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This paper investigates the influence of tunnel axis stress on strainburst. An experimental study of strainburst was conducted using a true-triaxial rockburst system. During the test process, a loading path that maintained one face free and applied loading along three axial directions on the other five faces was used to simulate the change of stres...
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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 interm...
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... Rockbursts are common dynamic rock failures that occur during deep underground excavation, and they are usually accompanied by intensive rock ejection (Huang et al. 2018;Li et al. 2008;Su et al. 2017b;Zhu et al. 2010). Due to their burstiness and randomness, rockbursts are a severe threat to engineering and construction devices during underground excavation. ...
... Generally, selfinitiated rockbursts are encountered approximately 1-3 days after excavation. In recent decades, numerous studies have been conducted to investigate the mechanisms (Cook 1963;Hua and You 2001;Huang et al. 2001;He et al. 2010;Su et al. 2017bSu et al. , 2020Zhao et al. 2014), prediction (Gong et al. 2019a;Xu et al. 2016), classification (Gong et al. 2019b;He et al. 2012;Kaiser 1996), and prevention (Cai 2013) of rockbursts. The results deepened the knowledge of rockbursts and promoted the development of research techniques and methods for predicting rockbursts. ...
Structural plane plays an important role in affecting the rockburst, especially when it is subjected to an external dynamic disturbance. To investigate the influence of weak dynamic disturbances on a structural rockburst, a series of rockburst tests were conducted using a true triaxial rockburst testing machine enhanced by a video monitoring system. The granite specimens were thin squares (200 mm × 200 mm × 30 mm) with a circular hole (diameter of 50 mm) in the center. These specimens were used to simulate the occurrence of structural rockbursts in tunnels. The structural rockburst process and the corresponding acoustic emission (AE) and energy release were systematically investigated. The results indicated that weak dynamic disturbance decreased the stress level and made the rockburst occur more easily. The type of structural plane changed the influence of the weak dynamic disturbance on the structural rockburst. For the specimens with unexposed structural planes, more rock split and collapsed but fewer local rockbursts occurred, the intensity of the final rockburst was smaller than that of the static loading, and the stress when the final rockburst occurred was approximately 9% lower compared with those without a weak dynamic disturbance. For the specimens with exposed structural planes, the stress when the final rockburst occurred was approximately 10% lower but with a higher rockburst intensity compared with those without a weak dynamic disturbance. The AE characteristics indicated that tensile fractures dominated the rock failure process during the weak dynamic disturbance. The proportion of shear fractures increased with the increasing number of weak dynamic disturbances. In addition, the characteristics of the energy release indicated that weak dynamic disturbance decreased the local energy release rate (LERR) of the unexposed structural planes but increased the LERR of the exposed structural planes. The dip angle of the structural plane had an obvious effect on the rockburst. As the dip angle of the unexposed structural planes increased, the rockburst intensity, variation in the AE parameters, and LERR initially increased and then decreased which indicated that the rockburst was more intense under the dip angles of 30° and 45°. For exposed structural planes, the rockburst intensity, variation of the AE parameters, and LERR increased with increasing dip angle (0°–60°), which indicated that the 60° dip angle was more dangerous.
... in axial stress; a gradual decrease, however, occurs when the axial stress is greater than 100 MPa. These variation trends in the present studies are very similar to that obtained by Mogi (1971), Wiebols and Cook (1968), Haimson and Chang (2000), Su et al. (2017), and also supports that failure criterion should account for the effect of σ 2 on strength when relating the borehole breakout dimensions to the prevailing in-situ stress (Vernik and Zoback 1995). Figure 15b shows the cumulative AE hit corresponding to different characteristic stresses. ...
