Rock Strength Research Articles

Failure behaviour of various rocks coated new thin spray-on liner with different curing time under uniaxial compression: Insights from AE and DIC

Thin spray-on liners (TSL) is increasingly popular in underground mine support operations. In this study, a new type of TSL material is proposed, and its macro-micro physical and mechanical properties are characterized. A systematic investigation on the deformability behaviour of six rocks (i.e., coal rock, soft red sandstone, marble, hard sandstone, granite, and limestone) covered TSL materials with different curing ages (i.e., 0-day, 3-day, 7-day, 14-day, 21-day, and 28-day) under uniaxial compression circumstance is performed using acoustic emission (AE) and digital image correlation (DIC) methods. The results show that TSL improves the peak strain of each rock sample, except hard sandstone. The impact of TSL on the peak compressive strength of coal rock, soft red sandstone, and marble is nearly negligible, while it diminishes the strength of hard sandstone, granite, and limestone. As the TSL ages, the peak strength and elastic modulus of limestone initially increase and then decrease, reaching the minimum at 14-day of age, with values 89.81 MPa and 23.13 GPa respectively. However, for other rock samples, the effect of TSL age is not significant. The AE signal produced inside the rock diminishes during the elastic-plastic stage, while the ratio of shear cracks increases upon failure of each rock specimen. The macroscopic failure modes of coal rock and limestone exhibit prominent axial splitting characteristics, whereas marble and hard sandstone undergo mixed failure processes involving shear, tension, and shear-tension. The test data highly correspond to the macroscopic fracture characteristics. The research findings can serve as guidance for TSL support in both soft and hard rock scenarios.

Tensile strength and failure mechanism of rock–cement sample: Roles of curing temperature, nano-silica and rock type

Understanding the tensile strength and failure mechanism of rock–cement interfacial transition zone (ITZ) is of vital significance to the sealing integrity of cement sheath under downhole condition. Taking advantage of multiple techniques, i.e., digital image correlation (DIC), nano-indentation, XRD-Rietveld analysis, 29Si MAS solid NMR, and SEM-EDX, this study is devoted to investigating the impacts of curing temperature, rock type, and the addition of nano-silica (NS), on the tensile strength and failure mechanism of rock–cement sample. The experimental results show that both the curing temperature and the addition of NS leads to the formation of more C-S-H, which densifies the ITZ microstructure and responsible for high tensile strength of rock–cement samples; the tensile strengths of shale-cement samples are consistently higher than those of sandstone-cement sample; the crack velocities for rock–cement samples under three-point bending tests are approximately 1 mm/s, the crack velocities for rock–cement samples are slowed down when the NS is incorporated in cement paste, but they are independent on the rock type and curing temperature.

Research Progress on the Mechanisms and Control Methods of Rockbursts under Water–Rock Interactions

Rock bursts are among the most severe and unpredictable hazards encountered in deep rock engineering, posing substantial threats to both construction safety and project progress. This study provides a comprehensive investigation into how moisture infiltration influences the propensity for rock bursts, aiming to establish new theoretical foundations and practical methods for their prevention. Through the analysis of meticulous laboratory mechanical experiments and sophisticated numerical simulations, we analyzed the variations in the physical and mechanical properties of rocks under different moisture conditions, with a particular focus on strength, brittleness, and energy release characteristics. The findings reveal that moisture infiltration significantly diminishes rock strength and reduces the likelihood of brittle fractures, thereby effectively mitigating the risk of rock bursts. Additionally, further research indicates that in high-moisture environments, the marked reduction in rock burst tendency is attributed to increased rock toughness and the suppression of crack propagation. This study advocates for the implementation of moisture control measures as a pre-treatment strategy for deep rock masses. This innovative approach presents a viable and effective solution to enhance engineering safety and improve construction efficiency, offering a practical method for managing rock burst risks in challenging environments.

