Search

The technical challenges associated with deep underground space activities have become increasingly significant. Among these challenges, one major concern is the assessment of rockburst risks and the instability of rock masses. Extensive research has been conducted by numerous scholars to mitigate the risks and prevent occurrences of rockburst through various assessment methods. Rockburst incidents commonly occur during the excavation of hard rock in underground environments, posing severe threats to personnel safety, equipment integrity, and operational continuity. Thus, it is crucial to systematically document real cases of rockburst, allowing for a comprehensive understanding of the underlying mechanisms and triggering conditions. This understanding will contribute to the advancement of rockburst prediction and prevention methods. Proper selection of an appropriate rockburst assessment method is a fundamental aspect in underground operations. However, there is a limited number of studies that summarize and compare different prediction and prevention methods of rockburst. This paper aims to address this gap by analyzing global trends using CiteSpace software since 1990. It discusses rockburst classification and characteristics, comprehensively reviews research findings related to rockburst prediction, including empirical, simulation, mathematical modeling, and microseismic monitoring methods. Additionally, the paper presents a compilation of current rockburst prevention measures. Notably, the paper emphasizes the significance of control strategies, which provide key insights into the effective utilization of stored energy within rock. Finally, the paper concludes by suggesting six directions for implementing intelligent management techniques to mitigate hazards during underground operations and reduce the probability of rockburst incidents.

Rockburst is one of the common geological disasters in deep underground areas with high stress. Rockburst prediction is an important measure to know in advance the risk of rockburst hazards to take a scientific approach to the response. In view of the fuzziness and uncertainty between quantitative indexes and qualitative grade assessments in prediction, this study proposes the use of a normal cloud model to optimize the theory of unascertained measures (NC-UM). The uniaxial compressive strength (σc), stress coefficient (σθ/σc), elastic deformation energy index (Wet), and brittleness index of rock (σc/σt) are selected as the index of prediction. After data screening, 249 groups of rockburst case data are selected as the original data set. To reduce the influence of subjective and objective factors of index weight on the prediction results, the game theory is used to synthesize the three weighting methods of Criteria Importance Through Intercriteria Correlation (CRITIC), Entropy Weight (EW), and Analytic Hierarchy Process (AHP) to obtain the comprehensive weight of the index. After validating the model with example data, the results showed that the model was 93.3% accurate with no more than one level of prediction deviation. Compared with the traditional unascertained measure (UM) rockburst prediction model, the accuracy is 15–20% higher than that of the traditional model. It shows that the model is valid and applicable in predicting the rockburst propensity level.

ABSTRACT Mixing time is one of the operational parameters that can affect the rheological and mechanical behaviors of cemented backfill. The objective of this study was to investigate the effect [...]

To reduce the energy release risk and optimise the crack network of tight shale reservoirs, this study experimentally investigated the coupled dynamic–static fracturing technique and its mechanism for the damage evolution and crack propagation in tight shale. First, an innovative coupled dynamic–static hydraulic fracturing test system with a fluid pressure loading strain rate of 10−4–100s−1 was developed, and physical model experiments were conducted. Then, the responses of the fluid pressure, acoustic emission (AE) parameter, and crack propagation pattern were obtained. Further, the mechanism of dynamic pulsation parameters, such as the pulsation amplitude (PA) and the upper pressure limit (UPL), on the damage evolution of shale matrix and the crack propagation patterns were analysed. The results show that during coupled dynamic–static fracturing, the initiation pressure and the transient initiation energy were significantly reduced by 0.65%–25.58% and 5.36%–31.51%, respectively, whereas the cumulative AE energy under dispersed release of energy increased by an average of 29.29%. In addition, the pulsation parameters mainly affected the cumulative damage of the shale matrix by controlling the microcrack propagation scale and number. The cumulative damage under increasing PA and UPL showed a trend of first increasing and then decreasing, implying that there are optimal PA and UPL for the fracturing process. Moreover, the dynamic pulsation parameters have control effects on the crack-propagation direction. With the increase in PA and UPL, the propagation of microcracks around the borehole presents a trend of ‘preferential propagation in the axis-section direction, alternate propagation in the axis-section and cross-section directions, and preferential propagation in the cross-section direction’, which induces the subsequent static propagation of hydraulic macrocracks along the direction of dominant microcracks. Based on the experimental results, a reasonable fracturing strategy was proposed to realise the coalescence of crack networks in the tight shale reservoirs.

