J. Cent. South Univ. (2020) 27: 2914-2929

DOI: https://doi.org/10.1007/s11771-020-4518-4

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

SI Xue-feng(司雪峰)¹, HUANG Lin-qi(黄麟淇)¹, GONG Feng-qiang(宫凤强)²,LIU Xi-ling(刘希灵)¹, LI Xi-bing(李夕兵)¹

1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;

2. School of Civil Engineering, Southeast University, Nanjing 211189, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: 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.

Key words: rockburst; loading rate; deep circular tunnel; true-triaxial test; V-shaped notch

Cite this article as: SI Xue-feng, HUANG Lin-qi, GONG Feng-qiang, LIU Xi-ling, LI Xi-bing. Experimental investigation on influence of loading rate on rockburst in deep circular tunnel under true-triaxial stress condition [J]. Journal of Central South University, 2020, 27(10): 2914-2929. DOI: https://doi.org/10.1007/s11771-020-4518-4.

1 Introduction

In deep underground engineering, rockburst is a dynamic failure phenomenon under high geostress conditions [1-6], which has a serious threat to construction personnel and equipment [7-15]. The occurrence of rockburst disasters is closely related to the high geostress, excavation unloading, and stress adjustment caused by dynamic disturbances in deep hard rocks [16]. However, there is no obvious precursor before the rockburst and it is sudden. There is great difficulty in monitoring and predicting rockbursts in actual engineering. To clarify the process of rockburst occurrence and the mechanism of rockburst induction, a multitude of experimental studies have been conducted. Under one-dimensional loading conditions, SALAMON [17] analyzed the relationship between specimen force and deformation, and discussed the stability of pillar workings, and introduced a design criterion to improve the mining of moderate and large depth pillars. SINGH [18, 19] observed that the brittleness, uniaxial compressive strengths and compressional wave velocity have strong influence on Burst Proneness Index. LI et al [20] and GONG et al [21] studied the mutual transformation mechanism among rockburst occurrences, dynamic strength increase and induced cracking under coupled static and dynamic load. Under the condition of biaxial compression, XU et al [22] carried out two- dimensional rockburst simulation tests with different specimen thicknesses, lateral pressure coefficients and opening methods by using similar materials with circular holes, and observed the phenomenon of rock fragment exfoliation.

To investigate the rockburst mechanism induced by excavation unloading and external disturbance or adjustment under three-dimensional stress, a host of true-triaxial unloading tests have been conducted [23-31]. HE et al [23, 24] studied the rockburst process and properties of limestone under true-triaxial unloading, and analyzed the acoustic emission (AE) energy characteristics and crack propagation. DU et al [25] obtained that occurrence of slabbing and rockburst is closely related to the rock type and stress path in true-triaxial unloading tests. LI et al [26, 27] investigated the influence of height-to-width ratios and intermediate principal stress on the failure modes and peak strength. SI et al [28] obtained that unloading induces an obvious strength-weakening effect on fine-grained granite, and revealed that the lower unloading rate is beneficial for improving the rock strength. SU et al [29, 30] studied the AE evolutionary features of rockburst processes in true-triaxial unloading tests. ZHAO et al [31] observed the instantaneous strainburst process of the granite samples on unloading surface. The above researches studied the rockburst process and mechanism induced by the destruction of rock materials from the aspects of stress environment, unloading effects and stress paths, and achieved many useful results. The rockburst process has significant spatial distribution and structural response characteristics [32-34], and the loading rate has a significant influence on rock mechanical properties [35-38]. In uniaxial or triaxial tests, rock strength increases with loading rate (strain rate) [39-42]. However, there are relatively few studies on the effect of loading rate on the rockburst process and mechanism of the cubic specimen hole sidewall under the action of true-triaxial stress. Different loading rates will lead to different increasing rates of vertical stress during the test, and then change the adjustment rate of maximum tangential stress of tunnel surrounding rock. Loading rate will affect the stress adjustment rate of tunnel surrounding rock. Therefore, the influence of stress adjustment rate on the severity of rockburst can be investigated by changing loading rate.

In this paper, to investigate the influence of loading rate on rockburst process and severity of the circular hole sidewall under true-triaxial stress conditions, rockburst simulation tests under different loading rates were conducted using a true-triaxial testing system on 100 mm×100 mm× 100 mm cubic sandstone specimens with a d50 mm circular perforated hole. During the test, a microcamera was used to monitor and record the rockburst process of the hole sidewall in realtime. The rockburst processes are analyzed in detail, and the influence of loading rate on the rockburst severity of the circular hole sidewall is discussed and investigated.