Investigation of Micro-Scale Damage and Weakening Mechanisms in Rocks Induced by Microwave Radiation and Their Associated Strength Reduction Patterns: Employing Meta-Heuristic Optimization Algorithms and Extreme Gradient Boosting Models

Microwave-assisted mechanical rock breaking represents an innovative technology in the realm of mining excavation. The intricate and variable characteristics of geological formations necessitate a comprehensive understanding of the interplay between microwave-induced rock damage and the subsequent deterioration in rock strength. This study conducted microwave irradiation damage assessments on 78 distinct rock samples, encompassing granite, sandstone, and marble. A total of ten critical parameters were identified: Microwave Irradiation Time (MIT), Microwave Irradiation Power (MIP), Longitudinal Wave Velocity prior to Microwave Treatment (LWVB), Longitudinal Wave Velocity post-Microwave Treatment (LWVA), Percentage Decrease in Longitudinal Wave Velocity (LWVP), Porosity before Microwave Treatment (PB), Porosity after Microwave Treatment (PA), Percentage Increase in Porosity (PP), and Uniaxial Compressive Strength following Microwave Treatment (UCSA). Utilizing the Pied Kingfisher Optimizer (PKO) alongside Extreme Gradient Boosting (XGBoost), we developed a PKO-XGBoost machine learning model to elucidate the relationship between UCSA and the nine additional parameters. This model was benchmarked against other prevalent machine learning frameworks, with Shapley additive explanatory methods employed to assess each parameter’s influence on UCSA. The findings reveal that the PKO-XGBoost model provides superior accuracy in delineating relationships among rock physical properties, microwave irradiation variables, microscopic attributes of rocks, and UCSA. Notably, PA emerged as having the most significant effect on UCSA, indicating that microwave-induced microscopic damage is a primary contributor to reductions in rock strength. Additionally, MR exhibited substantial influence; under identical microwave irradiation conditions, rocks with lower density demonstrated greater susceptibility to strength degradation. Furthermore, during microwave-assisted rock breaking operations, it is imperative to establish optimal MIT and MIP values to effectively diminish UCSA while facilitating mechanical cutting processes. The insights derived from this research offer a more rapid, cost-efficient approach for accurately assessing correlations between microwave irradiation parameters and resultant rock damage—providing essential data support for enhancing mechanical rock-breaking efficiency.

Grading scalping and sample size effects on critical shear strength of mine waste rock through laboratory and in-situ testing

Geotechnical stability analyses of mine waste rock (WR) piles require the critical friction angle (ϕcr) of the coarse blasted rock. However, due to the presence of oversized rock clasts, shear strength can only be characterized on small samples prepared using grading scaling techniques, such as scalping. Thus, considering a testing device able to handle samples of characteristic size D, the material should be scaled down to a maximum particle size dmax given by the minimum sample aspect ratio α = D/dmax. However, a practical concern about how far the size scale can be reduced while keeping representative results remains a matter of debate in the geotechnical community. International standards do not agree on the minimum recommended α, and its effects on the mechanical behavior remain poorly understood. This paper aims to investigate the grading effects and sample size effects on ϕcr of WR materials using the scalping technique, to provide insights on the minimum recommended α. Triaxial tests were conducted on loose and dense samples of diameters D = 150 and 300 mm. Samples were scalped from field material having dmax = 75 mm, to allow a range of α from 4 to 30. Additionally, one of the world largest in-situ direct shear boxes (120 × 120 × 38 cm3) was developed to test the same WR material. The results show that scalping is an appropriate technique to assess the critical shear strength of WR. The minimum α for ϕcr assessment in triaxial testing is not sensitive to grading nor sample size, but it is affected by sample density. The aspect ratio was found to be α ≥ 12 and α ≥ 16 for loose and dense samples, respectively. This finding advocates that α values recommended by worldwide standards, such as ASTM D7181-20, might be too low and should be revisited after comprehensive testing.

Research on Vault Settlement during Three-Step Tunnel Construction Process Based on Sandstone Rheological Experiment.

