Rock burst prediction based on in-situ stress and energy accumulation theory

International Journal of Rock Mechanics & Mining Sciences 83 (2016) 86–94

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Short communication

Rock burst prediction based on in-situ stress and energy accumulation theory Sheng-Jun Miao a,b,n, Mei-Feng Cai a,b, Qi-Feng Guo a,b, Zheng-Jun Huang a,b a

School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China Key Laboratory of High-Efficient Mining and Safety of Metal Mines (Ministry of Education of China), University of Science and Technology Beijing, Beijing 100083, China

b

art ic l e i nf o Article history: Received 18 June 2015 Received in revised form 29 October 2015 Accepted 1 January 2016 Keywords: Rock burst prediction In-situ stress measurement Rock burst proneness Energy accumulation Seismology

1. Introduction A rock burst is a strong and sudden brittle failure induced by high stresses. It occurs in a short period when the surrounding stress is redistributed during the excavation of tunnels and stopes in high geo-stress areas. The strong shock waves and blast followed by the outburst of broken rock can cause casualties and destroy underground constructions. Rock burst disasters tend to increase in mining that is further in scale and greater in depths.1 Therefore, the research on rock burst mechanism and prediction has become one of the key scientific and technical problems in rock mechanics.2 It is generally recognized that a high strength brittle rock and a high stress environment are two necessary conditions for a rock burst.3 To reduce and control rock bursts, extensive theoretical and experimental researches have been conducted on the failure mechanism, forecasting method, and prevention technology. Canada started two research programs, Canadian-Ontario-Industry Rockburst Project (1985–1990)4 and Canadian Rockburst Research Prograrm (1990–1995),5 for deep safe mining. Ortlepp and Stacey distinguished several different rock burst mechanisms in tunnels and shafts and drew a distinction between source and damage n Correspondence to: University of Science & Technology Beijing, No. 30 Xueyuan Road, Haidian District Beijing, Beijing 100083 CHINA. E-mail address: [email protected] (S.-J. Miao).

http://dx.doi.org/10.1016/j.ijrmms.2016.01.001 1365-1609/& 2016 Elsevier Ltd. All rights reserved.

mechanisms.6 Frid forecasted rock bursts by electromagnetic radiation energized by rock fracture and established electromagnetic radiation criterion for rock burst forecast in coal mines.7,8 Wang and Park predicted rock bursts based on an analysis of strain energy and discussed the relationship between rock burst and elastic energy criterion, brittle criterion, tangential stress, and RQD of rock.9 Feng and Zhao proposed a prediction method of rock bursts using a support vector machine model.10 Sharan established a finite element perturbation method for the prediction of rock bursts.11 Gong and Li established a distance discriminant analysis method for prediction and classification of rock bursts.12 Chen et al. conducted unloading confining pressure tests and provided rock burst criterion based on energy theory.13 He et al. studied rock burst processes in limestone and its acoustic emission characteristics under true-triaxial unloading conditions.14 Kaiser and Cai designed a rock support system under rock burst conditions.15 Adoko et al. built a knowledge-based and data-driven fuzzy logic model for rock burst prediction.16 But at present, rock bursts predicted by using energy accumulation and seismology theory are rare. Geo-stress is an important factor for rock burst prediction. In addition, the geo-stress data used in rock burst predictions are usually estimated by geological investigation, empirical formula, and numerical simulation, which make the predictions inaccurate and uncertain. To overcome these deficiencies, the traditional overcoring technique was improved and applied to in-situ stress

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Fig. 1. The hollow inclusion strain gauge with completely temperature compensation technique.

measurements at deep positions in Sanshandao Gold Mine. Based on the results, we predicted the potential location and intensity of rock bursts during future mining activities with the theoretical analysis and numerical simulation. The achieved results will provide new ideas and approaches for rock burst prediction in deep underground engineering.

2. Project overview Sanshandao Gold Mine is one of the most mechanized metal mines and one of the largest gold mines in China, with 10.25 million tons of available reserves, including 34.24 t of gold metal content, and has great prospective reserves. The mining depth of Sanshandao Gold Mine will reach and exceed 1000 m (a blind shaft has been constructed to
1180 m from level
600 m). The increase of mining depth and ground stress are causing surrounding rock deformation, collapse, roof fall, spalling, and other accidents in roadways and stopes. Additionally, the development work below the level of
600 m is facing potential threats from rock burst. Even with these risk factors, the Sanshandao Gold Mine is expecting to increase their production target to 8000 t per day. To guarantee the deep mining safety and achieve the expanding capacity, it is necessary to conduct systematic research on prediction and control of rock bursts. As shown in Table 1, physical and mechanical parameters of the main rocks in deep mining were obtained by laboratory tests.

