Search Results (19)

Tunnels frequently experience issues such as lining spalling and water leakage, making the stability of tunnel support critical for engineering safety. Given that tunnels are subjected to various ground stress disturbances and groundwater influences, it is essential to investigate the mechanical properties and damage mechanisms of tunnel support materials under different loading paths and saturation levels. Fiber-reinforced concrete (FRC) is widely used for tunnel support; in this study, uniaxial compression tests were conducted on FRC with different fiber contents (0%, 0.5%, 1.0%) under varying loading paths (monotonic, pre-peak cyclic loading, full cyclic loading). The stress–strain behavior, volumetric strain, and elastic modulus were analyzed. The results indicate that increasing fiber content enhances strength and stiffness, while higher water content leads to a significant water-weakening effect, reducing both parameters. To classify crack types, the logistic regression (LR) algorithm is employed based on the AF-RA features, identifying tensile damage (which accounts for 60–80%) as more dominant than shear damage. Using this classification, AE event distributions reveal the spatial characteristics of internal damage in FRC. Gaussian process regression (GPR) is further applied to predict the AE parameters, enabling the assessment of the tensile and shear damage responses in FRC. The location and magnitude of the predicted wave crest indicate extreme damage levels, which become more pronounced under a higher saturation condition. A damage constitutive model is proposed to characterize the post-peak softening behavior of FRC. The numerical verification demonstrates good agreement with the experimental results, confirming the model’s capability to describe the softening behavior of FRC under various fiber and water contents. Full article

As mining progresses, complexities arise, leading to potential coal-rock gas dynamic disasters triggered by mining disturbances. These dynamic phenomena are influenced by factors such as loading mode, coal properties, and the presence of gas. To gain a comprehensive understanding of the mechanical properties, deformation, and failure characteristics of gas-containing coal under cyclic loading, we conducted uniaxial compression tests. These tests varied in loading frequencies, amplitudes, and durations. By analyzing the peak stress variation of gas-containing coal and utilizing digital image correlation (DIC) technology, we captured the deformation characteristics of the loaded coal surface. Following the tests, we examined the fragmentation degree of gas-containing coal under different cyclic loading using fractal theory. This involved screening and crushing samples to assess the impact of varying loading on coal fragmentation. The results showed that peak stress is positively correlated with loading frequency and negatively correlated with loading amplitude and the number of cycles. Cyclic loading significantly affects the surface deformation morphology of gas-containing coal, and there is a correlation between the stress level of the coal sample and its surface deformation, with the formation and development of cracks corresponding to the stress level. Fractal theory can analyze the crushing characteristics of materials and quantitatively characterize their degree of crushing, and the fractal dimension is closely related to the mode of cyclic loading and comprehensively reflects various experimental factors. The results of our study aim to provide insights that can guide the prevention and control of coal mine dynamic disasters. Full article

Underground reservoir technology in coal mines enables the effective storage and utilization of water resources disturbed by mining activities. Owing to the effects of mining operations and water extraction/injection activities, the water head in underground reservoirs fluctuates dynamically. The total bearing capacity of a coal pillar dam is significantly reduced due to the combined effects of overlying rock stress, dynamic and static water pressures, and mining-induced stresses, which are critical for ensuring the safe operation of underground reservoirs. Based on the correlation between different water head heights and the corresponding water pressures on the coal pillar dam, a custom-made coal rock pressure water immersion test device was used to saturate the coal samples under various water pressure conditions. The mechanical deformation and failure characteristics of the samples and fracture propagation patterns under different water pressure conditions were studied using uniaxial compression, acoustic emission (AE), and three-dimensional X-ray microimaging. The results indicated that, compared with the dry state, the peak strain of the water-immersed coal samples increased to varying degrees with increasing water pressure. Additionally, the average porosity and the number of pores with diameters in the range of 0 to 150 μm significantly increased in water-immersed coal samples. Under the combined influence of water immersion pressure and uniaxial stress, loading the water-saturated coal samples to the fracture damage threshold significantly intensified deformation, failure, and fracture propagation within the samples, and the failure mode changed from tension to a composite tensile–shear failure. Full article

