Rockburst Tests Research Articles

Study on main Frequency precursor characteristics of Acoustic Emission from Deep buried Dali Rock explosion

Rock burst disaster events have become one of the most common disasters in the field of underground engineering, and it is of great practical significance to study the prediction and early warning signs of rock burst. Acoustic emission waveforms generated during a rock burst event, which are often characterized by parameter analysis, that analysis of the waveform spectrum can reflect the characteristics of rock damage and failure. Based on a single-axis strain-type rock burst test of deep buried marble, characteristics of the frequency spectrum, as well as the time-domain dominant frequency of acoustic emissions during the entire rock burst process, were analyzed. We find that the acoustic emission spectrum characteristics also have their own characteristics at different stages of rock burst. In general, The main frequency amplitude value presents an increasing trend, the peak shape of the acoustic emission spectrum shifts between unimodal and multimodal, and the secondary frequency typically appears in the plastic deformation stage. In the high-frequency domain (more than 200 kHz) of the initial loading of stress redistribution, the elastic phase of the energy build-up shows a relatively quiet period, the main frequency distribution of plastic deformation stage shows increasing tendency. Before and after the rock burst event, the dense area of the main frequency migrates from the high-frequency domain to the low-frequency domain, the main frequency in the high frequency field appears dense distribution on the eve of rock burst. These changes were distinguished as characteristic criteria for impending rock burst and can therefore be used for the prediction and early warning of rock burst events.

Strainburst characteristics under bolt support conditions: an experimental study

Strainburst, defined as a burst that occurs on the periphery of tunnels, is always associated with intact and hard brittle rocks and high geo-stress. In practical engineering, the prevention of strainburst by bolting does not achieve the desired effects. Deep insight into the strainburst process and mechanisms under bolt support conditions is necessary to ensure safe underground construction. In this study, strainburst characteristics under bolt support conditions were investigated using a true-triaxial rockburst testing system, which was equipped with an acoustic emission monitoring system. High-speed cameras were also used to capture the ejection failure process. Two indicators, namely kinetic energy and maximum decibel, were used to comprehensively evaluate the rockburst intensity. In addition, characteristic stresses, failure mode and cracking mechanisms under different bolt spacing conditions were investigated. The test results demonstrated that rockburst is greatly affected by the bolt arrangement. Rockburst changes from the way occurring in the form of a local failure in weak area to the way striking the bolt position and even the overall free face with the decrease in bolt spacing. The use of the bolt increases the rockburst intensity, but the intensity decreases as the bolt spacing decreases. Characteristic stresses including crack initiation stress, crack damage stress and peak strength increase monotonously with the use of bolt and the decreasing bolt spacing. During the strainburst, tensile failure dominates the cracking process regardless of bolt spacing; however, with the use of bolt and decrease in bolt spacing, tensile splitting near the free face become less obvious and the proportion of shear failure gradually increased.

Strainburst process of marble in tunnel-excavation-induced stress path considering intermediate principal stress

Strainburst is one type of rockburst that generally occurs in deep tunnel. In this study, the strainburst behaviors of marble specimens were investigated under tunnel-excavation-induced stress condition, and two stress paths were designed, a commonly used stress path in true triaxial unloading rockburst tests and a new test path in which the intermediate principal stress was varied. During the tests, a high-speed camera was used to record the strainburst process, and an acoustic emission (AE) monitoring system was used to monitor the AE characteristics of failure. In these two stress paths, all the marble specimens exhibited strainbursts; however, when the intermediate principal stress was varied, the rockburst became more violent. The obtained results indicate that the intermediate principal stress has a significant effect on rockburst behavior of marble. Under a higher intermediate principal stress before the unloading, more elastic strain energy was accumulated in the specimen, and the cumulative AE energy was higher in the rockburst-induced failure, i.e., more elastic strain energy was released during the failure. Therefore, more violent failure was observed: more rock fragments with a higher mass and larger size were ejected outward.

