True Triaxial Experiments Research Articles

Experimental research on hydraulic fracture evolution characteristics of multi-well pad fracturing in a tight conglomerate reservoir

Natural gas is considered a cleaner and more efficient energy pillar than oil and coal, so it has become one of the focuses of global energy exploitation. The tight conglomerate gas reservoir has a broad development prospect. However, the previous fracturing experiments focus on single horizontal well fracturing research, and it is difficult to realize the economies of scale development. Therefore, the true triaxial fracturing experiment of three horizontal wells was carried out with the large-size specimen (400 mm × 400 mm × 400 mm) to study the hydraulic fracture (HF) propagation mechanism and the effects of geostress, fracturing sequence, vertical well spacing and fracture initiation position on the HF evolution characteristics during the multi-well pad fracturing in the conglomerate reservoir. The results show that under the stress shadow, the later fracturing HF tends to avoid the stress shadow zone of the preceding HF, and the breakdown pressure gradually increases. A higher horizontal stress difference helps the HF tend to cross the gravel and increases the proportion of tensile events to 73.2 %, but intermediate HF height propagation is inhibited, and the upward propagation distance has a decline of about 31.8 %. The preferential fracturing of the intermediate fracturing well can enable the intermediate HF to extend fully, and the upward propagation distance has an increment of approximately 140.3 %, which improves the reservoir reconstruction effect between the bilateral HFs. Meanwhile, it can intensify the induced stress interference between the HFs and increase the proportion of shear failure to 43.6 %. A shorter vertical well spacing seriously inhibits the upward propagation of intermediate HF height by around 57.9 % and increases the proportion of shear failure to 38.5 % under the induced stress interference. The preferential fracturing in the intermediate fracturing well can enhance the gas productivity, but it is necessary to control the pumping rate or pumping schedule to reduce the frac hit risk. This study can provide theoretical guidance and suggestions for multi-well pad fracturing in field fracturing.

Experimental investigation on fractal characteristics of rockburst fragments for granite experiencing initial temperature damage

To reveal the influence mechanism of initial temperature damage on the failure characteristics of granite rockburst, the true triaxial strainburst experiments were carried out on granite samples experiencing different initial damage temperatures. First, the peak strength and failure intensity evaluated by the rockburst pit volume of the granite samples were investigated. Second, by calculating the fractal dimensions of the rockburst fragments, the impacts of the initial temperature damage on the scale parameters, surface morphology and microscopic cracks of the rockburst fragments were quantitatively analyzed. Finally, on the basis of the crack evolution characteristics inside the samples during rockburst, a link between the peak strength of the rockburst and the fractal dimension of the fragments was established. The results indicated that with increasing initial damage temperature, the peak strength and failure intensity of rockburst for granite samples initially increase and subsequently decrease. The samples experiencing an initial damage temperature of 200 ℃ present the highest fractal dimensions of scale parameters, surface morphology and microscopic cracks of rockburst fragments, indicating that 200 ℃ is the temperature threshold that affects the failure intensity of rockburst and the fragmentation degree of fragments. The peak strength of rockburst failure for granite samples experiencing different initial damage temperatures is positively correlated with the fractal dimensions of rockburst fragments, providing a scientific basis for revealing the incubation and evolution mechanism of granite samples during rockburst.

Effect of true triaxial principal stress unloading rate on strain energy density of sandstone