Table 1 Physical and mechanical parameters of the main rocks at deep position in Sanshandao Gold Mine. Lithology

Amphibolite tonalites Adamellite Biotite granulite Phyllic granite Phyllic Footwall granite

Density UCS

Tensile strength

(g/cm3) (MPa)

(MPa)

Cohesion Young's modulus

Internal friction angle (deg)

(MPa)

(GPa)

2.934

86.28 14.66

37.26

49.89

63.61

2.627 2.717

70.70 6.92 97.53 16.46

33.21 25.06

26.54 42.87

27.11 40.08

2.572 2.677 2.591

125.21 16.31 93.26 14.33 82.66 9.84

39.51 36.82 42.55

37.35 35.60 24.00

43.12 41.04 48.63

3. In-situ stress measurements 3.1. Improved overcoring technique of in-situ measurement Overcoring technique with a hollow inclusion strain gauge was used to measure the in-situ stress in deep mining of Sanshandao Gold Mine. Compared with the shallow position, the mechanical properties of the rock mass have significant nonlinearity, heterogeneity and anisotropy at deep positions.17 Before in-situ measurements, a horizontal borehole must be drilled at the surface of a roadway or chamber to install the hollow inclusion strain gauge. The borehole depth should reach certain requirements to ensure that the measurement point is indeed in the initial stress area where it cannot be affected by mining disturbance. The surrounding rock stress field around a deep roadway is more complex. Therefore, its borehole should be deeper than the shallow position. Besides, the rock mass at deep positions is of poor quality and in a high geothermal environment. These factors are not conducive to the strain gauge installation and increase the difficulty of in-situ measurement with the overcoring technique. In order to improve the reliability and accuracy of the measurement, Cai proposed and designed a completely temperature compensation technique incorporated in the stress relief method for the in-situ stress measurement (Fig. 1).18 Additionally, other improvements were considered in the in-situ measurement and implementation of the overcoring technique. A built-in camera was assembled on the installation equipment to monitor the damage of rock mass at different borehole depths and get the rock mass quality of the strain gauge installation site, which could effectively avoid the unsatisfied or invalid installation of the strain gauge because of over-broken rock. An orienting and positioning device was added during the installation to ensure that the strain gauge could be installed to the borehole in a preset direction and thus eliminate environmental and artificial influences. The currently used fifteen-lines access hollow inclusion strain gauge has twelve strain channels, a thermal sensitive channel and two ground wires, which can realize temperature compensation for the long wires. But there is a certain error effect on the actual input voltage because of the voltage consumption of ground wires. For example, there will be an error of about 10 με if the measurement value is 300 με.19 Only one ground wire is retained in the gauge for this in-situ measurement, which can better adapt to the high geothermal environment and completely realize temperature compensation with higher accuracy. But its implementation is relatively complicated. Based on the detail site investigation, nine in-situ stress measurement points were set up at six levels (Table 2) in Sanshandao

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Table 2 Measurement results of Principal stresses at 9 measurement points of Sanshandao Gold Mine. No.

1 2 3 4 5 6 7 8 9

Depth (m)

510 510 555 600 600 645 690 690 750

The maximal principal stress (s1)

The intermediate principal stress (s2)

The minimum principal stress (s3)

Value (MPa)

Direction (deg)

Dip angle (deg)

Value (MPa)

Direction (deg)

Dip angle (deg)

Value (MPa)

Direction (deg)

Dip angle (deg)

24.55 24.64 25.71 28.88 30.17 29.57 31.50 29.77 33.22

129
111
45 103 110 112
80
83 119

4 3
13 1
16
3 2 4
10

16.35 15.68 14.00 16.54 18.83 19.56 19.08 20.84 19.93


138 155 14 10 24
177 230
8
89

2 82 73 76
11
80
79
74
82

14.49 15.02 13.00 14.77 16.94 15.48 17.54 19.63 17.10

133 161 50 13 236
156 10 8 208


85
10
20
8
70
9
10 15
8

Channels

600

1 2 3 4 5 6 7 8 9 10 11 12

Strain (με)

500 400 300 200 100 0 0

2

4

6

8 10 Record times

12

14

Fig. 2. The initial stress relief curve of measurement point 7.