To investigate the failure process and characteristics of D-shaped tunnels under different maximum principal stress directions θ, true-triaxial tests were conducted on cubic sandstone samples with a through D-shaped hole. The test results show that the failure process can be divided into 4 periods: calm, buckling deformation, gradual buckling and exfoliation of rock fragment, and formation of a V-shaped notch. With an increase in θfrom 0° to 90°, the size of the rock fragments first decreases and then increases, whereas the fractal dimension of the rock fragments first increases and then decreases. Meanwhile, the failure position at the left side shifts from the sidewall to the corner and finally to the floor, whereas the failure position at the right side moves from the sidewall to the spandrel and finally to the roof, which is consistent with the failure position in underground engineering. In addition, the initial vertical failure stress first decreases and then increases. By comparing the results, the failure severities at different maximum principal stress directions can be ranked from high to low in the following order: 90°>60°>30°>45°>0°.

In this paper, an investigation into flow and heat transfer performance of supercritical methane (S-CH4) in a 180-degree curved circular duct has been numerically carried out. The mathematical description of energy equation induced by the Dean vortices under the Cartesian-coordinate is first deduced. After validating the dynamic computational fluid dynamics model and method against the public experimental data, the effect of mass flux on the thermodynamics properties of S-CH4is revealed. The calculation results show that due to the centrifugal force, the low temperature of S-CH4gathers near the outer wall generatrix. Meanwhile, owing to the existence of multiple Dean vortices, all thermophysical parameters on the 90° cross-section are symmetrically concave along the vertical axis. The core position of multiple Dean vortices inside the curved duct is closer to the inner wall generatrix, which makes the velocity fluctuation greater. The maximum value of circumferential heat transfer coefficient on different cross-sections differs, and the non-uniform flow development process occurs inside the curved duct. Compared to the straight duct, when the mass fluxes are respectively 300 kg/m2· s and 600 kg/m2· s, the magnitude of increase in heat transfer coefficient of curved duct presents 18.8% and 23.5%. In addition, the forced convection caused by the secondary flow inside curved duct is so strong that the natural convection by the gravity could be neglected. The research outcome is of vital importance for the optimization design of liquefied natural gas vaporizer.

Fast-cooling treatments are performed on heated granite specimens to induce thermal shock (TS), whereby the effect of TS on the physical and mechanical properties of granite is investigated through laboratory tests. Air-cooling and water-cooling methods are used to render different temperature changing rates in the TS process. Uniaxial compressive strength, tensile strength, and fracture toughness of the rock specimens are obtained in the laboratory tests. The aggravation of damage at higher temperature changing rates in the TS process is reflected in the test results. Through scanning electron microscopy, two typical fracture patterns reflecting the action of TS are observed in the specimens heated at temperatures higher than 200°C, which indicates that the TS effect has been successfully triggered. Based on the laboratory data and microscopic observations, the acting mechanisms of TS are discussed and corresponding mechanical behaviors are explained.

Pillar stability is always evaluated using the safety factor (SF), which is defined as the ratio of pillar strength to pillar stress. However, most researchers have estimated pillar stress using the pillar shape ratio (w/h), uniaxial compressive strength (UCS) of the intact rock mass, and pillar depth (H). In this study, the geological strength index (GSI) of hard rock pillars was considered as a new variable for predictive purposes. This index was developed by combining numerical simulation software (i.e., FLAC3D) and a backpropagation neural network (BPNN). A hard rock pillar stability analysis, based on three methods including deterministic method, sensitivity analysis, and Monte Carlo simulation (MCS), was performed. A new formula was proposed to estimate the SF values based on the predicted stress, considering the GSI variable in the deterministic method. The sensitivity analysis indicated that the variables impacting the SF from high to low are UCS, GSI, w/h, and H. In this study, pillar stability was analyzed mainly using the GSI and MCS techniques. The MCS results revealed that the GSI is also a major factor in pillar stability and has a greater effect on weak pillars than on strong ones. Furthermore, a pillar is more likely to be unstable when both the GSI and the UCS are decreased. This study provides several references and procedures for improving the design of stable pillars considering the GSI as an important factor.