Deep rock are often in a true triaxial stress state. Studying the impacts of varying unloading speeds on their strain energy (SE) density is highly significant for predicting rock stability. Through true triaxial unloading principal stress experiments and true triaxial stress equilibrium unloading experiments on sandstone, this paper proposes a method to compute the SE density in a true triaxial compressive unloading principal stress test. This method aims to analyze the SE variation in rocks under the action of true triaxial unloading principal stresses. Acoustic emission is used to verify the correctness of the SE density calculation method in this paper. This study found that: (1) Unloading in one principal stress direction causes the SE density to rise in the other principal stress directions. This rise in SE, depending on its reversibility, can be categorized into elastic and dissipated SE. (2)When unloading principal stresses, the released elastic SE density in the unloading direction is influence by the stress path and rate. (3) The higher the unloading speed will leads to greater increases in the input SE density, elastic SE density, and dissipative SE density in the other principal stress directions. (4) The dissipated SE generated under true triaxial compression by unloading the principal stress is positively correlated with the damage to the rock; with an increase in unloading rate, there is a corresponding increase in the formation of cracks after unloading. (5) Utilizing the stress balance unloading test, we propose a calculation method for SE density in true triaxial unloading principal stress tests.

A true triaxial experiment investigation of the mechanical and deformation failure behaviors of flawed granite after exposure to high-temperature treatment

Understanding the mechanical and deformation failure behaviors of heated flawed granite under true triaxial stresses is of great significance for evaluating the wellbore stability in developing hot dry rock resources. In this study, true triaxial experiments are conducted on flawed granite specimens of a varying flaw overlapping length lfo with and without high temperature (HT) treatment, with the emphases on the crack development under true triaxial stresses and its correlations with the strength, plastic anisotropy and failure forecast. Investigations demonstrate that HT, σ2 (intermediate principal stress) and lfo, significantly affect the mechanical properties of flawed granite. Due to strong 3D confinement, the loop crack, the secondary transverse crack and the far-field crack are frequently induced in flawed granite, unlike experiments of uniaxial, biaxial and conventional triaxial compression. When increasing lfo, the rock bridge undergoes the stress amplification which tendentiously drives the internal crack coalescence and typically causes a reduced strength. The PSIRs (Plastic Strain Increments Ratios) analysis, first extended to the true-triaxial experimental data, reveals that the 3D stress-induced extensional/shear fracturing from the flaw tips in σ2-direction is the mechanism of the plastic anisotropy generated. Besides, an attempt to anticipate the failure time in the framework of the time‐reversed Omori’s law suggests a poor predictability by using the acoustic emission but a higher-quality forecast (being more precise and less biased) by using the dissipated energy.

Investigation of fracture-plugging wellbore strengthening: Large scale true tri-axial experiments and field tests

Lost circulation caused by developed natural fracture occurs frequently in tight sandstone formations located in Sichuan Basin, China. Fracture-plugging wellbore strengthening by lost circulation materials (FPWSLCM) is a widely applied fluid loss control technique globally. The upper limit of the pressure-bearing capacity treated using FPWSLCM and the relevant engineering influencing factors needs to be investigated further. In this paper, a self-designed large-scale true tri-axial cell was developed to simulate the fracturing and sealing processes in a cubic sandstone sample (30 cm × 30 cm×30 cm) under anisotropic stress to investigate the effect of lost circulation materials (LCM) and the experimental processes on the formation pressure-bearing capacity. Three homogeneous cubic tight sandstone samples taken from Xujiahe Formation in Sichuan Basin with a central hole were used for the wellbore strengthening experiments with FPWSLCM, which was used to eliminate the heterogeneity effect of the rock. SRIPE (SINOPEC Research Institute of Petroleum Engineering) bridge plugging materials were used as LCM. The results show that the formation pressure-bearing capacity after treatment by FPWSLCM was affected by the initial injection pressure, the intrusion amount of LCM, the pressure holding time, and the injection rate. The formation pressure-bearing capacity did not decrease consistently with the increase of plugging zone instability times, but showed an obvious characteristic of fluctuations; the formation pressure-bearing capacity exceeded the fracturing pressure in some cases. The experimental results could be explained by the stress cage theory. Finally, the modified FPWSLCM was applied as a lost circulation control approach by drilling into the tight sandstone formation in the Shunbei oil field, which has a history of severe loss of fluid circulations. The result of the field test indicated that the modified approaches were more successful than the previous approaches used in other wells in this block. The research results are of great significance for improving the success rate of lost circulation control and a reduction in drilling costs.