Gold Mine. The deepest point was arranged at level
750 m. 3.2. In-situ stress measurement results The initial stress relief curves of the nine points were obtained from in-situ measurement (the stress relief curve of measurement point 7 (level
690 m) is shown in Fig. 2, and the channels’ order and direction are shown in Fig. 1). In addition, the borehole cores received from the in-situ test were used for temperature and confining pressure calibration tests. The cores were wrapped in plastic film and taken back to the laboratory to ensure their original state. In the temperature calibration test, a temperature chamber was used to simulate the in-situ temperature change and measure the additional strains in the process. Then the additional strains were eliminated them from the initial strains obtained through in-situ measurements. The modified strains would be used as the final stress relief data to calculate the in-situ stress. Young's modulus, Poisson's ratio, and coefficients k1, k2, k3 and k4 of the rocks at measurement points were obtained by confining pressure calibration tests with borehole cores.18 A special double iterative program was used to calculate the stress state of the measurement points, including the values and directions of three principal stresses (Table 2).

maximum principal stresses of the 9 measurement points are nearly horizontal and in the direction of NW–SE, which is consistent with the maximum principal stress's direction of regional tectonic stress field.20 The average ratio of the maximum horizontal principal stress (sh,max) to the vertical principal stress (sv) is 1.65, indicating that the in-situ stress field at deep positions in Sanshandao Gold Mine is dominated by the horizontal tectonic stress. The average ratio and the maximum ratio of the maximum horizontal principal stress (sh,max) to the minimum horizontal principal stress (sh,min) are 1.76 and 1.98, respectively. The difference of the two horizontal principal stresses is related to the maximum shear stress. This indicates that the shear stress is high in the horizontal plane, which could cause deformation, failure, and rock bursts in the roadways and stopes. The measurement results show that the sh,max, sh,min and sv of the mine area increase in a nearly linear relationship with depth. The in-situ stress linear regression equations were obtained by combining the nine measurement points with the other four points' results at shallow levels measured in 2002 (the data are shown in Fig. 3).21

σ h, max = 1.433 + 0.043H

3.3. In-situ stress field distribution On the basis of the in-situ stress measurement results of the nine measurement points (Table 2), the in-situ stress field distribution at deep positions in Sanshandao Gold Mine was determined as follows. Each measurement point has two principal stress directions close to the horizontal plane. The angles between the two principal stress directions and the horizontal plane are in general smaller than 10°, and several greater angles also do not exceed 20°. Another principal stress is close to the vertical direction, with a vertical angle that is no greater than 20°. The

Fig. 3. Regression lines of the principal stresses with depth.

(1)

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σ h, min = 0.536 + 0.024H

(2)

σv = 0.838 + 0.027H

(3)

where H is the vertical depth (m). The regression lines between the principal stresses and the depth are shown in Fig. 3. The linear correlation coefficients of the sh,max, sh,min and sv are 0.987, 0.963 and 0.954, respectively. The regression lines show that the sh,max (s1) of the mine area is greater than 30 MPa below the depth of
660 m. This provides the second necessary condition (a high stress environment) for the rock burst in deep mining of Sanshandao Gold Mine.

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testing equipment). Before the peak strength, the main strain of the high strength brittle rock is linear elastic strain with little plastic strain. And the elastic strain energy is released quickly after the rock failure. Therefore, we simply assume that the elastic strain could be completely recovered before the peak strength. The complete stress–strain curve of the high strength brittle rock can be divided into two parts, bound by the left and right half curves representing WE and Wp, respectively. And the impact property (R) can be calculated by the area ratio of the two parts. It is generally recognized that the rock burst will occur as RZ1; and the greater the R, the greater the impact tendency of rock burst.

