•SED index was employed for better understanding the energy characteristics of rockburst.•Rock failure in unloading/loading zone follows different regimes due to dynamic loading.•The possible ...locations of different failures are predicted in the numerical modelling.•The failure types and extend from simulated results agree well with field observations.
With an increase in mining depth, rockburst is becoming an important problem in roadways of Linglong gold mine, where the overburden is up to 1000m. Field observations indicate that rock failures are often triggered by blasting at the adjacent workface. In order to understand the rockburst mechanism, numerical investigation associated with an energy index, strain energy density (SED), is conducted to simulate the energy accumulation and dissipation characteristics of the failure process. The results show that rock failure follows different mechanisms under dynamic loading as a result of unloading and loading zones generated around the cavity. Numerical modelling demonstrates that strain burst occurs at the corner and the floor with the release of massive energy, while dynamic spalling occurs at the lower sidewall due to the reflected tensile stress wave at the free surface. The failure types and damage extend from simulated results agree well with field observations. The findings presented in this study will contribute to effective support design strategies in deep underground openings.
•Microseismic source location identification is conducted in deep mines.•Convolutional Neural Networks and Deep Learning algorithm are used.•The accuracy and performance are validated with blast ...experiment in a deep mine.
Recent years have witnessed a clear trend to develop deeper and longer tunnels to meet the growing needs of mining. Micro-seismic events location is vital for predicting and avoiding the traditional mine disasters induced by high stress concentration, such as rock burst, roof caving, water inrush and slope landslide. Deep learning has become a research hotspot within the field of artificial intelligence in recent years, which has achieved significant progresses and applications in the areas of image recognition, speech recognition, language processing and computer vision. The biggest difference between the deep learning and the traditional back propagation training method is that the deep learning can automatically and independently learn the characteristics of a large amount of data without human intervention. This paper uses Convolutional Neural Network (CNN) and deep learning techniques to develop a method for identifying the Time Delay of Arrival (TDOA) and subsequently the source location of micro-seismic events in underground mines. The power spectrum and phase spectrum of cross wavelet transform calculated from the recorded seismic waves due to micro-seismic events are used as inputs to CNN. The amplitude and phase information of the cross wavelet transform power spectrum are parameters that are used without manual manipulation to build the complex mapping to predict TDOA by deep learning network. Experimental data from the in-field blast tests and simulation tests show that the proposed approach can well identify TDOA and hence detect the event source locations of the field blasting tests. It is demonstrated that the proposed approach with the CNN and deep learning techniques gives more accurate micro seismic source identifications with the recorded noisy waveforms from in-situ blast tests, as compared to several typical existing methods.
•Hard rock fragmentation using a conical pick under high uniaxial confining stress was performed.•Rockburst triggered by pick penetration was reproduced in laboratory.•A process model was established ...to shed light on the rockburst mechanism.•Size statistics of the rock fragments produced in rockburst were analyzed.•Correlations between rockburst performances and rock properties were illustrated.
High excavation-induced stress concentration and strong mining/tunneling disturbance are prominent conditions in deep excavations, and often present a coupled process to induce rockbursts. This study aims to reproduce and shed light on the rockburst process triggered by hard rock fragmentation using a conical pick. A true triaxial test apparatus was used to apply confining stress and pick force on cubic rock specimen, where the real-time values of the stresses and pick forces were recorded. A high-speed camera system was equipped to shoot the processes of rock fragmentation and rockbursts. The size of fragments produced in rockburst was analyzed via image processing approach. The results show that the cuttability and the rockburst susceptibility of hard rock under high confining stress are correlated with strength parameter and brittleness index of the rock material. Rockburst process consists of three progressive steps: surface slabbing triggered by pick penetration, rapid ejection and violent burst of chips powered by high confining stress, and final shear failure. The mean size of fragments produced in rockbursts decreases as the rock brittleness increases, which suggests that higher rock brittleness will induce more intense rockbursts. Therefore, the precautions, such as timely backfilling of the adjacent mined-out areas, flexible and energy-absorbing rock supports, and pre-destress ahead of excavation, should be taken to prevent rockbursts in peninsula- or island-type hard rock pillar under high excavation-induced stress.