Study on crack propagation mechanism of coal under the different orientation between prefabricated slots and true triaxial stress

In engineering, the interrelationship between hydraulic slots and in-situ stress affects the efficiency of the relief stress of coalbeds. In this research, true triaxial experiments were carried out on coal with prefabricated slots parallel to the σ2 (FP-σ2) and the σ3 (FP-σ3). The results show that the modulus of elasticity and Poisson's ratio of the specimens change dynamically with the increase of cycles, and the peak strength and peak strain of the FP-σ3 specimen is larger than the FP-σ2 specimen. The cracks of FP-σ2 specimens were mainly distributed in the X-Z plane, while the cracks of FP-σ3 specimens were more uniformly distributed inside the specimens. For FP-σ3 specimens, a crack's stable expansion orientation of the slot exists, which influences the competitive extension of cracks in the slot-plane with the σ1-σ3 plane. In this orientation, the cracks are distributed in a complex manner. The effects of the horizontal stress difference Kn and the horizontal deflection angle ω of slot on the spatial distribution of cracks were further analyzed by numerical simulation. It can change the slot's horizontal deflection angle ω to make the cracks' distribution uniform. The optimisation method of hydraulic slotting in deep coalbeds is discussed based on research results.

Experimental study on dynamic impact of hydrothermal treated coal in different coal ranks

The new supercritical hydrothermal fracturing technology possesses promising applications in the field of coal and coalbed methane extraction. The research designed hydrothermal processing and true triaxial Hopkinson bar impact fracturing experiments on coals of different coal rank, measured the damage using acoustic waves and scanning electron microscopy. The results of the study showed that: 1. Hydrothermal treatment caused initial damage to coals in different coal ranks, resulted in the reduction of coal acoustic wave speed and dynamic strength. 2. As the coal rank advanced, the metamorphic degree of coal increased and its dynamic strength increased, and the dynamic strength of anthracite reached 149.79 MPa. 3. A pore expansion model of hydrothermally treated coal was established, which indicated that the porosity of hydrothermally fractured coal increased as a whole but the distribution of pore size was more concentrated in the process of hydrothermal fracturing in conjunction with adsorption resolution curves. 4. A dynamic damage constitutive model of coal after hydrothermal treatment under true triaxial circumferential pressure was proposed, which had great significance for evaluating the dynamic disaster safety after hydrothermal fracturing in coal mines.

Failure behaviors and acoustic emission characteristics of flawed granite of different grain sizes: Insights from true triaxial experiments with a free face

In this study, a group of experiments using true triaxial stress conditions with free face loading are performed on granite specimens with non-penetrating double flaws. The crack development and failure characteristics of these specimens are explored by integrating video of the free face and acoustic emission technology, with the combination of the four different intermediate principal stresses and three different grain sizes of the granite chosen as variables. First, surface flaws with a depth of 50 mm are found to have limited influence on the specimen’s failure mode. The intermediate principal stress has a greater effect as it inhibits the development of surface cracks; however, surface cracks develop much more readily in coarse-grained specimens than in other specimens. Second, when the intermediate principal stress exceeds 10 MPa, the rock burst failure phenomenon occurs in the specimen. Third, irrespective of changes in the intermediate principal stress and granite specimen grain size, the specimens primarily fail by tensile-shear failure and shear failure. Extending inward from the free face in the σ3 direction, according to the failure types, the failure types can be classified into tensile failure, shear failure, and tensile-shear failure. Finally, using the stress state synthesis method, the specimen’s stress state and crack distribution mechanism on the σ2 loading plane are analyzed.