4. Rock burst predictions

4.1.2. Linear elastic energy (WE) Under the uniaxial stress state, assuming that no damage appears before peak strength, the linear elastic energy stored in the rock specimen before rock failure is

4.1. Rock burst proneness analysis on loading tests

WE =

There are two necessary conditions for a rock burst. First, the rock mass must have the ability to store high strain energy (i.e., rock burst proneness) because there is a strong impact when a rock burst occurs. Second, the surrounding rock must be in a high stress environment, which can accumulate high energy and abruptly release it during rock burst. To predict the rock burst proneness and its potential intensity, many criteria and classifications16,22–24 were provided, such as Hoek, Turchaninov, Kidybiński, Russenes, et al. According to the results of physical and mechanical tests and in-situ stress measurements, multiple discriminant criteria2,9,25,26 (i.e., impact property, linear elastic energy, brittleness coefficient and Tao discriminant index) were used to analyze and evaluate the high energy storing ability (the first necessary condition) of the main rocks in Sanshandao Gold Mine.

where sc is the uniaxial compressive strength (UCS), MPa; Es is Young's modulus, MPa. The rock burst intensity can be divided into four levels: (1) Weak rock burst proneness as WE o 40 kJ/m3. (2) Moderate rock burst proneness as 40 kJ/m3 rWE o100 kJ/m3. (3) Strong rock burst proneness as 100 kJ/m3 rWE o200 kJ/m3. (4) Extra-strong rock burst proneness as WE Z 200 kJ/m3.

4.1.1. Impact property (R) Based on the complete stress–strain response, the discriminant index of impact property (R) can be calculated by

R = WE/WP

(4)

where WE is the elastic strain energy accumulated before rock failure; WP is the plastic strain energy consumed after rock failure. The ideal complete “stress–strain” curve is shown in Fig. 4. (It should be noted that after deformation localizes in the form of a failure plane, strain is no longer homogeneous, and the nominal stress and strain can no longer be said to correspond to constitutive behavior that is independent of the sample size and

Fig. 4. The ideal complete stress–strain curve.

σc2 2Es

(5)

4.1.3. Brittleness coefficient (B) A brittleness coefficient (B) can be defined as the ratio of uniaxial compressive strength (sc) to tensile strength (st) of rock:

B = σc /σt

(6)

The discriminant indexes of the brittleness coefficient are as follows: (1) No rock burst proneness as B 440; (2) Weak rock burst proneness as 26.7 oB r40; (3) Moderate rock burst proneness as 14.5 rB r26.7; (4) Strong rock burst proneness as B o14.5. 4.1.4. Tao discriminant index (α) Based on the stress reduction factor of the Q-system for rock mass classification developed by Barton, Tao conducted a large amount of research in China and put forward the following definition:

α = σc /σ1

(7)

where s1 is the maximum principal in-situ stress. The classification of rock burst submitted by Tao is as follows: (1) No rock burst proneness as α 4 14.5; (2) Weak rock burst proneness as 5.5 o α r14.5; (3) Moderate rock burst proneness as 2.5 o α r5.5; (4) Strong rock burst proneness as α r2.5. 4.1.5. The results of rock burst proneness Fig. 5 shows the complete stress–strain curves of the main rocks in Sanshandao Gold Mine. According to Fig. 4 and Eqs. (4)– (7), the rock burst proneness of the main rocks are calculated and listed in Table 3 based on the multiple discriminant criteria. In addition, the current mining activities of Sanshandao Gold Mine are concentrated at a depth of
600 m and several roadways are excavated at the level of
870 m. The maximum principal stresses at the depths of
600 m and
870 m were determined from Eq. (1) and used to calculate the Tao discriminant index α
600 and α
870. Although there are small differences in rock burst proneness discriminant results between the different criteria, the main rocks at deep position show a moderate or strong rock burst proneness

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80

100

80 60

80

40

40

60 40

20

20

0 0.0

Stress (MPa)

Stress (MPa)

Stress (MPa)

60

20

0.1

0.2 0.3 Strain (1%)

0 0.0

0.4

0.1

0.2

0.3 0.4 Strain (1%)

0.5

0 0.0

0.6

0.1

(b) Adamellite

(a) Amphibolite tonalities

0.2 0.3 Strain (1%)

0.4

0.5

0.4

0.5

(c) Biotite granulite

100

80

100

80

80

60

60 40

40

0.1

0.2 Strain (1%)

0.3

0.4

40

20

20

20 0 0.0

Stress (MPa)

60 Stress (MPa)

Stress (MPa)

120

0 0.0

0.1

(d) Phyllic granite

0.2 0.3 Strain (1%)

0.4

0.5

0 0.0

0.1

(e) Phyllic

0.2 0.3 Strain (1%)

(f) Footwall granite

Fig. 5. The complete stress–strain curves of the main rocks in Sanshandao Gold Mine.