Rockmass in deep mining is highly susceptible to large-scale collapses under high stress and blast-induced disturbances, leading to casualties and economic losses. To investigate the evolution characteristics of goaf instability and the types of seismic sources that induce instability, an experiment on goaf instability was designed under uniaxial compression conditions based on actual mining operations. The entire experimental process was monitored using digital image correlation analysis and acoustic emission monitoring. By calculating the digital speckle field on the surface of the rock specimen during the experiment, the evolution characteristics of the deformation and strain fields from the beginning of loading to complete failure were analyzed. The study explored the dynamic behavior of cracks from initiation to propagation and eventually inducing large-scale collapse. The results show that the instability process of the goaf begins with the formation of tensile cracks. As stress increases, shear cracks occur in the specimen, leading to macroscopic failure. Furthermore, based on the differences in overall microfracture types measured by RA-AF characteristic parameters during specimen failure, large amplitude acoustic emission events corresponding to the formation of dominant macroscopic cracks were selected, and the focal mechanisms of these events were inverted. The results indicate that shear failure sources are significantly more prevalent than tensile failure sources in acoustic emission events leading to goaf instability. These findings can provide useful guidance for the support design and the prevention and control of rockmass instability disasters. Full article

The backfill area of tunnel projects may deform or collapse due to the cyclic disturbance of groundwater and train loads. Hence, the anti-deformation and crack resistance performance of backfill materials under cyclic disturbance is critical to engineering safety. In this paper, concrete was produced by mixing 0.85 mm, 1–3 mm and 3–6 mm rubber particles instead of 10% sand, and tested to discuss the effect of rubber particle size on the deterioration of concrete material properties (compressive characteristics and energy dissipation) after bearing cyclic loading. The stress–strain curve and various parameters obtained through the uniaxial compression test and cyclic load test were used to explore the optimal grain size that can be applied to the tunnel engineering backfill area, and numerical simulation was adopted to calculate the deformation of the surrounding rock and the structural stress of different backfill materials. Research shows that the increase in particle size lessens the compressive strength, deformation resistance and cracking resistance of specimens, but after the cyclic loading test, the concrete material deterioration analysis indicates that rubber concrete has lesser and more stable losses compared to ordinary concrete, so the optimum rubber particle size is 0.85 mm. Numerical calculations show that RC-1 reduces the arch top displacement by 0.4 mm, increases the arch bottom displacement by 0.6 mm and increases the maximum principal stress by 11.5% compared to OC. Therefore, rubber concrete can ensure the strength and stability requirements of tunnel structures, which can provide a reference for similar projects. Full article

The deterioration of the surrounding rock at the tunnel bottom is a damage mechanics issue that occurs under disturbance load. To investigate the anisotropic characteristics of mechanical behavior and the AE response mechanism of layered sandstone, uniaxial compression tests and acoustic emission (AE) monitoring were conducted. The results show that the layer structure causes remarkable anisotropic characteristics in the wave velocities. The strain characteristics and mechanical parameters of layered sandstone exhibit obvious deterioration effects. The local strain and overall strain show a synergistic feature, with the local strain path being more complex and the deformation response being extremely sensitive. The peak stress and elastic modulus both exhibit V-type distribution rules, slowly decreasing first, then rapidly decreasing, and finally increasing rapidly, with the boundary points of the layer angle being 45° and 67.50°. The peak stress and elastic modulus show a nonlinear exponential correlation with the layer angle, and the sandstone belongs to the intermediate anisotropy level. The rupture pattern shows significant anisotropic characteristics, with the failure modes including tension failure, including tension failure I and tension failure Ⅱ, shear failure, and tension–shear composite failure. The fractal dimension shows a negative correlation with the layer deterioration effect. The AE activity exhibits a phased response characteristic to the aging deformation of layer structure. The more obvious the layer deterioration effect is, the longer the AE delay is. The AE intensity of tensile failure sandstone is generally greater than that of oblique shear failure. Full article