Experimental study on energy dissipation of fragments during rockburst

In this study, rectangular prismatic coarse-grain granites were analyzed through an application of a modified true-triaxial rockburst testing system. Various loading rates were considered in a process in which one face was kept free and loading conducted on the other five faces. Using a proposed surface energy per unit area measurement method, the energy dissipation due to the formation of rock fragments during the rockburst process was quantitatively analyzed. The experimental results show that rockburst occurrence depends on several conditions, including — specifically the tangential loading rate exceeding a certain threshold, the presence of considerable amounts of stored strain energy, the dissipation of energy through rock splitting on the free face, and the shear failure in the potential rockburst pit. With increases in the loading rate from 0.5 to 4.0 MPa/s, the fragmentation and energy dissipation of fragments decline linearly. Lamellar coarse fragments are found to be primarily induced by tension failure from the rockburst pit surface. Blocky medium, fine fragments and white powdery tiny particles are mainly induced by shear failure from the rockburst pit interior. Under various loading rates, the dissipated energy of fragments induced by shear is more than 90% of the total dissipated energy.

Experimental study of rockburst under true-triaxial gradient loading conditions

Due to the underground openings, the tangentially concentrated stress of the tunnel remains larger at excavation boundary and decreases toward the interior of the surrounding rock with a certain gradient. In order to study the effect of different gradient stress on rockburst, the true-triaxial gradient and hydraulic-pneumatic combined test apparatus were carried out to simulate the rockburst processes. Under the different gradient stress conditions, the rock-like specimen (gypsum) was tested independently through three principal stress directions loading--fast unloading of single surface--top gradient and hydraulic-pneumatic combined loading, which systematically analyzed the macro-mesoscopic damage phenomena, force characteristics and acoustic emission (AE) signals of the specimen during rockburst. The experimental results indicated that the rockburst test under the gradient and hydraulic-pneumatic combined loading conditions could perfectly reflect the rockburst processes and their stress characteristics; Relatively high stress loading could cause specimen failure, but could not determine its mode. The rockburst under the action of gradient stress suggested that the failure mode of specimen mainly depended on the stress gradient. When the stress gradient was lower, progressive and static spalling failure occured and the rockburst grades were relatively slight. On the other hand, shear fractures occurred in rockbursts accounted for increasingly large proportion as the stress gradient increased and the rockburst occurred more intensely and suddenly, the progressive failure process became unconspicuous, and the rockburst grades were moderate or even stronger.

Experimental Study of Influence of Support Failures on Rockbursts under True-Triaxial Condition

Supports can effectively reinforce the surrounding rock after excavation in underground engineering. However, a support failure may cause an extremely intense rockburst. Hence, the influences of support failures, including support forces, support failure timings, and support failure rates, on rockbursts were systematically investigated in the present study. Unloading tests on rock specimens, using a true-triaxial rockburst test system, were performed to simulate rockbursts induced by support failure. The experimental results indicate that increasing support forces increased the prepeak accumulated elastic strain energy, the kinetic energy of the ejection fragment, and the ratio between the kinetic energy and release strain energy, whereas the damage to the rock specimens declined. During the testing process, the longer it took to unload the minimum stresses was, which means that the later the support failed, the greater the prepeak accumulated elastic strain energy was, the kinetic energies of the ejection fragments were, and the ratio of the kinetic energy and release strain energy was. Furthermore, as the support failure rate incremented, the kinetic energies of the ejection fragments of the rockbursts linearly increased, the failure mode of the rock changed from static failure to dynamic rockbursts, and the intensities of the rockbursts increased.

Influence of deviatoric stress on rockburst occurrence: An experimental study

As mining delves deeper into the crust, it is necessary to investigate the complex rock responses associated with higher stress gradients. Therefore, it is essential to better understand the mechanisms associated with the rockburst phenomenon. However, due to the large-scale and difficult monitoring of real mining excavations, laboratory scale tests must be utilised to determine the conditions conducive to burst. To this end, this research focuses on the implementation of a new rockburst testing apparatus to replicate the stress conditions of a rock mass excavation at the time of bursting. This apparatus allows the determination of rockburst stresses and a relationship between deviatoric stress and in-situ pressure/depth. Using this relationship it is then possible to estimate the standardised stress levels for a certain rock type which might lead to rockburst occurrence. Furthermore, it is demonstrated that with increasing in-situ pressure, the likelihood (measured as a lower differential stress) and the extent (indicated by the increasing range of deviatoric stress) of rockburst increases. These findings provide valuable information about the conditions necessary for bursting in deep mining.

Rockburst mechanism research and its control

A new rockburst classification, innovative works in developing a “strainburst test machine” and an “impact-induced rockburst test machine” that can reproduce rockbursts in laboratory were researched. New concepts were proposed regarding the stress paths that take into account both the static and dynamic stresses analogous to that at excavation boundaries for generating artificially-induced strainburst and impact-induced rockburst. As an important method for rockburst control, a novel energy-absorbing bolt was developed, which has a constant-resistance under both static and impact loadings and a large-elongation capacity for containing large deformations of rock masses under burst-prone conditions.

