J. Cent. South Univ. (2021) 28: 582-594

DOI: https://doi.org/10.1007/s11771-021-4623-z

Excavation-induced microseismicity and rockburst occurrence:Similarities and differences between deep parallel tunnels with alternating soft-hard strata

FENG Guang-liang(丰光亮)^{1, 2}, CHEN Bing-rui(陈炳瑞)¹, JIANG Quan(江权)¹,

XIAO Ya-xun(肖亚勋)¹, NIU Wen-jing(牛文静)³, LI Peng-xiang(李鹏翔)³

1. State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and

Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China;

2. Guangxi Key Laboratory of Disaster Prevention and Engineering Safety, Guangxi University,Nanning 530000, China;

3. Key Laboratory of Ministry of Education of Safe Mining of Deep Metal Mines, Northeastern University, Shenyang 110819, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: Excavation-induced microseismicity and rockburst occurrence in deep underground projects provide invaluable information that can be used to warn rockburst occurrence, facilitate rockburst mitigation procedures, and analyze the mechanisms responsible for their occurrence. Based on the deep parallel tunnels with the maximum depth of 1890 m created as part of the Neelum–Jhelum hydropower project in Pakistan, similarities and differences on excavation-induced microseismicity and rockburst occurrence between parallel tunnels with soft and hard alternant strata are studied. Results show that a large number of microseismic (MS) events occurred in each of the parallel tunnels during excavation. Rockbursts occurred most frequently in certain local sections of the two tunnels. Significant differences are found in the excavation-induced microseismicity (spatial distribution and number of MS events, distribution of MS energy, and pattern of microseismicity variation) and rockbursts characteristics (the number and the spatial distribution) between the parallel tunnels. Attempting to predict the microseismicity and rockburst intensities likely to be encountered in subsequent tunnel based on the activity encountered when the parallel tunnel was previously excavated will not be an easy or accurate procedure in deep tunnel projects involving complex lithological conditions.

Key words: microseismicity; rockburst; soft and hard alternant strata; deep parallel tunnels; Neelum–Jhelum hydropower project

Cite this article as: FENG Guang-liang, CHEN Bing-rui, JIANG Quan, XIAO Ya-xun, NIU Wen-jing, LI Peng-xiang. Excavation-induced microseismicity and rockburst occurrence: Similarities and differences between deep parallel tunnels with alternating soft-hard strata [J]. Journal of Central South University, 2021, 28(2): 582-594. DOI: https://doi.org/10.1007/s11771-021-4623-z.

1 Introduction

Rockbursts can be one of the typical dynamic instability problems caused by high geostress during deep tunnel excavation as they often involve the detachment of rock fragments with very high velocities that are of considerable danger to both workers and tunneling equipment [1-3]. A large number of rockbursts tend to occur during the excavation of tunnels through rock that is deep underground and hard. In the past, such events have caused serious casualties, much mechanical damage, and delays to projects, leading to considerable economic losses. For example, seven fatalities and one injury, as well as the total destruction of a tunnel boring machine (TBM), were caused by an extremely intense rockburst that occurred on 28 November 2009 during the construction of the Jinping II hydropower station in China [4]. Another extremely intense rockburst occurred on 31 May 2015 in a deep tunnel of the Neelum–Jhelum (NJ) hydropower project in Pakistan. This event caused the tunnel’s shape to change, broke the support measures put in place, and led to a serious delay of roughly seven months in the project [5].

