True Triaxial Loading Conditions Research Articles

Analysis of failure properties of red sandstone with structural plane subjected to true triaxial stress paths

In order to study the mechanical behavior and failure characteristics of rock mass with the structural plane in complex deep stress environment, the QKX-YB200 true triaxial rockburst test system, acoustic emission, and digital video recorder were adopted to conduct true triaxial loading and unloading indoor tests on the red sandstone cube specimens. The results show that: (1) Under true triaxial loading conditions, all specimens propagate in the form of anti-wing crack, and the dip angle of the structural plane is linearly related to the average value of the anti-wing crack propagation angle; under true triaxial unloading conditions, the specimens with structural plane dip angles of 30°, 45°, and 60° propagate in the form of wing crack, while those with a 90° angle propagate anti-wing cracks, and the closure of structural plane is related to the the crack type at the tips of structural plane. (2) The unstable failure stages in the stress–strain curve under true triaxial unloading are longer and exhibit more pronounced fluctuations; the peak strength of the specimens under the conditions of true triaxial loading and unloading decrease first and then increase with the increase of the dip angle, reaching the lowest at 45°. (3) During the true triaxial unloading tests, the rockburst degree of the intact rock specimens is more violent than that of the specimens with a structural plane. Additionally, the dip angle of the structural plane influences the severity of rockbursts under the same stress path. (4) The crack propagation mechanism and the closure degree of structural planes under different true triaxial stress paths and dip angles are discussed. It is observed that the crack initiation mode around the structural plane tips and the crack mode near free surface are closely related to the stress environment in different areas of the specimens. (5) Based on the indoor test results and previous experimental data, the differences in peak strength characteristics between open structural planes and closed structural planes are preliminarily analyzed and compared.

Deep learning-based analysis of true triaxial DEM simulations: Role of fabric and particle aspect ratio

This study investigates the influence of micro-scale entities such as inherent and induced fabric anisotropy on the stress–strain behaviour of granular assemblies. In tandem with this exploration, our objective is to formulate a novel correlation that quantifies the evolution of fabric tensor across diverse loading paths. This correlation can be introduced to enhance the micro-mechanical insights in conventional constitutive models. Employing the Discrete Element Method (DEM), we simulate the drained and undrained responses of 680 transversely isotropic particulate assemblies with diverse initial fabrics and particle Aspect Ratio (AR) under true triaxial loading conditions. We consider a second-order fabric tensor based on inter-particle contact orientations to trace fabric evolution during loading. To account for fluid–solid interaction under undrained conditions, we adopted the DEM-Coupled Fluid Method (CFM). The simulation results highlight the significant influence of the Lode angle, particle shape and initial fabric on the stress–strain behaviour of granular materials, with fabric evolution primarily affected by the stress state, void ratio and particle AR. Lastly, we propose a new correlation for the quantification of the fabric tensor using a multi-layer feed-forward neural network. The satisfactory performance of the suggested correlation is demonstrated through a comparison between DEM data and predicted fabric tensor values.

Strength characteristics and energy evolution of cement stone under true-triaxial loading conditions

