Experiments on Rock Burst and its Control

Transcription

1 Experiments on Rock Burst and its Control M He 1 and L R Sousa 2,3 ABSTRACT Rock burst is common in deep underground excavations and is characterised by a violent ejection of block rocks from excavation walls. It is critical to understand the phenomenon of rock burst and particularly the failure mechanism. Laboratory experiments are one of the ways. For that purpose, a rock burst laboratory test system was developed at the State Key Laboratory for Geomechanics and Deep Underground Engineering at the China University of Mining and Technology, Beijing. A true triaxial test system, designated the Deep Underground Rock Burst Analogue Test Machine, was developed, which can transfer a stress state from three-direction-compression on six surfaces to the compression on five surfaces with one set free. The rock burst experimental system comprises an acquisition of acoustic emission signals, a high-speed recording in order to accurately record the kinetic characteristics of rock fragments ejected during a rock burst event and the possibility of infrared thermography showing the surface temperature of the samples. In this paper, analyses of typical results from rock burst tests are presented with a description of the obtained results. The mechanisms of rock burst are illustrated, and a rock burst classification is proposed based on these laboratory tests. For the control of rock burst, the constant resistance, large deformation (CRLD) bolt or anchor is presented for accommodating the large displacement of surrounding rock masses and absorbing the impact of the sudden release of rock burst while outputting a constant resistant force in response to the external load. The CRLD bolt or anchor has had practical applications in underground mines. Finally, in situ tests performed in a tunnel at Qingshui coalmine, China, are analysed in detail. INTRODUCTION Rock burst frequently occurs in a sudden or violent manner in the excavation face or on a working panel of an underground excavation at great depth. Although the evolution of a rock burst is generally recognised as a process of crack initiation, propagation and coalescence of internal fractures in the rock masses, a comprehensive understanding of the mechanisms of rock bursts have so far been illusive due to its sophisticated and non-linear nature. In recent years, rock burst phenomena have been extensively investigated by many researchers (CAMIRO, 2005; He, 2009; Kaiser, 2009) through a variety of experimental devices and theoretical approaches. It is critical to understand the phenomenon of rock burst, focusing on the patterns of occurrence of these events so as to save costs and possibly lives. There are several mechanisms that originate rock burst. The main source mechanisms are usually associated with local underground geometry and the existing geology (Ortlepp and Stacey, 1994; CAMIRO, 1995; Castro, Bewick and Carter, 2012; He et al, 2013). They normally occur in large-scale mining operations, but also in civil works. The most common phenomenon is strain bursting, although buckling and face crashing may also occur. In addition, impact-induced rock burst can occur in less stressed and deformed rock formations due to blasting and excavation of adjacent cavities. Some researchers have conducted experimental studies on rock burst using uniaxial compression tests, combined uniaxial and biaxial static-dynamic tests, true triaxial loading tests and conventional triaxial unloading tests. In recent years, some researchers have promoted the use of acoustic emission (AE) technology in rock mechanics, and considerable achievements have been made in the characterisation of rock failure and rock burst mechanisms (Yang and Wang, 2005; Zhao, 2006). A new method for rock burst studies involves the use of a modified true triaxial apparatus that can unload the samples being tested on one surface (He et al, 2011). It includes the development of a test system for simulating strain burst in a laboratory under deep ground excavation conditions, rock masses and an output in response to the external perturbed forces. In addition to understanding rock burst mechanisms, controlling rock burst is the most important issue for the safety of mining operations and other deep underground 1. Director, State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Beijing, 16 Tsinghua East Road, Haidian District, Beijing , China. hemanchao@263.net 2. Researcher, State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Beijing, 16 Tsinghua East Road, Haidian District, Beijing , China. sousa-scu@hotmail.com 3. Professor, University of Porto 1