During the development blasting of circular tunnels, the detonation of multiple blastholes arranged on concentric circles induces a complex dynamic response in the surrounding rocks. This process involves multiple blast loadings, static stress unloadings, and stress redistributions. In this study, the dynamic stresses of the surrounding rocks during development blasting, considering multiple blasting-unloading stages with exponential paths and triangular paths (linear simplified paths of exponential paths), are solved based on the dynamic theory and the Fourier transform method. Then, a corresponding discrete element model is established using particle flow code (PFC). The multiple-stage dynamic stress and fracture distribution under different in situ stress levels and lateral coefficients are investigated. Theoretical results indicate that the peak compressive stresses in the surrounding rocks induced by both triangular and exponential paths are equal, while the triangular path generates greater additional dynamic tensile stresses, particularly in the circumferential direction, compared to the exponential path. Numerical results show that the exponential path causes less dynamic circumferential tensile damage and forms fewer radial fractures than the triangular path in the first few blast stages; conversely, it exacerbates the damage and instability in the final blast unloading stage and forms more circumferential fractures. Furthermore, the in situ stress determines which of the two opposite effects is dominant. Therefore, when using overly simplified triangular paths to evaluate the stability of surrounding rocks, potential overestimation or underestimation caused by different failure mechanisms should be considered. Specifically, under high horizontal and vertical stresses, the static stress redistribution with layer-by-layer blasting suppresses dynamic circumferential tensile and radial compressive damage. The damage evolution of surrounding rocks in multi-stage blasting under different in situ stresses is summarized and classified according to the damage mechanism and characteristics, which can guide blasting and support design.

The size of mineral grain has a significant impact on the initiation and propagation of microcracks within rocks. In this study, fine-, medium-, and coarse-grained granites were used to investigate microcrack evolution and characteristic stress under uniaxial compression using the acoustic emission (AE), digital image correlation (DIC), and nuclear magnetic resonance (NMR) measurements. The experimental results show that the characteristic stress of each granite decreased considerably with increasing grain sizes. The inflection points of the b-value occurred earlier with an increase in grain sizes, indicating that the larger grains promote the generation and propagation of microcracks. The distribution characteristics of the average frequency (AF) and the ratio of rise time to amplitude (RA) indicate that the proportion of shear microcracks increases with increasing grain size. The NMR results indicate that the porosity and the proportion of large pores increased with increasing grain size, which may intensify the microcrack evolution. Moreover, analysis of the DIC and AE event rates suggests that the high-displacement regions could serve as a criterion for the degree of microcrack propagation. The study found that granites with larger grains had a higher proportion of high-displacement regions, which can lead to larger-scale cracking or even spalling. These findings are not only beneficial to understand the pattern of microcrack evolution with different grain sizes, but also provide guidance for rock monitoring and instability assessment.