In this study, structural model tests using cubic granite specimens of 100 mm × 100 mm × 100 mm with a centered horizontal circular hole of 50 mm diameter were used to investigate the influence of axial stress (5–160 MPa) on borehole strainburst under true-triaxial stress conditions. A camera and an acoustic emission (AE) monitoring system were used to capture the failure process. The experimental results indicate that the micro- and macroscopic failures are significantly influenced by the axial stress. As the axial stress increases, the vertical stress increment and the duration of particle ejection (from particle ejection to spalling) and spalling failure (from spalling to rockburst), as well as particle ejection to the occurrence of rockburst tend to decrease first and then increase. However, the cracking mechanisms around the V-shaped failure zone and the features of the trans-granular and the extensile micro-cracking in the cross-section perpendicular to the borehole axis are unaffected by the axial stress. For the specimens tested, the rock failure stresses corresponding to crack initiation, particle ejection, crack damage, spalling and rockburst, as well as the cumulative AE absolute energy during the entire test increase as the axial stress increases from 5 to 100 MPa and then they all decrease as the axial stress continues to increase from 100 to 160 MPa. It was also observed in our tests that an increase in the axial stress suppressed the propagation of radial cracks but promoted the development of axial extensile cracks. As a result, rock fractures were developed more in the axial direction in a smoother and more planar fashion and rock fragments with elongated dimension along the borehole axis were produced in the rockburst process. These results suggest that a higher axial stress enhances axial fracturing or spalling during strainburst.
... Feng (2017) divided the rockburst into four types: time-delay rockburst, immediate rockburst, strain rockburst and structural rockburst. Gong et al. (2018Gong et al. ( , 2019a and Su et al. (2017Su et al. ( , 2023 successfully simulated the rockburst and slabbing phenomenon encountered in deep-buried circular tunnel under three-dimensional stress conditions. He et al. (2021) conducted strain burst simulation tests by unloading single and double faces on red sandstone samples. ...
... Among them, the deep rock masses are most widely distributed with unfilled or micro-filled structural planes, which affects the stability of rockmass in underground engineering (Liu et al. 2017;Martin and Christiansson 2009;Pollard and Aydin 1988;Si et al. 2022). In recent years, structural instability and rockburst have been observed in many deep hard-rock tunnels (Gong et al. 2019a;Su et al. 2017;Wu et al. 2020). However, the different spatial distribution positions of these cracks have different impacts on the stability and failure process of the tunnel surrounding rock. ...
The failure mechanism of surrounding rock and the formation of rockburst in deep rockmass are influenced by various factors such as in-situ stress, geological conditions and excavation methods. In this paper, to investigate the impact of cross-sectional shape, principal stress direction and existing structural planes on the failure mechanism and rockburst process of tunnels, granite samples with horseshoe-shaped holes under different span-ratios and prefabricated cracks are made from the Longmen Mountain Tunnel, corresponding compression tests are carried out in different loading directions. According to the test results, adding the inverted arch and appropriately increasing the span-ratio are beneficial for improving the overall load-bearing performance of the tunnel. However, continuously increasing the flattening rate will lead to an increase in the excavation cross-sectional area, thereby reducing the overall strength of the rock mass. Gradually increasing the flatness of the hard-rock tunnel can effectively improve the stress concentration inside the surrounding rock, significantly reducing the frequency and intensity of rockburst occurrence. For a horseshoe-shaped tunnel with horizontal principal stress direction, the rockburst intensity will intensify, and the location of the occurrence shifts towards the arch bottom and vault. The deformation and failure of deep hard-rock tunnels are often the result of the combined action of tectonic stress and rock mass structural planes, and the existing cracks largely affect and control the fracture evolution process. The effect of prefabricated cracks parallel to compressive stress on granite samples is relatively small, while diagonal-arranged cracks have a significant impact on the failure mechanism of rock mass. If the prefabricated cracks are placed vertically at the haunches with concentrated compressive stress, these planes not only seriously reduce the bearing capacity of the rockmass, but also greatly weaken the strong rockburst disasters. The inclined structural planes lead to significant asymmetry in the failure mechanism and rockburst process of tunnels. Although the intensity of rockburst occurrence has decreased, the frequency of dynamic instability of the surrounding rock may even increase, and the risk of collapse in the upper rock mass will be enhanced.
... In comparison to uniaxial and conventional triaxial tests, true triaxial tests better replicate the actual stress state of the engineering rock mass. True triaxial test research on rockburst has mainly focused on immediate failure [17,20,21], while experimental research on time-delayed failure under fivesided loading and one face free conditions is rarely reported. ...
... The true-triaxial testing machine (see Figure 2A) designed by Su et al. (2017) can reach loading conditions with high performance, i.e., a maximum vertical loading of 5,000 kN and a maximum horizontal loading of 3,000 kN, and is specially equipped with independent loading systems in three directions (X, Y and Z). The failure processes of rock masses near excavation boundaries at different intermediate principal stresses can be accurately simulated with this machine. ...