A number of laboratory-based and in situ monitoring tests for rockburst research have been made recently. In laboratory tests, rockburst proneness index [6], mechanism of rockburst development process [7-10], influence of structural plane on rockburst [11, 12], etc. were studied. The in situ monitoring tests were mainly on the aspects of rockmass fracture monitoring [13, 14], mechanism of rockburst development process [15, 16], warning of rockburst [4, 17], etc. The evolution of rockbursts resulting from rock engineering activity can, in essence, be seen as a series of rockmass fracture events. Therefore, in situ microseismic (MS) monitoring of such fracturing processes has been widely used. The MS monitoring technique involves using a spatially- distributed set of MS sensors with different azimuths which subsequently capture the MS waves released when the rocks fracture. By analyzing the detected MS waves, the time, location, intensity, and type of rock fracture occurring can be deduced. MS activity or ‘microseismicity’ is essentially the MS events (rockmass fracture events) occurring within a given volume of rock over a certain period of time. Thus, excavation-induced microseismicity data are valuable and are often used to analyze rockburst mechanism, warn impending rockburst occurrence, and instigate steps to mitigate their effects. A great deal of progress has been made in this respect in deep mine engineering [18-22] and deep tunnel engineering [23-26]. For example, MS monitoring for rockburst warning was successfully carried out during the construction of the Jinping II hydropower station in China [23]. The process of intermittent rockburst development (i.e., more than one rockburst occurring in a particular area) has been investigated using excavation-induced microseismicity data recorded in the deep tunnels constructed as part of a railway line in China [24]. The characteristics of rockbursts occurring along a structural plane were studied using the technique during the construction of a tunnel forming part of the Hanjiang–Weihe river diversion project [26].

Excavation-induced microseismicity depends on numerous factors, e.g., geological conditions, in situ stresses present, properties of the rockmass, excavation technique, and support measures employed. As a result, the spatiotemporal characteristics and temporal evolution patterns associated with excavation-induced microseismicity during rockburst development are often very complicate. In order to obtain a better understanding of these complex development processes, it is clear that the excavation-induced microseismicity occurring under different conditions needs to be studied in greater detail. In this work, we consider the MS monitoring experiments conducted in the deep, parallel TBM tunnels excavated as part of the NJ hydropower project in Pakistan. These tunnels pass through interbedded layers of sandstone, siltstone, and mudstone and have a maximum depth of 1890 m. Rockbursts occurred frequently during their construction. Data were collected on the excavation-induced microseismicity occurring as the parallel tunnels were being excavated and the subsequent occurrence of rockbursts. Very little excavation-induced MS data have been collected under such complex lithological conditions, i.e., soft and hard alternant strata. Therefore, studying this data makes a very helpful contribution to the understanding of excavation-induced micro- seismicity and rockburst development in deep tunnel projects.

In the paper, we first present the excavation- induced microseismicity and rockburst data recorded in the deep, parallel NJ tunnels. We concentrate on a common range of chainage measuring some 2379 m in length in each tunnel and analyze the similarities and differences between the two. The excavation-induced microseismicity and rockburst characteristics in the parallel tunnels are then compared and analyzed. The data and results can help to improve our understanding of excavation-induced microseismicity and rockbursts in deep TBM tunnel projects involving complex lithological conditions.

2 MS monitoring of rockbursts in deep parallel NJ tunnels

2.1 Monitoring activity in tunnels

In situ MS monitoring was conducted in the NJ TBM tunnels for rockburst warning and risk mitigation purposes. Here, we concentrate on parallel tunnels (referred to as TBM696 and TBM697) that pass through strata that are alternately soft and then hard. The tunnels have a maximum burial depth of 1890 m and are both 8.53 m in diameter. The geology in the region of interest is characterized by a folded, heavily- tectonized sedimentary sequence belonging to the Lower Murree Formation. This formation consists mainly of interbedded sandstone, siltstone, and mudstone. The MS monitoring system consisted of 2-3 groups of MS sensors that were set up just behind the working face of each tunnel and manually removed, moved, and set up again as tunnelling proceeded. The MS system used (sensor layout, communication network, location of the monitoring center, etc.) has been described in detail elsewhere [4].

The parts of the two TBM tunnels that were monitored are also shown in Figure 1. In this work, we concentrate on two stretches of the parallel tunnels with the same chainage (corresponding to 8+005-5+626, i.e., 2379 m in each tunnel). The distance between the centers of the tunnels is 55 m throughout the region of interest and the tunnel TBM697 was excavated before TBM696. The support measures used in the two TBM tunnels are shown in Figure 1. Rockbolts, TH beams, steel mesh, and shotcrete which were deployed in the parallel tunnels as the tunnels were excavated. Further details of the excavation of these sections of the tunnels are shown in Table 1. The table shows that the time spent in excavating these sections and their average daily rates of advance are almost the same. However, the advance rates at a given chainage are often different between the tunnels, as shown in Figure 2. The values in Figure 2 were calculated every 100 m.