Geological exploration cores obtained from shale gas wells several kilometers deep often show different height-diameter ratios (H/D) because of complex geological conditions (core disking or developed fractures), which makes further standard specimen preparation for mechanical evaluation of reservoirs difficult. In multi-cluster hydraulic fracturing, shale reservoirs between planes of hydraulic fractures with different lengths could be simplified to have different H/Dratios. Discovering the effect of H/Don the mechanical characteristics of shale specimens with different bedding orientations will support mechanical evaluation tests of reservoirs based on disked geological cores and help to optimize multi-cluster fracturing programs. In this study, we performed uniaxial compression tests and acoustic emission (AE) monitoring on cylindrical Longmaxi shale specimens under five bedding orientations and four H/Dratios. The experimental results showed that both the H/D-dependent mechanical properties and AE parameters demonstrated significant anisotropy. Increasing H/Ddid not change the uniaxial compressive strength (UCS) evolution versus bedding orientation, demonstrating a V-shaped relationship, but enhanced the curve shape. The stress level of crack damage for the specimens significantly increased with increasing H/D, excluding the specimens with a bedding orientation of 0°. With increasing H/D, the cumulative AE counts of the specimens with each bedding orientation tended to exhibit a stepped jump against the loading time. The proportion of low-average-frequency AE signals (below 100 kHz) in specimens with bedding orientations of 45° and 60° increased to over 70% by increasing H/D, but it only increased to 40% in specimens with bedding orientations of 0°, 30°, and 90°. Finally, an empirical model that can reveal the effect of H/Don anisotropic UCS of shale reservoir was proposed, the anisotropic proportion of tensile and shear failure cracks in specimens under four H/Dratios was classified based on the AE data, and the effect of H/Don the anisotropic crack growth of specimens was discussed.

ABSTRACTIn forming procedure of the micro grooves in the flat micro-groove heat pipe, the tie rod is often observed to be broken and the multi-tooth cutter is damaged due to the sharp increase of the ploughing-pulling pressure. This paper theoretically analyzes the factors affecting the capillary heat transferring limit of the micro-groove heat pipe, and simulates the machining process using finite element to acquire the best processing parameters: the squeeze angle is 120°, the drawing depth is 0.25mm, and the ploughing-pulling velocity is 100mm/s. Then these parameters are verified by real manufacturing experiments. The experimental results show that the ploughing-pulling pressure of the micro-groove forming process is close to the strength limit of the rod or multi-tooth cutter, and the process makes little swarf during work. Thus, only using the appropriate machine tool parameters, forming parameters and forming methods can make the wicking structure of flat micro-groove micro-heat pipe with the best heat transferring performance.

Four groups of numerical models of Brazilian tests on rock-shotcrete interfaces were successfully conducted by PFC2D. The tensile strength and Young’s modulus of shotcrete were considered. Six different undulations of rock-shotcrete interface were set up. The influences of multiple parameters on the bearing characteristics of the rock-shotcrete interface were studied. The results showed that a better support performance can be obtained by increasing the Young’s modulus of shotcrete rather than the tensile strength of shotcrete. For different tensile strength and Young’s modulus, the increase of sawtooth height has different effects on the support performance. The failure mechanism of the rock-shotcrete interfaces was analysed in detail. The stress shielding effect and stress concentration effect caused by the shape characteristics of rock-shotcrete interface were observed. The influence of these parameters on the overall support performance should be fully considered in a reasonable support design.

As the advancement of deep earth energy engineering in the 21st century, the challenges imposed by dynamic loads and high temperatures will be increasingly regular. This paper studied the dynamic mechanical behaviors and fracture characteristics of defective red sandstone (single fissure inclination of 0°, 30°, 60°, and 90°) subjected to thermal treatments (25 °C, 200 °C, 400 °C, 600 °C, and 800 °C) under descending amplitude low cycle (DALC) impact loading. The experiments were conducted using a modified split Hopkinson pressure bar (SHPB) system, and the fracture surfaces of the failed specimens were characterized through the use of high-resolution 3D laser scanning to obtain point cloud data. Subsequently, the fractal, roughness, texture, and elevation features of fracture surfaces were analyzed using the boxing-counting method, grayscale co-occurrence matrix (GLCM), and elevation-based statistics. The scanning electron microscopy (SEM) was utilized to study the fracture micromorphology as well. Additionally, the mechanisms of damage and fracture evolution were discussed based on the results of X-ray diffractometry (XRD), synchronous thermal analysis (STA), discrete element method (DEM), and strain energy density theory. The results demonstrate that the peak stress and dynamic elastic modulus deteriorate with rising temperatures and increase with larger fissure angles. While the peak strain, strain rate, and peak stress increase factor (PIF) exhibit the opposite trend. The threshold values for the intensification of these events are 400 °C and 60°. The damage is dominated by tensile cracks, which transition to shear at 30° and 60° due to temperature dependence. Meanwhile, the fractal dimension (DC), joint roughness coefficient (JRC), second-order statistics of GLCM, and elevation eigenvalues have been modeled using multiple regression analysis to investigate their interaction with temperature and fissure angle. The best-fit models illustrate significant linear or non-linear correlations between the aforementioned parameters and the mechanical parameters. Furthermore, the magnitude and distribution pattern of thermal cracks, as well as the growth of shear fractures, are governed by the dehydration, thermal expansion, phase transition, plasticization, thermal fracture, and thermal decomposition. Finally, strain energy density theory manifests promising prospects for understanding and predicting DALC impact fracture patterns of specimens.