Influence of lithology and bedding orientation on failure behavior of “D” shaped tunnel

To explore the influence of lithology on the failure behavior of layered tunnel, true triaxial compression experiments were undertaken on cubical phyllite and yellow sandstone samples containing a “D” shaped hole. The failure progress of the hole sidewalls was monitored and captured using a miniature camera. The results reveal that the initial vertical failure stress of the phyllite samples presents a “U” shaped change as the bedding angle increases. At the bedding angle of 45°, compared with the initial vertical failure stress of the yellow sandstone tunnel, that of the phyllite tunnel is lower, resulting in larger rock fragments and deeper V-shaped grooves. The failure pattern of the phyllite tunnel is primarily manifested as extensive shear sliding failure, and the failure is more severe. The failure of the yellow sandstone tunnel is primarily characterized by the sequential laminar fracturing along the maximum principal stress direction, predominantly manifesting as tensile failure. The primary factors influencing the failure of two types of layered rocks are the significant variations in the clay mineral content within the rocks. For the phyllite, it contains nearly one-third of montmorillonite (a clay mineral). This results in the formation of weak bedding planes within surrounding rocks, which induces shear slip failures along these bedding planes. In contrast, the yellow sandstone has a lower clay mineral content, leading to the absence of distinct weak bedding planes within the surrounding rock. In this case, bedding planes present ignorable effect on the surrounding rock.

Mechanism of gas pressure action during the initial failure of coal containing gas and its application for an outburst inoculation

Faced with the continuous occurrence of coal and gas outburst (hereinafter referred to as “outburst”) disasters, as a main controlling factor in the evolution process of an outburst, for gas pressure, it is still unclear about the phased characteristics of the coupling process with in situ stress, which induce coal damage and instability. Therefore, in the work based on the mining stress paths induced by typical outburst accidents, the gradual and sudden change of three-dimensional stress is taken as the background for the mechanical reconstruction of the disaster process. Then the true triaxial physical experiments are conducted on the damage and instability of coal containing gas under multiple stress paths. Finally, the response characterization between coal damage and gas pressure has been clarified, revealing the mechanism of action of gas pressure during the initial failure of coals. And the main controlling mechanism during the outburst process is elucidated in the coupling process of in situ stress with gas pressure. The results show that during the process of stress loading and unloading, the original gas pressure enters the processes of strengthening and weakening the action ability successively. And the strengthening effect continues to the period of large-scale destruction of coals. The mechanical process of gas pressure during the initial failure of coals can be divided into three stages: the enhancement of strengthening action ability, the decrease of strengthening action ability, and the weakening action ability. The entire process is implemented by changing the dominant action of in situ stress into the dominant action of gas pressure. The failure strength of coals is not only affected by its original mechanical strength, but also by the stress loading and unloading paths, showing a particularly significant effect. Three stages can be divided during outburst inoculation process. That is, firstly, the coals suffer from initial damage through the dominant action of in situ stress with synergy of gas pressure; secondly, the coals with spallation of structural division are generated through the dominant action of gas pressure with synergy of in situ stress, accompanied by further fragmentation; and finally, the fractured coals suffer from fragmentation and pulverization with the gas pressure action. Accordingly, the final broken coals are ejected out with the gas action, initiating an outburst. The research results can provide a new perspective for deepening the understanding of coal and gas outburst mechanism, laying a theoretical foundation for the innovation of outburst prevention and control technologies.

Initiation and Propagation of Interface Cracks in Layered Composite Rocks: An Investigation Using True Triaxial Experiments

Initiation and Propagation of Interface Cracks in Layered Composite Rocks: An Investigation Using True Triaxial Experiments

Mechanisms and Operational Strategies of Multi-Lateral Steam-Assisted Gravity Drainage (SAGD) for Heterogeneous Reservoirs