2 M HE AND L R SOUSA structures. Bolts and anchors are efficient measures for the control of rock burst in underground excavations, performing different roles such as the reinforcement of the rock and to hold the retained reinforced rock mass. These supports were widely investigated by many researchers worldwide (Kaiser, 2009; He et al, 2011). Among them is the cone bolt, developed by a Canadian company. This bolt does not have the ability to adjust itself to the external loading to output a constant resistance. Its maximum deformation was only 120 mm and it is unable to adapt to the support requirements of larger deformation in mine roadways (greater than 200 mm). Another compressible rock bolt (Roofex) was developed by Atlas Copco in Australia, which was suitable for supporting a soft rock tunnel and capable of maintaining a constant resistance while the supported rock mass underwent large deformations. This type of bolt has a maximum extension value of up to 300 mm and a constant resistance force of 80 kn. However, as mining depths increase, the demands for anchor bolts with larger extension and higher loading capacity are growing. This paper introduces the state-of-the-art anchor and bolt technology, the constant resistance, large deformation (CRLD) bolt or anchor, developed by the State Key Laboratory for Geomechanics and Deep Underground Engineering (SKLGDUE) at the China University of Mining and Technology (CUMTB), Beijing. The CRLD bolt or anchor has the ability to accommodate larger deformations of the surrounding rock masses of tunnels at great depth in response to external forces. Field tests were performed and analysed in the last section of a coalmine in China. A LABORATORY SYSTEM FOR ROCK BURST The mechanism of rock burst The evolution of rock burst can be considered in a simplified way by two stages (He et al, 2007): the stress evolution and the plate structure evolution. Stress evolution refers to the in situ stress state of the rock mass, in which the stress state is transformed from a 3D six-surface state to a 3D five-surface state of stress due to an underground excavation (Figure 1). The so-called plate structure evolution refers to the structural response of the rock mass, which can be divided into the following three phases (Figure 2): 1. cracking through the surface parallel to the excavation 2. buckling of the surface near the excavation 3. the rock burst ejection process. The essence of the mechanical behaviour transformation process defined by the rock burst is an alteration of the FIG 2 Structural changes after rock burst. mechanical behaviour of the rock under external conditions, which has been proved by laboratory triaxial loading and unloading experiments. Experimental set-up Laboratory modelling of rock burst under the conditions in deep underground engineering is one of the major goals of SKLGDUE at the CUMTB. To test this, the Deep Underground Rock Burst Analogue Test Machine (DURAT M) was developed in 2006 (Figure 3). The testing system is a true triaxial testing scheme consisting of three main parts: a loading/unloading device, a high-speed data acquisition system and an AE detecting system. The loading/unloading device of the DURAT M is comprised of a main stand, hydraulic control apparatus and force-measuring transducers, which can provide dynamic loading and unloading independently in three principal stress directions. During the test, one surface of the specimen can be unloaded immediately from the true triaxial compression condition, simulating the stress condition for rock mass at the free excavation boundary in underground excavations. A series of physical modelling tests on the rock burst phenomena was conducted in the laboratory to calibrate the machine with realworld conditions in deep mining and to make adjustments to the physical and mechanical parameters of the DURAT M system (He et al, 2007). The DURAT M system is unique, with three main features (He et al, 2012b): 1. Transition of the mechanical behaviour of the rock mass at great depth can be simplified using this testing equipment. 2. Transformation of the stress state can be achieved with the DURAT M system. Rock masses located in deep ground are under compressive stress conditions in all FIG 1 Stress model for excavation-induced rock burst. FIG 3 Experimental system for simulation of rock burst process at great depth. 2

