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Multiple compression tests on rock-like samples of pre-existing cracks with different geometries were conducted to investigate the strength properties and crack propagation behavior considering multi-crack interactions. The progressive failure process of the specimens was segmented into four categories and seven coalescence modes were identified due to different crack propagation mechanisms. Ultimately, a mechanical model of the multi-crack rock mass was proposed to investigate the gradual fracture and damage evolution traits of the multi-crack rock on the basis of exploring the law of the compression-shear wing crack initiation and propagation. A comparison between theory and experimental results indicated that the peak strength of the specimens with multiple fractures decreased initially and subsequently increased with the increase in the fissure inclination angles; the peak strength of specimens decreased with the increase in the density of fissure distribution. [ABSTRACT FROM AUTHOR]

In the operations of underground rock engineering, such as mining, the formation of goafs is often accompanied by unloading and energy effects. In this study, a cyclic loading and unloading stress test is carried out to analyze the strength characteristics of the loaded samples under different loading and unloading ranges as well as different numbers of cycles. The rock force is accompanied by substantial energy changes. To better fit the energy analysis under cyclic loading and unloading conditions, thermal infrared radiation characteristic analysis is performed during rock loading and unloading. An infrared radiation camera is adopted to detect the infrared characteristics of the rock force process after cyclic loading and unloading. Multiangle detection is implemented on the temperature, temperature field, and frequency histogram. The analysis shows that cyclic loading and unloading first strengthen and then weaken the rock. Moreover, the failure caused by the local stress concentration leads to a sharp increase in the temperature. There are significant temperature fluctuations before and after failure, and the temperature field after failure can be divided into three zones, namely, the normal temperature zone, heating zone, and mutational temperature zone, to comprehensively reflect that rock compression failure which is accompanied by the process of energy accumulation and release. On the basis of infrared energy analysis, the index of the energy release rate is introduced, and the loading and unloading analysis model is constructed. The research results reveal that rock failure is accompanied by the process of energy accumulation and release, which provides evidence for the analysis of the spatial stability of the rock mass under cyclic loading and unloading conditions and engineering excavation. [ABSTRACT FROM AUTHOR]

In order to simplify the ratio decision process of cemented backfill in underground mines and achieve fine decision of filling ratio, the research on the energy matching between surrounding rock and cemented backfill in underground mines was conducted in this study. Based on the cubic function strength model of cemented backfill, the peak specific energy equation of backfill was improved by inversion analysis of the data of filling ratio experiment, and the functional relationship between the peak specific energy and the filling ratio was obtained by regression analysis. Then, based on the energy balance principle between the deformation energy released by the excavation of the underground rock mass and the peak specific energy of the cemented backfill, considering the physical and mechanical parameters of the surrounding rock of the goaf, including bulk density, elastic modulus, and burial depth, a ratio decision model of cemented backfill is established. The application results suggested that the calculation result of the model is reliable, and it can realize the rapid and accurate decision of the ratio of cement backfill in underground mines. [ABSTRACT FROM AUTHOR]

Chemical corrosion has a significant impact on the damage evolution behavior of rock. To investigate the mechanical damage evolution process of rock under a coupled chemical-mechanical (CM) condition, an improved statistical damage constitutive model was established using the Drucker-Prager (D-P) strength criterion and two-parameter Weibull distribution. The damage variable correction coefficient and chemical damage variable which was determined by porosity were also considered in the model. Moreover, a series of conventional triaxial compressive tests were carried out to investigate the mechanical properties of sandstone specimens under the effect of chemical corrosion. The relationship between rock mechanics properties and confining pressure was also explored to determine Weibull distribution parameters, including the shape parameter m and scale parameter F0. Then, the reliability of the damage constitutive model was verified based on experimental data. The results of this study are as follows: (i) the porosity of sandstone increased and the mechanical properties degraded after chemical corrosion; (ii) the relationships among the compressive strength, the peak axial strain, and confining pressures were linear, while the relationships among the elastic modulus, the residual strength, and confining pressures were exponential functions; and (iii) the improved statistical damage constitutive model was in good agreement with the testing curves with R2>0.98. It is hoped that the study can provide an alternative method to analyze the damage constitutive behavior of rock under a coupled chemical-mechanical condition. [ABSTRACT FROM AUTHOR]