Rock failure phenomena, such as rockburst, slabbing (or spalling) and zonal disintegration, related to deep underground excavation of hard rocks are frequently reported and pose a great threat to deep mining. Currently, the explanation for these failure phenomena using existing dynamic or static rock mechanics theory is not straightforward. In this study, new theory and testing method for deep underground rock mass under coupled static-dynamic loading are introduced. Two types of coupled loading modes, i.e. “critical static stress + slight disturbance” and “elastic static stress + impact disturbance”, are proposed, and associated test devices are developed. Rockburst phenomena of hard rocks under coupled static-dynamic loading are successfully reproduced in the laboratory, and the rockburst mechanism and related criteria are demonstrated. The results of true triaxial unloading compression tests on granite and red sandstone indicate that the unloading can induce slabbing when the confining pressure exceeds a certain threshold, and the slabbing failure strength is lower than the shear failure strength according to the conventional Mohr-Column criterion. Numerical results indicate that the rock unloading failure response under different in situ stresses and unloading rates can be characterized by an equivalent strain energy density. In addition, we present a new microseismic source location method without premeasuring the sound wave velocity in rock mass, which can efficiently and accurately locate the rock failure in hard rock mines. Also, a new idea for deep hard rock mining using a non-explosive continuous mining method is briefly introduced.

We study the three‐dimensional seismic attenuation (QPand QS) structure in the source area of the 2016 Kumamoto earthquake (M7.3) by using 18,296 tP*and 29,668 tS*data from 742 local earthquakes recorded by a dense seismic network consisting of 112 stations deployed in Kyushu Island. Our results show that significant low‐Q(high‐attenuation) anomalies exist in the crust and mantle wedge beneath the volcanic front and back‐arc area, which reflect hot and wet zones caused by convective circulation in the mantle wedge and dehydration of the young Philippine Sea (PHS) slab. The 2016 Kumamoto earthquake occurred in a high‐Qand high‐velocity (high‐V) zone in the upper crust, which is surrounded and underlain by low‐Qand low‐Vanomalies in the lower crust and upper mantle. These results suggest that the 2016 Kumamoto earthquake took place in a brittle seismogenic layer in the upper crust, but its rupture nucleation was affected by fluids and arc magma ascending from the mantle wedge. In addition, an obvious low‐Qzone is revealed in the fore‐arc mantle wedge, which reflects serpentinization of the fore‐arc mantle due to abundant fluids from the PHS slab dehydration. Seismic attenuation structure in the 2016 Kumamoto earthquake (M7.3) area is investigatedThe earthquake occurred in a high‐Qand high‐Vzone in the upper crust but is underlain by a low‐Qand low‐VzoneThe nucleation of the earthquake may be affected by arc magma and fluids