Biaxial compression is a typical stress state experienced by the surrounding rock near the excavation boundaries under deep underground engineering, frequently resulting in engineering geological disasters (spalling and rockburst). The motivation to mitigate the risk and damage of these disasters has led us to compressively examine the evolutionary characteristics of acoustic signals [microseismic (MS) events, sound and acoustic emission (AEs)] produced by granite under biaxial compression with different intermediate principal stresses. These characteristics include time (activeness and b value) and frequency (main frequency and proportion of the advantage frequency bands) domains. The results suggest that: 1) the signal properties-driven order of activeness under low and high intermediate principal stresses for the initial stresses were as follows: AE accounted for 37.4% and 43.5% of σ peak, MS for 61.1% and 66% of σ peak, and sound for 81.8% and 85.5% of σ peak. 2) The notable distinction in precursors of different acoustic signals before granite failure was confirmed: the sequential relationship in the continuous decrease rate of the b value (AE < MS < sound), the occurrence (only existing in AE signals) of a few signals with extremely high amplitude (the “quiescent period”) and the different frequency-change rule in the proportion of the advantage frequency bands. 3) The strong influences of intermediate principal stress on the signal precursors were determined; these precursors in the activeness, b value, and proportion are negative to intermediate principal stress, whereas that of the main frequency shows a positive correlation. Consequently, these findings can contribute integrated usage of the multifrequency signals in the prediction and warning of geological disasters under deep underground engineering.
... Furthermore, researchers have studied the lithology (He et al. 2012a), bedding orientation (He et al. 2012b), stiffness (He et al. 2019), loading path (Ren et al. 2020;Si and Gong. 2020), temperature (Akdag et al. 2018;Su et al. 2017a), intermediate principal stress (Su et al. 2017b), radial stress gradient (Su et al. 2017c), and number of unloading surfaces ) as test variables and obtained the influence law of each factor on the strain burst. The second type is the static unloading confining pressure test. ...
The effect of complex triaxial unloading confining pressure conditions on the mechanical behaviour and fracture mechanism of sandstone is investigated. Constant deviatoric stress direct and cyclic unloading confining pressure tests are carried out under two initial damage states. Based on the evolution law of deformation parameters, the influence of the initial damage and unloading rate on the mechanical properties of sandstone samples is expounded. The mechanism of tensile fracture development caused by the unloading of the confining pressure is proposed, and the engineering disasters that may be induced by this mechanism are discussed. The results show that the cyclic unloading confining pressure tests can replace direct unloading pressure tests to study the evolution process of the mechanical parameters of samples. The unloading confining pressure test under low damage conditions is more conducive to the study of unloading confining pressure failure. The unloading confining pressure produces obvious radial expansion and tensile cracks in the sample. With decreasing confining pressure, the bulk modulus of the samples under the two damage conditions gradually decreases. Under low damage conditions, the dilatancy angle decreases first and then increases, while under high damage conditions, it gradually increases. The unloading rate does not affect tensile crack development but does affect compression‒shear crack development, resulting in different strain responses of the two damaged specimens. The generation mechanism of macroscopic tensile cracks is as follows: the residual stress generated by the unloading of the confining pressure causes random tensile microcracks to nucleate, and the increase in microcrack density provides power for its development. The airfoil cracks gradually bend in the direction of the first principal stress, and the microcracks develop directionally and interconnect with each other to form macroscopic axial tensile cracks. The unloading of confining pressure produces tensile cracks perpendicular to the unloading direction, which may induce rock bursts and landslide disasters. The research results provide an important scientific basis for underground rock engineering construction and slope engineering prevention.
... He experimentally provided with a zone of confinement under the failure envelope where rockbursts are observed in intact rocks ( Figure 1). Many researchers conducted true triaxial unloading tests on intact rocks for laboratory simulation of rockbursts and study the potential of rocks for rockbursts with acoustic emissions (He et al., 2010), infrared thermography (Sun et al., 2017), high speed cameras and decibel meters (Su et al., 2017). ...
... The excavation of the tunnel changes the stress state of the rock mass near the excavation boundary where the tangential stress (r h ) increases gradually, the radial stress (r r ) decreases rapidly, and the axis stress (r a ) varies slightly as shown in Fig. 4 [61]. According to Kirsch's solution, the maximum tangential stresses (r hmax ) at the circular tunnel excavation boundary are a function of maximum and intermediate principal stresses as per Eq. ...
