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Acoustic emission/microseismic (AE/MS) source location plays a vital role in the safety monitoring of circular hole-contained structures (e.g., tunnels, pipelines, and pressure vessels). However, traditional methods may obtain a bad P-wave travel path and poor source location accuracy. To handle this, an analytical solution-based shortest P-wave travel path and a grid search-based AE/MS event source location method are proposed for circular hole-contained structures. Moreover, the objective function of AE/MS source location considers the P-wave arrival time system error. Both synthetic tests and pencil-lead break (PLB) application tests in 2D and 3D circular hole-contained structures show the outperformance of the proposed location methods. First, the analytical method obtains an equal or smaller P-wave travel distance than that of the Dijkstra and A* algorithms, which means the analytical method generates better theoretical data. Then, the average location errors of PLB events for the 2D circle-contained rectangle, 3D full circular hole-contained cuboid, and 3D part circular hole-contained cuboid are 0.57 cm, 1.87 cm, and 2.25 cm, respectively. These location results are better than results based on the shortest P-wave paths of the straight line, Dijkstra, and A* algorithms. Moreover, the proposed method solves that former analytical solutions do not consider the thickness of cylindrical shell structures. Highlights: An analytical solution-based shortest P-wave travel path has been proposed for circular hole-contained structures. The acoustic emission/microseismic source location objective function considers the P-wave arrival time system error. The average location errors of pencil-lead break events for circular hole-contained structures are within 2.25 cm. The analytical solution-based location method obtains better location results than methods based on the straight line, Dijkstra and A* algorithms. [ABSTRACT FROM AUTHOR]

In this study, mode I fracture tests on cracked straight-through Brazilian disc (CSTBD) and notched semi-circular bend (NSCB) shale specimens with different sizes were conducted to investigate the difference between maximum tangential stress fracture criterion and the size effect law (SEL) model in predicting apparent fracture toughness (Ka) of shale. In addition, the effects of specimen size and geometry on the Ka and the selection of fracture criterion on the prediction of the inherent fracture toughness (KIc) were also studied. The results show that the Ka increases with the increase of specimen size, and the difference between KIc of shale specimens with different sizes predicted by the fracture process zone length determined by the further improved maximum tangential stress (FIMTS) criterion is the smallest. For the prediction of Ka of NSCB specimen, the results predicted by the FIMTS criterion are the closest to the tested fracture toughness. However, the effect of SEL model applied to the prediction of Ka of NSCB specimens is poor. The effective establishment of SEL model requires high accuracy for test data, especially for the configuration with large variation of the dimensionless stress intensity factor (Y*) with normalized crack length (α). Highlights: Fracture toughness is closely related to the size and configuration of the specimen, which is ultimately attributed to the inconsistent tangential stress distribution at the crack tip. The further improved maximum tangential stress criterion has the best effect on apparent fracture toughness prediction, and higher-order terms of the Williams expansion should be considered in apparent fracture toughness prediction of small-size specimens. The fracture criterion for determining the fracture process zone length should be consistent with the prediction criterion of apparent fracture toughness. The effective establishment of size effect law requires high accuracy tested data, especially for the configuration with large variation of the dimensionless stress intensity factor with normalized crack length. [ABSTRACT FROM AUTHOR]

