Technical Note Burst Energy Release Index. S. P. Singh. Introduction

Transcription

1 Rock Mechanics and Rock Engineering 21, (1988) Rock Mechanics and Rock Engineering 9 by Springer-Verlag 1988 Technical Note Burst Energy Release Index By S. P. Singh School of Engineering, Laurentian University, Sudbury, Ontario, Canada Introduction The rockburst is a sudden manifestation of the release of strain energy stored in the rock mass. Three possible sources for the liberated energy are (Salamon, 1983) 1. Stored strain energy in the surrounding mass. 2. Change in the potential energy of the rock mass. 3. Minor slippage along rock contacts. This study has focussed on the rock's capacity to store and release elastic strain energy. It is noteworthy however that in addition to rock type, a large excavation at depth andor unfavorable geological features are also necessary to cause serious bursting conditions. Rockburst is an energy phenomenon accompanying rock failure in form of brittle fracture induced by mining (C o o k, 1966). Rockbursts in deep hard rock mines provide graphic illustrations of the phenomenon of explosive brittle fracture (H o ek and Brown, 198). Energy changes occur during the course of mining acitivities and a part of the energy is stored in the rock mass surrounding the excavation (Salamon, 1983). The ultimate stress state is created by the action of the pre-mining and mining induced stresses. The changes brought about by mining merely trigger latent seismic events that derive mainly from the strain energy produced by geological differences in the state of stress (Co ok, 1983). The only feature common to most of the rockbursts is that rock failure is sudden and the strain energy is released from a volume of stresssed rock (Ortlepp, 1983). The purpose of this study was to search for a parameter, which represents the energy released at the time of rock fracture and which can be used as a relative measure of the burst proneness of rocks. The study was conducted on rocks from "Sudbury Nickel Belt" in Canada.

2 15 S.P. Singh: Energy Released During Fracture The important feature of a rockburst is the violent fracture of the rock which accompanies the release of energy in amounts sufficient to damage the surroundings. This suggests the search for a parameter which represents the energy released at the time of rock fracture and which can be used as a relative measure of the burst proneness of rocks. It has been proposed by P e t u k h o v (1957) that the violent fracture of a rock specimen in a soft testing machine represents a laboratory simulation of the dynamic fracture of the stressed rock during a burst in a mine. In this context, the rock mass being fractured around an excavation behaves as a rock specimen and the strata surrounding this rock mass acts as a testing machine. The violent nature of the specimen fracture in the soft testing machine satisfies the necessary conditions for the occurrence of the rockbursts which have been depicted in Fig. 1. CONDITIONS FOR ROCKBURST ULTIMATE STRESS CONDITION CAUSED BY - Gravity loading -Mining induced stresses -Stresses due to previous temperature loading conditions - S t r e s s e s due to faulting, folding, metamorphism, etc. - Favourable rock characteristics CONDITION OF INSTABILITY CAUSED BY -Sudden stress change of sizeable magnitude -Discontinuous rock mass -Rocks in strain softening state -Some support system on the verge of coltapse -Geological discontinuities SOURCE OF SUFFICIENT ENERGY DUE TO THE ORK DONE BY -Mining induced stresses -Gravitational forces - Tectonic forces Fig. 1. Conditions for the occurrence of rockbursts In the light of the above discussion, the amount of energy released by rock specimens during their failure in a soft testing machine was determined by the extent of the vibrations generated in the machine. The vibrations were measured by a seismograph. The information of particular interest in this study was the vector sum of the peak particle velocities of the vibrations in the vertical, longitudinal and transverse directions. The vector sum is a measure of the energy released at the time of specimen fracture and will be referred to as "Burst Energy Release Index" (BERI) in this paper. The experimental set up is shown in Fig. 2. Burst Proneness Index (q): This was determined from the elastic hysterisis loop parameters in the uniaxial compression loading and unloading tests (Fig. 3). The ratio of the strain energy retained, ER, to the

3 EXTENSOMETEP. Burst Energy Release Index 151 l... L L I I" X-Y PLOTTER SEISMOGRAPH '~ P NON-STIFF TESTING MACHINE SAMPLE GEOPHONE Fig. 2. Experimental set up for determining the "Burst Energy Release Index" permanent strain energy, ED, provided the numerical value for the Burst Proneness Index (Peng, 1978; Kidbinski, 1981; Singh, 1986). 2, 161 no O9 "~ LLI n," I-- u') !iiii!iiiiii!ii!iiiiiiiiiii 9 ~i~i~i~!~it.~iiiiii!i~iiiiiiiii!i~iil..ii!ili!ii!i!iiiiiiiiiii:!:iiii?ii:l I 5 I.I.;i!i!i?iiil :!i!~i~i~ii!i!.:!i!iiiiii!iiiii?!il ::i?ii!i~iii~i!!i!iii?l STRAIN! -~ Fig. 3. Typical stress-strain curve for loading and unloading during uniaxial compression test I The values for compressive strength and the strain energy stored in the specimen up to failure were obtained by uniaxial compression test.