2 Experimental procedures

2.1 Specimen preparation

The experimental material was selected the sandstone from Linyi City, Shandong Province, China. The rock material used in this study is the same as that in Ref. [43]. The mineral composition of rock includes approximately 42% quartz, 35% plagioclase, 9% calcite, 8% zeolite, 5% K-feldspar, and 1% opaque minerals [43].

To conduct the simulation test of rockburst process of circular hole sidewall under different loading rates, the sandstone material was processed into 100 mm×100 mm×100 mm cube samples with a d50mm circular perforated hole. According to the International Society for Rock Mechanics (ISRM) suggested method, the standard tolerance is 0.0175 mm for the given dimensions of 100 mm× 100 mm×100 mm, and the perpendicularity tolerance is 0.025 mm for each side as a datum plane [44]. The basic physical and mechanical parameters of sandstone material are listed in Table 1. The sandstone material has moderate rockburst proneness according to the far field ejection mass ratio (FFEMR) and the residual elastic energy index [45].

2.2 True-triaxial testing machine

Figures 1(a) and (b) show the true-triaxial testing system. Detailed introduction of the experimental equipments can refer to GONG et al [32, 43] and LI et al [46]. To monitor the failure process of the circular hole sidewall in realtime, a d50 mm cylindrical hole was presented in x-direction loading block, and the microcamera (Figure 1(c)) was installed in the circular hole. The combination of the microcamera and the x-direction loading block is shown in Figure 1(d). The x- and y-direction are the horizontal axis and horizontal radial directions of the circular hole.

Table 1 Basic physical and mechanical parameters of sandstone material

Figure 1 Experimental equipments:

2.3 Loading path

To simulate the failure characteristics of circular hole sidewall under different loading rates, the initial stress state is calculated according to the fitted general far-field stress relationship at depth for China [47]. The in situ stress equations are expressed as follows:

                  (1)

where σ_v is the vertical stress; σ_hmax and σ_hmin are the horizontal maximum and minimum stress, respectively; H is the depth of the tunnel. When H is 500 m, the calculated results are σ_v=13.5 MPa, σ_hmax=17.7 MPa and σ_hmin=9.9 MPa. During the tests, the positional relationship between the circular tunnel and the three principal stress directions is shown in Figure 2. σ₁, σ₂ and σ₃ are the maximum, intermediate, and minimum principal stresses,respectively; σ_x, σ_y and σ_z are the x-, y- and z-direction stresses, respectively.

Figure 2 Schematic diagram of initial stress state

Figure 3 shows the loading path of the true-triaxial test. During the test, the cube sandstone specimen was installed in the loading box, and the loads in three (x, y and z) directions were loaded to the initial stress state by load control with a loading rate of 0.1 MPa/s. To eliminate the influence of the loading rate on the specimen during loading to the set stress state, the same loading rate (0.1 MPa/s) should be used for different sandstone specimens before loading to the set stress state. After loading to the set stress state, the loads in three directions are maintained for a period of time, then the z-direction load continued to increase with different loading rates (0.02, 0.10 and 0.50 MPa/s) for different sandstone specimens. To compare the severity of the sidewall failure, the vertical load is loaded to the same stress state at different loading rates, and the Z-direction load was maintained for a period of time. Ultimately, the x-, y- and z-direction loads were unloaded to 0 N.

Figure 3 Schematic diagram of stress path

3 Experimental results

3.1 Stress-time curves

Figure 4 shows the actual stress-time curves of three directions under three loading rates. The initial stress in z-direction was loaded to 17.7 MPa, and the constant loading times under three loading rates were 64.08 s (Figure 4(a)), 62.28 s (Figure 4(b)), 64.44 s (Figure 4(c)), respectively. To fully develop the failure of the hole sidewalls, when the vertical stress σ_z was loaded to 55.0 MPa, the constant loading times were 336.24 s (Figure 4(a)), 157.32 s (Figure 4(b)), 153.72 s (Figure 4(c)), respectively. Finally, three direction stresses were unloaded at the displacement rate of 20 mm/min to 0 MPa.