and the ability to store large amounts of elastic energy. The greater the depth, the stronger the rock burst proneness. Besides, the complete stress–strain curves present a strong brittle failure feature, which provides the first necessary condition (a high strength brittle rock) for rock burst. 4.1.6. The discussion of rock burst criteria for Shanshandao Gold Mine A great amount of theoretical researches, field investigation and in-situ measurement data, and laboratory test data have shown that the in-situ stress is greater than a certain percentage of the rock's UCS where rock burst occurs. Besides, the rock should be brittle, hard and intact (or relatively intact). Meanwhile, the elastic strain energy accumulated in rock should be much greater than the energy consumed after rock failure. The Chinese scholars also proposed several criteria based on engineering practices.27–29 Combining the rock physical–mechanical test results, rock burst proneness analysis, and the field investigations (for example, the first known rock burst occurred at roadway excavation of level
475 m) and measurements, and referring to other rock burst criteria and classifications, the authors provide a set of rock burst proneness criteria for the deep mining of Sanshandao Gold Mine.

σ1 ≥ 0.17σc (mechanical property)⎫ ⎪ ⎪ (brittleness) σc ≥ 13σt ⎬ RQD ≥ 62% (rock mass integrity) ⎪ ⎪ (energy) R ≥ 1.2 ⎭

(8)

Therefore, rock burst would occur when all requirements of Eq. (8) are met simultaneously in the deep mining of Sanshandao Gold Mine. Its potential intensity can be predicted by referring to other classifications. 4.2. Rock burst analysis on the unloading triaxial tests Currently, most criteria and classifications for rock burst were obtained by loading tests. However, this is not consistent with the stress variation as rock burst occurrence in the unloading process of rock excavation. Therefore, the unloading triaxial tests of Biotite granulite were conducted to study the rock burst. At first, the specimen was loaded to 85% of the peak strength (sp) of the conventional triaxial test (s3 ¼15 MPa, sp ¼225 MPa; s3 ¼ 30 MPa, sp ¼283 MPa). Then the confining pressure (s3) was unloaded with the axial strain (ε1) constant. The results are shown in Fig. 6.

Table 3 Rock burst proneness analysis of the main rocks in Sanshandao Gold Mine based on multiple criteria. Lithology

Amphibolite tonalites Adamellite Biotite granulite Phyllic granite Phyllic Footwall granite

Impact property

Linear elastic energy 3

Brittleness coefficient

Tao Zhenyu index

R

Proneness

WE (kJ/m )

Proneness

B

Proneness

α
600

Proneness

α
870

Proneness

1.8 2.5 2.6 3.1 2.8 2.3

Yes Yes Yes Yes Yes Yes

58.51 92.19 118.66 181.79 105.96 70.25

Moderate Moderate Strong Strong Strong Moderate

5.89 10.22 5.93 7.68 6.51 8.40

Strong Strong Strong Strong Strong Strong

3.17 2.60 3.58 4.60 3.42 3.04

Moderate Strong Moderate Moderate Moderate Moderate

2.22 1.82 2.50 3.22 2.40 2.13

Strong Strong Strong Moderate Strong Strong

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91

Fig. 6. Curves of unloading triaxial tests (unloading confining pressure with constant ε1) of Biotite granulite.

As shown in Fig. 6(a), in the initial stage of unloading confining pressure, the increase of circumferential strain (ε2) is small. There is an approximate linear relationship between the ε2 and s3, indicating that the ε2 is in the state of elastic deformation. With the reduction of confining pressure, the plastic deformation appears in the specimen. In this process, even a small decrease of s3 can cause an obvious increase in ε2. As shown in Fig. 6(b), the s1 gradually decreases with the reduction of s3, and the strength of specimen declines significantly. And the s1 of specimen is greatly reduced after failure, indicating that the effect of s3 on axial bearing capacity is remarkable. Besides, the test machine does not do work on the specimen as the confining pressure was unloaded with constant ε1. The elastic strain energy accumulated in the specimen is released during its yielding and failure process. From Fig. 6(c) and (d), we know that the rock failure will occur in a low stress state as the stress along one direction suddenly decreases. Therefore, actually, only small elastic strain energy will be absorbed by the rock, and most of them will be released to cause rock burst. Additionally, compared with rock specimens, the rock mass is large and full of fractures and joints, so the strength and rock burst proneness of the rock mass is weaker than the intact rock specimens. Therefore, a numerical model was built to perform the quantitative calculation of the rock burst risk prediction at deep positions in Sanshandao Gold Mine.