Oil well cement is a commonly used cementing material in oil and gas extraction engineering. Due to the change of stress conditions, it is prone to microcracks or micro annular gaps, leading safety concerns. To study the strength characteristics and energy evolution of oil well cement stone under three-dimensional stress states, a series of true-triaxial compression tests were carried out. Based on the experimental results, the mechanical properties and energy evolution mechanisms during the true-triaxial compression process of cement stone under different stress states were explored. The research results indicate that there is an intermediate principal stress effect on the true-triaxial compressive strength of cement stone specimens. When the minimum principal stress remains constant, the strength of cement stone first increases and then decreases with the increase of intermediate principal stress. When the minimum principal stress is 5 MPa, the specimen exhibits brittle failure, with ε1 at peak stress approximately 0.015. As the minimum principal stress increases to 20 MPa, the specimen gradually transits to plastic failure, with ε1 at failure exceeding 0.04. The type of failure is determined by the energy ratio at the time of failure; when there is more elastic energy, it results in brittle failure, otherwise it leads to ductile failure. Due to the high porosity of cement stone samples, most of the input energy is converted into dissipated energy. At minimum principal stresses of 5 MPa, 10 MPa and 20 MPa, the ratio of peak dissipated energy to total input energy fluctuates around 0.55, 0.73 and 0.85, respectively. There is a strong linear relationship between the peak elastic energy, peak dissipated energy, and total input energy, with fitting correlation coefficients of 0.847 and 0.996, indicating that the cement stone sample also follows the linear energy storage law. A strength criterion for cement stone was established based on the modified Lade criterion, which is more suitable for describing the strength of high porosity materials compared to traditional strength criteria. Finally, based on the interaction mechanism between energy accumulation and energy dissipation, a nonlinear model of energy evolution in cement stone was established. The results of the theoretical model are in good agreement with the experimental data.

Mechanical behavior and fracture characteristics of high-temperature sandstone under true triaxial loading conditions

To understand the stability of deep underground engineering rocks at high temperatures, it is necessary to conduct comprehensive tests on the mechanical properties of the rocks. In this study, a series of true triaxial tests (TTT) were conducted on sandstones using a self-developed true triaxial testing system combined with XRD, FE-SEM and CT analyses. The study systematically examined the microstructural characteristics, physical phase behavior, deformation properties and internal fracture development of the sandstone before and after heat treatment. The results of the study show that sandstone has temperature-dependent true triaxial mechanical properties after heat treatment. The stress-strain characteristic curves of the post-peak portion from room temperature to 800 °C show a significant stress-reducing segment, indicating the presence of brittle behavior. In addition, when the temperature exceeds 1000 °C, the sandstone exhibits brittle-ductile change characteristics, with a peak stress threshold observed at 800 °C. Notably, the peak stresses corresponding to 800 °C increased by 84.5 %, 63.1 %, 47 %, and 49.8 %, respectively, compared with the room temperature intermediate principal stresses(IPS) ranging from 20 MPa to 50 MPa, respectively. Importantly, the linear Mogi-Coulomb criterion remains valid for the peak strength of the high-temperature sandstone, while the internal fracture damage pattern of the specimens shows an inverted "Y" or inverted "W" configuration. The internal fracture network of rock samples above 800 °C is more complex. Intergranular crack is the main form of microcracking. This experimental study on the mechanical properties of sandstone under the combined effects of high temperature and real triaxial stresses provides valuable guidance for the selection of sandstone parameters and safety evaluation of coal underground gasification roofs.

Temporal and spatial distribution characteristics of hydraulic crack propagation under true triaxial loading using the direct current method

Hydraulic fracturing (HF) has been widely used in coal and rock dynamic disasters prevention and control and coalbed methane production in underground coal mines. However, how to effectively and accurately monitor and evaluate the hydraulic crack propagation process and impact range has not been resolved. We developed a novel experimental system for HF-direct current (DC) monitoring under true triaxial loading conditions. With the use of this system, we conducted HF-DC monitoring experiments under true triaxial loading conditions. The response characteristics of the emission current, apparent resistivity, and apparent resistivity difference contour map throughout the entire experimental process were studied. The results showed that the DC monitoring data contain a wealth of effective information on hydraulic cracks propagation. The emission current and apparent resistivity reflected the overall electrical conductivity of the rock. The contour map of the apparent resistivity difference between two measuring lines could accurately characterize the propagation process and spatial distribution of hydraulic cracks. The evolution process of the difference contour map showed that hydraulic cracks were more likely to spread into areas with low homogeneity. The change range and degree of the apparent resistivity were mainly affected by the spatial distribution, water-bearing state of the hydraulic cracks and wet area surrounding the hydraulic cracks. As the hydraulic crack network expanded, a highly conductive channels were formed inside the rock, which could affect the rock conductive properties and patterns. By comprehensively considering the temporal and spatial evolution characteristics of the apparent resistivity difference contour map and the regional apparent resistivity change curve, the dynamic expansion and spatial shape of hydraulic cracks in the rock could be accurately characterized via the DC monitoring method.