Studying how soft rocks behave dynamically in water-rich settings is vital for ensuring sustainable coal extraction from deep water-saturated soft rock mines. A dynamic disturbance loading system utilizing creep-impact dynamics was employed to analyze the mechanical traits and acoustic emission behaviors of both dry and fully saturated soft rock. Expanding on uniaxial compression tests as a foundational framework, additional experiments involving dynamic disturbances and acoustic emission observations were carried out on the aforementioned soft rock samples. These experiments encompassed a spectrum of cyclic disturbance amplitudes ranging from 2 kN to 10 kN. Experimental results indicated the following: (1) during dynamic disturbance, the hysteresis loop exhibits a “sparse to dense” variation. When subjected to the same number of disturbances, the hysteresis loop takes on a pointed leaf-like shape, which increases with the amplitude of the disturbances. (2) The pinnacle of intensity and the elastic modulus of the samples, when exposed to diverse amplitudes of disturbances, can be categorized into a strengthening phase and weakening phase. The reinforcement effect is highest for both samples under the effect of a perturbation of 4 amplitudes. (3) Under the action of disturbances at various amplitudes, the acoustic emission signals from the samples can be classified into four stages. In all stages, the maximum acoustic emission signals exhibited by the desiccated samples surpass those emanated from the saturated samples. In the fluctuation period (II), dry and saturated samples exhibit a cyclic strengthening effect, which becomes more pronounced as the amplitude increases. The study results offer theoretical support for understanding deformation and instability mechanisms in roadways of deep water-saturated soft rock mines, which is essential for ensuring sustainable coal resource development. Full article

Engineering rock structures containing joints and fissures are frequently subjected to discontinuous periodic disturbances of varying amplitudes. To attain the quantitative relationship between the crack angle and the mechanical and deformation properties of rock under complex stress paths, uniaxial cyclic loading and unloading tests with increasing stress gradients were conducted on sandstone specimens containing a single crack of different angles. Our results showed that the bearing capacity of the sandstone increased as the crack angle increased. The irreversible strain and elastic moduli of the rock presented a sudden increase when entering the next cycle of the stepped loading. However, the entire loading process can be divided into three stages according to their respective trends. These three stages correspond to the three stages of rock deformation, i.e., the pore crack compaction stage, the elastic deformation to the stable micro-elastic crack development stage, and the unstable crack development stage. In addition, the crack angle of sandstone showed a negative correlation with the irreversible strain, but a positive correlation with the elastic modulus. With the increasing crack angle, the failure mode of the rock changed from the tensile-shear failure to the shear failure, and then to the interlayer dislocation failure. Full article

In order to study the dynamic stress distribution and the fracture area of rock around the tunnel under different orientations of blasting disturbance, AUTODYN finite difference method software was used to conduct numerical simulation research. Gauge monitoring points were set around the numerical model of the tunnel to conduct real-time monitoring of the stress distribution, displacement and fracture area of the tunnel. Based on the analysis of the stress wave propagation law, the following conclusions are obtained: (1) under the condition of the same blasting loads, the stress and displacement of the tunnel is relatively small when the blasting disturbance source is located above the roof, i.e., the stress state of the tunnel is relatively stable and the fracture area around the tunnel is minimal; (2) from the uniaxial stress around the tunnel and the tunnel peripheral displacement, it can be seen that the displacement caused by horizontal direction stress of the tunnel is the largest, and the deformation is mainly concentrated above the floor and at the shoulder, while the vertical wall part has almost no deformation; (3) for brittle materials such as rock, the arch-shaped stress-bearing surface is more likely to disperse stress, while the straight wall and flat floor of the tunnel cannot well disperse stress, resulting in uneven stress on the stress-bearing surface, uncoordinated deformation and ultimately, failure. Full article