2.2 Description of rockbursts

Rockbursts occurred frequently as the TBMs moved through the alternating soft-hard strata. In fact, more than 800 rockbursts of varying intensity were encountered in total. In this work, we adopt the method of classifying rockburst intensity recommended by the National Standards Compilation Group of the People’s Republic of China in Code for Geological Investigations of Hydropower Engineering [27]. Four typical rockburst cases of different intensity that occurred in the tunnels are shown in Figure 3. The damage depth of the slight rockburst (Figure 3(a)) is ~0.3 m and the impact of the rockburst on construction is low. The damage degree of the moderate rockburst (Figure 3(b)) to surrounding rock is slightly larger. It has some impacts on the construction process, and the normal construction was resumed after a short period of time for risk elimination and support. The damage depth of the intense rockburst (Figure 3(c)) is more than 1 m, and the oil cylinder in the bolter and cutterhead were damaged. The extremely intense rockburst (Figure 3(d)) that occurred in tunnel TBM696 on the 31 May 2015 caused severe damage to the TBM and caused the excavation of that tunnel to be delayed by 7 months.

Figure 1 MS monitoring area and support measures in deep parallel NJ TBM tunnels

Table 1 Details of excavation of two sections of two TBM tunnels with same chainage (8005–5626) whose MS monitoring data are considered in this work

Figure 2 Average advance rate in different areas for two TBM tunnels

Figure 3 Photographs of rockbursts that occurred in deep NJ tunnels. The rockbursts shown correspond to intensities that are:

3 Excavation-induced microseismicity and rockburst occurrence in deep parallel NJ tunnels

Excavation-induced microseismicity and rockburst arise because of the stress changes occurring during tunnel excavation. The size of the stress present is substantially different in different rock formations. Therefore, excavation-induced microseismicity and rockburst occurrence are strongly affected by the varying lithology that occurs in the alternately soft-hard strata surrounding the NJ tunnels. The lithology, excavation-induced microseismicity, and rockburst occurrence along the parallel tunnels therefore need to be considered simultaneously, as we illustrate in Figure 4 as example. We can find that the lithology changes frequently along the tunnels and, for a particular chainage, the lithologies encountered in the parallel tunnels are only weakly related in the parts where the lithology changes frequently.

3.1 Similarity on excavation-induced micro- seismicity between parallel tunnels

Figure 4 shows that the microseismicity in the parallel tunnels are somewhat similar but also feature significant differences. They are similar because a large number of MS events occurred in each of the parallel tunnels during excavation. There are several reasons for this. Firstly, the tunnels are mainly composed of sandstone and siltstone which have high strengths. Sandstone and siltstone are strong and brittle. The average UCS of them are ~90 and ~70 MPa, respectively. And the average tensile strengths are ~8 and ~7 MPa, respectively. Secondly, the in situ stress in the deep tunnels is high. In fact, in situ stress measurements show that it is as high as 100 MPa in places [28]. Further evidence of the high in situ stress state in the engineering region is provided by the common observation of core discing and overcoring of the hollow inclusion cells when boreholes were drilled during these in situ stress tests (Figure 5). Thirdly, the lithology changes frequently along the two tunnels, which makes the stress gradient due to the adjacent lithologies large. For example, in Figure 4(a) (chainage 8005-7605), there are areas where the lithology changes 2-3 times in just 10 m of tunnel length. In the 10 m chainage range 7715-7705 in tunnel TBM696, the lithology changes according to the pattern sandstone– siltstone–sandstone–siltstone. We know that the difference in a single stress component between a strong layer and a weak layer is large. As a result, the stress gradient due to these adjacent lithologies is large. Such high strength, high stress and high stress changes make MS events occur more readily in the parallel tunnels.