Tunnel stability is influenced by the rheological properties of the surrounding rock. This study, based on the Ganshen high-speed railway tunnel project, examines the rheological characteristics of siltstone and sandstone through laboratory tests and theoretical analysis. Rheological curves and parameters are derived, revealing the time-dependent deformation mechanisms of the surrounding rocks. A numerical simulation model is created using these parameters to analyze deformation and stress characteristics based on different rock levels and inverted arch closure distances. Results indicate that sandstone follows the Cvisc model, with the Maxwell elastic modulus increasing under higher loads while the viscous coefficient decreases. The vault displacement is mainly affected by the surrounding rock strength; lower strength leads to greater displacement, which also increases with the closure distance of the inverted arch. These findings are crucial for determining the optimal closure distance of inverted arches in sandstone conditions.

Criterion for residual strength and brittle-ductile transition of brittle rock under triaxial stress conditions

The critical condition with respect to the brittle-ductile transition (BDT) is of great significance in understanding the distinction between brittle and ductile behavior for rocks. In this paper, we develop the two criteria for the residual strength and BDT point of brittle rock based on linear elastic fracture mechanics. The effect of the microcrack friction and slip friction of brittle rock on the residual strength and BDT point is analysed. Two models are derived to predict the critical confining stress and critical compressive stress at the BDT point for brittle rock, and a new brittleness index is proposed. The reliability of models is validated by using a great number of test data from previous publications. The results show that the stress of the BDT point increases with increasing microscopic friction on the microcrack, and decreases with increasing macroscopic sliding friction. The critical confining stress and critical compressive stress at the BDT point are predicted, which is qualitatively consistent with laboratory tests. The results also show that the two criterion have the capability to predict the residual strength and BDT point, which has provided useful insights into the mechanics of brittle-ductile transition in brittle rocks.

Investigating the influence of coarse aggregate characteristics and fiber dispersion on the flexural performance of ultra-high performance concrete (UHPC)

The flexural properties of ultra-high performance concrete containing coarse aggregate (UHPC-CA) strongly depend on the characteristics of coarse aggregates and the dispersion of fibers. However, the impact of the interaction between coarse aggregate and steel fibers on the bending performance of UHPC-CA is not well-understood. This study investigated the four-point bending properties of UHPC-CA to clarify the influence of coarse aggregates. Mechanical tests, laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), and image analysis were performed to investigate the impact of coarse aggregate on flexural properties, considering coarse aggregate properties, and steel fiber dispersion. The results indicate that coarse aggregates affect both bending strength and toughness. The type of coarse aggregate primarily influences bending strength, while the quantity of coarse aggregate determines toughness. The peak strength is closely related to the strength of the parent rock, whereas toughness is significantly affected by fiber dispersion. Coarse aggregate can adversely impact toughness by their inherent brittleness and worsening the fiber dispersion within the matrix. Additionally, this paper provides parameters for selecting suitable coarse aggregate for UHPC. It is recommended that coarse aggregate added to UHPC possess a strength approximately 1.5 times that of the matrix, with a particle size not exceeding 10 mm and a content below 30 %.

Compression-induced failure characteristics of brittle flawed rocks: Mechanical confinement-dependency

The flaw tips in brittle rocks are often the sources of crack initiation and growth due to the stress concentration, which commonly governs the rock strength. However, a unified framework identifying the compression-induced crack types, ultimate failure patterns and the cracking levels of brittle flawed rocks under different mechanical confinements is not yet available. This study conducts the laboratory compression experiments with the AE monitoring to explore the failure characteristics of flawed limestone and its confinement-dependency. Four new crack types including loop crack, secondary transverse crack, near-field transverse crack and far-field transverse crack are found experimentally, and then a modified crack type classification strategy is proposed. Four failure patterns including the σ1-axisymmetric flaw-disturbed spalling for uniaxial compression, the σ3-transverse-symmetric flaw-disturbed spalling for biaxial compression, the σ1 −axisymmetric flaw-disturbed shearing for conventional triaxial compression, and the mixed σ3-transverse-symmetric flaw-disturbed shearing and σ2-transverse-symmetric flaw-disturbed spalling for true triaxial compression, are documented for the first time. Moreover, an acousto-mechanics-based classification methodology of rock cracking levels is established, as well as an AF (average frequency)-RA (rising angle)-based Kernel density estimation method for interpreting the rock cracking nature and the strength mechanism. This paper gets insights into the mechanical confinement-dependency of the rock failure characteristics incorporating the pre-existing flaws and help interpret the field observations.

