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Rock failure due to energy release during unloading and application to underground rock burst control
Tunnelling and Underground Space Technology 16 Ž2001. 241᎐246
Technical note
Rock failure due to energy release during unloading and application to underground rock burst control An-Zeng Huaa,U , Ming-Qing Youb a
College of Architecture and Ci¨ il Engineering, China Uni¨ ersity of Mining and Technology, Xuzhou, Jiangsu 221008, PR China b Jiaozuo Institute of Technology, Jiaozuo, PR China Received 20 December 2000; received in revised form 4 June 2001; accepted 15 July 2001
1. Introduction In many engineering applications, a material deforms under increasing external loads from zero, and fails when the loads exceed the strength. However, in underground rock excavation, the rock is in a triaxial stress state before excavation. Excavating causes stress decrease in some directions, which results in rock failure. Uniaxial Ž 2 s 3 s 0. and triaxial Ž 2 s 3 / 0. compression experiments are designed to simulate the loading process, in which the axial stress and confining pressure are increased until specimens fail. In order to study the condition and process of rock failure in underground excavation where stress release is involved, laboratory experiments should be conducted to simulate stress decrease and the resulting rock failure process. The effect of loading path on rock properties has been studied by many researchers. However, the conclusions are not clear enough in understanding rock strength and circumferential deformation during stress release as in the situation of rock burst Že.g. Stacey, 1981; Li and Wang, 1993; Ortlepp and Stacey, 1994; Zhao, 2000; Ortlepp, 2001.. This study attempts to reveal the deformation and failure mechanism during confining pressure release through laboratory controlled experiments. There are two experimental methods dealing with confining pressure reduction tests. One is to keep axial stress or axial U
Corresponding author. Tel.: q86-516-399-5992; fax: q86-516399-5991. E-mail address: [email protected] ŽA.-Z. Hua..
deviator stress constant, which will cause brittle fracturing of the specimen ŽSwanson and Brown, 1971.. The other is to keep axial deformation constant to study the friction coefficient ŽShimamoto, 1985.. In this study, rock specimens are firstly compressed in a triaxial cell and then the confining pressure is reduced progressively while keeping the axial deformation constant. In the whole process, the axial and circumferential deformation is measured. Experiments on marble, siltstone and coal with decreasing confining pressure were carried out. The results indicate that rock fracturing in axial compression absorbs energy, while rock fracturing in decreasing confining pressure releases energy. The fracturing and failure mechanism during unloading, and application to underground rock burst control are discussed.
2. Experimental results The experiments were carried out on a MTS servocontrolled testing machine. The testing procedure is described as follows: 1. Hydrostatic triaxial pressure Žconfining pressure s axial stress . is applied and kept at a desired level. 2. Axial deviator stress is applied to a level that is higher than the uniaxial strength and lower than its triaxial strength corresponding to the applied confining pressure. 3. Confining pressure is decreased at a rate of approximately 2 MParmin while axial deformation is maintained at a constant level Ži.e. no additional axial deformation..
0886-7798r01r$ - see front matter 䊚 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 8 8 6 - 7 7 9 8 Ž 0 1 . 0 0 0 4 6 - 3
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
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Uniaxial and triaxial compression tests were also carried out to determine the uniaxial and triaxial strength of the rock material. Results of all the experiments are compared and discussed in this paper. During the testing, it was found that the circumferential deformation gauges were frequently damaged due to the excessively large circumferential deformation when the confining pressure is reduced. A new deformation measurement device was manufactured to overcome that problem and was successfully applied. Specimens will undergo elastic deformation, yield and failure during confining pressure reduction while the axial deformation is kept constant. Fig. 1 plots axial stress with confining pressure reduction for five marble specimens. At initial stage, as the confining pressure reduces, the axial stress decreases, following Hooke’s law, from which the Poisson’s ratio can be calculated as s
1 d1 s 0.14 2 d3
When the confining pressure decrease continues, the axial stress does not decrease linearly again. This means that the specimen has passed its yield point. Triaxial strengths of the rock specimen under constant confining pressure are also given in Fig. 1. Except for specimen No. 6 Žsuspected of having experimental error., the other specimens all failed in the two loading paths following the same strength criterion. That means, for
a particular rock, the failure condition for confining pressure reduction tests is almost the same as that for triaxial loading compression tests. The rock fails at the same axial stress for the same confining pressure for both tests. Experiments were also carried out on marble, siltstone and coal. Results are summarized in Tables 1᎐3. Figs. 2 and 3 show the complete axial stress᎐axial strain and confining pressure᎐circumferential strain curves of siltstone specimen No. 5, while Fig. 4 shows the relationship between strain energy and confining pressure during loading and unloading. In Fig. 2, the inclined lines are the deformation during loading stage, and the vertical line represents the confining pressure reduction stage, in which no additional axial deformation is allowed. During pressure reduction, the portion of few points Žaxial stress from 128 to 65 MPa. represents the fracturing of the rock specimen. After that, the axial stress is just the residual strength of the rock at that given confining pressure. Fig. 3 shows the change of circumferential deformation with confining pressure. The initial portion of the curve from origin to the peak indicates the increase of axial stress and confining pressure, as the rock specimen is compressed by hydrostatic pressure. The horizontal line indicates that during the increase of axial deviator stress while keeping the confining pressure constant, the specimen expands laterally. The remainder of the curves reflects the process associated with
Fig. 1. Change of axial stress with confining pressure reduction.