Stress conditions around deep underground mine openings can significantly influence rock fragmentation and stability, and thus the cuttability of the targeted rock. In this study, rock breakage ...experiments and associated regression analyses indicate nonlinear rock cuttabilities (decreasing followed by increasing) with increases in the differences between biaxial confining stresses and the values of uniaxial confining stresses. Rock breakages were found to be efficient and safe under low and no-stress conditions that require low indentation force and depth, cutting work and specific energy to completely split the rock wtih no rockburst risk. Stress concentration initially impeded rock breakage, although high uniaxial stress improved rock cuttability. Inducing high stress to fracture the rock and produce an excavation damage zone (EDZ) via stress release effectively transformed the stress condition into low confining stress or even the stress-free condition, improving rock cuttability significantly and preventing rockburst. Mining of rock in the EDZ around the pillar could be efficient, cost-effective and safe when using roadheaders, which showed high cutting efficiencies, low pick wear failures, high machine stabilities and no rockbursts. In addition, a binary linear regression model was proposed to determine the thickness variation of the EDZ correlated with the excavation span and a coupled index of rock properties and buried depth of opening. The results indicated that the thickness of the EDZ increases with increases in the buried depth of the opening, which can improve the applicability of non-explosive mechanized mining in deep mines.
In this study, we investigated the failure process and evolution mechanisms in circular tunnels under a complex stress path of deep in-situ stress + excavation unloading + stress adjustment using a ...simulation test based on loading first and then drilling (LFTD) mode. This test was performed on cubic sandstone specimens using a true-triaxial testing system combined with a self-developed drilling unloading test device. For comparison, another set of sandstone specimens, similar to the original specimens but with Φ25 mm circular through-holes, were processed to perform another set of simulation tests based on drilling first and then loading (DFTL) mode. Under constant lateral stress, compared with the DFTL test, the initial failure vertical stress was lower, and the surrounding rock was more prone to failure in the LFTD test, indicating that drilling unloading causes damage to the surrounding rock, thereby inducing a strength-weakening effect. Under constant lateral and vertical stresses, sidewall failure in the LFTD test was more severe. The sidewall's initial failure vertical stress difference between the LFTD and DFTL tests increases with increasing lateral stress or initial hydrostatic pressure. The effect of drilling unloading becomes more apparent under a higher initial stress state. Comparing the same types of simulation tests under different stress states, the initial failure vertical stress increases with the lateral stress or initial hydrostatic pressure, inducing a strength-strengthening effect in the surrounding rock. Comparing the strength-strengthening effect caused by increasing the lateral pressure or initial hydrostatic pressure and the strength-weakening effect caused by drilling unloading shows that the strength-strengthening effect is more evident.
Shale gas is becoming a new energy development focus worldwide with advances in horizontal wells and hydraulic fracturing, and spontaneous water-based working fluid imbibition is one of the strongest ...phenomena that can impact borehole stability and the gas extraction rate. Our present experimental work was carried out on shale Brazilian discs with 5 different bedding orientations (0°, 30°, 45°, 60° and 90°) after air-drying and spontaneous water imbibition using a split Hopkinson pressure bar. The results illustrate that under the same loading rate (100–900 GPa/s), spontaneous water imbibition obviously weakens the dynamic tensile strength of shale discs, except for shale discs with a bedding orientation of 90° when the loading rate exceeds 400 GPa/s, and does not change the anisotropic dynamic tensile strength variation trend versus bedding orientations. Models that can well fit test data and reveal the coupling effects of bedding orientation and loading rate on the dynamic tensile strength of shale after air-drying and spontaneous water imbibition are proposed. The rate-dependent failure mode only appears among shale discs with bedding orientations of 30°, 45° and 60°, and this phenomenon is exacerbated and more complex after spontaneous water imbibition. Bedding planes of shale discs after air-drying and spontaneous water imbibition both play three different roles in affecting the failure crack propagation, i.e., intersecting propagation, promoting propagation and promoting turning. Finally, the dynamic tensile strength responses of shale after spontaneous water imbibition are analyzed and discussed, and potential field applications in borehole stability, perforation design and hydraulic fracturing are suggested.
•Spontaneous water-based working fluid imbibition performs a promising influence on dynamic tensile behavior of shale.•Prediction models for dynamic tensile strength of shale before and after spontaneous water imbibition are established.•Dynamic tensile strength responses of shale after spontaneous water imbibition are discussed and explained.•Potential applications based on dynamic tensile behavior of shale after spontaneous water imbibition are proposed.
The D-shaped cross section is a commonly used tunnel cross section in underground engineering. To simulate the failure process of a D-shaped hole under deep three-dimensional (3D) high-stress ...conditions, true-triaxial tests were conducted on cubic granite specimens with a through D-shaped hole, and the failure process of the hole sidewall was recorded in real time. Results show that the spalling failure process of the D-shaped hole sidewall can be divided into four periods: calm, fine particle ejection, crack generation and propagation, and rock slab gradually buckling and spalling. Afterwards, symmetrical V-shaped notches were formed on both sidewalls between the corner and arch springing. The spalling failure shows tensile failure characteristics. Under high vertical stress and constant horizontal axial stress, increasing the lateral stress reduces the severity of the spalling failure and the depth of the V-shaped notch. The initial failure vertical stress of the D-shaped hole sidewall is higher than that of circular hole sidewall, and the failure of D-shaped hole sidewall is mainly characterized by static failure. The failure of the circular hole sidewall is a more severe dynamic failure. When the vertical applied stress is the maximum principal stress, the position of the V-shaped notch tip is 0.20–0.25 h (
h
is the height of the D-shaped tunnel) from the tunnel floor, whereas that in the circular tunnel is 0.5
d
(
d
is the diameter of the circular tunnel) from the tunnel floor. Specific support schemes should therefore be designed for tunnels with different cross sections according to the damage location, depth of failure zone, and severity of failure.