A new improved 3D Hoek-Brown criterion

The Hoek-Brown strength criterion has been widely used to estimate the strength of intact rocks and rock masses, and has been continuously developed. However, in the latest version of the standard, the criterion still ignores the influence of the intermediate principal stress. To study the different effects of intermediate principal stress on rock failure under triaxial or multiaxial stress states, this paper proposes a modified three-dimensional H-B strength criterion, which can be reduced to the H-B criterion, three-dimensional Priest criterion, Jiang’s (2012) criterion, and the Cai criterion. Multiaxial test data of six intact rocks were used for validation and applicability analysis. The results show that the proposed criterion can well describe the trend of multi-axial test data of six kinds of rocks, and the average mismatch of the six types of rocks is controlled within 10 MPa, which is much smaller than the fitting error of the original H-B criterion, Mogi criterion and Jiang’s criterion, indicating that the criterion has a good prediction effect on rock strength. The proposed criteria can provide a basic theory for the future construction of in-situ rock mass strength.

Utilization of Tunnel Waste Slag for Cement-Stabilized Base Layers in Highway Engineering.

The rapid expansion of highway infrastructure in the mountainous regions of China has led to a significant increase in tunnel construction, generating substantial amounts of tunnel waste slag. Concurrently, the development of transportation infrastructure has created a critical shortage of natural aggregates, necessitating the exploration of alternative sustainable sources. This study aimed to conduct a comprehensive evaluation of the physical and mechanical properties of tunnel waste slag and explore its potential for utilization in cement-stabilized base courses for highway engineering applications. The uniaxial compressive strength of the parent rock (tunnel waste slag) ranged from 81 MPa to 89 MPa in the desiccated state, indicating its suitability for use as a construction material. This study also determined the maximum dry density (2.432 g/cm3) and optimal moisture content (5.4%) of cement-stabilized mixtures incorporating recycled aggregates derived from tunnel waste slag. The splitting tensile strength of these mixtures at 28 days varied from 0.48 MPa to 0.73 MPa, demonstrating robust mechanical performance. Moreover, the unconfined compressive strength of these mixtures escalated from 7.0 MPa at 7 days to 11.0 MPa at 90 days, signifying a substantial enhancement in strength over time. These results validate the viability of utilizing tunnel waste slag in highway engineering and furnish valuable insights for designers, concrete manufacturers, and construction firms engaged in the development of cement-stabilized aggregate base courses.

Optimizing Deep Geothermal Drilling for Energy Sustainability in the Appalachian Basin

This study investigates the geological and geomechanical characteristics of the MIP 1S geothermal well in the Appalachian Basin to optimize drilling and address the wellbore stability issues encountered. Data from well logs, sidewall core analysis, and injection tests were used to derive elastic and rock strength properties, as well as stress and pore pressure profiles. A robust 1D-geomechanical model was developed and validated, correlating strongly with wellbore instability observations. This revealed significant wellbore breakout, widening the diameter from 12 ¼ inches to over 16 inches. Advanced technologies like Cerebro Force™ In-Bit Sensing were used to monitor drilling performance with high accuracy. This technology tracks critical metrics such as bit acceleration, vibration in the x, y, and z directions, Gyro RPM, stick-slip indicators, and bending on the bit. Cerebro Force™ readings identified hole drag caused by poor hole conditions, including friction between the drill string and wellbore walls and the presence of cuttings or debris. This led to higher torque and weight on bit (WOB) readings at the surface compared to downhole measurements, affecting drilling efficiency and wellbore stability. Optimal drilling parameters for future deep geothermal wells were determined based on these findings.