3 EXPERIMENTS ON ROCK BURST AND ITS CONTROL three principal directions. When an underground cavity is excavated, a parallelepiped volume element of the rock mass will transfer its compressive stress state in three directions into a compression state with one free surface. 3. One surface of the specimen can be unloaded immediately from the true triaxial compression condition. Ensuring that one surface was free when unloading the existing stress was crucial for the simulation of the in situ rock burst phenomenon. The design of the Single Face Unloading Device in the DURAT M was one of the difficulties during the initial period of experimentation (Figure 4). FIG 4 Sketch of dowel steel and pressure head. In conclusion, the information measuring system for the laboratory rock burst tests include data acquisition, AE monitoring, high-speed image recording and, optionally, infrared image recording. The data acquisition consists of sensors, amplifiers, data acquisition instruments, computers and appropriate software that can automatically collect, edit and process the test data. The AE released during the tests can be monitored using an acquisition card, a continuous current source, a preamplifier and an AE sensor. The system is also equipped with a high-speed digital camera that accurately records the kinetic characteristics of the rock fragments that are ejected during the rock burst tests. Finally, there is the possibility of incorporating infrared thermography, which shows changes in the surface temperature of the samples. These changes in temperature allow us to better understand the mechanism of rock burst. China, while the other relates to a rock burst test performed in a sample from Creighton mine, Canada. Rock burst test at Laizhou mine, China This section describes the results of a granite rock burst test from Laizou mine in Shandong province, China (He et al, 2012a). The dimensions of the sample were mm 3. X-ray diffraction analysis showed that the sample was 27 per cent quartz, 68 per cent feldspar and five per cent clay minerals. The loading/unloading path is presented in Figure 5. The in situ stresses were σ 1 = MPa, σ 2 = 59.8 MPa and σ 3 = 29.7 MPa. The stresses were kept constant for 15 minutes, before the load on the surface of σ 3 direction was removed suddenly. The sample failed with stresses of σ 1 = MPa, σ 2 = 58.5 MPa and σ 3 = 0.0 MPa, with the ejection of particles. The accumulated AE energy release is shown in Figure 6. Initially, AE energy releases at a low rate when the stresses are kept constant. Later, the AE accumulated was characterised by a sudden increase in the first unloading followed by a sharp increase with the occurrence of instantaneous burst. The movement process of the rock fragments ejected from the unloaded surface was recorded using the high-speed camera, as shown in Figure 7. It shows the surface of the sample before rock burst. The first falling down is also illustrated, and when the rock bursts occurred, a lot of fragments were ejected from the upper region of the sample. Granite sample from Creighton mine, Canada Rock burst tests were also performed at Creighton mine, which is located in southern part of Sudbury Basin, Canada (Camiro, 1995; He et al, 2014b). Creighton s sulfide orebodies are present in the lower sublayer of the hanging wall norites. CASES OF LABORATORY ROCK BURST TESTS General Since the first rock burst laboratory tests were performed in 2006, more than 200 tests have been carried out on samples from China, Italy, Canada and Iran by using the system developed at SKLGDUE. About 11 rock lithologies have been investigated (He et al, 2011). A database of 139 cases was created from the rock burst tests where complete information was obtained. A form was elaborated and a great number of fields were considered, including: location of the test, dimensions of the sample, rock material, main minerals and cracks, stresses before loading, stresses during tests, characteristics of rock burst test and critical depth. The results were analysed and several data mining techniques were applied in order to obtain models for the maximum principal stress obtained in the rock burst tests and for a rock burst risk index (He et al, 2014a). Two examples are described in the following sections. One relates to a test in Laizhou mine in the Shandong province, FIG 5 Loading/unloading path for a granite sample. FIG 6 Accumulate acoustic emission energy release of the burst. 3