True Triaxial Experimental Study of Rockbursts Induced By Ramp and Cyclic Dynamic Disturbances

A modified rockburst testing system was utilized to reproduce rockbursts induced by ramp and cyclic dynamic disturbances with a low-intermediate strain rate of 2 × 10−3–5 × 10−3 s−1 in the laboratory. The experimental results show that both the ramp and cyclic dynamic disturbances play a significant role in inducing rockbursts. In the tests of rockbursts induced by a ramp dynamic disturbance, as the static stress before the dynamic disturbance increases, both the strength of specimens and the kinetic energy of the ejected fragments first increase and then decrease. In the tests of rockbursts induced by a cyclic dynamic disturbance, there exists a rockburst threshold of the static stress and the dynamic disturbance amplitude, and the kinetic energy of the ejected fragments first increases and then decreases as the cyclic dynamic disturbance frequency increases. The main differences between rockbursts induced by ramp dynamic disturbances and those induced by cyclic dynamic disturbances are as follows: the rockburst development process of the former is characterized by an impact failure feature, while that of the latter is characterized by a fatigue failure feature; the damage evolution curve of the specimen of the former has a leap-developing form with a significant catastrophic feature, while that of the latter has an inverted S-shape with a remarkable fatigue damage characteristic; the energy mechanism of the former involves the ramp dynamic disturbance giving extra elastic strain energy to rocks, while that of the latter involves the cyclic dynamic disturbance decreasing the ultimate energy storage capacity of rocks.

True-Triaxial Experimental Study of the Evolutionary Features of the Acoustic Emissions and Sounds of Rockburst Processes

Rockbursts are markedly characterized by the ejection of rock fragments from host rocks at certain speeds. The rockburst process is always accompanied by acoustic signals that include acoustic emissions (AE) and sounds. A deep insight into the evolutionary features of AE and sound signals is important to improve the accuracy of rockburst prediction. To investigate the evolutionary features of AE and sound signals, rockburst tests on granite rock specimens under true-triaxial loading conditions were performed using an improved rockburst testing system, and the AE and sounds during rockburst development were recorded and analyzed. The results show that the evolutionary features of the AE and sound signals were obvious and similar. On the eve of a rockburst, a ‘quiescent period’ could be observed in both the evolutionary process of the AE hits and the sound waveform. Furthermore, the time-dependent fractal dimensions of the AE hits and sound amplitude both showed a tendency to continuously decrease on the eve of the rockbursts. In addition, on the eve of the rockbursts, the main frequency of the AE and sound signals both showed decreasing trends, and the frequency spectrum distributions were both characterized by low amplitudes, wide frequency bands and multiple peak shapes. Thus, the evolutionary features of sound signals on the eve of rockbursts, as well as that of AE signals, can be used as beneficial information for rockburst prediction.

Influence of Radial Stress Gradient on Strainbursts: An Experimental Study

Strainbursts, which are violent disasters that are accompanied by the ejection failure of rocks, usually occur in hard brittle rocks around highly stressed underground openings. The release of the radial stress at excavation boundaries is one of the major inducing factors for strainbursts in tunnels. After excavation, the radial stress usually exhibits different but apparent gradient variations along the radial direction near the boundary within a certain depth under different in situ stress conditions. In this study, the influence of the radial stress gradient on strainbursts of granite was investigated using an improved true-triaxial rockburst testing system, which was equipped with an acoustic emission monitoring system. The stress state and boundary conditions (i.e., one face free, other faces loaded and increasing tangential stress) of the representative rock element in the vicinity of the excavation boundary were simulated. High-speed cameras were used to capture the ejection failure processes during strainbursts, and the kinetic energy of ejected fragments was quantitatively estimated by analyzing the recorded videos. The experimental results indicate that with an increasing radial stress gradient, the strength increases, the apparent yield platform prior to the peak stress on the stress–strain curves decreases, the failure mode changes from strainburst characterized by tensile splitting to strainburst characterized by shear rupture, and the kinetic energy of ejected fragments during strainbursts significantly increases.