Figure 4 Excavation-induced microseismicity, rockburst occurrence, and geological condition variation along parallel TBM tunnels. The diagrams correspond to two parts of the tunnels (chainages):

Figure 5 Photographs illustrating phenomenon of core discing in NJ tunnels showing:

3.2 Difference on excavation-induced micro- seismicity between parallel tunnels

Most of the time, however, the excavation- induced microseismicity is different in the parallel tunnels. The main differences are:

1) As can be seen from Figure 4, there is generally no correspondence between the distribution of sites wherein the microseismicity aggregates spatially, MS events that release large MS energies, and MS events of large magnitude. The levels of microseismicity in the two tunnels, at the same chainage of some section, are ‘opposite’ to each other most of the time. For example, in the chainage region 7785-7685, the excavation- induced microseismicity in tunnel TBM696 is high with many MS events and much MS energy released, while in TBM697, it is low. In the following chainage (7685-7605), however, the relative microseismicity pattern is reversed in the two TBM tunnels (i.e., there are more MS activities in tunnel TBM697 than in tunnel TBM696). In the chainage region 6005-5625, the microseismicity in tunnel TBM697 is significantly greater than that in tunnel TBM696. The microseismicity remains active and is fairly concentrated spatially in TBM697, while it is relatively calm and sparse spatially in TBM696, on the whole.

2) The data can be analyzed statistically to extract further information about the number of excavation-induced MS events and energies occurring in the tunnels. The results are shown in Figures 6-8. Figure 6 shows that the number of MS events in TBM696 is significantly smaller than that in TBM697. However, the total MS energy released is virtually the same. This implies that the average MS energy per event is larger in TBM696 than it in TBM697. This is also apparent in Figure 7.

Figure 6 Numbers and energies of excavation-induced MS events in two tunnels

Figure 7 Proportion of MS events of different energy occurring in tunnels:

Figure 8 Variation of excavation-induced microseismicity along two tunnels showing:

The proportions of excavation-induced MS events of different MS energy are shown in Figure 7 in the form of pie charts. The figure shows that the proportion of high-energy MS events in TBM696 (that is, those with lg (E/J) values larger than 1.0) is 28.6% in total, which is larger than the corresponding value in TBM697 (18.8% in total). Thus, the behavior in TBM696 appears to be characterized by fracturing processes that are relatively fewer in number but larger in energy (while TBM697 is dominated by a relatively larger number of less energetic fracturing events).

3) Figure 8 shows the number of MS events and their energies plotted as a function of chainage in the tunnels. The values were calculated every 50 m. It is clear that the curves plotted fluctuate strongly, reflecting the fact that the excavation- induced MS events are strongly affected by the frequent changes in lithology and excavation in the tunnels. It can also be seen that the ways the excavation-induced MS events vary along the lengths of the two tunnels are different. That is, the number of MS events in TBM696 gradually decreases overall, while it decreases at first and then increases in TBM697 (see the fitlines in the figure). The reason for this phenomenon is given as follows.

The distribution of MS events can be used to divide the tunnels into three sections (labeled I, II, and III here), as shown in Figure 8(a). Section I (chainage 8005-7455) corresponds to the occurrence of many MS events in both tunnels (it is worth noting that the correlation between pairs of data points at any given chainage in Figure 8(a) appears to be very weak between the two tunnels). The lithology in Section I changes frequently and there is a relatively large amount of sandstone in both tunnels. The frequency at which the lithology changes in this section is 38 (50) in tunnel TBM696 (TBM697) and the proportion of sandstone is about 47% (48%). MS events are likely to occur in this section as the lithology changes frequently and there is a large amount of sandstone present, which is a strong and brittle rock that is highly stressed.

Section II corresponds to the chainage range 7455–6855. In this section, a large number of MS events also occurred, but not as many as in section I. The lithology in Section II changes frequently but not as frequently as in section I – to the extent that the change of lithology frequency is 25 (30) in tunnel TBM696 (TBM697). There is also a lot of sandstone in both tunnels, but less than in section I: 24% (32%) in tunnel TBM696 (TBM697). Therefore, the frequency at which the lithology changes and proportion of sandstone present are lower than in section I.