The effect of infilling on mechanical response and crack behavior of pre-stressed rock with double rectangular holes under dynamic load was investigated through a series of coupled static–dynamic loading tests on diorite specimens in three different filling states, i.e., no infilling, single infilling and double infilling, using a modified split Hopkinson pressure bar. The deformation, damage and fracture process of specimens were recorded and analyzed by low-speed and high-speed cameras with digital image correlation method. Test results reveal that under the same dynamic load, the strength of specimens increases first and then decreases with the increase of axial static pre-stress, and reaches the maximum under 25% UCS due to crack closure. The strengthening effect of double infilling on strength is much more significant than that of single infilling and increases with the pre-stress. Observations show that with the increase of pre-stress, the degrees of damage and strain localization of specimens increase, and the superficial damage on the free surface of sidewalls are more severe. The effect of infilling is significant on the crack initiation and propagation, especially on the inhibition of sidewall spalling, rock ejection and the axial displacement failure of rock septum between holes. With the increase of pre-stress, the failure pattern and the crack coalescence mode change from obvious tensile to shear, and the coalescence position changes from the inside to outside of the septum area. However, the change of filling state has little effect on the coalescence mode, only on the coalescence position. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A physical model test is carried out to simulate the blasting disturbance on the underground tunnel. The test including four blasting events with different blasting locations, where two blasting angles ( α ) and two blasting distances (D) are designed. The surface deformation characteristics of the physical model are observed by a DIC system. The deformation responses of four strain components during blasting are obtained. The influences of the blasting location on the surface deformation and failure characteristics are analyzed. The distributions of strain components εx, εy, εxy, and ε1 are obtained for different blasting locations. The different occurrences of failure are compared for cases where the blasthole is at varying locations from the tunnel. The results show that when the blasthole is far from the tunnel, the failure mainly occurs in the vicinity of the blasthole and the failure pattern of the physical model is conical. When the blasthole is close enough to the tunnel and is near the vertical wall, the surrounding rock fail in a V-shape. According to the failure patterns, the anti-disturbance ability of the arch is obviously stronger than that of the vertical wall. [ABSTRACT FROM AUTHOR]

Keywords: Preset angle shear; Direct shear; Triaxial compression; Cohesion; Internal friction angle; Shear strength parameters EN Preset angle shear Direct shear Triaxial compression Cohesion Internal friction angle Shear strength parameters 2505 2519 15 05/08/20 20200501 NES 200501 Introduction Estimation of shear strength parameters, which are generally known as the cohesion and internal friction angle ( I c i and I i ) in the Mohr-Coulomb (M-C) strength criterion, plays an essential role in rock mechanics and rock engineering (Zhao [34]; Kahraman et al. [14]; Alejano and Carranzatorres [1]; Gong et al. [7]). 6 Shear strength parameters of a marble and b red sandstone obtained by three different testing methods Comparison of Testing Methods for Rock Shear Strength Parameters Hereinbefore, data deviations were observed for marble and red sandstone under the three testing conditions. [Extracted from the article]

To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock. [ABSTRACT FROM AUTHOR]

To investigate the energy evolution characteristics of rock materials under uniaxial compression, the single-cyclic loading–unloading uniaxial compression tests of four rock materials (Qingshan granite, Yellow sandstone, Longdong limestone and Black sandstone) were conducted under five unloading stress levels. The stress–strain curves and failure characteristics of rock specimens under the single-cyclic loading–unloading uniaxial compression tests basically corresponded with those of under uniaxial compression, which indicates that single-cyclic loading–unloading has minimal effects on the variations in the loading–deformation response of rocks. The input energy density, elastic energy density and dissipated energy density of four rocks under five unloading stress levels were calculated using the graphical integration method, and variation characteristics of those three energy density parameters with different unloading stress levels were explored. The results show that all three energy density parameters above increased nonlinearly with increasing unloading stress level as quadratic polynomial functions. Meanwhile, both the elastic and dissipated energy density increased linearly when the input energy density increased, and the linear energy storage and dissipation laws for rock materials were observed. Furthermore, a linear relationship between the dissipated and elastic energy density was also proposed. Using the linear energy storage or dissipation law, the elastic and dissipated energy density at any stress levels can be calculated, and the internal elastic (or dissipated) energy density at peak compressive strength (the peak elastic and dissipated energy density for short) can be obtained. The ratio of the elastic energy density to dissipated energy density with increasing input energy density was investigated using a new method, and the results show that this ratio tends to be constant at the peak compressive strength of rock specimens. [ABSTRACT FROM AUTHOR]