Influence of temperature on the strainburst characteristics of granite under true triaxial loading conditions

The experimental study of the strainburst on granite treated with different high temperatures was conducted using a novel true-triaxial rockburst test system. During the test process, a loading path was given by keeping one free face and then loading on other five faces. The strainburst process, the failure patterns, the peak stress, the acoustic emission (AE) signal characteristics, the fragment characteristics and the kinetic energy of the ejected fragments were analyzed by employing the stress-strain curves, the high speed camera system and the AE technology. The experimental results show that temperature has significant influences on the strainburst. The critical temperature of these specimens is 300°C. With the temperature increasing from 30°C to 300°C, the peak stress, the peak strain and the AE process exhibit a slight variation, while the kinetic energy of the ejected fragments rapidly increases. When the temperature increases from 300°C to 700°C, the peak stress and the kinetic energy apparently decrease, but the peak strain swiftly rises. The time of the AE quiet period and the fragmentation increase with rising temperature. Compared with the specimens at 30°C, the specimens treated with temperatures from 100°C to 500°C have more intense rockburst, while the specimens treated with temperatures from 600°C to 700°C have a lesser kinetic energy of the ejected fragments.

Analysis of fractal characteristic of fragments from rock burst tests under different loading rates

Original scientific paper Rock burst is a common serious geological hazard in underground engineering, which seriously affects the progress of projects. The mechanism of rock burst can be explained by the distribution law of rock fragments and its fractal characteristics. A simulation experiment of rock burst was conducted with the granite samples under a biaxial loading machine system to analyze the fractal characteristics of the fragments from rock burst tests. The granite fragments were collected and divided into coarse, medium, fine, and micro grains by a screening method. The number and mass distribution of the fragments in different size ranges were also analyzed. The fractal dimensions of the rock fragments were calculated by the mass-granularity distribution method. Results show that the loading rate is proportional to the damage degree of rock burst; the mass of rock burst debris increases with the increase in loading rate, which indicates that a high loading rate leads to considerable rock damage. A high loading rate also results in small proportions of fine and medium grains and a large proportion of coarse grain, with no significant change in the micro grain. Under the high-loading-rate condition, the fractal dimensions of rock fragments are small, but the released energy of rock burst is large. The conclusions obtained in this study confirm the feasibility of reducing the risk of rock burst by adjusting the excavation rate in engineering practice and provide the basis for further study on the mechanism of rock burst.

Size effects on granite behavior under unloading rockburst test

Based on previous studies of rock behavior versus rock sample size and various research on rockburst simulation tests, a modified true-triaxial unloading testing machine was utilized to perform rockburst tests on granite specimens with changing heights for size effect investigation. The parameter of the height-to-thickness ratio (H/T) is proposed to characterize the rock scale, and it varies from 5 to 2 with a fixed thickness of 30 mm and changing heights from 150 to 60 mm. In this study, several testing methods were adopted to study the influence of H/T including non-destructive acoustic emission (AE) technology and high-speed camera recording systems as well as fragment kinetic energy analysis. The experimental results indicated that a size effect does exist during the rockburst simulation process and affects the rock failure strength and fracture mode. It can be seen that rock failure strength has an increasing trend with the decreasing of H/T. And at the higher H/T, extensile and splitting failures are dominant, and then the mixed failure mode including tensile and shear appears with decreasing H/T, until the single-plane shear becomes the main failure mode at the lowest H/T. AE energy release and kinetic energy of ejected fragments are not quite sensitive to sample size for H/T lower than 3. When the ratio of height and width is not lower than 3, the total AE energy during rock failure and kinetic energy of ejected fragments increased with the decreasing of sample size. With a further decrease of the ratio to 2, the total AE energy and kinetic energy of ejected fragments began to decrease.

Rockburst laboratory tests database — Application of data mining techniques

Rockburst is characterized by a violent explosion of a block causing a sudden rupture in the rock and is quite common in deep tunnels. It is critical to understand the phenomenon of rockburst, focusing on the patterns of occurrence so these events can be avoided and/or managed saving costs and possibly lives. The failure mechanism of rockburst needs to be better understood. Laboratory experiments are undergoing at the Laboratory for Geomechanics and Deep Underground Engineering (SKLGDUE) of Beijing and the system is described. A large number of rockburst tests were performed and their information collected, stored in a database and analyzed. Data Mining (DM) techniques were applied to the database in order to develop predictive models for the rockburst maximum stress (σRB) and rockburst risk index (IRB) that need the results of such tests to be determined. With the developed models it is possible to predict these parameters with high accuracy levels using data from the rock mass and specific project.