4 152 S.P. Singh: Results and Discussion The results for Onaping Norite, Creighton Granite and Schistose are presented in Table 1. It is of interest to examine the independence of the Burst Energy Release Index (BERI) and the Burst Proneness Index, the compressive strength and the strain energy stored in the specimen. Table 1. Burst Energy Release Index and Other Properties for Onaping Norite, Creighton Granite and Schistose Rock type Burst Energy Release Burst Proneness Compressive Energy store in the Index (BERI) Index (t) strength in MPa specimen in MJ Onaping Norite Creighton Granite Schistose O The bursts occur in mines where the rock has been stressed beyond its peak strength. The dependence of BERI upon the compressive strength of rocks is displayed in Fig. 4. Higher strength rocks contribute more to the problem of bursting, because they will accumulate large amounts of elastic strain energy prior to failure. The increase of BERI with the increase in the Burst Proneness Index is shown in Fig. 5. BERI is a measure of the energy released at the time of specimen fracture whereas Burst Proneness Index is an indicator of the rock's ability to store recoverable and irrecoverable strain energy. The linear relation between the two parameters indicates that they can be similarly related to rockbursting. The correlation between the BERI and the strain energy stored in the specimen is displayed in Fig. 6. The higher the maximum strain energy that can be stored in a given rock the more likely it will be subject to bursting. The amount of energy that a particular rock can release during fracture is a reliable indicator of its burst proneness because the source of

5 Burst Energy Release Index ]53 X LLI r"', (9 hi I- ra r~ m IA- ONAPING NORITE ~ CREIGHTON GRANITE D SCHISTOSE =~, ~ o,~ ~o ~ A 1.2 I I 5 I _5 3 COMPRESSIVE STRENGTH (MPo) t o Fig. 4. Burst Energy Release Index vs. compressive strength O 3.2_ :3. X 1.1..I o 2.8 uj 2,6 (3 ( hi nr )- ( CO rr" D 1.6 era NAPING NORiTE A CREIGHTON GRANITE O SCHISTOSE o O~o j 13 ~" " o I El I I " J 7 r,,, 1'6 1'8 ' 8 I BURST PRONENESS INDEX ('r) Fig. 5. Burst Energy Release Index vs. Burst Proneness Index seismic energy released by a burst is the strain energy stored in the rock prior to fracture. The analysis of the results provides enough confidence to use BERI as a relative measure to define the burst proneness of rocks.

6 154 S.P. Singh: X (r >- (.9 ILl I" 9 tw m ,2 o ONAPING NORITE A CREIGHTON GRANITE o SCHISTOSE o o 4 ~ o A O A o in I, I I I p I I I o.zoo.4.6 o.eoo l.ooo STORED ENERGY (MJ) Fig. 6. Burst Energy Release Index vs. strain energy stored in the specimen A However, it depends on the particular testing conditions. The comparison between BERI values for different rocks is only possible as long as the tests have been performed on the same machine or different machines with same stiffness and unloading characteristics. Conclusions Rockbursts occur under certain combinations of geologic and mining conditions. It is possible to assess burst prone rocks utilizing BERI during the planning, design and operating stages of the mine. The simplicity of the approach may encourage its use as a quantitative predictor in investigations on rockbursts. Acknowledgements The author wishes to thank NSERC for the funding through grant number A4945 and the mining companies in the Sudbury area for their assistance during this study. References C o o k, N. G.. (1983): Origin of rockbursts. Proc. Rockburst Prediction and Control, IMM, London, 1--9.

7 Burst Energy Release Index 155 C o o k, N. G.. (1966): The basic mechanics of rockbursts. J. South African Inst. Mining and Metallurgy, vol. 66, Hoek, E., Brown, E. T. (198): Underground Excavations in Rock. IMM, London, Jaeger, J. C., Cook, N. G.. (1969): Fundamentals of Rock Mechanics. London: Chapman and Hall. Kidbinski, A. (1981): Bursting liability indices of coal. Int. J. Rock Mech. Min. Sci. 18, Ortlepp,. D. (1983): The mechanics and control of rockbursts. Rock Mechanics in Mining Practice, Ch. 12. S. Budavari (ed.), SAIMM' Johannesburg, Peng, S. S. (1978): Coal Mine Ground Control. New York: John iley. Petukhov, I. M. (1957): Rockbursts in Kizel Coalfield Mines (in Russian), Perm. Publ., Perm, 143. S a 1 a m o n, M. D. G. (1983): Rockburst hazard and the fight for its alleviation in South African gold mines. Proc. Rockbursts Prediction and Control, IMM, London, Singh, S. P. (1986): Assessment of the rockburst proneness in hard rock mines. Proc. 5th Conf. on Ground Control in Mining, est Virginia University, Morgantown,