4 M HE AND L R SOUSA CG4# from the mine. The dimensions of the prismatic sample were mm 3. In the rock burst test, taking into consideration concentration factors, the in situ state of stress was supposed to be equal to σ 1 = MPa, σ 2 = 97.2 MPa and σ 3 = 90.6 MPa. Figure 9 indicates the loading path of the granite sample used for the rock burst test. The sample was first loaded with three principal directions loading step-by-step to simulate the original stress state referred. Next, the horizontal minimum principal stress on one surface of the sample was unloaded, and the rock burst process was simulated following the stress path indicated in Figure 9. The critical rock burst stresses were then obtained with the following values: σ 1 = MPa, σ 2 = 93.3 MPa and σ 3 = 0.0 MPa. FIG 9 Rock burst loading path for the sample from Creighton mine. FIG 7 Analysis of the burst process. The footwall rocks are mainly granite. The overall geotechnical parameters, estimated to a depth of about 2100 m, are the following: Young s modulus 30 GPa; Poisson ratio 0.25; strength parameters σ t = 0 MPa, c = 22 MPa and φ = 35. The samples for the rock burst tests were obtained to a depth of 2420 m. Figure 8 illustrates one of the tests for the sample FIG 8 Rock burst test for sample CG4# from Creighton mine. MECHANISMS OF ROCK BURST AND ITS CLASSIFICATION Indoor rock burst tests play an important role in understanding the mechanisms of rock burst, the calibration of numerical models, the evaluation of mechanical parameters and the identification of the stress state when a dynamic event may be initiated. Laboratory tests have been used by several researchers and include uniaxial compression tests (Badge and Petrosav, 2005; Zuo et al, 2006), combined uniaxial and biaxial static-dynamic tests, true triaxial loading tests (Chen and Feng, 2006; Cheon et al, 2006) and conventional triaxial unloading tests (Xu, 2003). However, the rock burst simulation tests could not provide correct in situ stresses on the near-face region during underground excavation. With respect to the triggering mechanisms, rock bursts may occur under high in situ stress conditions. For the surrounding rocks in lower stress states due to external disturbances (such as blasting, caving and adjacent tunnelling), rock bursts can also be triggered. In this paper, rock bursts are classified into two major types: strain bursts and impact-induced burst, as shown in Figure 10. Strain bursts are frequently encountered during tunnel excavation, and they are also associated with pillar and room mining cavities. According to different stress paths and failure locations, strain bursts can be divided into three subtypes: 1. instantaneous burst 2. delayed burst 3. pillar burst. After excavation, the surfaces of the cavities and pillars may also suffer rock bursts due to the impact waves generated by mining disturbances. According to their formation 4

5 EXPERIMENTS ON ROCK BURST AND ITS CONTROL A B FIG 10 Laboratory experimental methods based on rock burst classification (He et al, 2012a). mechanisms, impact-induced burst can be divided into three subtypes: 1. rock bursts induced by blasting or excavation 2. rock bursts induced by roof collapse 3. rock bursts induced by fault slip. Rock burst experimental equipment was briefly presented in the previous section. A new experimental system was developed in order to simulate impact-induced burst (Figure 11) (He et al, 2012a). It consists of a main stand, servo-controller and hydraulic power. As indicated in He et al (2012a), it can generate various types of disturbance wave signals. The following types of strain bursts can be simulated by this testing system: Instantaneous burst one surface of the sample is unloaded suddenly from a true triaxial stress state to simulate the strain bursts immediately after excavation. A schematic diagram of the loading/unloading path is shown in Figure 12a. Delayed burst one surface of the specimen is unloaded suddenly from a true triaxial stress state and then the vertically-imposed stress (σ 1 ) is increased based on the stress concentration to simulate the strain bursts that occur sometime after excavation due to stress redistribution. A schematic diagram of the loading/unloading path is presented in Figure 12b. Pillar burst due to excavation, the pillar size decreases, thus increasing the vertical stress. The horizontal stresses FIG 11 Illustrations for the new deep rock nonlinear mechanical system. (σ 2 and σ 3 ) can be gradually decreased to simulate the formation of a pillar until the burst occurs. A schematic diagram of the loading/unloading path is illustrated in Figure 12c. Rock burst induced by blasting or excavation can also be considered. Firstly, the static load stresses are applied on the sample to simulate the in situ stress state. Secondly, the disturbance wave is loaded in one, two or three directions and the burst phenomena is observed, in which the disturbance load is used to simulate site excavation, blasting, earthquakes or a mechanical vibration waveform. The schematic diagram of the loading/unloading path is presented in Figure 13. Different rock burst criteria were analysed in detail by He et al (2012a). Figure 14 shows three different stress paths for strain bursts using Hoek Brown criterion for the strength of the rock mass (Sonmez and Gokceoglu, 2006). Figure 14a shows the instantaneous burst path. The area Z1 is the potential zone for the occurrence of rock burst of this type. Point A represents the initial stress state before excavation. σ c and σ r are the uniaxial compressive strength and the long-term A B C FIG 12 Loading paths for strainburst. 5