As the SAGD steam chamber and production performance in heavy oil reservoirs under fluvial sedimentation environment are heavily impacted by reservoir heterogeneity, an innovative strategy was proposed in this study coupling rock dilation and multi-lateral wells in SAGD projects to break the mud barriers and achieve uniform steam chamber growth. True tri-axial experiments and numerical simulation were designed to validate the feasibility of this strategy, based on which the branches of the SAGD well pairs were designed and the operational parameters were optimized for different geologic heterogeneity conditions. The tri-axial experiment results indicate that the rock formations in the heavy oil reservoirs of the F oilfield exhibit significant shear dilation effects under low confinement pressure conditions, with a volumetric dilation capacity of up to 7%. The branches should be deployed in an interleaved manner, with a horizontal displacement of 20 m and a vertical displacement of 6 m. The optimal results are achieved when the branches intersect the interbeds, allowing for enhanced steam chamber conformance and enlarged volume. Dilation zones of 3–8 m can be created above the steam-injection horizontal wells and around the branches in the reservoir during the dilation of SAGD steam chambers. The maximum volume of dilation fluid used for hydraulic dilation is suggested to be less than 2000 m3. This strategy has been validated as being successful in the pilot SAGD well pair in the F oilfield, China, with the SAGD preheating time reduced by 50% and an incremental oil rate of 4.5 tones/day, indicating encouraging potentials in similar heavy oil reservoirs.

Cooperative acoustic emission locating with velocity tomography in true triaxial experiment

During true triaxial experiments, it is impractical to directly affix sensors to the rock sample, which consequently gives rise to a heterogeneous velocity structure extending from the source of acoustic emissions to the sensors. In pursuit of ascertaining the travel path of acoustic emission waves under anisotropic conditions and comprehending the evolution of wave velocity during triaxial loading, we conducted cooperative event localization in conjunction with velocity tomography. Initially, a numerical test was executed to validate the precision and stability of the employed fast-marching tomography method. Subsequently, true triaxial loading was applied to a granite rock specimen. The algorithm's accuracy was corroborated by the congruence between the final locating results and the observable crack positions. The tomography inversion results indicated that the wave velocity field remained relatively stable when the maximum principal stress was below 16 MPa, despite the occurrence of numerous acoustic emissions during this stage. The P-wave velocity exhibited a discernible upward trend with increasing maximum principal stress, particularly evident after reaching the 40 MPa threshold. Nevertheless, it's important to note that stress escalation does not invariably correspond to fracturing during the elastic deformation phase under confining pressure. The inversion method employed here enables the visualization of variable velocities and provides insight into the rock's deformation physics.

Experimental study on equilibrium initiation and extension of multiple clusters of fractures based on true triaxial physical simulation

In the context of tight sandstone reservoirs in the Ordos Basin characterized by compact and heterogeneous rock formations, conventional fracturing techniques yield monolithic fracture shapes, rendering 3D reservoir reconstruction unattainable. This study investigates the principles governing balanced initiation and propagation of fractures in multi-cluster fracturing within unconventional fracturing technology. Employing a large-scale true triaxial simulation experiment system, we utilize the dimensional analysis method (π theorem) to design a physical simulation experiment similarity criterion. Through various experimental adjustments involving proportioning, curing, and mechanical testing, we generate an artificially cured rock mass with mechanical parameters akin to the target layer. The rock mass is maintained at a size of 30 cm × 30 cm × 30 cm. Systematic physical simulation experiments on unconventional volume fracturing are carried out using the 30 cm-sized dense sandstone outcrop rock mass. Taking the conventional fracturing technology as a reference and manipulating experimental conditions and design parameters, we simulate the non-equilibrium initiation and extension behaviors of fracturing fractures under five unconventional volume fracturing technologies, namely hydraulic pulse pretreatment, temporary plugging between clusters, flow-limiting method, cyclic loading and unloading, and pulse intermittent fracturing. Through this, we elucidate the non-equilibrium initiation and extension laws governing multi-cluster fractures. Comparative analysis with conventional fracturing, known for inducing stress interference on fractures and inhibiting their expansion, revels that the five unconventional volume fracturing techniques mitigate stress interference in multi-cluster fracturing. This promotes uniform fracture initiation and expansion, facilitating the creation of complex fractures and larger reconstructed volumes. Among these techniques, inter-cluster block fracturing stands out for its exceptional ability to generate complex fracture networks. The research culminates in the development and refinement of a balanced fracture and extension control technique tailored for multiple cluster fractures in bulk fracturing. This technique significantly contributes to enhancing the degree of 3D reconstruction achievable in unconventional tight oil and gas reservoirs.