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
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Table 1 Experimental results on a marble Specimen number
1 2 3 4 5 6 7
Loading stage
Confining pressure reduction stage
Peak stress
Strain at peak
Energy absorbed ŽMJ my3 .
ŽMPa.
stress
1 s 150 2 s 3 s 0 1 s 218 2 s 3 s 10 1 s 321 2 s 3 s 30 1 s 238 2 s 3 s 20 1 s 342 2 s 3 s 40 1 s 283 2 s 3 s 45 1 s 319 2 s 3 s 45
1 s 0.007
0.53
1 s 0.01 2 s 3 s y0.0003 1 s 0.015 2 s 3 s y0.003 1 s 0.006 2 s 3 s y0.0002 1 s 0.01 2 s 3 s y0.002 1 s 0.006 2 s 3 s y0.005 1 s 0.006 2 s 3 s y0.006
1.11
Failure stress
Strain at failure stress
ŽMPa.
Note Energy released ŽMJ my3 .
2.69 0.66 1.75 1.07 1.23
1 s 135 2 s 3 s 4.32 1 s 141 2 s 3 s 9.46 1 s 111 2 s 3 s 5.2 1 s 71 2 s 3 s 16
1 s 0.006 2 s 3 s y0.008 1 s 0.01 2 s 3 s y0.01 1 s 0.006 2 s 3 s y0.007 1 s 0.006 2 s 3 s y0.003
y0.1 y0.4 y0.35 y0.34
Uniaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction
confining pressure reduction. It should be understood that the yield and failure of specimen is localized and elastic deformation will change into plastic deformation during axial stress decrease. The rapid increase of circumferential deformation in the middle portion of the rock specimen is also related to the axial plastic deformation. Also the failure of the specimen during confining pressure reduction is due to the rapid increase of circumferential deformation. Fig. 4 shows the complete curve of strain energy that specimen absorbed from the surrounding, which is calculated as
loading process of confining pressure and axial stress, but releases energy Ži.e. U decreases. in confining pressure reduction. When the confining pressure changes little but energy drops substantially Žat confining pressure approx. equal to 13.6 MPa., the specimen begins to fail. In other words, at this confining pressure, the restored axial elastic deformation gained during axial loading changes to the plastic deformation when the specimen fails. In the end, the axial elastic deformation gained from axial loading changes to the plastic deformation completely.
Us Ý1 ⌬ 1 q Ý2 ⌬ 2 q Ý3 ⌬ 3
3. Discussion on experimental results
where 1 is the axial stress, ⌬ 1 is the change of axial strain, 2 s 3 is the confining pressure, and ⌬ 2 s ⌬ 3 is the change of circumferential strain. The specimen absorbs energy Ži.e. U increases . in
The main difference between confining pressure reduction and axial compression loading is the energy change of specimens during failure. From Tables 1᎐3, it is clearly shown that specimen absorbs energy during
Table 2 Experimental results on a coal Specimen number
1 2 3 4 5
Loading stage
Confining pressure reduction stage
Peak stress
Strain at peak
ŽMPa.
stress
1 s 39.2 2 s 3 s 0 1 s 53.6 2 s 3 s 5 1 s 79.6 2 s 3 s 10 1 s 54 2 s 3 s 10 1 s 68 2 s 3 s 10
1 s 0.007 2 s 3 s y0.015 1 s 0.014 2 s 3 s y0.017 1 s 0.018 2 s 3 s y0.015 1 s 0.012 2 s 3 s y0.014 1 s 0.019 2 s 3 s y0.021
Energy absorbed ŽMJ my3 .