Effect of roughness and pore pressure on shear behavior of rock joint under true triaxial loading conditions

This paper presents the direct shear test to investigate the effect of the joint roughness and pore pressure on the shear behavior of rock joints. Recently, we have developed an experimental method for the direct shear test using true triaxial apparatus that can control three mutually perpendicular loads and pore pressure independently. The morphological properties of the joints were quantified using a non-contact 3D laser scanner to investigate the change in surface roughness due to shear and the effect of pore pressure. In addition, the time variation of shear load, normal displacement, and flow rate on the joint during shear sliding were measured, and the relationship with the change of the discontinuous surface after the experiment was discussed.

Validation of the Crack Mode-Changing Stress in Circular Tunnels Under Generalized Triaxial Tensile Stress States

In the past century, mining depths have increased profoundly from a few hundred meters to several kilometres following improvements in drilling techniques and instrumentation methods. In highly-stressed tunnels, the predominant failure mechanism of underground rock is controlled by stress-induced fractures parallel to the excavation boundaries in terms of spalling failures or, in some severe cases, a violent slab ejection that forms a V-shaped notch around the excavation walls. Such failures can seriously affect production and shut down the operation temporarily or even permanently. To better characterize, predict, and prevent these catastrophic failures, many laboratory tests and scaled model tunnels have been conducted under uniaxial and biaxial loading conditions, with limited studies also available under true triaxial stress states. This paper extends the previous studies on the impact of true triaxial loading conditions on the spalling failure in high porosity sandstone and investigates further the validation of a new Crack Mode-Changing Stress (CMCS) parameter, defined as the minimum principal stress required to change rock fracturing from splitting to a sliding failure mode. The results revealed that the minor principal stress can significantly affect the propagation of the stress-induced fractures of spalling failure along the tunnel sidewall.

Experimental Study on Damage Characteristics of Coal Samples under True Triaxial Loading and Dynamic Unloading

Abstract Coal bursts are one of the formidable hazards in underground coal mines, yet it is still not fully explored due to the complex stress environment that exists during mining. To better understand the bursting mechanism of coal under real-time mining conditions, it is necessary to develop an experimental method capable of reproducing in situ stress and loading-unloading paths of coal in in situ conditions. In this study, a self-developed true triaxial testing system was used to investigate the damage and failure characteristics of coal samples under true triaxial loading and dynamic unloading conditions. Acoustic Emission (AE) monitoring was used to capture the fracturing of the loaded coal. Passive Velocity Tomography (PVT) and Cumulative AE Energy Density (CAEED) were used to analyse damage evolution characteristics of the coal samples under true triaxial loading conditions. A high-speed camera was used to record the failure of the coal samples when the minimum principal stress σ3 was suddenly unloaded. It was found that continuous coal damage occurred primarily during the true triaxial loading period. The peak number of AE events in the coal samples increased and then dropped as σ1 levels increased. High and low wave velocity zones in the coal samples represent regenerations of the high-density zone and fracture emergence, respectively. Significant energy release zones transferred and expanded as the triaxial loading level increased. Under significant triaxial loads, the coal samples failed and were severely damaged, and the dynamic unloading of σ3 caused ejections of coal fragments at low velocities. The outcome of this study provides in-depth understanding of the failure mechanism of coal under in situ conditions.