Many deep underground excavation practices show that the size and distribution of in situ stress are the main factors resulting in the deformation and instability of the surrounding rock structure. The in situ stress measured by the Kaiser effect of rock is used by engineers because of its economy and convenience. However, due to the lack of quantitative judgment basis in determining the Kaiser point position, there is a large artificial error in the practical application. In response to the problem, this study systematically investigates the characteristics of rock acoustic emission curve on the basis of the fractal theory and establishes an accurate and simple interpretation method for determining the Kaiser point position. The indoor rock acoustic emission test was carried out by drilling a rock sample at a mine site. By using the conventional tangent method, the cumulative ringing count rate-time-stress curve of rock acoustic emission is analyzed to preliminarily determine the time range of Kaiser point appearance. Considering that the fractal dimension of the rock Kaiser point is lower than the adjacent point, the minimum point of the fractal dimension of this time range can be determined from the fractal dimension-time-stress curve. Such determined point is the Kaiser point. The size of the in situ stress is calculated using an analytical method. Based on the value of the in situ stress, the distribution of the in situ stress in the mining area is further analyzed using the geological structure of the mine. The maximum principal stress is 19.38 MPa, with a direction of N (30°-40°) E, and the minimum principal stress is 8.02 MPa with a direction of N (50°-60°) W. The maximum and minimum principal stresses are approximately in the horizontal plane. The intermediate principal stress is 11.73 MPa in vertically downward. These results are basically consistent with the distribution statistical law of the measured in situ stress fields in the world. The results presented in the study could provide a reference for the later mining, stability evaluation, and support of the surrounding rock. [ABSTRACT FROM AUTHOR]

We applied the finite frequency tomography method to S wave data recorded by 350 broadband stations beneath the South China Block (SCB) and its surroundings from earthquakes occurring between July 2007 and July 2010, to better understand upper mantle deformation. Differential travel-times in the pair of stations with appropriate weighting for each station are used in the inversion. Our results are consistent with previous tomography that show a high velocity anomaly beneath the Sichuan basin and a high velocity anomaly in the transition zone beneath the Yangtze Craton. However, the resolution of mantle heterogeneity provides new insight into the tectonic framework of subduction of Burmese lithosphere in the west part of the study region and subduction of oceanic lithosphere in the east. In the subduction realm, west of 107°E, a significant fast S-wave anomaly is located on the southeast of Sichuan Basin. East of 107°E, and two narrow and discontinuous fast S-wave anomalies occur at a depth of 400-600 km beneath the middle of the South China block overlain by the pronounced low S-wave anomalies at a depth of 100 and 400 km. If the fast anomalies located in the mantle transition zone represent stagnant slabs, their fragmented nature may suggest that they could be produced by different episodes of subduction beneath western Pacific island and the above slow velocity anomaly may associated with the back-arc regions of ongoing subduction. In addition, tomography also reveals an anomalously high S-wave velocity continental root extends eastward to a depth 400 km beneath the eastern Sichuan Basin. This anomaly may be related to eastern extrusion of Indian lithosphere associated with the collision of India and Eurasia. Moreover, our results also show large slow anomalies beneath the Red River fault region connected to deeper anomalies beneath the South China Fold Belt and South China Sea. All these observations are consistent with the scenario that the South China block has been built by both of subduction of Paleo-pacific plate and eastward subduction of Burma microplate. [ABSTRACT FROM AUTHOR]