Failure stress
Strain at failure stress
ŽMPa.
Note Energy released ŽMJ my3 .
0.16 0.37 0.68 0.30 0.56
1 s 16 2 s 3 s 1.61 1 s 47.7 2 s 3 s 8.04
1 s 0.012 2 s 3 s y0.021 1 s 0.019 2 s 3 s y0.024
y0.04 y0.06
Uniaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Failure by confining pressure reduction Failure by confining pressure reduction
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
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Table 3 Experimental results on a siltstone Specimen number
1 2 3 4 5 6 7 8 9 10 11
Loading stage
Confining pressure reduction stage
Peak stress
Strain at peak
ŽMPa.
stress
1 s 91 2 s 3 s 0 1 s 135 2 s 3 s 10 1 s 184 2 s 3 s 20 1 s 225 2 s 3 s 30 1 s 221 2 s 3 s 40 1 s 225 2 s 3 s 40 1 s 225 2 s 3 s 40 1 s 247 2 s 3 s 40 1 s 207 2 s 3 s 40 1 s 234 2 s 3 s 40 1 s 236 2 s 3 s 40
1 s 0.006 2 s 3 s y0.0016 1 s 0.012 2 s 3 s y0.0069 1 s 0.015 2 s 3 s y0.004 1 s 0.012 2 s 3 s y0.002 1 s 0.011 2 s 3 s y0.0006 1 s 0.011 2 s 3 s y0.0009 1 s 0.015 2 s 3 s y0.003 1 s 0.011 2 s 3 s y0.0005 1 s 0.011 2 s 3 s y0.0004 1 s 0.013 2 s 3 s y0.0007 1 s 0.015 2 s 3 s y0.0005
Energy absorbed ŽMJ my3 .
Failure stress
Strain at failure stress
ŽMPa.
Note Energy released ŽMJ my3 .
0.25 0.67 1.02 1.03 0.84 1.01 1.27 1.02 0.83 1.02 1.12
1 s 65 2 s 3 s 13.6 1 s 91 2 s 3 s 27.1 1 s 93 2 s 3 s 12.9 1 s 130 2 s 3 s 17.1 1 s 80 2 s 3 s 16 1 s 91 2 s 3 s 29 1 s 73 2 s 3 s 2.3
1 s 0.011 2 s 3 s y0.009 1 s 0.011 2 s 3 s y0.003 1 s 0.015 2 s 3 s y0.008 1 s 0.011 2 s 3 s y0.007 1 s 0.011 2 s 3 s y0.017 1 s 0.013 2 s 3 s y0.012 1 s 0.015 2 s 3 s y0.019
y0.31 y0.1 y0.29 y0.28 y0.64 y0.71 y0.41
Uniaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction
axial compression loading and releases energy during failure at confining pressure reduction. For uniaxial compression, loading and deformation are in the same direction, the strain energy absorbed by the specimen is increasing in the whole process. For triaxial compression, the axial loading and deformation are in the same direction, and specimen absorbs energy from the loading frame. On the other hand, loading and deformation are opposite in circumferential direc-
tion, the specimen releases energy to the confining oil. however, the strain energy in the specimen is still increasing since the amount of energy absorbed is greater than that released. In the confining pressure reduction test, the specimen releases energy to the confining oil during its circumferential expansion, but does not absorb energy from the loading frame as the axial deformation is kept constant. Therefore the specimen fails in the process of energy release. The amount of energy released is less than that absorbed in the loading process. A specimen will fail in confining pressure reduction at the point where the axial stress is greater than its strength.
Fig. 2. Typical axial stress and axial strain curve.
Fig. 3. Typical confining pressure and circumferential strain curve.
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
Fig. 4. Typical strain energy and confining pressure curve.
