Rockburst Process Research Articles

Experimental Investigation on the Influence of Depth on Rockburst Characteristics in Circular Tunnels

To investigate the influence of depth on the rockburst of surrounding rock in a circular tunnel, true-triaxial tests at different depths were carried out on cubic granite specimens with a circular through-going hole. A micro camera was used to monitor the rockburst process of the circular hole sidewall in real time. The test results show that the failure process at different depths can be divided into four periods: the calm period, the particle ejection period, the rock fragment exfoliation period, and the rock bursting period. With an increase in depth, the three-dimensional unequal stress state gradually increased; the failure range and the size of rock fragments increased, the initial failure vertical stress linearly increased, and the strength and stability of the surrounding rock were enhanced. Therefore, the support range of surrounding rock should be increased as the depth increased to improve the overall stability of surrounding rock and reduce the failure range.

Experimental simulation study of rockburst characteristics of Sichuan–Tibet granite: A case study of the Zheduoshan tunnel

Rockburst poses a great threat to the construction safety of deep tunnels under high geostress conditions. Recently, numerous deeply buried tunnels have been constructed in China, especially in the Sichuan–Tibet railway area. In this study, two rockburst experiments (strainbursts and impact-induced rockbursts) were conducted under different initial geostresses using the strainburst experimental apparatus and impact-induced rockburst experimental apparatus equipped with an acoustic emission (AE) monitoring system. A video acquisition system was used to capture the rockburst process. The results demonstrated that the rockburst process is similar under different initial geostresses, including grains ejection, rock plates spalling/buckling, and fragments ejecting violently. Based on the result of the AE characteristics, the large- amplitude AE signals were concentrated in the lower frequency band and the rockburst intensity was increased gradually with increase in the initial geostress. The b-value and βt-value fluctuated slightly at first, subsequently increased steadily, and then decreased abruptly when rockburst occurred. Furthermore, the displacement and divergence fields on the specimen surface were calculated using digital image correlation (DIC) technology. The result shows that the zone of large deformation and high divergence is consistent with the rockburst zone. Moreover, 80%σpeak is a critical stress state where rocks begin experiencing slight damage.

Effect of layered joints on rockburst in deep tunnels

The existence of joints in the surrounding rock mass has a considerable effect on tunnel rockbursts. Herein, we studied the effect of layered joints with different inclination angles and spacings on rockburst in deep tunnels and investigated the failure area, deformation process of the surrounding rock mass, stress change inside the surrounding rock mass, velocity of the failed rock, and the kinetic energy of the failure. The failure type of the surrounding rock mass can thus be determined. The results showed that the intensity of rockburst increases as rock quality designation (RQD) decreases, while the deformation rate of the surrounding rock mass first increases and then decreases. The deformation rate exhibits a turning point between RQD = 50 and 70, below which the deformation rate of the surrounding rock mass gradually decreases, ultimately ceasing to be a rockburst. Rockburst always occurs perpendicular to the direction of the joint. When σx = σy, as the joint inclination angle changes from 45° to 90°, the intensity of a rockburst first decreases (from 45° to 60°), and then increases (from 60° to 90°). When combined with the evolution law of stress and strain energy, the rockburst process can be divided into four stages.

О возможных проявлениях горных ударов при сооружении ПИЛ и методах их предотвращения

At great depths, the Yeniseiskiy section of the Nizhnekanskiy rock mass selected for the construction of an underground research facility (URF) may involve some areas of firm rocks with high internal stresses that may cause spontaneous rock bursts. The paper provides a qualitative description of the mechanism standing behind the rock burst process, the necessary and sufficient conditions for its occurrence and the conditions for its extinction. It considers some engineering methods dealing with rock bursts which may be applied given special requirements set out for the URF construction.