Section III covers chainage 6855–5655. In this section, the number of MS events in tunnel TBM696 is smaller than that in TBM697 (the trend in TBM696 is for the number of MS events to continue falling, while it starts increasing again in TBM697). In this case, the lithology in TBM696 changes much less often and is dominated by siltstone (accounting for over 90% of the rock in both tunnels). Thus, the amount of sandstone present is small. In TBM697, the lithology gradually starts to change more frequently and the proportion of sandstone also gradually increases along the length of the tunnel. In contrast to the number of events, Figure 8(b) shows that the trends apparent in the MS energy along the two tunnels are similar (generally decreasing with heavy fluctuation).

3.3 Similarity on excavation-induced rockbursts in parallel tunnels

Figure 4 also contains valuable information about the rockbursts occurring in the tunnels. Once again, there are some similarities in the occurrence of excavation-induced rockbursts in the two tunnels. These can be summarized as follows:

1) All of the rockbursts that occurred in the parallel TBM tunnels studied were of slight or moderate intensity; there was no intense or extremely intense rockburst.

2) Rockbursts only occurred in sandstone and siltstone, and the majority of them occurred in the sandstone components. For example, in Figure 4(a) (chainage 8005-7605), 16 rockbursts occurred in sandstone and only 2 in siltstone during the excavation of TBM696. Similarly, 18 rockbursts occurred in sandstone in TBM697 and 2 in siltstone.

3) The spatial distribution of rockburst is mainly concentrated near the rock interface and on the side of rock with a higher strength. For example, in Figure 4, rockbursts are mainly distributed near the rock interface, and most of them occurred in sandstone.

4) Rockbursts occurred most frequently in certain local sections of the two tunnels. Several examples are given in Table 2. Consider the high concentration of rockbursts encountered in tunnel TBM696 in the chainage range 6915-6875 (the locality with the highest rate of occurrence of rockbursts). Figure 9 gives a view of this interesting region. The figure shows that there is a large amount of microseismicity in the region 6935-6875 of this tunnel (many MS events and a great deal of MS energy released). This shows that there is a large amount of energy concentrated in the rockmass in this section of the tunnel. The figure also indicates that the microseismicity is concentrated in a region of sandstone which is a strong material capable of storing large amounts of energy. Therefore, when this region is excavated, the energy concentrated within the sandstone is violently released and this leads to the occurrence of several significant rockbursts. The daily advance of the workface during the excavation of this section is also shown in Figure 9. From December 31 to January 3, the workface advanced relatively slowly through the section wherein the rockbursts occurred. This suggests that the high concentration of rockbursts found in this section is not due to the daily advance rate being too high. Therefore, it is highly likely that the rockbursts occurred mainly because of the high concentration of energy stored in the sandstone. Clear cut marks are often left in the surface of the surrounding intact rockmass after TBM excavation. This indicates that the damage caused to the surrounding rock by TBM excavation is weak and that the energy concentrated within the rockmass is being released gradually during the TBM excavation process. When the TBM passes through an area with a high concentration of energy, much of the energy stored in the surrounding rock will be gradually released and some of it will appear in the form of rockburst. Therefore, a series of successive rockbursts can be readily generated when the TBM excavates a region in which energy is concentrated.

Table 2 Examples of regions containing high concentrations of excavation-induced rockbursts

3.4 Difference on excavation-induced rockbursts in parallel tunnels

There are also some differences in the characteristics of the rockbursts occurring in the parallel tunnels, namely:

1) Rockbursts (especially those characterized as slight) occurred much more frequently in tunnel TBM696 compared to TBM697, as highlighted in Figure 10.

2) The spatial correlation between rockbursts in the parallel tunnels is weak. This is highlighted in Figure 11, which shows the number of rockbursts occurring at different chainages along the length of the parallel TBM tunnels (calculated every 50 m giving a total of 48 data points). The numbers of rockbursts in the two tunnels exactly coincide in Figure 11 when no rockbursts occurred in either tunnel (20 data points). The other 28 data points feature rockbursts in at least one tunnel (10 correspond to rockbursts in both tunnels; 18 to rockbursts in one tunnel but not the other). Rockburst occurrence changes with the geological conditions and rockbursts are most likely to occur in regions where there are frequent lithology changes. The lithologies in the two tunnels are only found to be weakly related at the same chainage, especially when the lithology changes frequently. This may explain why the spatial characteristics of the rockbursts are different in the parallel tunnels.