6 M HE AND L R SOUSA used for the situations of impact by blasting, excavation, roof collapse or fault slip are analysed in detail in He et al (2012a). FIG 13 Loading/unloading path for rock burst induced by blasting or excavation. peak strength respectively. Instantaneous burst occurs with the release of σ 3 if the maximum principal stress σ 1c is greater than σ c. Figure 14b shows the delayed burst path. Area Z2 is the potential burst-prone zone of this type. Point B represents the initial stress state before excavation. As σ 1 is lower than σ c, the delayed burst will not occur when σ 3 is suddenly released unless there is enough energy that can be released in the form of kinetic energy or other. The increase of σ 1 may be attributed to the tangential stress concentration due to excavation and to the damage of the surrounding rocks by field engineering disturbances, such as excavation and blasting. Figure 14c shows the stress path for pillar burst. The areas Z1 and Z2 can be the potential burst-prone zones of the pillar. Points C1 and C2 represent the initial stress states in the areas Z1 and Z2 before excavation respectively. Increasing σ 1 and decreasing σ 3 will result in the occurrence of pillar burst. Figure 15 shows schematic diagrams of impact-induced burst criteria. Theoretical considerations about the criteria CONSTANT RESISTANCE LARGE DEFORMATION BOLTS Bolts are a major method used in rock support. However, in high-stressed rock masses, bolts frequently break because they cannot adapt to large deformations. A bolt with constant working resistance and steady large deformation, under large deformations and impact loads, was developed at SKLGDUE (He et al, 2011). The CRLD bolt or anchor device consists of two parts, the constant resistance element and the bolt rod (Figure 16). The constant resistance element is composed of a slide track sleeve and a constant-resistant body. Figure 17 illustrates the supporting principle of the CRLD bolt over the following three different stages (A is the anchored segment, B is the wall of the drilling hole, C is the constant resistance element and D is the baffle plate): 1. Elastic deformation (Figure 17a) the deformation energy of the surrounding rocks could be converted to the bolt rod in the bolt assembly through the baffle plate and inner anchorage segment. In the case that a relative deformation for the surrounding rocks and the axial force loaded by the rock deformation is less than the rated constant resistance for the CRLD bolt, the bolt will not elongate by the displacement of the constant-resistance element, but will resist to the deformation and failure of the rock, solely relying on the elastic deformation of the bolt rod itself. 2. Structural deformation stage (Figure 17b) with the deformation building up, the axial force on the build rod will be increasing and may be equal to or larger than the rated constant resistance for the CRLD bolt, leading to the frictional-sliding displacement of the constant-resistant body along the sleeve track (ie the CRLD bolt elongates). While elongating, the bolt will be keeping the constant- A B C FIG 14 Schematic diagram of strain bursts criteria. A B C FIG 15 Schematic diagrams of impact-induced burst criteria. 6