Evolution of anisotropy during sandstone rockburst process under double-faces unloading

Evolution of anisotropy during sandstone rockburst process under double-faces unloading

Combined finite-discrete element modellings of rockbursts in tunnelling under high in-situ stresses

Rockburst is one of the severe geotechnical problems that may occur while tunnelling under high in-situ stresses. Despite numerous studies in the relevant literature, the evaluation and prediction of the rockburst still remain a common challenge worldwide. In this paper, a self-developed combined finite-discrete element method (FDEM) is used to numerically investigate the rockbursts in deep tunnelling under high in-situ stresses. The rockbursts in the drainage tunnel at the Jinping II Hydropower Station are modelled and the effects of the geological and geotechnical characteristics of the site on the occurrence of rockbursts are investigated. The FDEM numerical modelling vividly simulates the fracture initiation and propagation, as well as the fragment expulsion, ejection and flyout resulting in the rockburst process that could be difficult to capture on the site or via the conventional continuum modellings. The effect of the pre-existing fault on the site is studied, and the critical roles of the location and the dip angle of the fault in determining the rockburst development around the tunnel are highlighted. Then, the effects of in-situ stresses on rockburst development are investigated by applying different lateral pressure coefficients in the model. It is found that based on the Jinping site condition, a low lateral pressure coefficient could contribute to the alleviation of the rockburst around the tunnel but the alleviation effect is relatively limited, while a high lateral pressure coefficient could result in the rockburst that breaks more rock masses on the tunnel surface. Moreover, the influences of tunnel shapes on the rockburst incidents are also studied by further modelling rockbursts around tunnels in square shape and horseshoe shape. It is found that the square shape is inadequate in controlling rockbursts at the roof and floor areas, and the most suitable tunnel shape to resist rockbursts in such field condition is the circular shape. By vividly reproducing the rockburst processes in deep tunnelling under high in-situ stresses and elaborating the effects of several critical geological and geotechnical characteristics on the rockburst development around the tunnel, this study contributes to the understanding of the associated rockburst mechanisms and is also of guiding importance for certain tunnelling designs and constructions in practice. It is also expected that the proposed method can be used to evaluate and predict the rockbursts in many other scenarios in future studies.

Optimal Proportion of Similar Materials and Rockburst Tendency of White Sandstone

To explore the tendency of rockburst, a similar material ratio was optimised based on white sandstone. Quartz sand, iron powder, gypsum, cement, retarder, and a water‐reducing agent were used as the main materials. The orthogonal test design principle was used to determine the four‐factor and four‐level orthogonal test design with the quartz sand content, iron powder content, gypsum‐cement ratio, and sand particle size as the influencing factors. Uniaxial compression tests and tensile tests were conducted on similar material models. The tensile strength and elastic modulus were analysed, the significance of each influencing factor was investigated, and the test results of the similar materials were fitted. The optimal ratios of the similar materials of white sandstone were found to be quartz sand content of 36%, iron powder content of 1.9%, gypsum‐cement ratio of 1.8 : 1, and sand particle size of 2–4 mm. The physical and mechanical properties of the similar materials were consistent with those of white sandstone. The mechanical properties of the similar materials were compared with those of the original rock. By judging the rockburst propensity and verifying the index, it is concluded that the similar materials can effectively simulate the characteristics of white sandstone, which is an ideal similar material of rockburst, and they all show strong rockburst propensity. The rock specimens with optimal proportions were produced, and the internal energy changes and rockburst mechanisms of the model at different temperatures were discussed. The results show that the rockburst process is closely related to energy, such as thermal energy and elastic strain energy, and the rock failure process can be divided into three main stages: energy accumulation, microcrack formation and propagation, and crack penetration and bursting. It provides an experimental basis for the preparation of rockburst similar materials that are more in line with the actual situation of the project and provides a basis for discussing the energy criterion of rockburst.