Phase transition in monodisperse granular materials: How to model it by using a strain hardening visco‐elastic‐plastic constitutive relationship

AbstractDuring heating (loading characterised by a progressive increase in strain rate) and cooling (loading characterised by a progressive decrease in strain rate) numerical tests, performed by using Discrete Element codes, granular materials experience phase transition phenomena, named in this paper ‘dry liquefaction’ (when from solid the material starts behaving like a fluid) and ‘dry resolidification’ (freezing, when the material from fluid starts behaving like a solid). The aim of this paper consists in reproducing phase transition phenomena by using a strain hardening visco‐elastic‐plastic model based on the critical state concept and kinetic theories of granular gases. The authors demonstrate that crucial is the role of isotropic softening/hardening, which describes the size of the elastic domain and the capability of the solid skeleton of storing elastic energy according to permanent force chains. The main ingredients of the model are: (i) the additivity of quasi‐static and collisional stresses, (ii) the energy balance equation governing the evolution of the granular temperature, interpreted this latter as an additional internal variable for the system for the collisional contribution, (iii) the mixed isotropic and kinematic hardening characterising the quasi‐static incremental constitutive relationship. The model has been both calibrated and validated on Discrete Element Method (DEM) numerical results, obtained by testing dry assemblies of monodisperse spheres under true triaxial loading conditions.

Laboratory stands for hydraulic fracturing simulation in a nonuniform stress field

The paper considers the capabilities of well-known laboratory facilities for hydraulic fracturing modeling under the true triaxial loading conditions. Of greatest interest are the stands that allow you to create a crack in large cubic samples with an edge length of 200 mm or more. In this case, it is possible to reduce the influence of edge effects from the boundaries of the sample on the propagation path of the discontinuity. The review includes research results obtained using 10 different facilities located in major scientific centers.

Effect of the Minor Principal Stress on Crack Initiation Stress Threshold

The rapid expansion of urban and mining infrastructures worldwide over the last two decades has seen an escalation of development in hard rock, including urban tunnels, deep open pits, and underground mines. However, despite many success stories, the pervasiveness of rock burst and spalling remains yet a major unresolved ground control problem in many deep mines and tunnels. Spalling is characterised as a sudden explosion-like rock failure that occurs spontaneously and could affect both the short-term and long-term viability of mining operations. Previous studies strongly show that the spalling strength in low-porosity rocks can be associated with the initiation of internal cracks in the sample. However, very limited studies are available that looks at the effect of all the principal stresses on rock crack initiation stress threshold. This paper investigates the previously ignored effect of the minor principal stress on the crack initiation stress level of sandstone cubes tested under true triaxial loading conditions. The results reveal that the minor principal stress has a profound effect on the crack initiation point. As a result of this study, a new crack mode-changing stress (CMCS) concept is also introduced which is defined as the corresponding minimum principal stress required to change rock fracturing from splitting to a sliding failure mode.

Fractal characteristics of acoustic emission of gas-bearing coal subjected to true triaxial loading

As coal mining proceeds deeper, coal and rock are subjected to growing stresses and gas pressures. Consequently, coal and rock gas dynamic disasters pose more and more considerable threats and hazards which should be warned in advance. Coal and rock in the field is actually in the true triaxial stress state. Study on deformation characteristics and precursory information of coal and rock under this state is meaningful for disaster warning. In this paper, experiments on the deformation, failure and acoustic emission (AE) characteristics of gas-bearing coal under true triaxial loading conditions were carried out. Besides, the variation law and fractal characteristics of AE under different gas pressures and confining stresses were analyzed. Furthermore, the effects of gas pressure and confining stress on the deformation, failure and fractal characteristics of coal were discussed. The results show that the process of AE variation under true triaxial loading conditions can be divided into two stages, namely the slow growth stage and the accelerated growth stage. A higher gas pressure corresponds to a shorter duration of slow growth stage, while a higher confining stress corresponds to a longer duration of slow growth stage. AE time series has fractal characteristics, and the correlation dimension can characterize the damage degree of a loaded coal sample. The dynamic changes, i.e., fluctuation-increase-decrease, in correlation dimension can accurately reflect the damage evolution process of a coal sample. In addition, the gradual reduction of correlation dimension can be used as the precursor information of coal sample instability and damage. The research results boast instructive significance for preventing the occurrence of coal and rock gas dynamic disasters and for reducing casualties and property loss in coal mines.