The measurement of in-situ stress is a critical prerequisite for the mining design and the stability analysis of surrounding rock in deep mining engineering. In this study, a novel re-oriented core ...acoustic emission (RCAE) method incorporating the non-oriented core ground re-orientation and AE techniques was proposed for in-situ stress measurement. First, the principle of non-oriented core ground re-orientation is established according to the spatial coupling relationship between the drilling trajectory and the core surface characteristics. Then, an innovative apparatus was designed for recovering the original spatial orientation of non-oriented cores. Furthermore, the measurement procedures of RCAE method are described in detail, involving suitability analysis, outdoor operations, core re-orientation, specimen preparation, AE test and stress calculation. Using this method, a case study for in-situ stress measurement was performed in an ultra-deep borehole (2360 m) of the Sanshandao gold mine, China. Meanwhile, the verifications of measurement accuracy were conducted based on the tectonic stress inversion and the underground overcoring method. The results show that the measurement accuracy of RCAE method is acceptable, especially the horizontal maximum principal stress and its azimuth measured are well matched with those measured by other methods. In addition, the field applications indicate that the in-borehole procedure of RCAE method only requires drilling trajectory measurement without excessive complex operations, and the adequate measuring depth of this method in the case area is up to 3841 ± 521 m according to the damage stress threshold analysis. Therefore, the RCAE method could break through the challenges caused by the restricted measuring space and large measuring depth when no deep underground access exists. Moreover, the non-oriented cores can be rapidly re-oriented on the ground, which could expand the application value of ordinary geological boreholes in the field of deep geostress and tectonic measurement.
Combined dynamic and static loading tests with equal impact energy (about 150 J) were conducted on granite after high temperature by an improved split Hopkinson pressure bar (SHPB) test device. ...Effect of axial compression ratios (ACR) on the strength, dynamic elastic modulus (
E
d
), failure behavior, dissipated energy, and fractal dimension of heat-treated samples was investigated. Results indicated that post-peak strain of samples before 300 °C began to rebound with increasing ACR, belonging to type II. After 300 °C, the ACR would not change the curves’ type, belonging to type I. The sensitivity of dynamic uniaxial compression strength (DUCS) and combined strength to temperature was consistent, but their sensitivity to ACR was different. Empirical equations for variations of strength with ACR and temperature indicated that same strength corresponded to multiple combinations of ACR and temperature, but samples’ crushing degree existed differences. As the ACR or temperature increased, the
E
d
climbed up and then declined. As the temperature rose or the ACR declined, the percentage of dissipated energy during sample failure increased. When the ACR was 0.8 and temperature was within 300 °C, the dissipated energy was negative, i.e., the sample released elastic strain energy for debris ejection instead of absorbing energy. When the temperature or ACR increased, mass of small-sized sample debris increased, and the crushing degree enhanced. Fractal dimension
D
well characterized the crushing degree of samples. Scanning electron microscope (SEM) images explained the diminishing mechanical properties of samples with increasing temperature. Results provided a reference for the construction of deep high-stress and high-temperature rock engineering.
Micro cracks inside rocks can open or close under variable external forces, which significantly affects the rock modulus and wave velocity. However, conventional simulations typically ignore or ...homogenize micro cracks primarily because of statistical intricacy and implementation difficulties. A micro-cracked rock model was proposed—characterized by the inclusion of unbonded contacts with an initial gap, based on the flat-joint contact model—to study the mechanisms of micro-cracked rocks and rock masses. The relationship between the static modulus and crack parameters in the proposed model was estimated based on the energy additive theory. Additionally, the macromechanical properties affected by the crack intensity and width were investigated using uniaxial compression, wave velocity measurements, Brazilian splitting, and direct shear simulations. Based on the simulated results, a new calibration procedure was formulated to efficiently calibrate the crack-closure stage under uniaxial compression. Furthermore, the relationships between the P-wave velocity, cracks, and biaxial stresses were investigated. The results indicate that existing micro cracks can transition between open and closed states under variable biaxial-stress conditions, influencing wave propagation and resulting in anisotropy. The performance observed during the transient unloading of a tunnel validated the capability of the proposed method for dynamic analysis in jointed-rock-mass engineering.