Figure 4 Stress-time curves:

3.2 Failure process of circular hole sidewall

The failure processes of both sidewalls of specimens S-200, S-1000 and S-5000 are shown in Figures 5-7. Taking specimen S-200 as an example (see Figure 5), the failure process and characteristics of both sidewalls of circular holes are introduced and analyzed in detail. At 1493.32 s, the vertical stress σ_z is loaded to 41.18 MPa, and fine particle ejection occurs on right sidewall, as shown in Figure 5(a). With increasing the vertical stress to 43.57 MPa (at 1613.56 s), the rock fragments on the right sidewall are spalled (see Figure 5(b)).

Figure 5 Failure process of circular hole sidewall of specimen S-200

Figure 6 Failure process of circular hole side wall of specimen S-1000

Figure 7 Failure process of circular hole side wall of specimen S-5000

During the period of 1493.32 s and 1613.56 s, the failure of right sidewall gradually developed to the side close to the miniature camera. Compared with Figure 5(b), the vertical stress in Figure 5(c) is increased by 2.27 MPa, and right sidewall failure is significantly expanded, accompanied by a large amount of particle ejection. With increasing σ_z, the particle ejection and rock fragment exfoliation of the right sidewall occur many times, and the rock fragment is peeled off accompanied by a large number of particle ejections, as shown in Figure 5(d). Before the rock fragments are peeled off, the rock fragments gradually open accompanied by a large number of particle ejections with increasing σ_z (see Figure 5(e)). When the vertical stress is 50.21 MPa at 1945.44 s, the rock fragment is peeled off at one position on the right sidewall and the rock fragment is opened rapidly (see Figure 5(f)). This opening rock fragment is peeled off at 1954.04 s (50.37 MPa), as shown in Figure 5(g). From Figures 5(h) and (i), the failure on the right sidewall will continue to occur with increasing the vertical stress. At 2104.32 s, the vertical stress was loaded to 53.38 MPa, and particle ejection occurred on the left sidewall (see Figure 5(j)). When σ_z was loaded to 55.00 MPa for a period of time, during maintaining the load, the failures on both sidewalls continued to occur. The rock fragment of left sidewall gradually opened (see Figure 5(k)). The rock fragment of left sidewall continued to open and the rock fragment of right sidewall peeled off (see Figure 5(l)). During the test, the failure of hole sidewall was not concentrated at a certain point, but V-shaped failure zone was formed. Different failures occurred at different locations in the failure zone, such as particle ejection and rock fragment exfoliation (see Figures 5(d) and (j)), particle ejection and rock fragment opening (see Figure 5(e)), and rock fragment exfoliation and rock fragment opening (see Figures 5(f),(i) and (l)).

Figures 6 and 7 show the rockburst processes of the hole sidewalls corresponding to the loading rates of 0.10 and 0.50 MPa/s, respectively. After comparison, rockburst processes are similar at different loading rates, but the failure severity is different. The detailed analysis is discussed below. From Figures 5-7, the failure process of the hole sidewall experienced calm period, pellet ejection period, the rock fragment exfoliation period, and finally formed the V-shaped notch. The failure process is the same as that of the circular hole sidewall in Ref. [48].

4 Discussions

4.1 Influence of loading rate on sidewall failure

The initial failure stress of the hole sidewall at different loading rates is listed in Table 2. The trend of the initial failure stress with the loading rate is shown in Figure 8. The initial failure stress increases with the loading rate. However, increasing extent of initial failure stress of the hole sidewall gradually decreases with increasing the loading rate. This indicates that the sensitivity of the initial failure stress to the loading rate gradually decreases, which is consistent with the results of many researchers on relationship between the peak strength and loading rate (strain rate) [49, 50]. In underground engineering, according to the kirsch solution of the stress distribution in the circular tunnel surrounding rock, the maximum tangential stress σ_θmax can be obtained as follows:

σ_θmax=3σ_z-σ_y                                              (2)

where σ_θmax is the maximum tangential stress; σ_z is the vertical stress; and σ_y is the horizontal radial stress. The maximum tangential stress σ_θmax of hole sidewall was calculated using Eq. (2), and the results were listed in Table 2. σ_θmax of the initial failure increases with the loading rate, and σ_θ/σ_c is higher than 1 under different loading rates, which indicates that σ_θmax is higher than the uniaxial compressive strength of sandstone.