It is essential to investigate focal mechanisms of induced seismicity for understanding the rock fracturing, the failure mode, and the hazard evolution in underground mines. But the conventional methods using empiricism to infer the source mechanisms usually lead to ambiguous results for individual events. An optimized moment tensor inversion method using full waveforms was employed to quantitatively determine the rock fracturing orientation and the type of rupture process. Source parameters including the scalar moment, the moment magnitude, the full moment tensor, and the fault plane solutions were resolved of a seismic sequence in fault zones. Results show that the shear failure events in the fault F1 vicinities have similar focal mechanisms and suggest that the fault F1 is a reverse fault. The resolved strikes and dips are basically constant with the orientation of the fault. The events in the fault F2 vicinities are mainly dominated by shear-tensional failure. But the shear events experienced shear rupture and crack opening simultaneously, resulting in slippages not along the actual fault plane. There are more events in the fault F3 area which are characterized by complicated focal mechanisms. Three of the non-shear events are dominated by compressional failure and related to rock collapse, while the other non-shear events are dominated by tensional failure and related to crack opening. The shear dominated events experienced dual effects of shear failure and compression failure. The resolved fault plane solutions cannot reflect the actual geometry of the fault. It is proved that the moment tensor inversion is able to quantitatively analyze the focal mechanism of mining-induced seismicity in fault zones and it provides beneficial understandings of mining-induced fault slips. [ABSTRACT FROM AUTHOR]

Dynamic impact tests were conducted on rectangular marble specimens with flaws to investigate the effect of the flaw angles and ligament angles on the rock dynamic mechanical properties, fracturing behavior, and energy evolution characteristics. A 75-mm diameter split Hopkinson pressure bar (SHPB) device was used in the tests. The experimental results show that the flaw geometries have a strong effect on the dynamic mechanical properties and fracturing behavior of rocks. In general, the effects of the flaw angle and flaw number on the dynamic strength are more significant than the effect of the ligament angle. A high-speed camera was used to monitor and record the crack initiation and coalescence and the failure processes of the marble specimens in real-time. The crack propagation can be divided into two major stages, i.e., the formation of white patches and the generation of macrocracks. The white patches usually appear prior to the peak stress of the specimen. The shear cracks usually initiate at or near the flaw tips and propagate to form X or half-X shear belts that dominate the macroscopic failure under impact loading. Nine types of crack coalescence behaviors are identified for the marble specimens with double flaws, and the macro failure modes of the specimens is usually combined by some of them. The energy absorption properties of the marble specimen are also significantly affected by the pre-existing flaws. The energy absorption ratio can be maximized by setting a certain number of pre-existing flaws with an appropriate inclination angle and a proper ligament angle to effectively improve the fragmentation efficiency of rocks. [ABSTRACT FROM AUTHOR]

To evaluate tunnel ground vibrations induced by fault-slip in underground mine, it is crucial to resolve the peak ground velocity (PGV) and the peak ground acceleration (PGA). To quantify PGV and PGA in three-dimensional space, the forces acting on the fault and the seismic wave radiation pattern were investigated. A qualitative method for ground vibration of tunnels induced by fault-slip in underground mine was proposed and applied to the Yongshaba Phosphate Mine, China. Firstly, the moment tensor of the significant fault-slip event was obtained using a full waveform source inversion method based on the recorded seismograms. It describes the forces acting on the fault and the seismic wave radiation pattern. Then, the inverted source was used as a reference to evaluate the ground vibrations under possible larger seismic magnitudes and probable slip models. The vibrations along the tunnels resulted from different slip models were calculated using the representation theorem by the convolution of the Green's function and the corresponding moment tensor. The velocity series and the acceleration series were obtained. Results show that the spatial distribution of the ground motions are closely related to sensor-source distance, the sensor-source azimuth, as well as the strike, dip, and rake configuration of the slip model. The seismic magnitude has an exponential effect on the distribution of the PGVs and PGAs. The PGV and PGA have similar spatial distribution patterns. The PGV values of the north and vertical components show nearly 5 times higher than the east component. The vertical vibrations affect the widest area. The ground support principles were suggested considering the parameters including the PGVs, the PGAs, the rock mass rating, and the dynamic loads. [ABSTRACT FROM AUTHOR]