In the engineering practices, it is increasingly common to encounter fractured rocks perturbed by temperatures and frequent dynamic loads. In this paper, the dynamic behaviors and fracture characteristics of red sandstone considering temperatures (25 °C, 200 °C, 400 °C, 600 °C, and 800 °C) and fissure angles (0°, 30°, 60°, and 90°) were evaluated under constant-amplitude and low-cycle (CALC) impacts actuated by a modified split Hopkinson pressure bar (SHPB) system. Subsequently, fracture morphology and second-order statistics within the grey-level co-occurrence matrix (GLCM) were examined using scanning electron microscopy (SEM). Meanwhile, the deep analysis and discussion of the mechanical response were conducted through the synchronous thermal analyzer (STA) test, numerical simulations, one-dimensional stress wave theory, and material structure. The multiple regression models between response variables and interactive effects of independent variables were established using the response surface method (RSM). The results demonstrate the fatigue strength and life diminish as temperatures rise and increase with increasing fissure angles, while the strain rate exhibits an inverse behavior. Furthermore, the peak stress intensification and strain rate softening observed during CALC impact exhibit greater prominence at increased fissure angles. The failure is dominated by tensile damage with concise evolution paths and intergranular cracks as well as the compressor-crushed zone which may affect the failure mode after 400 °C. The second-order statistics of GLCM in SEM images exhibit a considerable dependence on the temperatures. Also, thermal damage dominated by thermal properties controls the material structure and wave impedance and eventually affects the incident wave intensity. The tensile wave reflected from the fissure surface is the inherent mechanism responsible for the angle effect exhibited by the fatigue strength and life. Ultimately, the peak stress intensification and strain rate softening during impact are determined by both the material structure and compaction governed by thermal damage and tensile wave.

Some open pits in China are severely threatened by hidden cavities under benches which are always inaccessible and unmapped. The first thing to be well considered is safe and precise cavity detection, as the conventional detection methods cannot output a clear cavity vision, a three-dimensional laser measurement system is employed to perform detection, which is deployed from the surface through boreholes. The results from the scanner demonstrated very well the detailed level of information that can be collected in a cavity using this method, with the cavities’ layout under various benches being fully mapped. As detected cavities are always in horizontal and vaulted roof shape, two different theoretical calculation methods are proposed to analyze the cap rock stability for different roof shape. And also, the three-dimensional solid model generated through scanned laser data is used for stability numerical simulation by converting the data format into the one that recognized in corresponding software. Furthermore, acoustic emission technique is adopted to carry out long term real time rupture monitoring in cap rock, and four kinds of typical monitoring results are discussed which represent the rupture behavior in cap rock of unstable cavity, stable cavity, cavity with large working drill above and cavity beside explosion site. Thus, a complete safety evaluation system for such cavity will be established to ensure safe operation above.

To achieve a smooth fracture plane with minimal damage to a rock block or remaining rock mass, a water-coupling cutting blasting technique was applied in a granite quarry. The mechanism of water-coupling cutting blasting is demonstrated with the specification of prerequisites for crack growing and arresting. Using the LS-DYNA program, the maximum principal stress distribution and rock damage evolution process were observed, and the validity of crack extension till coalescence was verified by comparing the calculated and theoretical water pressure needed to induce fracture. The optimum blasting parameters for achieving the best cutting blasting performance were obtained by simulations under different coupling ratios and blasthole spacings. The successful water-coupling cutting blasting practice in the granite quarry has great potential to be applied in other similar engineering.

Abstract: Stress conditions are critical in deep hard rock mining and significantly influence hard rock cuttability. The peak cutting force (PCF), cutting work (CW), and specific energy (SE) can reflect rock cuttability and determine the feasibility and saving of mechanized mining to some extent. In this paper, the influence of uniaxial lateral stress on rock cuttability was investigated by an indentation experiment on cuboid rock using a conical pick, and a theoretical model was proposed to analyze the PCF and associated factors. The PCF, CW, and SE were used as indices to measure hard rock cuttability. The regression analyses show that rock cuttability presents as decreasing followed by increasing as uniaxial lateral stresses increases. The theoretical model was established by simplifying rock fragments into three-dimensional ellipse cones, and a formula was derived based on the elastic fracture mechanics theory. The error between the calculated and experimental values is 3.8%, which confirms the accuracy of the prediction formula. Finally, rock fragmentation by using conical picks was successfully applied on the field mining stope by inducing high geostresses to promote adjustments in stress and improve ore-rock cuttability. Highlights: The influences of uniaxial lateral stress on rock cuttability have been investigated. The peak cutting force, cutting work and specific energy can reflect the rock cuttability. A new theoretical model has been proposed to analyze the peak cutting force. The rock fragmentation using conical picks was successfully applied in deep hard rock mining.