Non-linear anisotropic damage rheology model: Theory and experimental verification

We extend the isotropic non-linear damage rheology model with a scalar damage parameter to a more complex formulation that accounts for anisotropic damage growth under true triaxial loading. The model takes account of both the anisotropy of elastic properties (associated with textural rock structure) and the stress- and damage-induced anisotropy (associated with loading). The scalar, isotropic model is modified by assuming orthotropic symmetry and introducing a second-order damage tensor, the principal values of which describe damage in three orthogonal directions associated with the orientations of the principal loading axes. Different damage components, accumulated under true triaxial loading conditions, allows us to reproduce both stress-strain curves and damage- and stress-induced seismic wave velocity anisotropy. The suggested model generalization includes a non-classical energy term similar to the isotropic non-linear scalar damage model, which allows accounting for the abrupt change in the effective elastic moduli upon stress reversal.For calibration and verification of the model parameters, we use experimental stress-strain curves from deformation of dry sandstone under both conventional and true triaxial stress conditions. Cubic samples were deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage, changes in ultrasonic P-wave velocities in the three principal directions were measured, together with the bulk acoustic emission output. The parameters of the developed model were constrained using the conventional triaxial test data, and provided good fits to the stress-strain curves and P-wave velocity variations in the three orthogonal directions. Numerical simulation of the true triaxial test data demonstrates that the anisotropic damage rheology model adequately describes both non-linear stress-strain behavior and P-wave velocity variations in the tested Darley Dale sandstone.

Experimental and theoretical analysis of hydraulic fracturing and gas fracturing of rock under true triaxial compressions

A test system was developed for true triaxial hydraulic fracturing (HF) and gas fracturing (GF) combined with acoustic emission (AE) monitoring and 3D computerized tomography (CT) scanning reconstruction to carry out a series of HF and GF tests under different true triaxial loading conditions. The critical breakdown pressure characteristics, the AE activities, and the fracture propagation and spatial morphologies were obtained during the fracturing process. The test results showed that the GF breakdown pressure was approximately 29.0–31.3% less than the HF breakdown pressure. Moreover, for the GF specimen, the gas pore pressure induced a large number of new microfractures before critical fracture. However, both HF and GF under true axial compression can only produce simple main longitudinal fractures (LFs, along the borehole) and transverse fractures (TFs, across the borehole) that are strictly dependent on the initial true triaxial stress field. Based on the experimental observations, a hypothesis for the initiation of the HF and GF under true triaxial compression was proposed by using the maximum tangential stress on the borehole wall and the maximum axial stress along the borehole. Accordingly, analytical solutions of the critical breakdown pressure for the HF (impermeable) and GF (permeable) specimens were derived. The theoretical predictions agree well with the observations from the 3D numerical simulations and tests. Lastly, a novel pulsed GF method (dynamic loading fracturing) was proposed to overcome the dependence of the in-situ crustal stress field associated with traditional HF and GF methods (static loading fracturing). Laboratory test results showed that the dynamic pulsed GF at moderate pressure can generate a complex and random fracture network independent of the in-situ stress field in rock.

Rockburst characteristics of several hard brittle rocks: A true triaxial experimental study

Abstract Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation. Rock lithology is a control factor of rockburst. In this paper, rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks, i.e. granodiorite, granite, marble, basalt, sandstone and limestone, under one-free-face true triaxial loading conditions. With the use of high-speed cameras, an acoustic emission (AE) system and a scanning electron microscope (SEM), rockburst of different rocks was investigated. The results show that the strainbursts of granodiorite, granite and marble were accompanied by tensile splitting near the free face, and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release. For basalt, sandstone and limestone, failure was primarily dominated by shear rupture. The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release; while no burst failure occurred on limestone due to its relatively low peak strength. Rock strength, fracturing and fragmentation characteristics, and failure modes of different rocks can significantly affect rockburst proneness and magnitude. The AE evolution coupled with SEM analysis reveals that the differences in the inherent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.