4. Mechanism of energy release The strain energy stored in the specimen varies with stress condition. The change of stress condition causes the change of energy, reflected by energy absorbing or releasing. In the experiments of confining pressure reduction, the specimen releases energy. The process is not elastic unloading and hence is different from that in axial compression. In other words, the maximum principal stress decreases passively while the minimum principal stress decreases actively. However, the effect of the stress decreasing on circumferential deformation is the same as that during axial compression ŽFig. 3.. That is, the specimen fractures as the circumferential plastic deformation increases, which results in the energy release of the specimen to the confining oil.
5. Applications to underground rock burst control The stress induced rock failure process such as rock burst in tunnel excavation, is a phenomenon of stress decrease and energy release. It could not be simulated and explained by the mechanical characteristics of rock during triaxial compressive loading. However, rock burst has been studied by conducting brittle failure tests in compression with a testing machine of low stiffness. Some researchers attempted to classify rock burst potential with complete stress᎐strain curves of rock material obtained from compression tests. It should be noted that the complete stress᎐strain curve reflects the combined characteristics of the testing machine, loading rate and rock material. It is not an intrinsic property of rock material. Therefore, it should not be used to explain rock burst failure mechanisms. From the exper-
245
iments of confining pressure reduction, it shows that rocks fail during the process of confining stress reduction, i.e. rocks can also fail during the change of stress condition, not only during the loading process. Strain energy stored in rock material is sufficiently large to cause failure when it is released. Therefore, based on the experimental results presented in this paper, rock will fail in excavation so long as the maximum in situ stress is greater than the uniaxial compressive strength of the rock. Rock will be stable when the maximum in situ stress is lower than the uniaxial compression strength. Also rock will outburst when the maximum in situ stress is excessively greater than the uniaxial compression strength. It is often difficult to measure the maximum in situ stress. But the core drilling can often show the stress state of the rock. For example, schistose and disc effects indicate that stress in the rock body is very high and outburst may happen. To prevent rock burst, the basic method is to release the strain energy in the rock mass before excavation. After that, the rock will not have enough energy stored to cause failure. The authors have used the pre-boring method to release the strain energy and to prevent the outburst in coal and gas mines and other situations ŽHua, 1989; Hua et al., 1999..
6. Conclusions Tests have been conducted in a triaxial compression system to simulate the failure process of stress released, by confining pressure reduction methods. The results show that rocks fail during confining reduction, if the initial axial stress is greater than the uniaxial strength of the rock. As shown in the experiments, rocks fail during the process of confining stress reduction, and the failure is associated with strain energy release. Strain energy is absorbed and stored during the loading stage in rock. The energy is sufficiently large to cause failure when it is released. To prevent rock burst, one of the basic methods is to release the strain energy in rock before excavation, so the rock will not have a large amount of energy stored and released to cause failure. Pre-boring has been successfully used to release the strain energy and to prevent the outburst in mines.
Acknowledgements The study is supported by a project funded by the National Natural Science Foundation of China. The authors would like to thank their colleagues of the
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A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
CKSP Geotechnical Engineering research group at the China Mining and Technology University for technical discussion and comments. References Hua, A.Z., 1989. Techniques and theoretical analysis of vertical shaft through burst prone seams. Chin. J. Rock Mech. Eng. 8 Ž4., 322᎐329. Hua, A.Z., Shen, W.A., Li, J.S., 1999. Outburst prevention technology of rock stratum containing fuel gas in the air shaft of Haishiwan mine. J. China Coal Soc. 24 Ž1., 52᎐55. Li, T.B., Wang, L.S., 1993. An experimental study on the deformation and failure features of basalt under unloading condition. Chin. J. Rock Mech. Eng. 12 Ž4., 321᎐327.
Ortlepp, W.D., Stacey, T.R., 1994. Rockburst mechanisms in tunnels and shafts. Tunnelling Underground Space Technol. 9 Ž1., 59᎐65. Ortlepp, W.D., 2001. The behavior of tunnels at great depth under large static and dynamic pressures. Tunnelling Underground Space Technol. 16 Ž1., 41᎐48. Shimamoto, T., 1985. Confining pressure reduction experiments. Int. J. Rock Mech. Mining Sci. 22 Ž4., 227᎐236. Stacey, T.R., 1981. A simple extension strain criterion for fracture of brittle rock. Int. J. Rock Mech. Mining Sci. 18, 469᎐474. Swanson, S.R, Brown, W.S., 1971. An observation of loading path independence of fracture in rock. Int. J. Rock Mech. Mining Sci. 8 Ž3., 227᎐231. Zhao, J., 2000. Applicability of Mohr᎐Coulomb and Hoek᎐Brown strength criteria to dynamic strength of brittle rock materials. Int. J. Rock Mech. Mining Sci. 37, 1115᎐1121.