The Earth's deep rock mass is subjected to various complex temperature and stress perturbations. To study the effect of temperature and dynamic disturbance on the damage mechanical properties of these rocks, a series of laboratory tests was carried out by means of a modified split Hopkinson pressure bar system. Specimens were heat treated from room temperature up to a maximum of 800 °C and then cooled to room temperature. During repeated loading tests, the dynamic incident energy was kept constant in each cycle. For samples under the same treated temperature, the dynamic strength and deformation capacity degraded gradually with increasing impact number. Furthermore, rock strength decreased with increasing treated temperature, with the number of impacts before failure reduced accordingly. Since damage can be initiated by thermal treatment and then aggravated by dynamic disturbance, according to the observed decrease in rock strength and increase in strain rate under the different temperature conditions, a temperature of 400 °C was found be a significant failure threshold. The maximum strain was employed to describe damage evolution during cyclic impact loading, indicating that fatigue damage is gradually accumulated and maintains a three-segment growth with an increase in repeated impacts. Under the coupled effect of temperature and cyclic impact loading, specimens exhibited two different failure modes: split tensile failure and unloading failure. The micro-properties of fracture morphology in granite after different temperature and repeated impact were also discussed in detail. [ABSTRACT FROM AUTHOR]

Induced seismicity in underground mining zones reflects inner changes of the rock structure caused by human activities, thus providing potential information for hazard assessment. Precise hypocenter parameters and magnitudes of the induced micro-earthquakes as well as their spatial distributions are essential for understanding the seismicity. We first accurately relocate 834 micro-earthquakes recorded by a 28-sensor micro-seismic monitoring system from an underground phosphate mine, using an iterative pseudo-bending ray-tracing method and a 3-D velocity structure obtained by seismic tomography. We then determine the local magnitudes of the events by a well-recalibrated formula, and finally analyze the seismicity for different parts of the mine divided by the fuzzy c-means clustering method according to the spatial distribution of the micro-earthquakes. The bimodal distribution of the recalibrated local magnitudes indicates two fundamentally different processes of rock failure may exist in the mine. They are possibly fracture-dominated rupture and friction-dominated slip, i.e., tensile failure and shear failure of the surrounding rock. The clustering result shows that mining practices cause very different seismic features in different parts of the mine. Furthermore, we find that most of the large events are located in low-V zones, and that events occurred in the lower velocity zones tend to have a correspondingly larger magnitude, as revealed by the seismicity analysis and seismic tomography. [ABSTRACT FROM AUTHOR]

To investigate the mesodamage evolution of rock specimens containing a pre-existing opening under integrated static and dynamic loading, the nuclear magnetic resonance technique was utilised in this study. The changes in the transverse surface relaxation time (T2) distribution, peak area, and porosity of the specimens prior to and due to varying pre-stresses under an identical dynamic loading were systematically analysed. The experimental results show that as the pre-stress level increased the size and the amount of the pores, the peak area and the porosity increase, indicating that more damage is induced under a high pre-stress. Under identical loading conditions, the specimens having 30° and 60° square openings are more susceptible to damage than those having a 0° square opening. However, compared with the specimens containing a circular opening, the specimens with a square opening are more seriously damaged. The hoop stress concentration surrounding the opening induced by the pre-stress and subsequent dynamic loading was further demonstrated. The result indicates that the initiation of rock fracture occurs in the maximum hoop stress concentration area. [ABSTRACT FROM AUTHOR]

High geostresses and stress gradients are the predominant stress conditions in deep excavation-disturbed rock masses. The aim of this study is to determine the triaxial compressive strength properties of hollow cylindrical granite specimens under a radially non-uniform confining stress field with different radial stress gradients determined by coupled external and internal confining stresses. Triaxial compression testing of hollow cylindrical rock specimens was performed to investigate the influence of the radial stress gradient, external confining stress and specimen length-to-diameter (L/D) ratio on the triaxial compressive strength. The experimental results and regressed failure criteria indicate that the triaxial compressive strengths of the hollow cylindrical granite specimens increase with the external confining stresses, but decrease with an increase in the radial stress gradients. The calculated goodness of fit (R2) and root-mean-squared error suggest that the nonlinear failure criterion based on the Hoek-Brown model is more accurate than the linear failure criterion based on the Mohr-Coulomb model for determining the influences of the external confining stress and radial stress gradient on the triaxial compressive strength. In addition, the triaxial compressive strength increases with a decreasing L/D ratio due to the strengthening end effect of the hollow cylindrical granite specimens and the change in the failure pattern of these specimens from shear to slabbing. [ABSTRACT FROM AUTHOR]