Abstract: Numerical simulations were performed using a 2D particle flow code (PFC2D) to study the dynamic behavior of two deep buried tunnels subjected to blasting disturbance. Depending on the distances between the two tunnels and blasthole, the two tunnels were divided into adjacent tunnels and nonadjacent tunnels, respectively. The propagation characteristics of the blasting stress wave were first examined. The evolution of strain energy around the two deep buried tunnels was then evaluated. The results show that at different locations around the tunnels, the variation characteristics of strain energy are different. Moreover, in the region where the strain energy accumulates, the safety factor decreases, while in the region where the strain energy releases, the safety factor increases. The effects of tunnel spacing and tunnel shape on the nonadjacent tunnel were further investigated. For the nonadjacent tunnel, the safety factor at the left and right sidewalls is positively related to the tunnel spacing, while the safety factor at the roof tends to increase first and then decrease. When the tunnel shape subsequently changes from circular to U-shaped and rectangular, the safety at the right sidewall and roof show the trend of decreasing and increasing respectively, and the safety factor at the left sidewall decreases first and then increases. Article highlights:

The stress evolution characteristics are closely related to the location of surrounding rock.

The tunnel setting is closely related to the stability of non-adjacent tunnel.

The variation of strain energy is negatively correlated with the variation of the safety factor of tunnel.

Filled inclusions in rock discontinuities play a key role in the mechanical characteristics of the rock and thereby influence the stability of rock engineering. In this study, a series of impact tests were performed using a split Hopkinson pressure bar system with high-speed photography to investigate the effect of interlayer strength on the wave propagation and fracturing process in composite rock-mortar specimens. The results indicate that the transmission coefficient, nominal dynamic strength, interlayer closure, and specific normal stiffness generally increase linearly with increasing interlayer stiffness. The cement mortar layer can serve as a buffer during the deformation of composite specimens. The digital images show that tensile cracks are typically initiated at the rock-mortar interface, propagate along the loading direction, and eventually result in a tensile failure regardless of the interlayer properties. However, when a relatively weaker layer is sandwiched between the rock matrix, an increasing amount of cement mortar is violently ejected and slight slabbing occurs near the rock-mortar interface.

Abstract: The crustal structure of the northwestern sub-basin area of the South China Sea was modeled by inverting a wide-angle seismic survey line across the entire region and on both sides of its bounding continental margins. The survey line extended over 484 km. A total of 14 Ocean Bottom Seismometers (OBS) were deployed at intervals of 30 km to record air-gun array sources with a combined volume of 5160 in3. The crustal velocity structure of the northwestern sub-basin area was acquired through the integration of multi-channel seismic data. OBS data were processed and modeled initially using ray tracing inversion techniques. Results indicate that crustal thickness under the continental slope decreases from 21 to 11 km, crustal thickness of the northwestern sub-basin is 7.7 km, and the depth to the Moho ascends from 21 km under the upper continental slope to 11 km under the middle basin. The crust of the northwestern sub-basin is similar to that of the eastern sub-basin in its oceanic crustal structure. This structure has a thicker layer 1 (sedimentary layer) and a thinner layer 2. These characteristics are especially clear in the eastern sub-basin, which differs somewhat from typical oceanic crust. The tectonic geometry and velocity structure of the northwestern sub-basin and its margins comprise a symmetrical conjugate and indicate a pure shear mode with regard to the continental margin rifting mechanism. We did not find clear seismic signals from high velocity layers under the lower crust of the continental margin in the northern part of the northwestern sub-basin, which provides new evidence for the idea that the western part of the northern continental margin of the South China Sea constitutes non-volcanic crust. Because the seafloor spreading period of the northwestern sub-basin was short, layer 2 might have experienced asymmetrical basalt magma flows, which may have blurred the magnetic anomaly lineations of the northwestern sub-basin.