4.3. Rock burst risk prediction of deep mining based on energy accumulation condition 4.3.1. The second necessary condition for rock bursts A rock burst is a dynamic phenomenon of rock failure caused by energy that accumulates and releases. The rock mass accumulates a certain amount of elastic strain energy in the initial state. And the rock burst would occur when the stored energy attains a certain limit. So the occurrence and intensity of rock bursts have a direct relationship with the amount of energy that has accumulated in the rock mass. Namely, rock burst occurrence must have another necessary condition as well: the mechanical system composed by the rock mass must be able to provide a stress environment that accumulates and abruptly releases high energy. 4.3.2. Establishment of the numerical model The FLAC3D program was used to calculate and analyze the distribution and accumulation of energy in surrounding rock during the orebody excavation. According to the occurrence environment of lithology and orebody, and the mining method (Room-and-pillar method) at deep positions in Sanshandao Gold Mine, the numerical model was set up from the elevation of
380 m to
1180 m. In the model's coordinates, it took the strike direction and thickness direction of the orebody and the vertical direction as x-axis, y-axis and z-axis, respectively. The model's

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Table 4 Physical and mechanical parameters assigned in the model. Lithology Rock mass

Orebody

Hanging wall Footwall Fault

Bulk modulus (GPa)

Shear modulus (GPa)

Cohesion (MPa)

Internal friction angle (deg)

Tensile strength (MPa)

3.37 5.48 0.595 2.51

2.8 3.45 3.78 1.35

11.4 42.8 0.128 21.5

31 37 18 33

4.37 5.98 0.0198 3.44

length in the direction of x-axis and y-axis and the height in the direction of the z-axis are 400 m, 800 m and 800 m, respectively. The model is divided into 239,508 units and 256,878 nodes. Considering geological and hydrogeological conditions, the model is divided into four groups: the rock mass around the hanging wall, footwall, fault, and orebody. The in-situ stress field model received from the in-situ stress measurement was used to calculate the sh,max, sh,min and sv, then loaded the stress gradient constraints on the model boundary and the gravitational field to the model. Finally, the model constrained the z direction displacement of the bottom surface, the x direction displacement on the two surfaces perpendicular to the x-axis, and the y direction displacement of the two surfaces perpendicular to the y-axis.

-5.1180e+004 to -4.0000e+004 -4.0000e+004 to -2.0000e+004 -2.0000e+004 to 0.0000e+000 0.0000e+000 to 2.0000e+004 2.0000e+004 to 4.0000e+004 4.0000e+004 to 6.0000e+004 6.0000e+004 to 8.0000e+004 8.0000e+004 to 9.9585e+004

-2.0000e+004 to 0.0000e+000 to 2.0000e+004 to 4.0000e+004 to 6.0000e+004 to 8.0000e+004 to

4.3.3. The elastic strain energy distribution of surrounding rock in deep mining The elastic strain energy parameters were defined in FLAC3D to simulate the energy accumulation and distribution in surrounding rock during the orebody excavation. The elastic strain energy of the surrounding rock can be calculated according to

-2.0000e+004 to 0.0000e+000 to 2.0000e+004 to 4.0000e+004 to 6.0000e+004 to 8.0000e+004 to 1.0000e+005 to

0.0000e+000 2.0000e+004 4.0000e+004 6.0000e+004 8.0000e+004 1.0000e+005

-2.5000e+004 to 0.0000e+000 to 2.5000e+004 to 5.0000e+004 to 7.5000e+004 to 1.0000e+005 to 1.2500e+005 to 1.5000e+005 to

The physical and mechanical parameters of the model, obtained by the rock mechanics tests, appropriately reduced considering the size effect based on Hoek–Brown criterion. The parameters of the rock mass around the hanging wall, footwall, fault, and orebody assigned in the model are shown in Table 4.