Experimental Study on the Influence Mechanism of the Structural Plane to Rockbursts in Deeply Buried Hard Rock Tunnels

During the excavation of a large number of deeply buried tunnels and mining projects, rockburst disasters occur frequently due to the complex geologic environment in deep underground, including high initial geostress, adverse tectonic actions, and excavation disturbance. Many rockbursts have been found to be induced by some small‐scale structural planes in the area around the tunnels during the construction of Jinping II hydropower station. In order to study the influence mechanisms of the structural plane to rockbursts, the physical simulation tests of rockbursts under biaxial stress conditions are carried out using marble samples by considering different relative positions of the structural plane with tunnels, namely, in tunnel spandrel, in tunnel sidewall, and at the intersection with the tunnel. The digital image correlation (DIC) technique is used to trace the evolution of the deformation on the surface and the rockburst process of the marble sample. The results reveal that three types of rockbursts are identified, namely, fault‐slip rockburst, split bulking rockburst, and shear rupture rockburst, and their evolution processes are reproduced. The presence of small‐scale structural planes in the vicinity of deep tunnels could be one of the major influence factors in triggering rockbursts. The findings could provide helpful references for predicting the development process and the design of burst‐resistant measures for this type of rockbursts.

Suppressing rockburst by increasing the tensile strength of rock surface: An experimental study

Suitable rockburst support is critical for the prevention of rockburst disasters. In the present study, rockburst process and mechanism are investigated first in detail based on laboratory tests. It is considered that the splitting and buckling are two stages that precede the occurrence of rockburst, and buckling is the necessary prior step and precondition of rockburst. Therefore, in this study, it is proposed to use a layer of material with high tensile strength adhered to the rock surface to increase the tangential tensile strength of the surrounding rock surface and then suppress the buckling, hence achieving the purpose of suppressing the rockburst. In our experiments, cement mortar, acrylate, polyurea and basalt fabric were used to create the surface reinforcement layer. To verify the effectiveness of the proposed support strategy, true-triaxial rockburst tests were carried out, with high-speed cameras and an acoustic emission (AE) system used to monitor the failure process. The results show that increasing the surface tensile strength effectively increases the allowable buckling deformation or yielding range of rock, and the severity of rockburst reduces as the surface tensile strength increases. Characteristics of deformation, rock fragmentations and AE show that with the increase of the surface tensile strength, the internal cracking activity before the final failure is more intense, resulting in more small rock fragments formed, and more energy is consumed and dissipated in the fracturing process (continuous expansion, splitting and buckling) with the energy released in a more gradual manner and hence less kinetic energy is available to cause violent rockburst. Therefore the proposed solution is demonstrated to be an effective ground support scheme to suppress rockburst in rock excavations.

Experimental study of energy and frequency characteristics of acoustic emissions induced from tensile and shear cracks in tunnel rockburst process

Studying the characteristics of tensile and shear cracks in a rockburst event is crucial for understanding the rockburst mechanism. In this study, several specimens were tested under biaxial compression, and the released acoustic emissions (AEs) of the resulting tunnel rockburst event were recorded. Based on the AE energy and dominant frequency parameters, the different AE characteristics and dominant frequency evolution law of tensile and shear cracks during the rockburst were analysed. According to the results, there are four types of AE signals in a rockburst event: low-frequency and low-energy signals, low-frequency and high-energy signals, medium-frequency and low-energy signals and high-frequency and low-energy signals. The low-frequency and low-energy signals are mainly generated by tensile cracks, and the low-frequency and high-energy signals originate from both tensile and shear cracks. The medium-frequency and low-energy signals and high-frequency and low-energy signals are often generated by tension cracks. The rockburst process can be classified into four stages: calm period, small-grain ejection period, sheet-like peeling period and violent ejection period. Before the occurrence of rockburst, both tensile and shear cracks produce low-frequency and high-energy signals. The low-frequency and high-energy (amplitude) AE signals are crucial for monitoring and early warning procedures for rockburst events.

Experimental investigation on failure process and spatio-temporal evolution of rockburst in granite with a prefabricated circular hole