Initiation mechanism of landslides in cold regions: Role of freeze-thaw cycles

Freeze-thaw cycles are recognized as one of the key triggers for some major landslides in cold regions around the world. Though the effects of freeze-thaw cycles on the rock strength degradation have been studied extensively, little effort has been made to qualitatively evaluate how it contributes to the evolution from a stable rock slope to a large-scale mass movement. In this study, we use a discrete element-based numerical model to simulate the entire process of the initiation of landslide under the action of freeze-thaw cycles in a slope with randomly distributed initial cracks. The main goal of this work is to quantitatively describe the landslide evolution process regarding the slope displacement, crack propagation, stress chain and load-bearing structure. Our results show the essence of the displacement evolution of a landslide subjected to freeze-thaw cycles; namely frost heave pressure induces the generation of new cracks, leading to the failure and reconstruction of the load-bearing structure of the slope. Deep-seated landslides can occur when the slope is crossed by a fault; otherwise, the slope is prone to surface erosion or shallow landslides.

Crack propagation process in double-flawed granite under compression using digital image correlation method and numerical simulation

The existence of flaws seriously weakens the rock strength, directly affects the crack expansion morphology, and indirectly affects the slope stability. In this study, uniaxial compression tests were carried out on double-flawed granite to investigate the effects of flaw angle on its compressive strength and crack coalescence based on DIC technology and PFC2D numerical simulation. The result indicates that the UCS values of flawed specimens at angles of 15°, 30°, 45°, and 60° are approximately 25.5–51.7% lower than that of the intact specimen. The failure strength of the double-flawed sample increases with the increase of flaw inclination angle, and the fracture morphology shifts from no expansion to split expansion. When the flaw tip strain of each flaw sample exceeded 0.6%, the stress concentration was generated at the flaw tip, and the sample began to appear macroscopic large cracks. DIC technique can well observe the crack initiation and propagation process, recording inclined shear strain bands at flaw tips, tensile strain bands in the middle of flaws, and tensile shear O-shaped strain ring in rock bridge areas. Numerical simulations using PFC2D software were carried out and showed a good agreement with the physical results. In addition, the effect of structural inclination on specimen failure strength is clearly explained by the theory of compressive damage based on the projected size of flaws. The deformation of rock mass is not a simple material deformation but is composed of material deformation and structural deformation. These experimental and numerical results enhance our understanding of crack initiation and coalescence characteristics, aiding in the analysis of rock structure stability in scenarios such as excavated underground openings, slopes, and tunneling construction, where pre-existing cracks or step-path fractures are pivotal to structural integrity.

Numerical stability assessment of a mining slope using the synthetic rock mass modeling approach and strength reduction technique

Numerical stability assessment of a mining slope using the synthetic rock mass modeling approach and strength reduction technique

Effects of rock pore and micromorphology on electromagnetic radiation characteristics

Electromagnetic radiation (EMR) is a crucial tool for monitoring and early warning of underground engineering disasters. Investigating the inherent pore characteristics of rocks is essential for a comprehensive understanding of the EMR phenomenon. The EMR was monitored during various types of rock splitting failures. The pore structure and micromorphology of rocks are studied using quantitative methods such as mercury intrusion porosimetry, fractal analysis, and the Gray Level Co-occurrence Matrix (GLCM). Results indicate that the fractal dimension of red sandstone is significantly lower than the other three rocks. The fractal complexity increases sequentially from red sandstone to marble, granite, and limestone. As the fractal dimension decreases, the signal waveform characteristics of the four rocks become more complex before the main fracture, with a significant increase in signals during the compaction and elasticity stages. Higher fractal dimensions lead to a shift in energy and count from elasticity stage to the post-peak stage. The main fracture amplitudes of the four rocks generally exhibited a consistent pattern, following the sequence of granite > marble > limestone > red sandstone. The main fracture amplitude decreases with increasing complexity of the rock's pore micromorphology. Rock pore characteristics affect frequency domain characteristics by influencing rock strength and crack expansion. An increase in the average pore diameter tends to decrease both the main and center frequencies.