Table 2 Stress states and sidewall failure stresses under different loading rates

Figure 8 Variation in initial failure stress of hole sidewall with loading rate

To further study the effect of loading rate on rockburst severity of hole sidewall, within 20 s from the time of the initial failure stress, the failure stresses and photos of sidewall under different loading rates are given every 5 s, and 5 failure photos are given under each loading rate, as shown in Figure 9. When the loading rate is 0.02 MPa/s, within 20 s from the initial failure time (1493.32 s) of the hole sidewall, the failure on both sidewalls is similar to the severity of the initial failure, and no obvious failure occurred (see Figure 9(a)). This indicates that both sidewalls were in calm period within 20 s, and the surrounding rock accumulated energy. At the loading rate of 0.10 MPa/s, within 20 s from the initial failure time (549.00 s) of the hole sidewall, the failure of the right sidewall develops from the initial particle ejection to form an approximately perforated crack along the hole axial direction. During the period of time, the crack propagated rapidly, and the rock fragment were exfoliated, as shown in Figure 9(b). The failure degree of the right sidewall slightly increases. Compared with the failure development of sidewall at the loading rate of 0.02 MPa/s, the failure development is faster at the loading rate of 0.10 MPa/s. Figure 9(c) shows the development of the failure degree of the sidewall from the initial failure within 20 s at the loading rate of 0.50 MPa/s. The failure severity of the sidewall increases with the time and vertical stress. When the vertical stress increases to 55.00 MPa, the load is maintained, and the failure of the sidewall continues to increase during the constant loading. After repeated particle ejections and rock fragment exfoliations, the left and right sidewalls of the circular hole caused serious failure. Compared with the failure development of sidewall at the loading rate of 0.10 MPa/s, the failure development is faster with a loading rate of 0.50 MPa/s. After the initial failure of sidewall, there will be a calm period for the accumulation of energy. When the accumulated energy exceeds the rock capacity, the hole sidewall is further failure. With increasing the loading rate, the time of calm period decreases. This indicates that rockburst severity is significantly increased in the same period of time.

Figure 9 Comparison of failure severity of hole sidewall in same time period under different loading rates:

When the vertical stresses are 50.00, 52.00 and 55.00 MPa, the failure states of the hole sidewall under different loading rates are shown in Figure 10. When the vertical stress is 50.00 MPa and the loading rate is 0.02 MPa/s, obvious failure occurs on the right sidewall, and the specimen is approximately penetrated along the hole axial direction. When the loading rate is 0.10 MPa/s, only one crack along the axial direction occurred on the right sidewall, and no rock fragment exfoliation occurred. At the loading rate of 0.50 MPa/s, there is no obvious failure on both sidewalls. By comparison, rockburst severity decreases with increasing the loading rate when the vertical stress was maintained at a higher stress level. The same results can be obtained by comparing the failure severity under different loading rates when the vertical stress is 52.00 MPa or 55.00 MPa.

After the end of the tests, the overall failure of the specimens and both sidewalls of the circular hole at different loading rates are shown in Figure 11. From Figure 11, slight failure occurs on the left sidewall, and serious failure occurs on the right sidewall at the loading rate of 0.02 and 0.10 MPa/s. When the loading rate was 0.50 MPa/s, there was slight failure on both sidewalls. The results indicate that the depth of the V-shaped notch on the right sidewall is the largest at the loading rate of 0.02 MPa/s, and the failure is the most serious. There is no obvious V-shaped notch on the left sidewall, and the failure is similar to that at the loading rate of 0.10 MPa/s. The failures on the left and right sidewalls are the lightest at the loading rate of 0.50 MPa/s. The results further prove that the failure severity of the sidewall decreases with increasing loading rate when the vertical stress is maintained at a higher stress level.

Figure 10 Comparison of failure severity of hole sidewall under different loading rates with constant vertical stress (50.00, 52.00 and 55.00 MPa):

Figure 11 Failure of overall specimen and left and right sidewalls of circular tunnel:

In general, the failure severity of the sidewall increases with increasing the loading rate in the same time period after the initial failure.When the vertical stress is constant at a high level, the failure of the sidewall at a higher loading rate is lower, and the failure at a lower loading rate is more serious. Therefore, the failure severity of the hole sidewall decreases with increasing the loading rate. The main reasons that the loading rate affects the severity of rockburst are as follows:

Rock is a viscoelastic body, which is sensitive to time. The development degree of viscous deformation is less, and the plastic deformation is smaller at a higher loading rate, so the rock shows obvious brittleness. In rock mechanics, rock has a significant loading rate effect. The failure strength of the specimen is lower at a lower loading rate, which indicates that the specimen is more likely to failure, so the initial failure stress of the hole sidewall is lower at a lower loading rate. During the same period of time from the initial failure, due to the low initial failure stress of tunnel sidewall under the low loading rate, the energy stored in rock is low. The micro cracks and other defects in the surrounding rock propagate and coalescence, which leads to the consumption of much energy stored in rock during the loading process, that is, the proportion of dissipated energy is large. Therefore, the energy released of sidewall failure is low, and the kinetic energy converted during the sidewall failure is relatively small, which is manifested by the lower severity of the hole sidewall failure at low loading rate in the same period of time.