Solid materials such as rocks can contain primary defects, and internal defects are activated in the event of mining disturbance, which causes rock damage and destruction. Therefore, it is of great significance for rock engineering to study the mechanical properties and failure mechanism of damaged rock. In this study, damaged prefabricated crack sandstone specimens were prepared with the cyclic loading-unloading test, and the uniaxial loading test was carried out with damaged specimens. The evolution law of peak strength, elastic modulus, and peak strain of specimens with different damage degrees was studied, the quantitative relationship between the P-wave velocity and the damage degree was obtained, and the acoustic emission (AE) count and energy evolution characteristics of specimens with different damage degrees were analyzed. The energy evolution law of damaged specimens was revealed, and with the increase in damage degree, the elastic energy stored in the specimens can be converted into crack propagation more quickly, and the dissipated energy density increases rapidly, resulting in complete rock failure. The research results can provide theoretical support for the stability analysis and control of underground engineering rock mass in the event of multiple disturbances. Full article

Understanding accumulated damage effects is essential when undertaking deep underground rock engineering, as complex in situ environments and intense engineering disturbances realistically affect the physical and mechanical properties of rocks. Accumulated damage mainly causes the extension of micro-cracks and the sprouting of specific defects in the rocks, altering the microstructural parameters. In this investigation, loading and unloading tests were used to simulate the damage states of the deep underground coal measures sandstone. The accumulated damage factor was formed by combining the P-wave and energy damage variables. The effect of accumulated damage on the bearing capacity and deformation behavior of sandstone was particularly pronounced after experiencing impact loading. The experimental results demonstrate that the accumulated damage factor can depict the initial damage state of sandstone as well as the subsequent dynamic and progressive damage. There is a mutually governing effect between accumulated damage and strain rate. In contrast, accumulated damage significantly extends the range of strain rates, which is fed back into the dynamic uniaxial compressive strength of the sandstone. There is a negative correlation between dynamic fracture energy and accumulated damage, which strongly agrees with the sandstone’s deformation mechanism. The combination of accumulated damage and impact loads can be used to assess the long-term safety of deep underground rock engineering. Full article

Phyllite is affected by its own bedding, stress environment and water-saturated conditions. There are great differences in its deformation and failure in engineering, and its creep characteristics are an important basis for evaluating the long-term stability of phyllite engineering. Therefore, this study carried out creep tests of water-saturated phyllite under different bedding angles and confining pressures, studied the coupling effect of factors that affect the creep characteristics of phyllite, and investigated and analyzed the deformation characteristics of a phyllite roadway support on site to provide basic support for phyllite roadway mine disaster control and collaborative mining research. The results showed the following: (1) When the bedding dip angle was 30~60°, under the control of the bedding, the sliding deformation along the bedding suddenly increased under the low-stress condition and the specimen did not undergo structural damage. It could continuously bear multi-level stress and generated creep deformation. In this case, a phyllite roadway should adopt the support method of combining flexibility and rigidity. (2) In the process of multi-stage stress loading, the creep instantaneous stress was directly proportional to the initial stress. When the stress was loaded to 50% of the failure strength, the instantaneous stress tended to be stable and maintained a linear, slightly increasing relationship with the stress. When the bedding angle was 30~60°, the creep deformation accounted for more than 50% of the total deformation. The bedding angles of 0° and 90° were dominated by the instantaneous strain during the stress loading process. For the flexible support of the roadway, the deformation caused by disturbance stress should be fully considered. (3) The uniaxial creep specimen mainly displayed compression shear tensile failure, with a small number of parallel cracks along the main fracture surface. The triaxial creep fracture mode changed to single shear failure. The confining pressure showed greater inhibition of the creep of the specimen with a bedding inclination of 0° and 90°. The strength design of the rigid support should refer to the original rock stress value of the roadway. The creep deformation and failure of the specimen with a bedding inclination of 30~60° were mainly controlled by the bedding. The included angle between the bedding dip angle and the maximum principal stress should be kept within 30~60° as far as possible in the roadway layout. Full article

