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Impact rockburst is induced by the combined static and dynamic loads after excavation.There are two types of impact rockburst experiments，one is induced by changing the static load and another is induced by changing the amplitude of dynamic load.The sandstone fragments after these two experiments were collected.Meanwhile，the sandstone fragments generated from the similar compression experiment under static load were collected too.And the mass， length，width and thickness of each fragment has been measured respectively.The quantities and mass distribution of fragments with coarse，medium，fine and micro grains in different size ranges，and size distribution were further analyzed.The crushing level and shape characteristic of fragments were obtained.Then the fractal dimensions were calculated.The results show that the degree of crushing level of fragments from impact rockburst is higher and with obvious slice shape；the ways of combined static and dynamic loads have a little effect on the crushing level and scale characteristic of fragments from impact rockburst experiments；energy dissipation from impact rockburst experiments is higher than that of similar compression experiment.

To explore the effect of rapid excavation unloading under high pressure on delayed rockburst, a true triaxial double-sided rapid unloading rockburst experiment and a biaxial compression failure experiment without unloading were carried out. The results show that the rapid unloading of excavation before the occurrence of rockburst will cause unloading damage, and the axial strain, acoustic emission (AE) count, and AE energy will suddenly increase at the unloading point. The AE source shows that the rapid unloading produces many tensile cracks, mainly distributed near the free surface. The axial peak stress of the rockburst and the stored strain energy before failure are higher than those of biaxial compression, so the ejection velocity of free surface fragments is also higher at failure. [ABSTRACT FROM AUTHOR]

Rockburst may cause damage to engineering equipment, disrupt construction progress, and endanger human life. To this day, the occurrence of rockburst remains complex and difficult to predict. This study proposes the D-P-Transformer algorithm to address this issue by improving the embedding structure of the Transformer for specific applications to rockburst data. To reduce the computational requirement, sparse self-attention is adopted to replace self-attention. A distilling operation and multiple layer replicas are simultaneously used to enhance the robustness and speed up the algorithm's process. Taking all relevant rockburst factors into consideration, multiple experiments are conducted on seven large-scale rockburst datasets with different training ratios to verify the reliability of the proposed D-P-Transformer rockburst prediction algorithm. As compared to the original algorithm, the proposed algorithm shows average reductions of 24.45%, 46.56%, 17.32%, and 48.11% in the mean absolute error (MAE), mean square error (MSE), root mean square error (RMSE), and mean absolute percentage error (MAPE), respectively. The results indicate that the novel D-P-Transformer rockburst prediction algorithm is superior to the Transformer prediction algorithm, and could be used for coal mine rockburst prediction analysis. [ABSTRACT FROM AUTHOR]

This study examines the effects of the rock block percentage (RBP) on the cyclic behaviors in soil rock mixture (SRM) by conducting cyclic triaxial tests at a constant stress amplitude and low strain level. The SRM samples were prepared using compaction testing with different RBPs. For the evaluation of the mesoscopic damage and stress dilatancy behaviors, the in-situ X-ray computed tomography (CT) technique was employed. It was revealed that the hysteresis loop presents a sparse-dense-sparse pattern that is caused by the plastic strain accumulation. Adding rock block to SRM influences the shape of the hysteresis loop, and the area of the hysteresis loop decreases with increasing RBP for samples at the same stress amplitude, the energy loss becomes smaller for SRM samples with a high RBP. On the other hand, after extracting cracks from the original CT images, it shows that damage of the SRM with different RBPs is different under the same loading cycle, the geometric parameters of crack decrease as the RBP increases. Localized deformation in the SRM sample results in the differential stress dilatancy behavior, there is interlocking among rock blocks and restriction of localized bands, which is affected by the different RBP in the SRM sample. [ABSTRACT FROM AUTHOR]

To study the effect of excavation unloading on hard rock failure, a series of true-triaxial compression tests, biaxial compression tests, and true-triaxial unloading compression tests (two different unloading rates) at different confining pressures was conducted on red sandstone cube samples. The strength and failure characteristics and their relationship for red sandstone unloading at different unloading rates and confining pressures were analyzed. Based on the test results, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were explored, and a reasonable explanation for unloading-induced spalling in hard rock tunnels was presented. The results show the stress-strain curve of highly stressed red sandstone exhibits a stress step during unloading, and the higher the unloading rate, the lower the stress level required for a stress step. The rock strength-weakening effect induced by unloading was confirmed. The mechanical properties of red sandstone become more unstable and complicated after unloading. After the red sandstone is unloaded to a two-dimensional stress state, with increasing confining pressure, the strength increases first and then decreases; the failure mode changes from a low-confining pressure tensile-shear failure to a high-confining pressure tensile failure; and the geometries of the slabs change from large thick plates and wedges to medium- and small-sized thin plates. At equal confining pressures, the higher the unloading rate, the lower the strength (i.e., the strength-weakening effect is more pronounced), the thinner the slab, and the lower the confining pressure required for the failure mode to change from tensile-shear failure to tensile failure. The unloading rate and confining pressure affect the strength and failure characteristics by affecting the crack initiation type and propagation direction in hard rock. For deep hard rock tunnels with high unloading rate and axial stress, neglecting the effects of unloading rate and axial stress will lead to a dangerous support design. For deep hard rock ore, if the maximal horizontal principal stress exceeds the critical confining pressure, the mining surface should be perpendicular to the direction of the minimal horizontal principal stress. The results of this study are of great engineering significance for guiding deep hard rock tunnel construction and mining. [ABSTRACT FROM AUTHOR]