Microwave-induced rock fracture is one of the promising approaches of achieving non-blasting continuous mining and assisted mechanical rock breaking. It is of great theoretical and practical significance to study the temperature effect and fracture characteristics of rocks of different sizes under microwave heating; however, there are few studies in this field. Microwave heating of φ 50 × 100 mm, φ 50 × 50 mm, and φ 50 × 25 mm sandstone samples with different heating powers and times was performed to measure the temperature of the sample, the microwave energy absorbed, the mass, and the P-wave velocity before and after heating. The results show suppress that (i) under the same heating conditions, the mass difference and the temperature increase range of φ 50 × 100 mm and φ 50 × 50 mm samples are larger than that of the φ 50 × 25 mm samples; (ii) the wave velocity change rate and the damage factor of samples increase with the increase of heating power and time; (iii) different size specimens have different crack- propagation modes. The main crack of φ 50 × 100 mm specimens usually starts from the middle of the height of the specimen; for the φ 50 × 50 mm specimens, it usually starts from the middle or bottom-end surface of the specimen height; the main crack of φ 50 × 25 mm specimens starts from the vertical surface of the specimen. With an increase in the heating time, the length and width of the main crack continuously increase and secondary cracks are generated. The fracture mode of the sample is also related to the size of the sample. The fracture mode can be divided into three parts: melt fracture, thermal-expansion fracture, and secondary thermal-expansion fracture. The relationship between the sample temperature and the absorbed microwave energy is approximately linear. [ABSTRACT FROM AUTHOR]

This paper evaluates the effect of freeze–thaw (F-T) cycles on the fracture mechanical properties of rocks with pre-fabricated cracks through experimental and numerical simulation. Three-point bending tests are conducted to determine the fracture characteristics of cyan sandstone with varying pre-fabricated crack heights. Subsequently, the failure characteristics of cyan sandstone with pre-fabricated cracks under F-T cycles are examined. A numerical model using parallel bond modeling (PBM) is established and validated against experimental results to further investigate the fracture behavior of sandstones with pre-fabricated cracks. Results demonstrate that both axial peak force and fracture toughness (K IC) linearly decrease with increasing freeze–thaw (F-T) cycles, and the axial peak force reduces with the rise in pre-fabricated crack height (H c). The deformation damage process is categorized into four stages: compression-density, elastic deformation, pre-peak destabilization, and post-peak destabilization damage. Energy evolution is closely tied to microcrack initiation and propagation, with the majority of energy dissipating as microcrack surface energy. Both strain energy and dissipation energy decrease linearly with increased F-T cycles. These findings offer valuable insights for investigating the fracture mechanism of fractured rock mass in cold regions. [ABSTRACT FROM AUTHOR]

To reveal the energy-dissipation mechanism of the rock deformation and destruction process under the influence of freeze–thaw cycles and investigate the essence of rock freeze–thaw damage, the evolution characteristics of the total strain energy, elastic energy, and dissipated energy in the sandstone failure process are analysed according to the energy principle, number of freeze-thaw cycles, and uniaxial compression. The results reveal that: (1) the evolution behaviours of the total strain energy, elastic energy, and dissipative energy of sandstone under different numbers of freeze–thaw cycles are similar; that is, the total strain energy and elastic energy gradually increase with the increasing strain, and the elastic energy decreases sharply after reaching the peak limit. The dissipative energy gradually increases and thereafter remains stable; finally, it rapidly increases. (2) With increasing freeze–thaw cycles, the growth rates of the total strain energy, elastic energy, and dissipated energy gradually decrease, whereas the energy value at the peak first increases and then decreases. The energy distribution during rock loading is affected by the number of freeze-thaw cycles (defined as N). When N is <30, the proportion of dissipation energy in the compacting section first increases and then decreases linearly; the decreasing rate decreases gradually with an increase in N. When N is larger than 30, the dissipation energy ratio linearly decreases as the strain increases, and the decreasing rate increases as N increases. (3) According to the dissipative energy ratio evolution characteristics of rock failure, the peak strength and peak strain damage evolution models of frozen−thawed rocks are established. The fitting results indicate that the model has a good correlation with the experimental data and can accurately reflect the peak strength and peak strain evolution of frozen−thawed rocks. This paper studies the freeze-thaw damage of rock based on the principle of energy dissipation. The research has the following highlights that readers might be interested in: • The variation characteristics of U, Ue and Ud of rock after different freeze-thaw cycles are obtained； • The relationship between energy distribution and rock deformation process is analysed； • A new freeze-thaw damage factor based on energy dissipation ratio is defined for frozen rock； • The evolution model of peak strength and peak strain of freeze-thaw rock based on energy mechanism is established. [ABSTRACT FROM AUTHOR]