7 EXPERIMENTS ON ROCK BURST AND ITS CONTROL FIG 16 Layout of the constant resistance, large deformation bolt. A B FIG 18 Comparison among different bolts or anchors. C FIG 17 Supporting principle for a constant resistance, large deformation bolt. (A) elastic deformation stage; (B) structural deformation stage; (C) ultimate deformation stage. resistant characteristics and resist the deformation and failure of the rock mass by its elongation (ie the structural deformation of the constant-resistant element). 3. Ultimate deformation stage (Figure 17c) after undergoing the elastic deformation of the bolt rod itself at the first stage and large deformation that equals the elongation of the constant-resistant element, the deformation energy for the rock mass in abutment to the excavations has been fully released. In this case, the external load will be smaller than the rate of constant resistance, and the constant-resistant body will stop sliding due to fractional drags. Therefore, the surrounding rock mass for the excavation has been stabilised. Consequently, under conditions of deformation of the surrounding rock masses, the bolt can absorb the deformation energy of the rock mass, which will release the energy stored in the surrounding rocks. Over the structural deformation stage, the bolt is still able to elongate steadily while keeping its working resistance constant in response to the external forces. Thus, the stabilisation could be realised for the CRLD boltsupported rock masses adjacent to excavations, mitigating potential disasters such as rockfall, collapse, slabbing and splitting and floor heaving. The development of the CRLD bolt or anchor has been tested in situ, in the laboratory and in some practical applications. The maximum extension of bolt rock for CRLD is about 1000 mm, which can fully accommodate the displacement extent of the rock mass adjacent to the deep underground excavations. In comparison with currently existing largedeformation anchors from abroad, this novel bolt has much longer extension length under the same external pulling force. At the same time, its maximum load-carrying capacity is much larger, as illustrated in Figure 18. FIG 19 Static tension test set-up for the constant resistance, large deformation bolt. The nominal length of the test CRLD bolts is 1 m, the diameter of the bolt rod is 22 mm and the inner diameter of the slidetrack sleeve tube is 34 mm. The bolt was fastened on the two ends of the testing machine by the holding device (Figure 20). Profiles of the resistance against displacement for a 20 t CRLD bolt and two anchors (35 and 85 t) are indicated in Figure 18. These are compared with the Canadian cone bolt and the Australian Roofex bolt, with maximum displacements of about 120 mm and 300 mm respectively. Dynamic impact tests are also performed at SKLGDUE. The testing system can be used to evaluate the performance of resistance and absorbing impacting energy for CRLD bolts or anchors by measuring the extension length of the bolt (anchor) rod body itself and the radial deformation of the rod Static and dynamic tests were developed at SKLGDUE for this type of bolt. The aim of the static tests was to evaluate the static characteristics of the CRLD bolts, including the constant resistance, magnitudes of the resistance and maximum elongation extent. The equipment is illustrated in Figure 19. FIG 20 Detail of the experimental set-up of the static tension testing system. 7

8 M HE AND L R SOUSA body. The testing system is shown in Figure 21, and has the following technical specifications: maximum impact energy J effective range of the impacting height m hammer height with five grade kg allowed diameter of the bolt 34 mm and 22 mm length 1500 m and 2500 m. A B FIG 21 Dynamic impact test set-up. The purpose of the dynamic impact test is to evaluate the energy absorbing capacity for the bolts under dynamic loading. Before the impact, the testing bolt was fastened to the test machine with the holding device. The baffle plate was then installed and the initial displacement was set up. After setting the maximum dropping height and confirming that all the procedures were ready, the test was conducted with a cyclic-loading scheme until the completion of all the cycles. Experimental results are illustrated in Figure 22, which shows the time domain curves of impacting load versus deformation of bolt under dynamic loading with an impacting height of 10 mm. Figure 22a shows the whole time domain waveform figure with an impacting height of 10 mm for a bolt and Figure 22b shows the time domain waveform figure after first impact. Advancements in Hopkinson dynamic tests were also developed for the CRLD bolts. Experimental studies with one and two bolts are still being analysed, complemented by the use of numerical models using LS-Dyne software. IN SITU TESTING RESULTS General In situ tests with CRLD anchors were performed at Hongyang coalmine in Liaoning province, China, using a blasting method in order to simulate rock burst that had occurred frequently in this mine. An abandoned roadway tunnel was used for the location of the tests at a depth of about 780 m (Figure 23). The geological stratigraphy near a section of the roadway is illustrated in Figure 24, showing that there is a mudstone layer above the tunnel and medium sandstone layer below. The average thickness of the coal seam is about 2.6 m. The roadway tunnel was used for ventilation purposes and has a rectangular cross-section (Figure 25). The supporting conditions were generally good, with a stable roof, and the tunnel was over 100 m from other tunnels. In situ program tests were performed in the roadway tunnel and included two kinds of tests using different support methods. The first type of support contained normal bolts of 2.2 m in length and 20 mm in diameter, anchors of 6.5 m in length and 21.7 mm in diameter and wire mesh. The second type added 35 t CRLD anchors with the same length of 6.5 m instead of normal anchors. These supports were submitted to impact tests. The FIG 22 Time domain waveform figure with an impacting height of 10 mm: (A) the whole time domain waveform; (B) time domain waveform figure after first impact. total length of the testing area was 40 m, which was divided into four sections of 10 m each (Figure 26). Sections I and IV used the first type of supports, while sections II and III used supports with CRLD anchors. Chambers were excavated between sections I and II and between sections III and IV, with dimensions of 4.5 m in length, 3.0 m in width and 2.0 m in height. Figure 27 presents an overview of the testing method. Figure 27a presents a side view of the testing area, while Figure 27b provides a view from up to down as well as the chambers (caves). In the chambers, two blastholes of about 7 8 m in length were drilled in both sides in the coal seam for the placement of the explosives, as illustrated in Figures 27 and 28. The explosive loads were 4.6 kg and 6.0 kg, which approximately represented the seismic events occurring in this mining area. In the chamber between sections III and IV, the explosives were applied again, therefore the explosives were doubled with two times blasting. Monitoring and results A monitoring plan was established with the purpose of manually measuring surface displacements at the roadway tunnel in the four sections. Later, a real-time monitoring system, photographs, forces and elongations for normal and CRLD anchors were used. The real-time monitoring system used BeiDou satellites, and the information was transmitted in real-time to SKLGDUE in Beijing (Figure 29). 8