Technical note
Rock failure due to energy release during unloading and application to underground rock burst control An-Zeng Huaa,U , Ming-Qing Youb a
College of Architecture and Ci¨ il Engineering, China Uni¨ ersity of Mining and Technology, Xuzhou, Jiangsu 221008, PR China b Jiaozuo Institute of Technology, Jiaozuo, PR China Received 20 December 2000; received in revised form 4 June 2001; accepted 15 July 2001
1. Introduction In many engineering applications, a material deforms under increasing external loads from zero, and fails when the loads exceed the strength. However, in underground rock excavation, the rock is in a triaxial stress state before excavation. Excavating causes stress decrease in some directions, which results in rock failure. Uniaxial Ž 2 s 3 s 0. and triaxial Ž 2 s 3 / 0. compression experiments are designed to simulate the loading process, in which the axial stress and confining pressure are increased until specimens fail. In order to study the condition and process of rock failure in underground excavation where stress release is involved, laboratory experiments should be conducted to simulate stress decrease and the resulting rock failure process. The effect of loading path on rock properties has been studied by many researchers. However, the conclusions are not clear enough in understanding rock strength and circumferential deformation during stress release as in the situation of rock burst Že.g. Stacey, 1981; Li and Wang, 1993; Ortlepp and Stacey, 1994; Zhao, 2000; Ortlepp, 2001.. This study attempts to reveal the deformation and failure mechanism during confining pressure release through laboratory controlled experiments. There are two experimental methods dealing with confining pressure reduction tests. One is to keep axial stress or axial U
Corresponding author. Tel.: q86-516-399-5992; fax: q86-516399-5991. E-mail address: [email protected] ŽA.-Z. Hua..
deviator stress constant, which will cause brittle fracturing of the specimen ŽSwanson and Brown, 1971.. The other is to keep axial deformation constant to study the friction coefficient ŽShimamoto, 1985.. In this study, rock specimens are firstly compressed in a triaxial cell and then the confining pressure is reduced progressively while keeping the axial deformation constant. In the whole process, the axial and circumferential deformation is measured. Experiments on marble, siltstone and coal with decreasing confining pressure were carried out. The results indicate that rock fracturing in axial compression absorbs energy, while rock fracturing in decreasing confining pressure releases energy. The fracturing and failure mechanism during unloading, and application to underground rock burst control are discussed.
2. Experimental results The experiments were carried out on a MTS servocontrolled testing machine. The testing procedure is described as follows: 1. Hydrostatic triaxial pressure Žconfining pressure s axial stress . is applied and kept at a desired level. 2. Axial deviator stress is applied to a level that is higher than the uniaxial strength and lower than its triaxial strength corresponding to the applied confining pressure. 3. Confining pressure is decreased at a rate of approximately 2 MParmin while axial deformation is maintained at a constant level Ži.e. no additional axial deformation..