The back-filling body in the gob-side entry retaining is subject to continuous disturbance due to repeated mining. In this study, uniaxial and cyclical loading tests of back-filling concrete samples were carried out under laboratory conditions to study damage evolution characteristics with respect to microscopic hydration, deformation properties, and energy evolution. The results showed that, due to the difference in the gradation of coarse and fine aggregates, the cemented structure was relatively loose, and the primary failure modes under cyclical loading were tensile and shearing failure, which significantly decreased its strength. With an increasing number of loadings, a hysteresis loop appeared for the axial strain, and the area showed a pattern of decrease–stabilization–increase. This trend, to a certain extent, reflected the evolution of the cracks in the back-filling concrete samples. The axial, radial, and volumetric plastic strain curves of the back-filling concrete samples showed a “U” shape. The plastic strain changed in three stages, i.e., a rapid decrease, stabilization, and a rapid increase. A damage parameter was defined according to the plastic strain increment to accurately characterize the staged failure of the samples. The plastic strain and energy dissipation of the samples were precursors to sample failure. Prior to the failure of the back-filling samples, the amount and speed of change of both the plastic strain and energy parameters increased significantly. Understanding the characteristics of plastic strain, damage evolution, and energy dissipation rate of the back-filling samples are of great reference value for realizing real-time monitoring of back-filling concrete in the gob-side entry retaining and providing early warning of failure. Full article

To investigate the mechanical properties and damage characteristics of granite under frequent disturbance loads in the process of underground engineering construction, laboratory uniaxial compression tests were conducted on granite under combined dynamic and static loading conditions. The following conclusions were reached: (1) under a dynamic disturbance, the failure stress of granite grows gradually as the initial stress and disturbance load rise due to the coupling of damage and strain-rate effect; (2) the characteristic stresses of granite specimens grow with the increasing amplitude of disturbance Δσ under the disturbing loads; with the same Δσ, the characteristic stresses show an increase trend with the increasing initial stress σ_m; (3) the particle size distribution of rock fragments broken under the disturbance load follows the fractal law, and the fractal dimension F gradually enlarges with the growth of Δσ, indicative of an increased degree of fragmentation; and (4) the damage variable grows rapidly at first, then steadily, and, finally, shows a rapid growth trend again under the disturbance loads. The Δσ significantly influences the number of cycles and rate of change of the damage variable during the steady increase. This research has certain theoretical significance and engineering guidance value for dynamic disaster recognition and control. Full article

Engineering rock mass is normally subject to cyclic–dynamic disturbances from excavation, blasting, drilling, and earthquakes. Natural fractures in rock masses can be reactivated and propagated under dynamic and static loadings, which affects the stability of rock mass engineering. However, fractured rock mass failure induced by cyclic-impact disturbances is far from clear, especially considering varying angles between the rock mass and the direction of impact loadings. This work investigated rock deformation and failure characteristics through cyclic impact tests on granite samples with cracks of different angles. A Hopkinson bar was employed for uniaxial cyclic impact tests on granite samples with the crack inclination angles of 0–90°. The magnetic resonance imaging technique was used to determine rocks’ porosity after cyclic impacts. The stress–strain curves, porosity, strength, deformation modulus, failure modes, and energy density of samples were obtained and discussed. Results showed that the crack inclination angles significantly affected the damage evolution and crack morphology of rocks. Under the constant cyclic impact, the dynamic deformation modulus and dynamic strength of rock samples first increased and then decreased with the increase in crack inclination angle. The failures of granite samples for inclination angles of 0 and 90° were dominated by tensile cracking, while those for the inclination angles of 30–60° were dominated by shear cracking. The energy density per unit time gradually decreased with the increase in impact cycles. The results can provide references for the stability analysis and cyclic-impact-induced failure prediction of fractured rock masses. Full article