Figure 9 Expanded view of chainage 6975-6865 in TBM696 tunnel showing high concentration of excavation-induced microseismicity, geological condition, rockburst locations, and daily advances made during tunneling

Figure 10 Numbers of excavation-induced rockbursts of different intensity detected in parallel TBM tunnels with same chainages studied

3.5 Mechanism analysis for differences

The occurrence of excavation-induced microseismicity and rockbursts is dependent on many factors including the local geological condition, local stress conditions, and daily advance of the working face. The local geological conditions are different in the parallel tunnels and the local stress conditions are also different due to the differences in the local geological conditions. These differences will be particularly large in the deep NJ parallel tunnels as the lithological conditions are very complex (Figure 4). Moreover, in the parts where the lithology changes frequently, the type of lithology in one tunnel is only weakly related to that in the other tunnel, at the same chainage. Furthermore, although the average daily advance is virtually the same in the two tunnels, the daily advance at a given chainage is different in each tunnel (Figure 2).

The abovementioned factors (e.g. local geological condition, local stress conditions, and daily advance) will strongly affect the excavation- induced microseismicity and rockbursts occurrence in the parallel tunnels. Therefore, the excavation- induced microseismicity and rockbursts occurrence in the parallel tunnels will generally be different in many ways (spatial distribution, intensity distribution, etc.). As a result, significant differences were observed in the spatial distributions of the MS events, the number of MS events, the distribution of MS energy magnitudes, and the variation in the microseismicity along the two tunnels. The frequency and spatial characteristics of the rockbursts occurring along the tunnels are also different. Thus, attempting to predict the microseismicity and rockburst intensities likely to be encountered in tunnel TBM696 based on the activity encountered when tunnel TBM697 was previously excavated will not be an easy or accurate procedure.

4 Discussion

In the two deep tunnels, the evolution laws of the number of MS events and the MS energy show different characteristics. The number of MS events in TBM696 gradually decreases overall, while it decreases at first and then increases in TBM697 (see the fitlines in Figure 8(a)). In contrast to the number of events, Figure 8(b) shows that the trends apparent in the MS energy along the two tunnels are similar (generally decreasing with heavy fluctuation). It means that in TBM696, the evolution laws of the number of MS events and the MS energy are almost the same, i.e., gradually decreases. However, in TBM697, the evolution laws of the number of MS events and the MS energy are quite different.

Figure 11 Spatial variation of numbers of excavation-induced rockbursts along two tunnels

It was found that rockmass structure is one of the important factors affecting the correlation between the number of MS events and the MS energy. Figure 12 shows the MS energy distribution in 44 rockburst development processes that occurred with and without the presence of structural planes in the deep tunnels of Jinping-II Hydropower Station, China [23]. It can be seen from Figure 12 that the proportion of low-energy MS events, i.e., whose logarithm of MS energy (measured in Joules) is smaller than or equal to 1.09, is relative higher when with structural plane than without it. However, the proportion of high-energy MS events, i.e., whose logarithm of MS energy (measured in Joules) is larger than 3.83, is relative lower when with structural plane than without it.

Figure 12 Effect of structural planes on MS energy distribution in rockburst development processes in deep tunnels of Jinping-II Hydropower Station, China

When the integrity of rockmass is good and belongs to an integral structure, rockmass failure will be difficult. The number of MS events generated under excavation will be small even under high stress environment. However, due to its high energy storage capacity and the high stress environment, some of the fractures in the rockmass will be strong and the MS energies release associated with the fractures will be high. In this situation, the microseismicity will be characterized by a small number of MS events but with high MS energies. When the integrity of the rockmass is poor and belongs to a layered structure, the rockmass failure will likely to occur along the fracture when with high stress environment. However, due to the low energy storage capacity of rockmass, the majority of the MS events generated will be with low energies. In this situation, the microseismicity is characterized by a large number of MS events but with low MS energies. It can be seen that the correlation between the number of MS events and the MS energy is generally weak in the above two situations. When the integrity of the rockmass is good and belongs to a massive structure or a thick layer structure, the rockmass will have a certain energy storage capacity and there will be a certain number of structural planes. In this situation, the correlation between the number of MS events and the MS energy will be generally strong.