Under the condition of low loading rate, it takes a long time to load to the same vertical stress, and micro cracks in the surrounding rock have sufficient time to expand and penetrate. The sidewall failure is relatively severity. When the loading rate is high, the strength of rock is high, and the deformation of sidewall failure will be reduced. Also, yield stage is shorter, and the deformation speed is faster than the elastic stage, and the energy consumed is less. Therefore, the elastic deformation energy accumulated in rock is higher. Releasable strain energy stored in rock is used as the source of rockburst, and multiple energy released often induces sudden and severe rockburst. The larger the loading rate is, the smaller the elastic energy consumed by the surrounding rock before the sidewall failure, and the higher the elastic strain energy released when the rockburst occurs, showing obvious elastic failure. Therefore, rockburst is energy process of small consumption and large release at a high loading rate.

4.2 Rock fragment characteristics of sidewall failure under different loading rates

In underground engineering, the characteristics and laws of fractals were investigated on the distribution of microcracks, crack propagation and damage evolution during rock mass breaking. To analyze the characteristics of rock fragments produced during the failure of circular hole sidewalls under different loading rates, fractal theory was introduced to analyze and study the rock fragments. The rock fragments were sieved, and the diameters of the sieve holes were 8.00, 6.70, 4.75, 2.36, 1.00, 0.55 and 0.075 mm, respectively. The sieved rock fragments are shown in Figure 12, and the masses of rock fragments were weighed by electronic scale, which were listed in Figure 12. From Figure 12, when the particle sizes of the rock fragments under different loading rates are greater than 2.36 mm, the rock fragments presents a long strip shape. On the contrary, the length and width of the rock fragments are not significantly different. The larger rock fragments generated by the hole sidewalls are mainly from the surfaces of the V-shaped notches formed on both sidewalls, and the failure modes are primarily tensile and split. The smaller rock particles are fine powder, which are from the inside of the V-shaped notches. The failure mode is mainly shear and slide. Comparing the test results under different loading rates, the failure of the hole sidewalls is more serious at the loading rate of 0.02 MPa/s, and the sidewall produces more rock fragments and the quality of the rock fragments is higher. With increasing the loading rate, the quality of rock fragments decreases.

Figure 12 Rock fragment distribution with different particle sizes under different loading rates:

According to the fractal theory, the ratio between the rock fragment mass with a diameter r smaller than R and the total mass of rock fragments produced by the rockburst can be expressed by Eq. (3) [51].

                     (3)

Taking the natural logarithm to Eq. (3), it can obtain Eq. (4) as follows:

            (4)

where r is the grain of size; R is a specific measuring scale, which is the diameter of the sieve; D is the fractal dimension; M is mass under the condition of rT is the total mass of the particle; R_L is the diameter of the largest sieve. Using the data in Figure 12, the M (rT and R_L under different loading rates can be calculated, which are listed in Table 3. Using the data in Table 3 and Eq. (4) to perform linear fit, Figure 13 can be obtained. The slope of the fitting line is 3-D, therefore, fractal dimension D of rock fragments generated by rockbursts of the hole sidewall under different loading rates can be obtained. Figure 14 shows the relationship between the fractal dimension D and the loading rate. From Figure 14, the fractal dimension D increases with the loading rate.

Table 3 Calculated results under different loading rates

4.3 Engineering significance of loading rate

In underground engineering, different mining (drilling) methods have significant impact on production efficiency, and fully mechanized mining technology is an important technical means to improve production efficiency [52]. LI et al [53] reputed that the loading rate studied in the laboratory can be reflected as the tunnel driving speed and the working surface propulsion speed under field conditions. Theory and practice show that too slow advancing speed is not conducive to safe production. With the gradual maturity of fully mechanized mining technology, the advancing speed of the working surface tends to accelerate [52, 54]. The mining rate during fully mechanized mining will affect the change of the loading rate of surrounding rock, and the elastic deformation energy of the rock mass will also increase with the loading rate [55], so the advancing speed of fully mechanized mining face is not the faster, the better. At the same time of improving productivity with high speed propulsion in some mines, it is found that the impact proneness in mining is also increasing, which brings great hidden danger to safety production [56, 57]. Therefore, only a proper increase in the fully mechanized mining speed can be beneficial to working surface management and safe production [52, 55, 58].