To study the mechanism of rockburst and its spatio-temporal evolution criterion, a rockburst simulation experiment was performed on granite specimens, each with a prefabricated circular hole, under different lateral loads. Using micro camera, acoustic emission (AE) system, and infrared thermal imager, the AE characteristics and thermal radiation temperature migration were studied during the rockburst process. Then, the failure mode and damage evolution of the surrounding rock were analyzed. The results demonstrate that increasing the lateral load can first increase and then reduce the bearing capacity of the hole. In this experiment, the hole failure process could be divided into four periods: quiet, particle ejection, stability failure and collapse. Correspondingly, the AE signals evolved from a calm stage, to have intermittent appearance; then, they were continuous with a sudden increase, and finally increased dramatically. The failure of the surrounding rock was mainly tensile failure, while shear failure tended to first increase and then decrease. Meanwhile, damage to the hole increased gradually during the particle ejection period, whereas damage to the rockburst mainly occurred in the stability failure period. The thermal radiation temperature migration exhibited warming in shallow parts, inward expansion, cooling in the shallow parts with free surface heating, inward expansion, a sudden rise in temperature of the rockburst pits, and finally specimen failure. The initial reinforcement support should fully contribute to surface support. Furthermore, an appropriate tensile capacity and good energy absorption capacity should be established in support systems for high-stress roadways.

Experimental investigation on influence of loading rate on rockburst in deep circular tunnel under true-triaxial stress condition

To investigate the influence of loading rate on rockburst in a circular tunnel under three-dimensional stress conditions, the true-triaxial tests were conducted on 100 mm×100 mm×100 mm cubic sandstone specimens with d50 mm circular perforated holes, and the failure process of hole sidewall was monitored and recorded in real-time by the microcamera. The loading rates were 0.02, 0.10, and 0.50 MPa/s. The test results show that the rockburst process of hole sidewall experienced calm period, pellet ejection period, rock fragment exfoliation period and finally formed the V-shaped notch. The rockburst has a time lag and vertical stress is high when the rockburst occurs. The vertical stress at the initial failure of the hole sidewall increases with loading rate. During the same period after initial failure, the rockburst severity of hole sidewalls increased significantly with increasing loading rate. When the vertical stress is constant and maintains a high stress level, the rockburst of hole sidewall under low loading rate is more serious than that under high loading rate. With increasing loading rate, the quality of rock fragments produced by the rockburst decreases, and the fractal dimension of rock fragments increases.

Experimental investigation on rockburst process and failure characteristics in trapezoidal tunnel under different lateral stresses

In general, the in-situ stress increases with the increase of excavation depth, and it significantly affects the safe and long-term stability of tunnels and chambers. To study the influence of the in-situ stress on the rockburst process and failure characteristics, three different lateral stresses were loaded to perform biaxial compression experiments on sandstone specimens with a trapezoidal opening. The results indicate that the high lateral stress accelerates the formation of localized damage around the trapezoidal opening, which can effectively consume the elastic strain energy. The deep surrounding rock at the roof and floor are basically in a compressive stress state, and the tangential compressive stress in the deep surrounding rock of the sidewall is greater than that in the deep surrounding rock of the roof and floor, which can cause the deep surrounding rock of the sidewall to be damaged more severely. For the corners of the trapezoidal opening, the high lateral stress can restrain the rotation of the principal stress, which lead to the reduction of the crack density around the corners. AE signals and CT scanning show that the failure mode of the model specimen shifts from tensile fracture to mixed tensile and shear fracture with the increase of the lateral stress. Under different lateral stresses, the outlines of failure zones on both sidewalls of trapezoidal opening are V-shaped, and the damage degree at the sidewall of the trapezoidal opening increases with the increase of the lateral stress when the rockburst occurs. Additionally, the failure zones on both sidewalls are larger than those of circular opening, which indicate that the supporting area on the sidewall of trapezoidal opening needs to be larger than that on the sidewall of circular opening.

Rockburst prediction and stability analysis of the access tunnel in the main powerhouse of a hydropower station based on microseismic monitoring

The access tunnel in the main powerhouse of the Shuangjiangkou hydropower station in China has complex geological conditions and is subject to high in situ stress in deep buried sections. Microseismic activity in the surrounding rock mass of the tunnel was monitored by a microseismic monitoring system, and rockburst was effectively predicted. Based on abundant data obtained from the microseismic monitoring, statistical parameters, which include cumulative apparent volume, the energy index, cumulative released energy and the Es/Ep value, were used to analyze the microseismic activity before and after rockburst to determine a more accurate early warning period and construction safety period. A sharp decrease in the energy index and a rapid increase in the cumulative apparent volume indicated a deterioration of the surrounding rock mass stability. The change characteristics of Es/Ep values revealed that the rockburst process underwent a transformation of compression-shear damage, tension-shear mixed damage and tension damage. Finally, based on the number of daily events N and the b value of the microseismic events, lgN/b was first established to evaluate the rockburst risk of tunnels. When the value of lgN/b was greater than 1, rockburst was more likely to occur; the larger the lgN/b value, the more severe the rockburst was. The research results provide an important reference value for the prediction of rockbursts in deep tunnels and the regulation of site construction progress.

