs raises new challenges in understanding the fracture mechanism of granitic materials. Triaxial hydraulic fracturing experiments were conducted to examine the influence of coarse grains on the failure behaviour of impermeable granites based on the associated seismic responses. The waveform frequencies in laboratory hydraulic fracturing are predominantly below 600 kHz, with a dominant frequency around 200 kHz. Source mechanism analysis reveals that tensile cracks constitute the largest proportion among all crack types while have smaller magnitudes than non-tensile cracks. The micro-cracks are induced prior to fracture initiation which delays the immediate macro rupture, following peak injection pressure by the dilatancy hardening effect. The tensile cracks contribute to the increase in damage volume while the non-tensile ones lead to a reduction effect. Overall, the total damage volume escalates as average grain size increases. Granites have event magnitude less than -6.0 and b-values greater than 2. Small magnitude seismic events take less dominance of proportion with increasing grain size. Larger average grain size and lower grain size heterogeneity weaken the cementation between mineral grains, triggering larger boundary cracks. These cracks result in greater offset distance between multiple fractures and increase fracture tortuosity, also leading to longer failure duration, stronger seismic detectability, and a larger slip plunge range and average degrees.

Slip rockburst are triggered by dynamic slip along pre-existing fault or newly generated shear fracture zone. To study its disaster-causing mechanism, based on the Froude similarity theory, this paper uses the detonating fuse as the explosion source, and using the geotechnical multi-functional test device to carry out the dynamic instability model test of the surrounding rock of the tunnel with fault under the action of the explosion plane wave. According to the data of acceleration, strain and displacement from the model test, the vibration and deformation characteristics of tunnel surrounding rock, the process of the slip-type rockburst and the models of dynamic failure are analyzed. The results show that under the action of explosive ground impact, vibration and deformation characteristic parameters such as acceleration, strain and displacement at the fault increase significantly, with the fault becoming the primary energy dissipation zone of the tunnel. Based on the principle of conversion measurement of cantilever beam deflection and strain, a cantilever beam structure displacement transducer was designed, which provides a new technology for the monitoring of fault slip. The disaster process of slip-type rockburst in tunnels with fault was successfully monitored by using a motion camera, and the disaster mechanism was summarized from the perspective of energy dissipation. According to the dynamic characteristics of slip-type rockburst, the process can be divided into several stages: the initiation of cracks on the fault, crack propagation, fault slip, block or particle ejection, and return to calm. Finally, by comparing and analyzing the characteristic parameters of vibration and deformation, as well as the process of slip-type rockburst and the dynamic failure results of the two models, it is concluded that when a fault passes through the arch foot and the straight wall of the tunnel is more likely to trigger severe slip-type rockbursts. The research findings provide valuable insights for the safe excavation and effective support of deep underground engineering projects.

Reservoir rocks are highly heterogeneous as they are formed by minerals and natural fractures with varying concentration, distribution, and orientation. Precise identification of these inclusions is essential for optimizing reservoir stimulation, energy production from geothermal wells, or storage of greenhouse gases in the subsurface. The estimation of the concentration of fractures (or inclusions) relies on evaluating the elastic properties of a rock through ultrasonic (dynamic) and uniaxial (static) measurements. The measured effective elastic properties are subsequently employed to estimate fracture density using effective medium theories (EMTs). This study identifies the primary factors influencing the accuracy of higher-order EMT models when estimating the elastic properties of fractured or heterogenous reservoirs. Finite element simulation is used to calculate the static and dynamic elastic properties of two-dimensional rocks with natural cracks exhibiting vertical transversely isotropic (VTI) symmetry. The cracks are horizontally, vertically, or randomly oriented and filled with water. The calculated properties are then compared with two EMT models, namely, the self-consistent approximation (SCA) and the differential effective medium theory (DEM) for varying crack densities. The calculations show that the effective elastic properties match those of EMT only for specific cases that we identify by defining three dimensionless factors: the scattering $\beta$ ratio, the distance D ratio, and the homogeneity H ratio. The $\beta$ ratio is defined as the wavelength over the crack size, the D ratio is defined as the wavelength over the distance between cracks, and the H ratio is defined as the ratio of the longitudinal to the transverse distance between the cracks multiplied by the inverse of the aspect ratio. Results show that the ultrasonic models match EMTs for higher values of $\beta$. The D ratio exerts minimal influence on the accuracy of elastic property estimations with respect to EMT. Moreover, lower frequencies of the ultrasonic pulse led to the closest match between measured and estimated elastic properties from EMT. Static calculations align with those of EMT, provided that the cracks are homogenously distributed within a sample. The findings highlight the cases for which it is appropriate to use EMT for rock property inference and the percent deviation expected for heterogeneous reservoirs.

Grouting is widely used in tunnel construction as a measure to reduce water seepage through rock fractures. Fresh cement-based grout often comes into contact with flowing water after being injected into rock fractures, especially in post-excavation grouting scenarios in rock tunnels or pre-excavation grouting in deep tunnels and remedial grouting in dam foundations. The flowing water can cause erosion of the fresh grout and viscous fingering in the grout, which reduces the efficiency of the grouting. In the present study, experimental tests using a simulated fracture were carried out to investigate grout erosion and viscous fingering in the time period after the injection stops until the grout has gained sufficient strength. The aim of the tests was to evaluate the validity of the existing criteria used to determine grout erosion and viscous fingering. The test results showed significant grout erosion and viscous fingering caused by the flowing water despite these behaviors not being expected according to the existing criteria. The reduction in the grouted area was up to 50% after 10 min and up to 64% after 60 min. Based on these results, the mechanism of grout erosion and viscous fingering between water and grout is discussed with respect to grouting design strategy. The present study provides a deeper understanding of grout erosion and viscous fingering after the grouting is completed, indicating complex mechanisms of these behaviors and oversimplification in the existing criteria. The results are useful for the design of grouting in fractures with flowing water.