The failure modes and peak unloading strength of a typical hard rock, Miluo granite, with particular attention to the sample height-to-width ratio (between 2 and 0.5), and the intermediate principal stress was investigated using a true-triaxial test system. The experimental results indicate that both sample height-to-width ratios and intermediate principal stress have an impact on the failure modes, peak strength and severity of rockburst in hard rock under true-triaxial unloading conditions. For longer rectangular specimens, the transition of failure mode from shear to slabbing requires higher intermediate principal stress. With the decrease in sample height-to-width ratios, slabbing failure is more likely to occur under the condition of lower intermediate principal stress. For same intermediate principal stress, the peak unloading strength monotonically increases with the decrease in sample height-to-width. However, the peak unloading strength as functions of intermediate principal stress for different types of rock samples (with sample height-to-width ratio of 2, 1 and 0.5) all present the pattern of initial increase, followed by a subsequent decrease. The curves fitted to octahedral shear stress as a function of mean effective stress also validate the applicability of the Mogi-Coulomb failure criterion for all considered rock sizes under true-triaxial unloading conditions, and the corresponding cohesion C and internal friction angle φ are calculated. The severity of strainburst of granite depends on the sample height-to-width ratios and intermediate principal stress. Therefore, different supporting strategies are recommended in deep tunneling projects and mining activities. Moreover, the comparison of test results of different σ2/σ3 also reveals the little influence of minimum principal stress on failure characteristics of granite during the true-triaxial unloading process. [ABSTRACT FROM AUTHOR]

To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks. [ABSTRACT FROM AUTHOR]

High geostress is a prominent condition in deep excavations and affects the cuttability of deep hard rock. This study aims to determine the influence of confining stress on hard rock fragmentation as applied by a conical pick. Using a true triaxial test apparatus, static and coupled static and dynamic loadings from pick forces were applied to end faces of cubic rock specimens to break them under biaxial, uniaxial and stress-free confining stress conditions. The cuttability indices (peak pick force, insertion depth and disturbance duration), failure patterns and fragment sizes were measured and compared to estimate the effects of confining stress. The results show that the rock cuttabilities decreased in order from rock breakages under stress-free conditions to uniaxial confining stress and then to biaxial confining stress. Under biaxial confining stress, only flake-shaped fragments were stripped from the rock surfaces under the requirements of large pick forces or disturbance durations. As the level of uniaxial confining stress increased, the peak pick force and the insertion depth initially increased and then decreased, and the failure patterns varied from splitting to partial splitting and then to rock bursts with decreasing average fragment sizes. Rock bursts will occur under elastic compression via ultra-high uniaxial confining stresses. There are two critical uniaxial confining stress levels, namely stress values at which peak pick forces begin to decrease and improve rock cuttability, and those at which rock bursts initially occur and cutting safety decreases. In particular, hard rock is easiest to split safely and efficiently under stress-free conditions. Moreover, coupled static preloading and dynamic disturbance can increase the efficiency of rock fragmentation with increasing preloading levels and disturbance amplitudes. The concluding remarks confirm hard rock cuttability using conical pick, which can improve the applicability of mechanical excavation in deep hard rock masses. [ABSTRACT FROM AUTHOR]

Dynamic impact tests are performed on prismatic marble specimens containing a single flaw using a modified split-Hopkinson pressure bar device. The effects of pre-existing flaws with different flaw angles and lengths on the dynamic mechanical properties are analyzed. The results demonstrate that the dynamic strength of marble is influenced by the flaw geometry. The dynamic fracturing process of flawed specimens is monitored and characterized with the aid of a high-speed camera. Cracking of marble specimens with a single pre-existing flaw under impact loading is analyzed based on experimental investigations. Cracking involves two major stages: formation of white patches and development of macrocracks. Six typical crack types are identified on the basis of their trajectories and initiation mechanisms. The presence of an artificial flaw may change the failure mode of marble from splitting-dominated for an intact specimen to shear-dominated for a flawed specimen under dynamic loading. Nevertheless, the geometry of the flaws appears to have a slight influence on the failure modes of flawed specimens under impact loading. [ABSTRACT FROM AUTHOR]

The article presents an investigation on the impact of hole diameter of the ring-shaped specimens including sandstone on the dynamic mechanical properties of the specimens using the dynamic Brazilian test. It is determined that the modified split Hopkinson pressure bar (SHPB) is used to apply dynamic loading and the deformation behaviour of loaded specimen is investigated upon the digital image correlation (DIC) technique based on the image recorded using the high-speed (HS) camera.