Analysis of rockburst triggered by hard rock fragmentation using a conical pick under high uniaxial stress

High excavation-induced stress concentration and strong mining/tunneling disturbance are prominent conditions in deep excavations, and often present a coupled process to induce rockbursts. This study aims to reproduce and shed light on the rockburst process triggered by hard rock fragmentation using a conical pick. A true triaxial test apparatus was used to apply confining stress and pick force on cubic rock specimen, where the real-time values of the stresses and pick forces were recorded. A high-speed camera system was equipped to shoot the processes of rock fragmentation and rockbursts. The size of fragments produced in rockburst was analyzed via image processing approach. The results show that the cuttability and the rockburst susceptibility of hard rock under high confining stress are correlated with strength parameter and brittleness index of the rock material. Rockburst process consists of three progressive steps: surface slabbing triggered by pick penetration, rapid ejection and violent burst of chips powered by high confining stress, and final shear failure. The mean size of fragments produced in rockbursts decreases as the rock brittleness increases, which suggests that higher rock brittleness will induce more intense rockbursts. Therefore, the precautions, such as timely backfilling of the adjacent mined-out areas, flexible and energy-absorbing rock supports, and pre-destress ahead of excavation, should be taken to prevent rockbursts in peninsula- or island-type hard rock pillar under high excavation-induced stress.

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.

Dynamic analysis and criterion evaluation on rockburst considering the fractured dissipative energy

As one of the dynamic disasters of deep mining, rockburst significantly affects the safety of underground environment especially at great depth. First, the elastic energy and dissipative energy are deduced to characterize rockburst process separately employing the theoretical analysis and test technique. Then, a series of split-Hopkinson pressure bar system tests are conducted. After calculation, the percentage of fractured dissipative energy occupied in total input dynamic energy is obtained. Subsequently, the dynamic mechanical parameters of rock specimens and dynamic impact factors are fitted. Finally, a three-dimensional model is developed using the mechanical property of the host rock, constitutive model, and in situ geostress. The contours of energy and the damage conditions of host rock are predicted, and the tendentiousness of rockburst is estimated.

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.

Failure Prediction of Two Types of Rocks Based on Acoustic Emission Characteristics

The stability analysis of rock is an important basis to ensure the safe exploitation of underground resources and the reliable operation of space engineering. Uniaxial compression and acoustic emission (AE) tests were carried out on two common rock samples with strong rock burst tendency. The relationship between mechanical characteristics, AE characteristics, and rock burst tendency in the failure process of rock with different body types and the evolution of fractal characteristics of AE parameters were discussed. Based on the cusp mutation theory, the catastrophe model of AE characteristic parameters was established to quantify the instability mechanism of rock mass. The results show that the AE mutation rate (AEMR) of the cubic specimens increase from a low level to a high level gradually in the stable fracture stage, while that of the cylindrical specimens increase sharply to the maximum when the specimens are near failure. The AE cumulative energy curves of cubic specimens show a “step” rise, while that of cylindrical specimens show a “gradual” rise, and the rock burst process of cubic specimens is faster. The fractal dimension evolution mode of AE characteristic parameters of cubic specimens during uniaxial compression text is decline‐rise‐decline, while that of cylindrical specimen is decline‐rise‐decline‐steeply rise. According to the periodic change of AE cumulative energy curve, combined with the rock failure cusp mutation model, the occurrence of rock burst can be well predicted, providing certain theoretical guidance for the stability analysis of underground engineering rock mass.