Mechanical Properties and Failure Behavior of Dry and Water-Saturated Anisotropic Coal Under True-Triaxial Loading Conditions

In underground coal mining, coal failure generally occurs due to the relatively weak strength of the coal and the high applied mining-induced stresses. The outer complex geological conditions (i.e., tectonic structure and water intrusion) and internal structural anisotropy of the coal introduce uncertainty in predicting its mechanical properties and failure behavior. In this study, laboratory investigations of the mechanical properties and failure behavior of dry and water-saturated anisotropic coal samples subjected to different true-triaxial loading stresses were conducted. The effects of water weakening, intermediate stress, and structural anisotropy on the mechanical properties and failure behavior of the coal were systematically studied. The results indicate that the presence of water significantly reduced the strength, elastic modulus, and strength anisotropy of the coal. The maximum stress at failure first increased and then decreased with increasing intermediate stress. The residual strength-to-peak strength ratios and failure plane angles of the coal showed a linear increase with increasing intermediate stress. When the coal samples were loaded in the bedding plane direction, the brittleness of the coal was higher than when they were loaded in the other two cleat plane directions. In addition, when the coal samples were loaded in the butt cleat plane direction, the brittleness of the coal decreased with increasing intermediate stress. Two typical failure modes of the dry and water-saturated coal samples were observed: shear and mixed splitting and shear failures. The dominant failure mode of the coal also varied with the loading direction relative to the weakness planes, which could be well recognized and predicted by the acoustic emission (AE) characteristic curves. To further reveal the fracture mechanism, the microcrack patterns of the coal were further identified based on the AE parameters.

Experimental simulation analysis of the process and failure characteristics of spalling in D-shaped tunnels under true-triaxial loading conditions

Spalling is a typical failure mode encountered on the sidewalls after the tunnel face excavation of deep D-shaped tunnels or caverns. It significantly affects the safe and efficient construction and long-term stability of tunnels. To study the process and failure characteristics of sidewall spalling in D-shaped tunnels, four different three-dimensional initial stress states were established to perform a series of true-triaxial tests on cube specimens with a D-shaped hole. During the tests, a wireless micro camera was used to monitor and record the failure process of the sidewall in real time. Based on the test results, the process and failure characteristics of sidewall spalling were summarised, and the influence of cross-section shape on tunnel failure characteristics and tunnel stability were discussed. The results indicate that both sidewalls perpendicular to the maximum principal stress are cut into approximately parallel rock slabs, demonstrating typical spalling features. Failure zones on both sidewalls are located primarily between the corner and spandrel, and their outlines are “V” shaped (i.e., V-shaped notch). Spalling slabs are thin in both wings, and thick in the middle; they are cut into multilayer thin slices that are not completely detached from one another. With the increase in failure depth, the shape of the cracks first evolved from a straight shape into an arced shape, and subsequently evolved into a “V” shape. When the maximum principal stress is perpendicular to the tunnel axis, tunnel excavation along the minimum principal stress is conducive to the prevention of spalling. Compared with the circular tunnels, the D-shaped tunnels can effectively reduce the severity of rock failure and transform the rockburst into a static spalling. In a relatively low stress environment, the D-shaped cross section can improve the stability of the tunnel, while it is not conducive to the stability of the tunnel in a high stress environment. The result of this study benefits the design and prevention of spalling and rockburst of hard rock tunnels.