0.0000e+000 2.5000e+004 5.0000e+004 7.5000e+004 1.0000e+005 1.2500e+005 1.5000e+005 1.5825e+005

0.0000e+000 2.0000e+004 4.0000e+004 6.0000e+004 8.0000e+004 1.0000e+005 1.0918e+005

-2.5000e+004 to 0.0000e+000 to 2.5000e+004 to 5.0000e+004 to 7.5000e+004 to 1.0000e+005 to 1.2500e+005 to

0.0000e+000 2.5000e+004 5.0000e+004 7.5000e+004 1.0000e+005 1.2500e+005 1.3689e+005

-7.4467e+004 to -5.0000e+004 -5.0000e+004 to 0.0000e+000 0.0000e+000 to 5.0000e+004 5.0000e+004 to 1.0000e+005 1.0000e+005 to 1.5000e+005 1.5000e+005 to 2.0000e+005 2.0000e+005 to 2.3784e+005

Fig. 7. The elastic strain energy distribution in surrounding rock during the future mining process (unit: J/m3). (a) level -780m to -825m, (b) level -825m to -870m, (c) level -870m to -915m, (d) level -915m to -960m, (e) level -960m to -1005m and (f) level -1005m to -1050m.

S.-J. Miao et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 86–94

We =

1 (σ1ε1 + σ 2 ε2 + σ 3 ε3 ) 2

log E = 4.8 + 1.5 M

where E represents the energy accumulated in rock mass in Eq. (10), and M is the earthquake magnitude corresponding to the rock burst. The potential mine earthquake magnitudes (Table 5) caused by rock bursts can be calculated according to the accumulated energy in the surrounding rock at different sub-levels using Eq. (11). The research and prediction results show that rock bursts will most likely occur when the mining depth exceeds
825 m. Mine earthquakes caused by rock bursts could reach and exceed magnitude 3 below
825 m, and the mining in sub-levels of
1005 m to
1050 m could induce mine earthquakes of magnitude 3.4. This should draw significant attention and cause the necessary measures to be taken to monitor and control rock bursts in future deep mining activities. 4.3.6. The prevention measures of rock burst Room-and-pillar method is still used in the deep mining of Sanshandao Gold Mine. During the deep excavation of roadways or stopes, the lateral stress of surrounding rock is removed and the stress field is redistributed. According to the unloading triaxial tests results, the unloading effect causes the rapid decrease in the bearing capacity of surrounding rock, which could lead to rock failure in a low stress state. The rock burst will occur with the quick and sudden release of elastic strain energy accumulated in the surrounding rock. Therefore, timely and effective support should be carried out in the deep excavation to prevent a sudden decrease in the confining pressure of surrounding rock. In addition, many studies have shown that the greater the volume of one excavation, the greater the energy release; the faster the excavating speed, the higher the energy release rate.13 Especially, when the excavation face is close to discontinuities, the stress concentration of interface is more remarkable. In the meantime, the rock burst easily occurs because of the increase of the strain energy and the decrease of the maximum energy storage capability of surrounding rock. Therefore, reasonable and effective mining method, support, and excavation manner and speed would be chosen to avoid various predisposing factors that cause rock bursts in the deep mining of Sanshandao Gold Mine.

4.3.4. Energy accumulation of surrounding rock in deep mining The simulation results show that the elastic strain energy increases with mining from the higher sub-level to lower sub-level. The energy accumulation zone is mainly distributed in the surrounding rock around two sides and the handing wall of the stope. The maximum energy value appears at the junctions of two sides and hanging wall of the stope. The research30 shows that the rock burst is likely to occur when the strain energy accumulated in the surrounding rock is greater than 1.0  105 J/m3. The maximum elastic strain energy of the surrounding rock is about 9.9585  104 J/m3 during the mining of sub-level
780 m to
825 m. Therefore, below the depth of
825 m, the maximum elastic strain energy during mining is greater than 1.0  105 J/m3, which could create the high-energy environment for rock burst occurrence in the surrounding rock. According to the stress and strain distribution from the simulation, the energy stored in the surrounding rock can be calculated with: n

∑ i=1

(11)

(9)

where We is the elastic strain energy of the surrounding rock; s1, s2, s3 and ε1, ε2, ε3 are the principal stresses and the principal strains, respectively, in a rock element. Currently, the deepest development of the Sanshandao Gold Mine is at the level of
780 m, so the mining area from level
780 m to
1050 m was chosen. The current sub-level height, 45 m, was assigned as the sub-level height of the model. On the basis of the existing modules of FLAC3D, Fish language was programmed to simulate and reveal the accumulation and distribution of elastic strain energy in the surrounding rock after the deep orebody excavation. The elastic strain energy contours of the surrounding rock around the stopes during the future mining activities are shown in Fig. 7.