Figure 13 Fitting relationships with different particle sizes under different loading rates

Figure 14 Relationship between fractal dimension D and loading rate

The results of this study showed that the tunnel (cavern) surrounding rock has lower failure strength under lower loading rate, and the surrounding rock is more prone to failure. When the loading rate is higher, the failure strength of the surrounding rock is higher, and higher elastic energy is accumulated in the surrounding rock. Once the surrounding rock occurred rockburst, the rockburst severity is stronger. Therefore, compared with the low loading rate, the tunnel (cavern) surrounding rock has greater rockburst grade under high loading rate, and the surrounding rock is more prone to severe rockburst disasters. In actual engineering, the reasonable design of roadway driving speed and working surface advancing speed (controlling the loading rate of surrounding rock) is not only beneficial to improve production efficiency, but also can effectively reduce the possibility of rockburst occurrence and the severity of rockburst occurrence. Furthermore, it can promote the good and orderly production and construction of underground engineering to the maximum extent, which has important reference value and guiding significance for actual engineering.

5 Conclusions

In this study, the true-triaxial loading tests were conducted using true-triaxial testing machine on cubic sandstone specimens with a circular perforated hole under different loading rates. The rockburst process was monitored and recorded in real-time using microcamera. By analyzing and discussing the experimental results, the primary conclusions are as follows.

The rockburst process of the circular hole sidewalls under different loading rates is basically similar, and experienced calm period, pellet ejection period, the rock fragment exfoliation period and finally formed the V-shaped notch.

Comparing the development of sidewall failure in the same time period from the initial failure under different loading rates, the initial failure stress increases with increasing the loading rate. During the same period after the initial failure, the rockburst severity of hole sidewalls increased significantly with increasing the loading rate. When the vertical stress is constant and maintains a high stress level, the rockburst of circular hole sidewall under low loading rate is more serious than that under high loading rate.

With increasing the loading rate, the quality of rock fragments produced by the rockburst decreases, and the fractal dimension of rock fragments increases.

Contributors

SI Xue-feng provided the methodology, conducted the tests, wrote original draft, and revised and edited the manuscript. HUANG Lin-qi revised and edited the manuscript. GONG Feng-qiang provided the idea and methodology, and revised and edited the manuscript. LIU Xi-ling revised and edited the manuscript. LI Xi-bing revised the manuscript, and provided financial support.

Conflict of interest

SI Xue-feng, HUANG Lin-qi, GONG Feng-qiang, LIU Xi-ling and LI Xi-bing declare that they have no conflict of interest.

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(Edited by HE Yun-bin)

中文导读

真三轴应力条件下加载率对深部圆形隧道岩爆影响的试验研究

摘要：为了研究加载率对真三轴应力条件下圆形隧道围岩岩爆的影响，对含有d50 mm圆形贯穿孔洞的100 mm×100 mm×100 mm立方体砂岩试样开展了真三轴试验，采用微型摄像机实时监测并记录洞壁的破坏过程。加载率分别为0.02，0.10和0.50 MPa/s。试验结果表明，岩爆过程经历了平静期、颗粒弹射期、岩片剥落期，最终形成V形槽。岩爆具有一定的时滞性，发生岩爆时垂直应力较高。洞壁初始破坏时的垂直应力随着加载率的增加而增加。在初始破坏之后的同一时间段内，岩爆的剧烈和严重程度随着加载率的增加而增强。当垂直应力恒定并保持在较高的应力水平时，低加载率下洞壁的岩爆破坏程度要比高加载率下的严重。随着加载率的增加，岩爆产生的岩片的质量减小，岩片的分形维数增大。

关键词：岩爆；加载率；深部圆形隧道；真三轴试验；V形槽

Foundation item: Projects(11972378, 41630642) supported by the National Natural Science Foundation of China; Project(2019zzts310) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-06-22; Accepted date: 2020-09-04

Corresponding author: LI Xi-bing, PhD, Professor; Tel: +86-13974870961; E-mail: xbli@csu.edu.cn; ORCID: https://orcid.org/0000- 0002-1191-9544