... (1). At the excavation boundary, the radial stress is relieved completely while it increases by going far from the face of the excavation [61]. Furthermore, the excavation triggers shear stresses (s) on the surfaces due to the constraint of surrounding rock. ...
... Stress state of representative rock mass before and after tunnel excavation[61] ...
As the global push towards clean energy intensifies, the demand for critical minerals has driven deep excavation in hard rock formations, posing significant challenges related to rockburst and spalling. Spalling refers to explosion-like rock fractures under high geo-stresses. Despite several successful studies and practical models, the mechanisms governing spalling propagation under polyaxial stress states remain inadequately understood, particularly in weaker and high-porosity rocks. This study introduces a novel Crack Mode-Changing Stress (CMCS) concept, which defines the minimum principal stress required to change the crack mode from shear to tensile failures when rock spalls. The concept was validated using cubed sandstone samples containing centric circular holes subjected to a range of loading conditions including uniaxial, biaxial, generalized triaxial compression, generalized triaxial tensile, and true triaxial loading stress states. Our results highlight the significance of the out-of-plane minor principal stress on the crack initiation threshold and the CMCS, emphasizing the need for careful consideration when designing openings in highly stressed environments.
... In our study, the true-triaxial rockburst testing machine developed at Guangxi University, China, is used to simulate the rockburst process. As shown in Figure 21, a loading path, "one free face, five stressed faces" is used, and the tested cuboid specimen has dimensions of 100 mm (width) × 100 mm (length) × 200 mm (height) [39]. The loading plan can be described as follows: σ x and σ y are increased to 30 MPa and 5 MPa; simultaneously, σ z is increased at a rate of 0.3 MPa/s until the rock specimen fails. ...
... In our study, the true-triaxial rockburst testing machine developed at Guangxi U versity, China, is used to simulate the rockburst process. As shown in Figure 21, a load path, "one free face, five stressed faces" is used, and the tested cuboid specimen has mensions of 100 mm (width) × 100 mm (length) × 200 mm (height) [39]. The loading p can be described as follows: σx and σy are increased to 30 MPa and 5 MPa; simultaneou σz is increased at a rate of 0.3 MPa/s until the rock specimen fails. ...
... As σ z gradually increases until the rock specimen loses its bearing capacity, a large amount of strain energy is stored inside the rock specimen from loading to this stage. According to existing studies [39], most of the strain energy stored in a rock specimen that passes through an internal shear dislocation dissipates. At this time, many shear cracks, which are the main cause of the energy dissipation, form, and a macroscopic oblique shear zone that penetrates the rock specimen is formed. ...
Sound signals generated during rock failure contain useful information about crack development. A sound-signal-based identification method for crack types is proposed. In this method, the sound signals of tensile cracks, using the Brazilian splitting test, and those of shear cracks, using the direct shear test, are collected to establish the training samples. The spectrogram is used to characterize the sound signal and is taken as the input. To solve the small sample problem, since only a small amount of sound signal spectrogram can be obtained in our experimental test, pre-trained ResNet-18 is used as a feature extractor to acquire deep characteristics of sound signal spectrograms. Gaussian process classification (GPC) is employed to establish the recognizing model and to classify crack types using the extracted deep characteristics of spectrograms. To verify the proposed method, the tensile and shear crack development processes during the biaxial test are identified. The results show that the proposed method is feasible. Moreover, this method is used to investigate the tensile and shear crack development during the rockburst process. The obtained results are consistent with previous research results, further confirming the accuracy and rationality of this method.
... Reference source not found. (Su et al., 2017). Kirsch's solution suggested that the maximum tangential stress ( ) at the excavation boundary of the tunnel is a function of maximum and intermediate principal stresses (≈ 3 1 − 3 ). ...
... However, despite the effect of all principal stresses on the induced in-situ spalling failure, to some extent, the biaxial loading condition can be used to simulate the spalling failure in a laboratory model. (Su et al., 2017) As we go deeper, not only the stresses increased but also the in-situ temperature; the average rise in temperature is ranging between 25 and 30 degrees Celsius per kilometre (Hohmeyer, 2008). Besides the geothermal gradients, other factors also have an influence in escalating the in-situ temperature at depth such as complex geological formations (i.e. ...