Engineering properties of rock salt and simplified closed-form deformation solution for circular opening in rock salt under the true triaxial stress state

Underground structures and storage caverns are increasingly being constructed in rock salt due to its unique self-healing ability and impervious nature. Rock salts are likely to experience and develop time-dependent behavior/deformation. Hence, the characterization of rock salt for its properties, including viscosity, are pertinent for design of these structures. In addition, the material in underground openings experiences true-triaxial stresses and hence it is vital to study the behaviour of such structures under true-triaxial loading conditions. This paper presents results of extensive laboratory testing carried out on specimens of Khewra rock salt for its physico-mechanical properties. Physical and engineering properties such as mineralogy, density, sonic wave velocity, resistivity, point load index and shear strength properties were measured. A simplified closed-form solution which deploys three-dimensional strength criterion (Mogi-Coulomb) with Maxwell viscoelastic behaviour is developed to assess the stress and deformation behaviour of circular opening in plane strain condition. The closed-form solution assumes that the material exhibits the Maxwell behavior when the stresses are lower than the yield stress and the material behavior is the combination of Maxwell and Mogi-Coulomb elastoplastic material, when the stresses are equal to the yield stress. In addition, a cylindrical wellbore problem is solved with the proposed closed-form solution and the deformation of the wellbore was estimated for the different time intervals.

Effects of different fluids on microcrack propagation in sandstone under true triaxial loading conditions

AbstractThe mechanical behavior of rock under conditions of multiphase fluids is currently a key scientific issue in the field of geotechnical engineering. In this work, we conducted triaxial compression experiments of initially water‐saturated samples, which were flooded with CO2 using the semipermeable separator method; comparatively investigated the mechanical behaviors of sandstone under the conditions of mono‐phase fluids CO2/N2/H2O and biphase fluid CO2‐H2O; and analyzed the stress‐strain characteristics, internal crack expansion, and macrocrack failure patterns of sandstones. Compared with H2O, CO2 and N2 could lead sandstone cracks to show a pattern of multiple fractures, displaying a double shear failure mode and multiple branch cracks along principal fractures. Moreover, the cracking effect of supercritical CO2 was more evident. Under the conditions of pore pressure of 10 MPa and temperature of 323 K, the ratio of work done by CO2, N2 and H2O during crack expansion was 23.5:24.1:1, with transient pressure drops to 7.82 MPa, 9.25 MPa, and 0.1 MPa, respectively. Overall, the study analyzed the mechanisms of physicochemical interactions of fluid types and the number of phases in sandstone minerals at the microscale and revealed the mechanical mechanism between fluids and rock minerals, as well as the impacts of these fluids on the strength, internal crack expansion and deformation of sandstone. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

True-Triaxial Experimental Study of the Evolutionary Features of the Acoustic Emissions and Sounds of Rockburst Processes

Rockbursts are markedly characterized by the ejection of rock fragments from host rocks at certain speeds. The rockburst process is always accompanied by acoustic signals that include acoustic emissions (AE) and sounds. A deep insight into the evolutionary features of AE and sound signals is important to improve the accuracy of rockburst prediction. To investigate the evolutionary features of AE and sound signals, rockburst tests on granite rock specimens under true-triaxial loading conditions were performed using an improved rockburst testing system, and the AE and sounds during rockburst development were recorded and analyzed. The results show that the evolutionary features of the AE and sound signals were obvious and similar. On the eve of a rockburst, a ‘quiescent period’ could be observed in both the evolutionary process of the AE hits and the sound waveform. Furthermore, the time-dependent fractal dimensions of the AE hits and sound amplitude both showed a tendency to continuously decrease on the eve of the rockbursts. In addition, on the eve of the rockbursts, the main frequency of the AE and sound signals both showed decreasing trends, and the frequency spectrum distributions were both characterized by low amplitudes, wide frequency bands and multiple peak shapes. Thus, the evolutionary features of sound signals on the eve of rockbursts, as well as that of AE signals, can be used as beneficial information for rockburst prediction.