To investigate the specimen shape effect on rockburst proneness of rock materials, a string of conventional and single-cycle loading-unloading uniaxial compression tests was performed with cylindrical and cuboid red sandstone specimens. Despite similar development paths on stress-strain curves for the specimens with two shapes, the cuboid specimens generally show a higher uniaxial compressive strength than the cylindrical specimens. The energy evolution laws inside the two shaped specimens were explored. The results show that the input energy density (IED), elastic energy density (EED), and dissipated energy density (DED) of the two shaped specimens increased in a quadratic relationship with the increment of unloading level. Moreover, the linear relationships between the EED, DED, and IED were further confirmed for two shaped specimens, which were defined as the linear energy storage and dissipation laws, respectively. The energy storage coefficients and energy dissipation coefficients (the slopes of the linear relationships between the EED, DED, and IED, respectively) are almost independent of the specimen shape. According to the linear energy storage and dissipation laws, the peak EED and peak DED of every specimen can be calculated accurately. Finally, combining the failure process of rock specimens recorded by a high-speed camera, the elastic energy index ( W ET ), the residual elastic energy index (AEF), and the far-field ejection mass ratio (MF) of each specimen were adopted to assess the rockburst proneness of the red sandstone sampled in cylindrical and cuboid. The results show that cuboid specimens exhibited stronger rockburst proneness than cylindrical ones, which favorably agreed with the actual failure phenomena. [ABSTRACT FROM AUTHOR]

The work aims to experimentally investigate the anisotropic fatigue characteristics of marble with different bedding orientations using uniaxial fatigue experiments and posttest computed tomography (CT) scanning. The impacts of interbed orientation on the strength, deformation, fatigue life, and stimulated fracture pattern were systematically analysed. Results show that the axial, lateral, and volumetric strain curves present a three‐stage deformation and are influenced by the interbed orientation. The volumetric strain is maximum and minimum for a marble sample with 30 and 90° orientation, respectively, which is closely related to the failure mode. The crack pattern in the CT images is related to the interbed orientation; the crack density and scale are the largest for a 90° orientation sample and are the smallest for a 30° orientation sample. Good correlation has been established between the fatigue lifetime and the interbed orientation. It is suggested that anisotropic cyclic behaviours of marble is structurally dependent. [ABSTRACT FROM AUTHOR]

Rockburst disaster is one of the prominent problems faced by deep underground engineering. In this study, rockburst in four elliptical holes with different axial ratios in sandstone under biaxial loading is studied as an analogue for underground roadways. Video and acoustic emission (AE) equipment is used to monitor the biaxial loading tests. Experimental results indicate that each of the elliptical holes goes through four stages: quiet period, small particle ejection, spalling, and rockburst. The duration of quiet and spalling periods increased with increasing axial ratio of ellipse. The duration of the ejection and rockburst periods remains unchanged. All the four elliptical holes have V-shaped pits after rockburst occurs. The fragments produced during rockburst are divided into coarse, medium, fine, and micro grains. The quantity of coarse and medium grains increases with increasing axial ratio. The mass ratio of coarse and medium grains increases and that of fine and micro grains decreases. The depth, angle, and area of the V-shaped pits decrease with increasing axis ratio. Tensile cracks play an important role in rockburst failure. Tensile cracks are the dominant crack types formed during rockburst and account for over 70% of all cracks in the samples. The number of tensile cracks increased and the number of shear cracks decreased. This paper has some reference value for practical engineering design and prevention of rockburst. [ABSTRACT FROM AUTHOR]

The ability of fracture propagation (fracability) is a key parameter of evaluating the fracture network generation during hydraulic fracturing. In order to consider the in situ conditions of the shale formation, four factors should be taken into consideration, which are brittleness, fracability, in situ stresses and natural fractures. The current shale brittleness and fracability evaluation methods rarely involve all the above factors. In this paper, coupling brittleness, stress-strain data and fracture morphology, a new shale fracability evaluation method is proposed. The brittleness is calculated by the mineral components and elastic parameters; the stress-strain data obtained by triaxial compression experiments are used to describe the shale breaking process; and the fracture morphology is evaluated by the fractal dimension method. This method is used to calculate the fracability index of the Longmaxi shale in Sichuan, China. The results show that: (1) the shale fracability under 5 MPa confining pressure is higher than that of under 20 MPa confining pressure; (2) the fracability in 7 coring angles shows a significant difference under the same confining pressure, the highest fracability is at 15∘ and the lowest is from 60∘ to 75∘. Compared with Jin and Rickman’s models, our model matches to the fracture morphologies and the fragments distribution closely. [ABSTRACT FROM AUTHOR]