Fracture Propagation Mechanism of Tight Conglomerate Reservoirs in Mahu Sag

Conglomerate reservoirs are usually formed in sag slope belts, which have the characteristics of near-source rapid deposition, rapid sedimentary facies change, and distinct reservoir heterogeneity. Therefore, it is difficult to carry out treatments of stimulation because of insufficient understanding of the propagation mechanism of the unique “gravel-bypassing” and “gravel-penetrating” characteristics of fracture morphologies in Mahu conglomerate reservoirs. In order to study the law of hydraulic fracture propagation in conglomerate reservoirs, based on Brazilian splitting test results for conglomerates with different gravel particle sizes and different cementation degrees, true tri-axial fracturing experiments conducted in the laboratory were performed to conduct experimental research on natural conglomerate outcrops and analyze the effects of gravel size, fracturing fluid viscosity, and pumping rate on hydraulic fracture propagation morphology. The results show that: (1) the gravel cementation strength of fracture pressure is higher and the pressure drops preferably after fracturing. The fracture is more inclined to “pass through the gravel” to propagate in large-particle-size gravel. The poor gravel cementation of fracture pressure is relatively low-level and the pressure after fracture drops slightly, and fractures tend to occur at the margin of gravel; (2) using slick water for fracturing tends to initiate and propagate fractures at multiple points on the wellbore, which is conducive to the formation of complex fracture networks and the improvement of volume stimulation effects. Guanidine-gum fracturing has a higher fracture-forming efficiency and higher net pressure; and (3) a low pumping rate will increase the interaction degree between fractures and gravel, and gravels will cause a change in fracture roughness, resulting in small local fracture widths.

Experimental study on the true triaxial fracturing of tight sandstone with supercritical CO2 and slickwater

Under the effect of the goal of carbon peaking and carbon neutrality, the use of supercritical CO2 has gradually become a research hotspot as a new waterless fracturing fluid, supercritical CO2 fracturing is expected to become an effective techniques to solve the problem of fracturing fluid backflow in low pressure sandstone reservoirs. The outcrop of the Shaximiao Formation in the Sichuan Basin was selected as the research object, the true triaxial fracturing experiments with supercritical CO2 and slickwater were carried out by the large true triaxial fracturing experimental equipment, the computed tomography (CT) scanner and three-dimensional laser scanner to investigate the initiation, propagation patterns, fracture tortuosity and fractal dimension of fractures produced in tight sandstone via supercritical CO2 and slickwater fracturing and to clarify the influences of the different fluids, displacement rates, and in-situ stress differences on the propagation of fractures in tight sandstone. The results showed that the supercritical CO2 fracturing had a lower fracturing pressure and a longer rock fracturing time than slickwater fracturing. Furthermore, the supercritical CO2 fracturing produced fractures with greater tortuosity and fractal dimension as well as a more complex spatial morphology. In addition, the increase in the displacement rate and the decrease in the in-situ stress difference for supercritical CO2 fracturing was more conducive to the multi-point initiation and reorientation of the hydraulic fractures, increasing the complexity of the reservoir fractures and improving the flow effect. It can provide the important theoretical foundation for the creating plans for supercritical CO2 fracturing of tight sandstone with the experimental knowledge.