0886-7798r01r$ - see front matter 䊚 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 8 8 6 - 7 7 9 8 Ž 0 1 . 0 0 0 4 6 - 3
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
242
Uniaxial and triaxial compression tests were also carried out to determine the uniaxial and triaxial strength of the rock material. Results of all the experiments are compared and discussed in this paper. During the testing, it was found that the circumferential deformation gauges were frequently damaged due to the excessively large circumferential deformation when the confining pressure is reduced. A new deformation measurement device was manufactured to overcome that problem and was successfully applied. Specimens will undergo elastic deformation, yield and failure during confining pressure reduction while the axial deformation is kept constant. Fig. 1 plots axial stress with confining pressure reduction for five marble specimens. At initial stage, as the confining pressure reduces, the axial stress decreases, following Hooke’s law, from which the Poisson’s ratio can be calculated as s
1 d1 s 0.14 2 d3
When the confining pressure decrease continues, the axial stress does not decrease linearly again. This means that the specimen has passed its yield point. Triaxial strengths of the rock specimen under constant confining pressure are also given in Fig. 1. Except for specimen No. 6 Žsuspected of having experimental error., the other specimens all failed in the two loading paths following the same strength criterion. That means, for
a particular rock, the failure condition for confining pressure reduction tests is almost the same as that for triaxial loading compression tests. The rock fails at the same axial stress for the same confining pressure for both tests. Experiments were also carried out on marble, siltstone and coal. Results are summarized in Tables 1᎐3. Figs. 2 and 3 show the complete axial stress᎐axial strain and confining pressure᎐circumferential strain curves of siltstone specimen No. 5, while Fig. 4 shows the relationship between strain energy and confining pressure during loading and unloading. In Fig. 2, the inclined lines are the deformation during loading stage, and the vertical line represents the confining pressure reduction stage, in which no additional axial deformation is allowed. During pressure reduction, the portion of few points Žaxial stress from 128 to 65 MPa. represents the fracturing of the rock specimen. After that, the axial stress is just the residual strength of the rock at that given confining pressure. Fig. 3 shows the change of circumferential deformation with confining pressure. The initial portion of the curve from origin to the peak indicates the increase of axial stress and confining pressure, as the rock specimen is compressed by hydrostatic pressure. The horizontal line indicates that during the increase of axial deviator stress while keeping the confining pressure constant, the specimen expands laterally. The remainder of the curves reflects the process associated with
Fig. 1. Change of axial stress with confining pressure reduction.
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
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Table 1 Experimental results on a marble Specimen number
1 2 3 4 5 6 7
Loading stage
Confining pressure reduction stage
Peak stress
Strain at peak
Energy absorbed ŽMJ my3 .
ŽMPa.
stress
1 s 150 2 s 3 s 0 1 s 218 2 s 3 s 10 1 s 321 2 s 3 s 30 1 s 238 2 s 3 s 20 1 s 342 2 s 3 s 40 1 s 283 2 s 3 s 45 1 s 319 2 s 3 s 45
1 s 0.007
0.53
1 s 0.01 2 s 3 s y0.0003 1 s 0.015 2 s 3 s y0.003 1 s 0.006 2 s 3 s y0.0002 1 s 0.01 2 s 3 s y0.002 1 s 0.006 2 s 3 s y0.005 1 s 0.006 2 s 3 s y0.006
1.11
Failure stress
Strain at failure stress
ŽMPa.
Note Energy released ŽMJ my3 .
2.69 0.66 1.75 1.07 1.23
1 s 135 2 s 3 s 4.32 1 s 141 2 s 3 s 9.46 1 s 111 2 s 3 s 5.2 1 s 71 2 s 3 s 16
1 s 0.006 2 s 3 s y0.008 1 s 0.01 2 s 3 s y0.01 1 s 0.006 2 s 3 s y0.007 1 s 0.006 2 s 3 s y0.003
y0.1 y0.4 y0.35 y0.34
Uniaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction
confining pressure reduction. It should be understood that the yield and failure of specimen is localized and elastic deformation will change into plastic deformation during axial stress decrease. The rapid increase of circumferential deformation in the middle portion of the rock specimen is also related to the axial plastic deformation. Also the failure of the specimen during confining pressure reduction is due to the rapid increase of circumferential deformation. Fig. 4 shows the complete curve of strain energy that specimen absorbed from the surrounding, which is calculated as
loading process of confining pressure and axial stress, but releases energy Ži.e. U decreases. in confining pressure reduction. When the confining pressure changes little but energy drops substantially Žat confining pressure approx. equal to 13.6 MPa., the specimen begins to fail. In other words, at this confining pressure, the restored axial elastic deformation gained during axial loading changes to the plastic deformation when the specimen fails. In the end, the axial elastic deformation gained from axial loading changes to the plastic deformation completely.