Analysis of tensile failure mechanism in intact, foliated, and cracked rocks using distinct element method: Influence of anisotropy

A thorough numerical analysis was conducted to understand the failure mechanisms and behaviour in intact rocks, foliated rocks and rocks with pre-existing cracks using the Brazilian tensile strength tests. The dominant anisotropy in rocks, especially foliated rocks like phyllites, slates and schists, as well as sedimentary rocks like sandstone, limestones and shale, leads to complex failure mechanism. The presence of cracks leads to anisotropy causing different mechanisms of failure and variation in tensile strengths of rock mass. Numerical simulations were conducted based on experimental studies of foliated phyllite and dolomitic limestone from the Tejam Group, Lesser Himalaya. The variations in failure behaviour and strength of foliated rocks were studied for samples at different foliation angle using the particle distinct element model. The simulation results show that with increase in foliation angle to the loading direction (from 0° to 90°), the peak tensile strength of rocks remains almost same for specimens with θ = 0° to 30° and then increases linearly till 90°. The influence of cracks as anisotropy was also studied by changing crack length (from 5 mm to 30 mm) and angle (from 0° to 90°). The development of failure patterns in rocks with pre-existing cracks leads to a better understanding of the failure modes and makes it possible to interpret failure behavior, pattern, and strength parameters. With increase in crack length, the tensile strength of rocks decreases. Overall, this study depicts the influence of anisotropy and microparameters on failure modes and behavior in Brazilian tensile strength tests on foliated and pre-cracked rocks.

Effect of artificially induced microcracks near the rock surface on granite fragmentation performance under heating treatment

Various novel assisted drilling technologies enhance rock fragmentation performance by introducing microcracks on the rock surface to weaken rock strength. However, the quantitative relationship between artificially induced microcracks and rock fragmentation characteristics is not clear. In this study, we induced artificial microcracks of varying degrees on the rock surface through one-dimensional heat conduction. With the aid of the fluorescent resin, we visualized the microcrack patterns and quantitatively assessed the artificially induced microcracks. Subsequently, we performed quasi-static indentation tests on granite samples containing microcracks to establish the quantitative relationship between microcracks and rock fragmentation performance. The results indicate that the release of crystal water within the temperature range of 200 °C–300 °C is the primary factor leading to a significant increase in microcracks. Load drop signals correlate with the propagation of microcracks, including the competitive interactions between mechanically induced microcracks and artificially induced microcracks. Artificially induced microcracks require a certain initial length to continue propagating under mechanical stress, and excessively short microcracks are detrimental to subsequent propagation. A higher density of microcracks implies a more complex microcrack network, facilitating the merging of cracks to form rock chips under smaller mechanical loads. The consistency between the length density and number density of microcracks in influencing the crater parameters reflects their equal importance in affecting rock fragmentation performance. These findings could help determine the extent of rock weakening by artificially induced microcracks and reveal the mechanisms of rock fracture behavior influenced by microcrack, holding significant implications for the optimization of the process parameters of various assisted rock drilling techniques.

ELECTRIC PULSE METHOD OF ROCK DESTRUCTION AND BLAST WELLS DRILLING

The paper proposes an electric pulse method for rock destruction and drilling blast holes based on microwave radiation generated by a magneto-cumulative generator. A description of an electric pulse method installation with a magneto-cumulative generator, protected by a Russian patent, is provided. Relationships are given for an engineering calculation method during the periods of mastering a new technology and it is shown that the introduction of an innovative technology based on the proposed electric pulse method will allow us to talk about a low dependence of the efficiency of electric pulse drilling on the strength of rocks.

Residual strength and critical state of strain softening–hardening considering thermal effect for brittle rocks

Since the post failure of rock has a significant effect on the performance of excavation, the residual strength of rock considering the thermal effect, is important for deep rock engineering, such as sequencing in underground excavation and tunnel support design. From the micromechanical perspective, this paper presents novel models of residual strength and critical state of strain softening–hardening, considering the thermal effect for brittle rocks. The model of the residual strength for brittle rocks was established by considering the thermal effect. Then, the criterion was proposed under for critical state of stress softening and strain hardening by considering the thermal effect. The new models include independent parameters with physical meaning. Finally, the reliability of the proposed model and criterion was verified by comparing theoretical predictions and experimental data. It was found that the proposed models can provide an effective means to predict the residual strength behavior and critical state of strain softening–hardening, considering the thermal effect for brittle rocks.