Mechanical response and gas flow characteristics of pre-drilled coal subjected to true triaxial stresses

Many gas pre-drainage boreholes are generally constructed in coal seams to ensure safe underground operation. True triaxial experiments were conducted to study the mechanical response and gas flow characteristics of pre-drilled coal to reveal the in-situ behavior at pre-drilled regions in coal seams. The mechanical parameters, failure modes, and flow rate evolutions of pre-drilled coal samples were collected. The gas flow rate of pre-drilled coal samples showed an S-shaped trend with the increase of strain, and no obvious descending progress was found in the flow rate-strain curves. The typical failure mode for pre-drilled coal samples was shear-type during the true triaxial stress compression. V-shaped macro-fractures around the boreholes were observed when boreholes were perpendicular to bedding planes and parallel to face cleats. In contrast, single shear macro-fractures around the boreholes were observed when boreholes were parallel to butt cleats. The pre-drilled coal samples exhibited lower peak strength values when boreholes were parallel to butt cleats, induced by the relatively low cohesion parameter. Pre-drilled coal's initial flow rates perpendicular to bedding planes and parallel to butt cleats directions were only 33.33% and 41.67% of values measured along face cleats. More complex macro-fractures were formed, and up to a 39.30% decrease in strength was observed in pre-drilled coal samples under true triaxial unloading stress paths. The variations in volumetric strain level and the flow rate evolution were further discussed with an anisotropic conceptual failure model. These mechanical responses and flow characteristics of pre-drilled coal would guide the design of borehole layout and prevention of coal dynamic disasters.

Deformation characteristics and novel strain criteria of strainbursts induced by low‐frequency cyclic disturbance

AbstractStrainbursts induced by cyclic disturbance with low frequency (termed as cyclic‐induced strainbursts) are major dynamic disasters during deep excavation and mining. There is currently no quantitative criterion available for the prediction of such disastrous events. In this study, based on true triaxial experiments, we analyzed the deformation characteristics, established two novel strain criteria for the cyclic‐induced strainbursts, and explained the physical meaning of these criteria. Characteristic strains for the cyclic‐induced strainbursts were defined, including the control strain εctr, the strain caused by the combined dynamic and static loading εsd, and the ultimate strain εu after strainbursts. As indicated by the results, the deformation evolution of the cyclic‐induced strainbursts shows remarkable fatigue characteristics, which resemble that of rock subjected to cyclic loading and unloading. In other words, there are three stages during deformation evolution, namely, initial rapid growth, uniform velocity growth after several periods of disturbance, and sudden sharp growth preceding the burst. The ultimate strain εu is insensitive to the tangential static stress and disturbance amplitude, but it changes nonlinearly with disturbance frequency. From the perspective of deformation, the occurrence of a cyclic‐induced strainburst is controlled by the control strain εctr. Thus, a control strain criterion is proposed; that is, when the stain εsd is larger than the control strain εctr, a strainburst will be induced by cyclic disturbance. Moreover, based on the statistical results, a strain ratio criterion is proposed; that is, when the strain ratio εsd/εu is greater than 30%, a cyclic‐induced strainburst will be induced.

Research on stress evolution law and failure characteristics of top coal in fully mechanized caving of extra-thick coal seams

In this study, based on the engineering background of the No. 8222 working face of the Tashan coal mine in Shanxi Province, China, true triaxial experiments were carried out on Geotechnical Consulting and Testing Systems rock mechanics testing machine using cubic coal samples, and the numerical simulation was conducted through the FLAC3D software. Moreover, the stress evolution law of different layers of extra-thick coal seams and the failure mechanism of top coal in the fully mechanized caving were investigated, and the failure structure model of top coal was established. The results show that there are differences in the principal stress values at different layers of the top coal.

Cracking Behaviors and Acoustic Emission Characteristics in Brittle Failure of Flawed Sandstone: A True Triaxial Experiment Investigation

Cracking Behaviors and Acoustic Emission Characteristics in Brittle Failure of Flawed Sandstone: A True Triaxial Experiment Investigation