Us Ý1 ⌬ 1 q Ý2 ⌬ 2 q Ý3 ⌬ 3
3. Discussion on experimental results
where 1 is the axial stress, ⌬ 1 is the change of axial strain, 2 s 3 is the confining pressure, and ⌬ 2 s ⌬ 3 is the change of circumferential strain. The specimen absorbs energy Ži.e. U increases . in
The main difference between confining pressure reduction and axial compression loading is the energy change of specimens during failure. From Tables 1᎐3, it is clearly shown that specimen absorbs energy during
Table 2 Experimental results on a coal Specimen number
1 2 3 4 5
Loading stage
Confining pressure reduction stage
Peak stress
Strain at peak
ŽMPa.
stress
1 s 39.2 2 s 3 s 0 1 s 53.6 2 s 3 s 5 1 s 79.6 2 s 3 s 10 1 s 54 2 s 3 s 10 1 s 68 2 s 3 s 10
1 s 0.007 2 s 3 s y0.015 1 s 0.014 2 s 3 s y0.017 1 s 0.018 2 s 3 s y0.015 1 s 0.012 2 s 3 s y0.014 1 s 0.019 2 s 3 s y0.021
Energy absorbed ŽMJ my3 .
Failure stress
Strain at failure stress
ŽMPa.
Note Energy released ŽMJ my3 .
0.16 0.37 0.68 0.30 0.56
1 s 16 2 s 3 s 1.61 1 s 47.7 2 s 3 s 8.04
1 s 0.012 2 s 3 s y0.021 1 s 0.019 2 s 3 s y0.024
y0.04 y0.06
Uniaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Failure by confining pressure reduction Failure by confining pressure reduction
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Table 3 Experimental results on a siltstone Specimen number
1 2 3 4 5 6 7 8 9 10 11
Loading stage
Confining pressure reduction stage
Peak stress
Strain at peak
ŽMPa.
stress
1 s 91 2 s 3 s 0 1 s 135 2 s 3 s 10 1 s 184 2 s 3 s 20 1 s 225 2 s 3 s 30 1 s 221 2 s 3 s 40 1 s 225 2 s 3 s 40 1 s 225 2 s 3 s 40 1 s 247 2 s 3 s 40 1 s 207 2 s 3 s 40 1 s 234 2 s 3 s 40 1 s 236 2 s 3 s 40
1 s 0.006 2 s 3 s y0.0016 1 s 0.012 2 s 3 s y0.0069 1 s 0.015 2 s 3 s y0.004 1 s 0.012 2 s 3 s y0.002 1 s 0.011 2 s 3 s y0.0006 1 s 0.011 2 s 3 s y0.0009 1 s 0.015 2 s 3 s y0.003 1 s 0.011 2 s 3 s y0.0005 1 s 0.011 2 s 3 s y0.0004 1 s 0.013 2 s 3 s y0.0007 1 s 0.015 2 s 3 s y0.0005
Energy absorbed ŽMJ my3 .
Failure stress
Strain at failure stress
ŽMPa.
Note Energy released ŽMJ my3 .
0.25 0.67 1.02 1.03 0.84 1.01 1.27 1.02 0.83 1.02 1.12
1 s 65 2 s 3 s 13.6 1 s 91 2 s 3 s 27.1 1 s 93 2 s 3 s 12.9 1 s 130 2 s 3 s 17.1 1 s 80 2 s 3 s 16 1 s 91 2 s 3 s 29 1 s 73 2 s 3 s 2.3
1 s 0.011 2 s 3 s y0.009 1 s 0.011 2 s 3 s y0.003 1 s 0.015 2 s 3 s y0.008 1 s 0.011 2 s 3 s y0.007 1 s 0.011 2 s 3 s y0.017 1 s 0.013 2 s 3 s y0.012 1 s 0.015 2 s 3 s y0.019
y0.31 y0.1 y0.29 y0.28 y0.64 y0.71 y0.41
Uniaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Triaxial compression failure at loading stage Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction Failure by confining pressure reduction
axial compression loading and releases energy during failure at confining pressure reduction. For uniaxial compression, loading and deformation are in the same direction, the strain energy absorbed by the specimen is increasing in the whole process. For triaxial compression, the axial loading and deformation are in the same direction, and specimen absorbs energy from the loading frame. On the other hand, loading and deformation are opposite in circumferential direc-
tion, the specimen releases energy to the confining oil. however, the strain energy in the specimen is still increasing since the amount of energy absorbed is greater than that released. In the confining pressure reduction test, the specimen releases energy to the confining oil during its circumferential expansion, but does not absorb energy from the loading frame as the axial deformation is kept constant. Therefore the specimen fails in the process of energy release. The amount of energy released is less than that absorbed in the loading process. A specimen will fail in confining pressure reduction at the point where the axial stress is greater than its strength.