Fractal characteristics of cracks and fragments generated in unloading rockburst tests

True triaxial rockburst experiments with four different unloading rates were performed on four prism specimens of granite sampled from Beishan, China. The damage evolution in the rockburst test was investigated from two aspects including fracture surface crack and fragment characteristics. The scanning electron microscopy was used to observe the micro crack information on fragment surface. Combing binarization and box counting dimensions, the fractal dimensions of cracks were obtained. Meanwhile, the fragments were collected and a sieving experiment was conducted. We weighed the fragments qualities, counted the amount of fragments and measured the fragments length, width and thickness. Utilizing four methods to calculate damage fractal dimensions of fragments, the trend of fractal value changing with unloading rates can be roughly described. It can be concluded from these experiments that the fractal dimension either for crack or for fragment holds a decreasing trend with the decreasing unloading rate, indicating a reduction of damage level.

Fracture Angle Analysis of Rock Burst Faulting Planes Based on True-Triaxial Experiment

The aim of this paper is to estimate fracture angles in deep-seated rock bursts encountered in intact hard rock tunnels. The fracture angles of fault planes in rock burst failure are analytically formulated by employing stress analysis based on Mohr’s circle construction. Mohr’s circle construction suits well for representing the rock burst stress states including the static loading and dynamic unloading processes existing at or near the excavation surface. Four fracture angles can be precisely predicted using the proposed mathematical models, including two minimum angles for two conjugate planes where the shear stress is equal to the maximum static shear stress τmax while the normal stress approaches to zero, and two maximum angles for two conjugate planes where the normal stress is reduced from σ1 to σ1/2 while shear stress increases markedly from ±(σ1–σ3)/2 to the maximum dynamic shear τdmax = ±σ1/2. For validation of the analytical solutions to fracture angles, rock burst experiments on Laizhou granite were conducted using a modified true-triaxial apparatus. The predicted fracture angles are compared very well with the results obtained from the laboratory rock burst tests and are in good agreement with the in situ observations. The proposed solutions to the fracture angle are a function of the static stresses only which can be known a priori from a field survey.

Fragmentation characteristics analysis of sandstone fragments based on impact rockburst test

Impact rockburst test on sandstone samples with a central hole is carried out under true triaxial static loads and vertical dynamic load conditions, and rock fragments after the test are collected. The fragments of sandstone generated from strain rockburst test and uniaxial compression test are also collected. The fragments are weighed and the length, width and thickness of each piece of fragments are measured respectively. The fragment quantities with coarse, medium, fine and micro grains in different size ranges, mass and particles distributions are also analyzed. Then, the fractal dimension of fragments is calculated by the methods of size-frequency, mass-frequency and length-to-thickness ratio-frequency. It is found that the crushing degree of impact rockburst fragments is higher, accompanied with blocky characteristics observably. The mass percentage of small grains, including fine and micro grains, in impact rockburst test is higher than those in strain rockburst test and uniaxial compression test. Energy dissipation from rockburst tests is more than that from uniaxial compression test, as the quantity of micro grains generated does.

Analysis of Cracking Processes of Sandstone in Rockburst Test

High speed camera system is used to record crack propagation on rock surface during the lab rocburst experiment. This paper presents the specific analytical method of crack propagation with high speed images. Crack propagation types and directions are briefly described, and the lengths, growth rate and fractal dimension of cracks are calculated. The results show that the crack grows mainly along the horizontal line on the unloading surface, at a low speed before rockburst and reaches a very high speed abruptly near the burst point. The fractal dimensions demonstrated that the crack turns to be rough when rockburst happens. The study on the crack propagation is benefit to the understanding on the mechanism of rockburst.

Studies on classification, criteria and control of rockbursts

This paper attempts to present the findings involving rockbursts classification, rockburst failure criteria, and related control measures. Experimental investigations were performed using the strainburst testing machine and impact-induced rockburst testing machine. According to the stress paths and experimental methods, rockbursts were classified into two major groups, i.e. the strainbursts and impact-induced bursts. The mechanisms and criteria of rockburst obtained from experimental investigations were discussed. Then, the developments of constant-resistance and large-deformation bolt (CRLDB), which can adapt itself to the external loading at a constant resistance by elongating continually, were introduced. The deformation energy of country rocks with large deformation can be absorbed by CRLDBs. Finally, the principles and the experimental results for control and prevention of rockburst using the CRLDBs were presented.