To understand the effects of increasing temperature and loading rate on the flexural tensile strength of Laurentian granite, dynamic flexural tensile strength experiments were carried out by means of a semi-circular bend specimen with a modified split Hopkinson pressure bar system. The tests were performed at different loading rates, specimens were treated from room temperature up to 850 °C, and a high-speed camera was utilized to monitor the failure process of the specimen. For samples in the same temperature group, a loading rate dependence of the flexural tensile strength was observed; it increased consistently with the increase of loading rate. Temperature effects on rock mechanical properties were investigated from the microscopic viewpoint, and the dynamic flexural tensile strength decreased with the treatment temperature. A formula relating dynamic flexural tensile strength to loading rate and temperature is presented to quantify the results. It was found that the change regulation of the dynamic flexural tensile strength of rock is very similar to that of its crack growth along with the increase of loading rate, which indicates that the essence of rock failure is the initiation and propagation of the internal cracks. Compared with our earlier work on dynamic tensile tests using the Brazilian test, it was observed that the flexural tensile strength is higher than the tensile strength. Non-local failure theory can be adopted to explain this discrepancy at low temperature conditions, but it is no longer effective at high temperatures. Under high loading rates, rock failure is initiated at the centre of the half circular disc, and finally it is separated completely into two equal parts. [ABSTRACT FROM AUTHOR]

Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens. [ABSTRACT FROM AUTHOR]

Seismic events and blasts generate seismic waveforms that have different characteristics. The challenge to confidently differentiate these two signatures is complex and requires the integration of physical and statistical techniques. In this paper, the different characteristics of blasts and seismic events were investigated by comparing probability density distributions of different parameters. Five typical parameters of blasts and events and the probability density functions of blast time, as well as probability density functions of origin time difference for neighbouring blasts were extracted as discriminant indicators. The Fisher classifier, naive Bayesian classifier and logistic regression were used to establish discriminators. Databases from three Australian and Canadian mines were established for training, calibrating and testing the discriminant models. The classification performances and discriminant precision of the three statistical techniques were discussed and compared. The proposed discriminators have explicit and simple functions which can be easily used by workers in mines or researchers. Back-test, applied results, cross-validated results and analysis of receiver operating characteristic curves in different mines have shown that the discriminator for one of the mines has a reasonably good discriminating performance. [ABSTRACT FROM AUTHOR]

True triaxial tests have been carried out on granite, sandstone and cement mortar using cubic specimens with the process of unloading the minor principal stress. The strengths and failure modes of the three rock materials are studied in the processes of unloading σ and loading σ by the newly developed true triaxial test system under different σ, aiming to study the mechanical responses of the rock in underground excavation at depth. It shows that the rock strength increases with the raising of the intermediate principal stress σ when σ is unloaded to zero. The true triaxial strength criterion by the power-law relationship can be used to fit the testing data. The 'best-fitting' material parameters A and n ( A > 1.4 and n < 1.0) are almost located in the same range as expected by Al-Ajmi and Zimmerman (Int J Rock Mech Min Sci 563 42(3):431-439, 2005). It indicates that the end effect caused by the height-to-width ratio of the cubic specimens will not significantly affect the testing results under true triaxial tests. Both the strength and failure modes of cubic rock specimens under true triaxial unloading condition are affected by the intermediate principal stress. When σ increases to a critical value for the strong and hard rocks (R4, R5 and R6), the rock failure mode may change from shear to slabbing. However, for medium strong and weak rocks (R3 and R2), even with a relatively high intermediate principal stress, they tend to fail in shear after a large amount of plastic deformation. The maximum extension strain criterion Stacey (Int J Rock Mech Min Sci Geomech Abstr 651 18(6):469-474, 1981) can be used to explain the change of failure mode from shear to slabbing for strong and hard rocks under true triaxial unloading test condition. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on several physical properties and the tensile strength of Laurentian granite (LG) are measured in this study. Brazilian disc LG specimens are treated at temperatures of up to 850 °C. The physical properties such as grain density, relative volume change per degree, and P-wave velocity are investigated under the effect of heat treatment. The results indicate that both the density and the P-wave velocity decrease with the increase in heating temperature. However, the relative volume change per degree is not sensitive below 450 °C, while a remarkable increase appears from 450 to 850 °C. All cases are explained by the increase in both number and width of the thermally induced microcracks with the heating temperature. Brazilian tests are carried out statically with an MTS hydraulic servo-control testing system and dynamically with a modified split Hopkinson pressure bar (SHPB) system to measure both static and dynamic tensile strength of LG. The relationship between the tensile strength and treatment temperatures shows that static tensile strength decreases with temperature while the dynamic tensile strength first increases and then decreases with a linear increase in the loading rate. However, the increase in dynamic tensile strength with treatment temperatures from 25 to 100 °C is due to slight dilation of the grain boundaries as the initial thermal action, which leads to compaction of rock. When the treatment temperature rises above 450 °C, the quartz phase transition results in increased size of microcracks due to the differential expansion between the quartz grains and other minerals, which is the main cause of the sharp reduction in tensile strength. [ABSTRACT FROM AUTHOR]