5 Conclusions

Excavation-induced microseismicity and rockbursts occurring in a pair of parallel and deep tunnels with soft and hard alternant strata in Pakistan have been analyzed in this paper. Their similarities and differences between the parallel deep tunnels are revealed. The main conclusions are as follows.

1) A large number of MS events occurred in each of the parallel tunnels during excavation. The correspondence between the microseismicity in the two TBM tunnels (spatial distribution and number of MS events, distribution of MS energy, and pattern of microseismicity variation) is weak. The microseismicity in tunnel TBM696 is dominated by fracturing events that release larger amounts of energy (compared to those in tunnel TBM697). The number of excavation-induced MS events and their energy change considerably along the length of the two TBM tunnels. The patterns of evolution of the number of MS events along the tunnels are also different.

2) The rockbursts were all slight and moderate in intensity and only occurred in sandstone and siltstone (most of them were in the sandstone). Rockbursts occurred most frequently in certain local sections of the two tunnels. However, there is no obvious spatial correspondence between the rockbursts occurring in the parallel tunnels. Rockbursts (especially those characterized as slight) occurred much more frequently in tunnel TBM696 compared to the subsequent tunnel TBM697.

3) Attempting to predict the microseismicity and rockburst intensities likely to be encountered in subsequent tunnel TBM696 based on the activity encountered when tunnel TBM697 was previously excavated will not be an easy or accurate procedure in deep tunnel projects involving complex lithological conditions.

Acknowledgments

The authors appreciate the help and assistance from the Beijing Vibroflotation Engineering Co., Ltd., Neelum-Jhelum Consultant, China Gezhouba Group Co., Ltd., and Beifang Investigation, Design & Research Co., Ltd. The authors would like to thank Prof. FENG Xia-ting, CEng Gary Peach, and Mr. TAN Shuang for their kindly help during the research.

Contributors

FENG Guang-liang carried out the onsite experiment, organized the study and wrote the content. CHEN Bing-rui designed and took part in the onsite experiment. JIANG Quan and NIU Wen-jing modified the content. XIAO Ya-xun and LI Peng-xiang took part in the onsite experiment study. All authors have read and approved the final manuscript.

Conflict of interest

FENG Guang-liang, CHEN Bing-rui, JIANG Quan, XIAO Ya-xun, NIU Wen-jing, and LI Peng-xiang declared that they have no conflicts of interest to this work.

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(Edited by YANG Hua)

中文导读

开挖诱发的微震活动与岩爆：深埋软硬交互地层平行隧洞的异同

摘要：深部地下工程开挖诱发的微震活动和岩爆实录可为岩爆预警、岩爆防控策略制定及岩爆发生机理分析提供非常宝贵的信息。本文依托巴基斯坦尼勒姆-杰勒姆水电站深埋平行隧洞(最大埋深1890 m)，聚焦开挖诱发的微震活动和岩爆信息，对软硬交互地层平行隧洞之间的异同进行了研究。结果表明，开挖过程中，平行隧洞均产生了大量微震事件，岩爆频发于隧洞局部洞段。同时，平行隧洞由开挖诱发的微震活动性(微震事件的空间分布和数量、微震能量分布和微震活动性变化规律)和岩爆特征(频次和空间分布)均存在显著差异。针对复杂岩性条件的深埋平行隧洞，仅根据先行隧洞开挖诱发的微震活动和岩爆信息，难以有效预测后续平行隧洞对应区域开挖过程中将发生的微震活动和岩爆强度。

关键词：微震活动；岩爆；软硬交互地层；深埋平行隧洞；尼勒姆-杰勒姆水电站

Foundation item: Projects(41972295, U1965205) supported by the National Natural Science Foundation of China; Project(2019ZDK034) supported by the Guangxi Key Laboratory of Disaster Prevention and Engineering Safety, China

Received date: 2020-07-11; Accepted date: 2020-09-22

Corresponding author: FENG Guang-liang, PhD, Associate Professor; Tel: +86-15827043587; E-mail: glfeng@whrsm.ac.cn; ORCID: http://orcid.org/0000-0001-9231-0732