• The rockburst infrared precursors of sandstone with different bedding dip angles were investigated; • The low-frequency fluctuations of the horizontal Fourier spectrum can be used as the precursor of rockburst in inclined bedding sandstones; • The NARX model, less dependent on the data length, was used to predict rockburst stress by using AISD. In order to determine the infrared precursors of rockburst in sandstone with different bedding dip angles (β = 0°, 30°, 60°, and 90°), four groups of true triaxial rockburst tests were carried out. An infrared thermal imager was used to obtain the infrared temperature field variation process on the free face of the sample. Additionally, the rockburst characteristics were analyzed from the stress–strain curve, failure mode of rock, Average Infrared Temperature Difference (AITD), Average Infrared Standard Deviation (AISD), and infrared thermal images, and the corresponding spectrums. Finally, based on the Nonlinear Autoregressive Model (NARX), the rockburst stress was predicted by using the AISD. The results show that with the increase in β , the failure mode of the sandstone changes from shear failure (β = 0°) to splitting failure (β = 30°, 60°, and 90°). The AITD and AISD curves show a steep increase before rockburst, and the corresponding time are before rockburst 33 ms, 762 ms, 567 ms, and 316 ms, respectively for the β are 0°, 30°, 60°, and 90°. Furthermore, the horizontal Fourier spectrums of the sandstone with inclined bedding planes (β = 30° and 60°) show evident low-frequency fluctuations in a short period before the amplitude of the spectrum increases. Overall, the NARX network predicts peak stress with relatively high accuracy (greater than 85 %); however, the prediction errors of the sandstone with horizontal and vertical bedding planes (β = 0° and 90°) are larger than that for the inclined bedding planes. [ABSTRACT FROM AUTHOR]

Objective: The aim of this network meta-analysis (NMA) was to explore the optimal energy density (ED) for relief of orthodontic-related pain by photobiomodulation therapy. Background: Orthodontic treatment corrects the deformity of teeth, dental arch, jaw, and face using various orthodontic devices under the action of biomechanics. Materials and methods: A computerized literature search was conducted within the MEDLINE, EMBASE, Scopus, Cochrane Library, Web of Science, CNKI, Wanfang, VIP, and CBM databases to identify randomized controlled trials (RCTs) that used photobiomodulation therapy to relieve pain during orthodontic treatment. Particular inclusion and exclusion criteria were connected to recognize relevant articles. The information was extricated autonomously by two reviewers, and a quality assessment was carried out by utilizing the Cochrane Collaboration "risk of bias" tool. The RevMan 5.4 software and STATA 15.0 were utilized for data analysis. Also, the NMA was performed with mvmeta commands in Stata (StataCorp, TX). Results: Of the 1024 recognized articles, 21 RCTs were at long last included. Within the risk-of-bias assessments, 2 studies displayed a high risk, 17 an unclear risk, and 2 a low risk. The global and local inconsistency of the NMA in 1 and 2 days was not significant; however, the NMA in 3 days showed a significant inconsistency. Laser with lower ED (0-10 J/cm 2 ) appeared to be the most noteworthy possibility to become the optimal intervention, and it also had the highest mean rank in the rankogram and the biggest value of surface under the cumulative ranking in all three NMA structures. Conclusions: On account of the limited evidence of included trials, laser with lower ED (0-10 J/cm 2 ) appeared to be more valid than the remaining treatment modalities (laser with higher ED, placebo, and control) for pain reduction in 1, 2, and 3 days after orthodontic procedures were applied. In the future, more high-quality research with consistency in research design is needed for further evaluation.

Objective: To assess the viability of photobiomodulation therapy (PBMT) in decreasing orthodontic-related pain after different orthodontic procedures. Methods: A computerized literature search was conducted within the MEDLINE, EMBASE, Scopus, Cochrane Library, Web of Science, CNKI, WanFang, VIP, and CBM databases to identify randomized controlled trials (RCTs), which used PBMT to relieve pain during orthodontic treatment. Particular inclusion and exclusion criteria were connected to recognize relevant articles. The information was extricated autonomously by two reviewers, and a quality assessment was carried out by utilizing the Cochrane Collaboration "risk of bias" tool. Meta-analysis was conducted with fixed- or random-effects models as suitable. Statistical heterogeneity was also examined. The RevMan 5.4 software was utilized for data analysis. Results: Of 1024 recognized articles, 25 RCT were at long last included. Within the risk-of-bias assessments, 1 study displayed a high risk, 22 an unclear risk, and 2 a low risk. The meta-analysis appeared that in patients treated with photobiomodulation (PBM) versus control there was a distinction in support of PBMT in placement of elastomeric separators, canine retraction, and archwire placement during 1 week. Conclusions: PBMT demonstrated to be effective in advancing a reduction pain after elastomeric separators placement, canine retraction, and archwire placement within 1 week. In the future, more high-quality research with consistency in research design is needed for further evaluation.