Open joints have significantly different effects on stress wave propagation across them as compared to closed and filled joints. In the present study, a theoretical model is developed based on the analysis of interaction process between stress waves and a single open joint. The analytical solutions to the energy transmission coefficient are mathematically derived for stress waves across an open joint. Parametric studies are conducted to evaluate the effects of various parameters on the energy transmission coefficient. It is found that the energy transmission coefficient follows a similar trend for all types of stress waves, but stress waves with different waveforms have different values. The energy transmission coefficient increases with the increase in the wave amplitude and duration, but decreases with the gap width. It also increases at the outset, and then decreases gradually with the increase in the incident angle for rectangle, symmetric triangle and ascending triangle waves, but it decreases with the incident angle for sine and descending triangle waves. Furthermore, different-shaped stress waves have different critical gap widths for wave transmission. The optimal incident angle and the critical gap width increase as the wave duration increases, but the critical stress amplitude decreases. [ABSTRACT FROM AUTHOR]

A split Hopkinson pressure bar (SHPB) system with a special shape striker has been suggested as the test method by the International Society for Rock Mechanics (ISRM) to determine the dynamic characteristics of rock materials. In order to further verify this testing technique and microscopically reveal the dynamic responses of specimens in SHPB tests, a numerical SHPB test system was established based on particle flow code (PFC). Numerical dynamic tests under different impact velocities were conducted. Investigation of the stresses at the ends of a specimen showed that the specimen could reach stress equilibrium after several wave reverberations, and this balance could be maintained well for a certain time period after the peak stress. In addition, analyses of the reflected waves showed that there was a clear relationship between the variation of the reflected wave and the stress equilibrium state in the specimen, and the turning point of the reflected wave corresponded well with the peak stress in the specimen. Furthermore, the reflected waves can be classified into three types according to their patterns. Under certain impact velocities, the specimen deforms at a constant strain rate during the whole loading process. Finally, the influence of the micro-strength ratio ( $${{\tau_{\text{c}} } \mathord{\left/ {\vphantom {{\tau_{\text{c}} } {\sigma_{\text{c}} }}} \right. \kern-0pt} {\sigma_{\text{c}} }}$$ ) and distribution pattern on the dynamic increase factor (DIF) of the strength DIF were studied, and the lateral inertia confinement and heterogeneity were found to be two important factors causing the strain rate effect for rock materials. [ABSTRACT FROM AUTHOR]

Rocks in underground projects at great depth, which are under high static stresses, may be subjected to dynamic disturbance at the same time. In our previous work (Li et al. Int J Rock Mech Min Sci 45(5):739-748, ), the dynamic compressive behaviour of pre-stressed rocks was investigated using coupled-load equipment. The current work is devoted to the investigation of the dynamic tensile behaviour of granite rocks under coupled loads using the Brazilian disc (BD) method with the aid of a high-speed camera. Through wave analyses, stress measurements and crack photography, the fundamental problems of BD tests, such as stress equilibrium and crack initiation, were investigated by the consideration of different loading stresses with abruptly or slowly rising stress waves. The specially shaped striker method was used for the coupled-load test; this generates a slowly rising stress wave, which allows gradual stress accumulation in the specimen, whilst maintaining the load at both ends of the specimen in an equilibrium state. The test results showed that the tensile strength of the granite under coupled loads decreases with increases in the static pre-stresses, which might lead to modifications of the blasting design or support design in deep underground projects. Furthermore, the failure patterns of specimens under coupled loads have been investigated. [ABSTRACT FROM AUTHOR]

The effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) was investigated in this work. Notched semi-circular bend (NSCB) LG specimens are heat treated at temperatures up to 850 °C. The micro-cracks in the rock samples induced by thermal treatment are examined by scanning electron microscope (SEM). The microscopic observations are consistent with the subsequent P-wave velocity measurements, which shows that the P-wave velocity decreases with the treatment temperature monotonically when the temperature is higher than 250 °C. Dynamic fracture toughness measurements are then carried out on these samples with the dynamic load exerted by a modified split Hopkinson pressure bar (SHPB) system. The relationship between fracture toughness and treatment temperature is investigated. Experimental results show that fracture toughness increases with the loading rate but decreases with the treatment temperature. However, when the heating temperature is below 250 °C and above 450 °C, the dependence of dynamic fracture toughness on the temperature is different from other temperatures, which can be explained by the physical processes at the microscopic level of the rock due to heating. At treatment temperatures below 250 °C, the thermal expansion of grains leads to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weakness such as grain boundaries and phase transition of silicon are depleted, and as a result the decrease in fracture toughness is not as significant as other treatment temperature ranges. [ABSTRACT FROM AUTHOR]

large number of tests have recently been conducted with the Split Hopkinson pressure bar (SHPB) method to determine the characteristics of rock dynamics. However, it is still impossible to get test results at a perfect constant strain rate from this set-up owing to the rate dependency of rock materials. For instance in most cases, dynamic behavior of rock can only be described with an average strain rate. The results from these methods, including rich strain rate information, frequently tend to be inexplicable or self-contradictory. The obtained stress-strain curves can then never be directly treated as constitutive curves as in static tests. In this paper, the reasons behind the controversial stress-strain results with current methods are analyzed. In addition, the requirement for the rock specimen to deform at a constant strain rate is demonstrated after theoretical analysis of correlations among specimen, deforming stress, incident stress, reflected stress and transmitted stress. With test results from SHPB by pulse shaper and special shape striker methods, the requirement is verified. Finally, the method of 3D scattergram considering stress-strain-strain rate simultaneously is brought up to get constitutive relationships of rate-dependent rock. The new method gives reasonable predictions for constitutive relationships of rock at different strain rates. At the same time, the new method has fewer requirements and has a wider application scope for SHPB tests. [ABSTRACT FROM AUTHOR]

The blast pressure acting on the borehole wall is the basis of the dynamic analysis of rock blasting. The history of blast-induced borehole wall pressure (BWP) is difficult to measure owing to the complex interaction between the blasting load and the borehole wall, and the limitations of the general test system. In this work, the complete history of BWP under air-deck charge was measured for the first time, and the stress-response process was analyzed. PVDF (polyvinylidene fluoride) gages and a charge mode for the test circuit were employed. To improve the testing accuracy, the matching relationships among the parallel capacitor, the equivalent capacitance of PVDF gages, and the parasitic capacitance of the coaxial cable were effectively addressed. The experimental data within the span of 0–0.65 m from the explosive cartridge center show that there are two obvious peaks in the complete history of BWP, the first is a μs-level air shockwave load, followed by a ms-level explosion gas load. The peak of BWP is dominated by the peak of explosion gas (Pg) within the scope of about 0–0.35 m from the explosive cartridge center, while is mainly determined by the peak of air shockwave (Ps) in a farther range. The specific impulse distribution suggests that the explosion energy is mainly consumed in the area within the range of about 0–0.25 m from the explosive cartridge center. The strain rate in the borehole wall induced by air shockwave demonstrates a dynamic response (range of 131.9 ~ 2922.6 s−1), while indicating a quasi-dynamic loading (range of 0.1 ~ 2.1 s−1) when induced by the explosion gas. The findings contribute to expanding the understanding of the stress characteristics of the air shock wave and explosion gas and provide accurate initial loads for numerical simulation related to rock blasting of non-fluid–solid coupling. [ABSTRACT FROM AUTHOR]