9 EXPERIMENTS ON ROCK BURST AND ITS CONTROL FIG 23 Hongyang Mine; location of the testing area. FIG 24 Hongyang Mine; geological formations near the roadway. FIG 25 Section of the roadway and location of constantresistance and large deformations anchors. Figures 30, 31 and 32 are before and after photographs of normal section I and the CRLD-supported sections II and III respectively. In the normal section, it is clear that the upper part of the roadway tunnel collapsed after blasting, with failure of the existing supports. Following the blast, this section of the tunnel was no longer available for safety reasons. The other two sections supported by CRLD anchors remained stable after the large deformations, with only the steel mesh being broken. Some monitored results were obtained. Figure 33 illustrates measured displacements at sections II and III from the heads of the CRLD anchors for the first explosion. The minimum values were about 25 mm and the larger values were almost 60 mm, but the average values were between 25 mm and 30 mm. Other measurements included forces for the types of anchors. 9

10 M HE AND L R SOUSA CONCLUSION FIG 26 In situ testing area. SKLGDUE has developed a true triaxial apparatus that is able to reproduce the rock burst phenomenon in a laboratory. The initial feature lies in a single-face unloading that simulates the stress paths existing in the face of underground excavations. The experimental set-up comprised several systems, including devices for loading/unloading, high-speed acquisition, AE detecting and infrared thermography. The distribution law of the dominant frequency bandwidth for different rock samples under varied stress paths during rock burst simulation tests can also be obtained based on timefrequency analysis and the discrete Fourier transformation from the AE waveform data. The investigations performed have laid a solid foundation for further development of the state-of-the-art theories and experimental devices developed by SKLGDUE. Also, a database with a large number of rock burst tests was created. The information was analysed with several data mining techniques in order to obtain models for the maximum stresses obtained in the tests and for a rock burst risk index. The rock burst triaxial laboratory was adapted in order to simulate impact-induced bursts by generating different types of disturbance wave signals analogous to the impact produced by the drill-and-blast method. Based on these rock burst testing devices, a new classification for rock burst was proposed, including the experimental procedures, stress paths imposed and the released energy-based rock bursts criteria. A B FIG 27 Overview of the testing method. FIG 28 Location of the blastholes and explosives. 10

11 EXPERIMENTS ON ROCK BURST AND ITS CONTROL FIG 29 Automatic monitoring transmission system. FIG 30 Photographs of a normal support section before and after blasting. FIG 31 Photographs of constant resistance, large deformation-supported section II before and after blasting. 11