Rock drilling machine, INSTRON testing system, and SHPB device are updated to investigate the characteristics of rocks at great depth, with high loads from overburden, tectonic stresses and dynamic impacts due to blasting and boring. It is verified that these testing systems can be used to study the mechanical properties of rock material under coupled static and dynamic loading condition and give useful guidance for the deep mining and underground cavern excavation. Various tests to determine the rock strength, fragmentation behavior, and energy absorption were conducted using the updated testing systems. It is shown that under coupled static-dynamic loads, if the axial prestress is lower than its elastic limit, the rock strength is higher than the individual static or dynamic strength. At the same axial prestress, rock strength under coupled loads rises with the increasing strain rates. Under coupled static and dynamic loads, rock is observed to fail with tensile mode. While shear failure may exist if axial prestress is high enough. In addition, it is shown that the percentage of small particles increases with the increasing axial prestress and impact load based on the analysis of the particle-size distribution of fragments. It is also suggested that the energy absorption ratio of a specimen varies with coupled loads, and the maximum energy absorption ratio for a rock can be obtained with an appropriate combination of static and dynamic loads.

The Gutenberg‐Richter (G‐R) relationship has been regarded as the fundamental description of the size distribution from large‐scale‐earthquake to small‐scale‐laboratory rock ruptures. However, because deviation from the G‐R relationship has often been reported, especially when the amplitude distribution is used for bvalue estimation in rock acoustic emission testing, the effect of attenuation should be considered. Here, we perform a detailed analysis on the deviation of the size distribution from a G‐R law and discuss modification of the attenuation on the doubly truncated size distribution from a more general perspective. We find that the attenuation will not modify the size distribution, and the bvalue is theoretically verified to be unchanged within an interval specified by the minimum and maximum amount of attenuation. Based on these discussions, we propose a new bvalue estimation procedure for rock acoustic emission testing and apply it to a dilation rupturing test; the results confirm that bvalue depends on material heterogeneity and stress. The Gutenberg‐Richter relationship is an intrinsic characteristic of the frequency‐size distribution of earthquake and laboratory rock ruptures, indicating that the number of small‐scale ruptures is much greater than the number of large‐scale ones. Nevertheless, due to many reasons such as the statistical methods, inadequate data acquisition, and data truncation in cumulative frequency distribution, deviation from a Gutenberg‐Richter relationship of the frequency‐size distribution is inevitable in practice. In order to understand the deviation of the frequency‐size distribution in the laboratory rock fracture testing, we perform a detailed analysis of the statistical methods and the attenuation effect on the Gutenberg‐Richter relationship of the frequency‐size distribution and propose a new procedure for scaling parameter estimation of Gutenberg‐Richter relationship. This newly proposed procedure shows its reliability in a dilation rupturing test and can well ensure the robustness of the scaling parameter estimation. The inferred bvalue for acoustic emission apparent amplitude is equal to that at source irrespective of the form of attenuation lawThe finite size of the sample leads to a finite range of applicability of the Gutenberg‐Richter lawA new method is used to determine the finite range, and its application confirms that bdepends on material heterogeneity and stress

Under certain extreme conditions in rock engineering works, fast change in temperature in the load-bearing rocks can happen. Known as thermal shock (TS), such process involves rapid temperature rise or drop, which causes fracturing in the rock material and thus can pose as a threat to the stability of the rock structures. To investigate the influence of thermal shock caused by fast cooling on the mechanical property of rock, laboratory tests are performed on heated granite which are cooled with different methods, with the highest cooling rate reaching 167.4 °C/min. The dynamic loading tests are performed on the heated granite specimens utilizing the split Hopkinson pressure bar (SHPB) system. The test results show that the dynamic compressive strength drops with the increase in heating level or cooling rate. This pattern is explained by the nuclear magnetic resonance (NMR) test data that the pores inside the heated granite increase both in size and quantity as heating level or cooling rate rises. Damage patterns of the tested granite specimen fragments are analyzed based on the observation with scanning electron microscope (SEM), and the mechanisms of thermal shock in granite are also discussed.