Fig. 2. Typical axial stress and axial strain curve.
Fig. 3. Typical confining pressure and circumferential strain curve.
A.-Z. Hua, M.-Q. You r Tunnelling and Underground Space Technology 16 (2001) 241᎐246
Fig. 4. Typical strain energy and confining pressure curve.
4. Mechanism of energy release The strain energy stored in the specimen varies with stress condition. The change of stress condition causes the change of energy, reflected by energy absorbing or releasing. In the experiments of confining pressure reduction, the specimen releases energy. The process is not elastic unloading and hence is different from that in axial compression. In other words, the maximum principal stress decreases passively while the minimum principal stress decreases actively. However, the effect of the stress decreasing on circumferential deformation is the same as that during axial compression ŽFig. 3.. That is, the specimen fractures as the circumferential plastic deformation increases, which results in the energy release of the specimen to the confining oil.
5. Applications to underground rock burst control The stress induced rock failure process such as rock burst in tunnel excavation, is a phenomenon of stress decrease and energy release. It could not be simulated and explained by the mechanical characteristics of rock during triaxial compressive loading. However, rock burst has been studied by conducting brittle failure tests in compression with a testing machine of low stiffness. Some researchers attempted to classify rock burst potential with complete stress᎐strain curves of rock material obtained from compression tests. It should be noted that the complete stress᎐strain curve reflects the combined characteristics of the testing machine, loading rate and rock material. It is not an intrinsic property of rock material. Therefore, it should not be used to explain rock burst failure mechanisms. From the exper-
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iments of confining pressure reduction, it shows that rocks fail during the process of confining stress reduction, i.e. rocks can also fail during the change of stress condition, not only during the loading process. Strain energy stored in rock material is sufficiently large to cause failure when it is released. Therefore, based on the experimental results presented in this paper, rock will fail in excavation so long as the maximum in situ stress is greater than the uniaxial compressive strength of the rock. Rock will be stable when the maximum in situ stress is lower than the uniaxial compression strength. Also rock will outburst when the maximum in situ stress is excessively greater than the uniaxial compression strength. It is often difficult to measure the maximum in situ stress. But the core drilling can often show the stress state of the rock. For example, schistose and disc effects indicate that stress in the rock body is very high and outburst may happen. To prevent rock burst, the basic method is to release the strain energy in the rock mass before excavation. After that, the rock will not have enough energy stored to cause failure. The authors have used the pre-boring method to release the strain energy and to prevent the outburst in coal and gas mines and other situations ŽHua, 1989; Hua et al., 1999..
6. Conclusions Tests have been conducted in a triaxial compression system to simulate the failure process of stress released, by confining pressure reduction methods. The results show that rocks fail during confining reduction, if the initial axial stress is greater than the uniaxial strength of the rock. As shown in the experiments, rocks fail during the process of confining stress reduction, and the failure is associated with strain energy release. Strain energy is absorbed and stored during the loading stage in rock. The energy is sufficiently large to cause failure when it is released. To prevent rock burst, one of the basic methods is to release the strain energy in rock before excavation, so the rock will not have a large amount of energy stored and released to cause failure. Pre-boring has been successfully used to release the strain energy and to prevent the outburst in mines.
Acknowledgements The study is supported by a project funded by the National Natural Science Foundation of China. The authors would like to thank their colleagues of the
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CKSP Geotechnical Engineering research group at the China Mining and Technology University for technical discussion and comments. References Hua, A.Z., 1989. Techniques and theoretical analysis of vertical shaft through burst prone seams. Chin. J. Rock Mech. Eng. 8 Ž4., 322᎐329. Hua, A.Z., Shen, W.A., Li, J.S., 1999. Outburst prevention technology of rock stratum containing fuel gas in the air shaft of Haishiwan mine. J. China Coal Soc. 24 Ž1., 52᎐55. Li, T.B., Wang, L.S., 1993. An experimental study on the deformation and failure features of basalt under unloading condition. Chin. J. Rock Mech. Eng. 12 Ž4., 321᎐327.
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