12 M HE AND L R SOUSA FIG 32 Photographs of constant resistance, large deformation-supported section III before and after blasting. Candaian Mining Industry Research Organization (CAMIRO), Rockburst Research Handbook, volume 6, 977 p (CAMIRO Mining Division: Sudbury). Castro, L M, Bewick, R and Carter, T G, An overview of numerical modelling applied to deep mining, in Innovative Numerical Modeling in Geomechanics (eds: L R Sousa, E Vargas Jr, M M Fernandes and R Azevedo), pp (CRC Press: London). Chen, J T and Feng, X T, True triaxial experimental study on rock with high geostress, Chinese Journal of Rock Mechanics and Engineering, 25(8): (in Chinese). Cheon, D S, Jeon, S, Park, C and Ryu, C, An experimental study on the brittle failure under true triaxial conditions, Tunnelling and Underground Space Technology, 21(3): Control of rock burst is a very important issue. Therefore, the CRLD bolt or anchor, which permits constant resistance and large deformations, was developed at SKLGDUE. The performance of these bolts was experimentally verified with the development of static and dynamic equipment. In situ tests were performed at a coalmine in Liaoning province, China. The feasibility of the new bolt or anchor was successfully verified, which is expected to have a significant role in control and the prevention of the occurrence of rock burst in deep underground engineering. ACKNOWLEDGEMENTS This work was supported by the Key Project of Natural Science Foundation of China (No ) and the General Program of National Natural Science Foundation of China (Nos and ). REFERENCES FIG 33 Elongation of constant resistance, large deformation anchors in sections II and III. Bagde, M N and Petorsa, V, Fatigue properties of intact sandstone samples subjected to dynamic uniaxial cyclical loading, International Journal of Rock Mechanics and Mining Sciences, 42(2): He, M, Sousa, L R, Miranda, T and Zhu, G, 2014a. Rockburst laboratory tests database: application of data mining techniques, J Engineering Geology for Geological and Geotechnical Hazards (in press). He, M, Xia, H, Jia, X, Gong, W, Zhao, F and Liang, K, 2012a. Studies on classification, criteria and control of rockbursts, J of Rock Mechanics and Geotechnical Engineering, 4(2): He, M C, The mechanism of rockburst and its countermeasure of support, Int consultation report for the key technology of safe and rapid construction for Jinping II Hydropower Station high overburden and long tunnels, Jinping II, pp He, M C, Gong, W, Wang, J, Qi, P, Tao, Z and Du, S, Development of a novel energy-absorbing bolt with extraordinarily large elongation and constant resistance, Int J Rock Mechanics Min Science, 67: He, M C, Jia, X N, Coli, M, Livi, E and Sousa, L R, 2012b. Experimental study on rockbursts in underground quarrying of Carrara marble, Int J Rock Mechanics Min Science, 52:1 8. He, M C, Jia, X N, Gong, W L, Liu, G J and Zhao, F, A modified true triaxial test system that allows a specimen to be unloaded on one surface, in True Triaxial Testing of Rocks, (eds: M A Kwasniewski, X Li and M Takahashi), pp (CRC Press: London). He, M C, Miao, J L, Li, D J, et al., Experimental study on rockburst processes of granite specimen at great depth, Chinese Journal of Rock Mechanics and Engineering, 26(5): (in Chinese). He, M C, Wang C G, Feng, J L and Sousa, L R, 2014b. Investigations on gas flow in cracked granite samples, J Soils and Rocks, 37(1):

13 EXPERIMENTS ON ROCK BURST AND ITS CONTROL Kaiser, P K, Failure mechanisms and rock support aspects, Int consultation report for the key technology of safe and rapid construction for Jinping II Hydropower Station high overburden and long tunnels, Jinping II, pp Ortlepp, W D and Stacey, T R, Rockburst mechanisms in tunnels and shafts, Tunnelling and Underground Space Technology, 9(1): Somnmez, H and Gokceoglu, C, Discussion of the paper by E Hoek and M S Diederichs Empirical estimation of rock mass modulus, Int J of Rock Mechanics and Mining Science, 43(5): Xu, L S, Research on the experimental rock mechanics of rockburst under unloading condition, Journal of Chongqing Jiaotong University, 22(1):1 4 (in Chinese). Yang, J and Wang, L J, Study on mechanism of rock burst by acoustic emission testing, Chinese Journal of Rock Mechanics and Engineering, 24(20): (in Chinese). Zhao, X D, Experimental study on earthquake bearing model of rock fracture based on sound emission detection, PhD dissertation, Northeastern University, Shenyang (in Chinese). Zuo, Y J, Li, X B, Tang, C A, et al Experimental investigation on failure of rock subjected to 2D dynamic-static coupling loading, Chinese Journal of Rock Mechanics and Engineering, 25(9):

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