W=

93

1 ( σ1ε1 + σ 2 ε2 + σ 3 ε3 ) × Ve 2

(10)

where W is the energy stored in rock mass elements; Ve is the element volume; n is the number of elements. As shown in Fig. 7, the maximum elastic strain energy concentrates on the corner of the stope and the elastic energy in surrounding rock is mainly distributed within the range of 10 m around the stope. Therefore, the above range of the surrounding rock was outlined to calculate the gross accumulated energy around the stopes in the different depths (Table 5).

5. Conclusions The overcoring technique with hollow inclusion strain gauge adapted for in-situ stress measurements at great depth was used to obtain the three-dimensional stress states of nine measurement points at six levels in Sanshandao Gold Mine. Based on the results of in-situ stress measurements, uniaxial tests and unloading triaxial tests, multi criteria were used to qualitatively analyze and evaluate the rock burst proneness in deep mining. Finally, combined with the energy accumulation, distribution, and development, potential rock burst locations and the magnitudes induced by the future deep mining were predicted. The main conclusions were drawn as follows.

4.3.5. The prediction of rock burst magnitude A rock burst is essentially a sudden release of strain energy accumulated in a rock mass during the excavation in a high in-situ stress field. In essence, the rock burst is a kind of artificially induced earthquake.31 According to the seismological theory, the energy released through seismic waves can be used to calculate the earthquake magnitude with:

Table 5 Energy accumulation status and rock burst caused mine earthquake magnitude in deep mining. Sub-level


780 m to
825 m

The maximum elastic strain energy 9.9585 × 104 We,max (J/m3) The accumulation elastic energy in the / range of 10 m around stope (J) Mine earthquake magnitude /


825 m to
87 0m


870 m to
915 m


915 m to
960 m


960 m to
1005 m


1005 m to
1050 m

1.0918 × 105

1.2078 × 105

1.3689 × 105

1.5825 × 105

2.3784 × 105

3.9742 × 109

4.3964 × 109

4.9828  109

4.9828 × 109

8.6574 × 109

3.1

3.2

3.2

3.3

3.4

94

S.-J. Miao et al. / International Journal of Rock Mechanics & Mining Sciences 83 (2016) 86–94

The in-situ stresses are at a high level in deep positions, and the horizontal tectonic stress is in the dominating position in the mine area. The sh,max, sh,min and sv increase in a nearly linear relationship with depth. The maximum ratio of the sh,max to the sh,min is 1.98, which provides favorable conditions for rock failure and rock burst. The discriminant results obtained from multiple criteria show that the main rocks at deep positions in Sanshandao Gold Mine have a tendency towards rock burst. Especially, the greater the depth, the stronger the rock burst proneness. It satisfies the first necessary condition for a rock burst: the rock must have the ability to store large amounts of elastic energy. In addition, a set of rock burst proneness criteria for the deep mining of Sanshandao Gold Mine is provided. Curves of unloading triaxial tests show that the rock failure will occur in a low stress state as the stress along one direction suddenly decreases. Actually, only small elastic strain energy will be absorbed by the rock, and most of them will be released to cause rock burst. Energy accumulation and distribution during the deep mining process calculated by numerical simulation with FLAC3D show that the high-energy accumulation regions (the elastic energy density is greater than 1.0  105 J/m3) exist below the mining of level
825 m in Sanshandao Gold Mine. This satisfies the second necessary condition for the rock burst: a stress environment that adapts to accumulate high strain energy. Rock bursts will most likely occur when the mining depth exceeds
825 m in Sanshandao Gold Mine, and the mine earthquakes caused by rock burst could exceed magnitude 3. Therefore, monitoring techniques and safety measures should be taken to pay particular attention to the potential rock burst regions. In addition to the two necessary conditions, other various predisposing factors also affect the rock burst, such as mining method, support conditions, excavation manner and speed, hydrogeological conditions, and so on, which induce uncertainty of rock burst prediction. Therefore, it is necessary to combine with engineering practice, in-situ measurement and monitoring to prevent and control the occurrence of rock burst in deep mining of Sanshandao Gold Mine.

Acknowledgments This research is financially supported by National Natural Science Foundation of China (No. 51574014, No. 11002021 and No. 51534002), Beijing Higher Education Young Elite Teacher Project (No.YETP0341), National Key Basic Research Program of China (973 Program) (No. 2015CB060200), Fundamental Research Funds for the Central Universities (No. FRF-TP-15-037A2). Prof. Joseph F. Labuz is gratefully acknowledged for his review of this paper.

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