Layered rocks, commonly encountered in underground engineering, often contribute to weak and fractured geological environments. Sericite phyllite, a typical thin-layered rock with a well-developed foliation, exhibits significant anisotropic mechanical properties. The aim of this study is to investigate the anisotropic mechanical behavior of sericite phyllite. Triaxial compression tests were conducted on specimens with foliation angles of 0°, 45°, and 90°, under dry and saturated conditions, and different confining pressures. The results show that the strength curves and strain curves of sericite phyllite exhibit a U-shaped distribution across foliation angles, while the strain curves under saturated conditions with confining pressure follow an A-shaped pattern. Specimens with a 45° foliation angle may fail due to frictional sliding between layers, and the disconnection between layers leads to a sudden decrease in modulus, while specimens with a 90° foliation angle fail due to vertical foliation buckling. A foliation-related modulus (FRM) is proposed to address the sudden changes in modulus for inclined foliation and the decelerating modulus growth under increasing confining pressure. An enhanced piecewise strength criterion (EPSC) based on Jaeger's Plane of Weakness model and the Mohr–Coulomb criterion is introduced, allowing for piecewise fitting across different angle ranges to improve accuracy. These findings enhanced the design and risk assessment of underground engineering by providing quantitative predictions of strength reduction and deformation patterns in layered rocks, particularly valuable for tunnel boring and cavern excavation where varying foliation angles critically affect stability and support requirements.

The anisotropic behaviour of jointed soft rocks subjected to cyclic loading corresponding to small earthquake events and the affecting parameters is not studied extensively. In the present study, the anisotropic strength response of jointed soft rocks under cyclic loading is investigated under varying pre-heating temperatures (i.e., 30 °C–300 °C), specimen porosities (i.e., at 60% w and 80% w) and infill conditions (i.e., cement and bio-concrete mix). The jointed specimens have prepared using Plaster of Paris (PoP) material with a flaw intersected at various inclinations with horizontal in the middle. Further, the joint along specimens is filled with different grouting materials, i.e., cement and sand-cement mortar with the bio-concrete mix, to examine their efficacy in improving this response of specimens. The specimens are tested under dynamic loading conditions using the cyclic test machine at stress control fatigue. The experiments are coupled with high-speed camera to perform Digital Image Correlation (DIC) analyses of specimens to investigate the underlying fracturing mechanisms along specimens. The strength behaviour is represented in terms of number of stress cycles sustained by the specimens. The jointed specimens exhibit the anisotropy in their cyclic response (or, number of cycles), with minimum strength observed for the flaw orientation of 30° due to co-incidence of flaw with the fracture. The presence of grouts has inhibited the anisotropic response of specimens possibly due to the convergence of their behaviour towards intact specimens. Similarly, the anisotropic response of specimens in general reduced with the increasing pre-treatment temperature due to the dominance of thermal cracks in the fracturing of specimens. The fracturing along the specimens is dependent upon the prevailing experimental conditions. The fractures have been observed to be initiating from flaw tips, except for some grouted specimens, propagated mostly parallel to the loading direction under room temperature conditions. The fracturing has been observed to be more dispersed for the pre-heated specimens due to the presence of thermal cracks.

This study aimed to enhance the quantitative comprehension of the mechanisms underlying coal–gas compound dynamic disasters (referred to as “compound disasters”) from an energy perspective. First, theoretical analysis and modeling methods were employed to propose a framework for elucidating the energy on control function of compound disasters. For this reason, a refined gas expansion energy model was derived, and its superiority was verified. An elastic energy model incorporating the effect of gas was established, allowing the comparison of elastic energy and gas expansion energy in the same coordinate system. Laboratory experiments were conducted on destabilizing gas-containing coal–roof systems under different gas pressure levels. The experimental results validated the control function of energy in compound disasters and revealed the distribution characteristics of ejected coal powder. Finally, the critical gas pressure of compound disasters in Xin’an Coal Mine was analyzed based on the control function of energy on compound disasters, and the influence of various factors on compound disasters was discussed quantitatively. The results demonstrated that differences in gas expansion energy and elastic residual energy determined the type of disaster. When these energy differences lacked an order-of-magnitude distinction during disaster initiation, the event was classified as a compound disaster. Coal powder distribution in compound disasters and coal and gas outbursts was categorized into near, middle, and far zones. A significant difference in the distribution of extreme coal powder levels was observed between the two types of disaster within the middle region: compound disasters exhibited less extreme coal powder levels than coal–gas outbursts. The critical gas pressure for compound disasters in Xin'an Coal Mine, determined using the derived energy model, ranged from 0.4 to 0.74 MPa, which aligned with actual observations. The energy control mechanism of compound disasters could effectively identify compound disasters and facilitate the application of targeted comprehensive measures.