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Computer simulation of open pit bench blasting in jointed rock mass
e
Pergamon
PH:
Int. J. Rock Mech. Min. Sci. Vol. 35, No. 4/5, p. 476, Paper No. 121, 1998 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0148-9062(98)00137-5 ISBN: 0080433332 ISSN: 0148-9062/98 $19.00 + 0.00
Computer Simulation of Open Pit Bench Blasting in Jointed Rock Mass z. JIAt G.CHENt s. HUANGt
Paper No. 12It Full paper on enclosed CD-ROM Geological discontinuities, such as bedding planes and joint planes, have significant effects on rock blasting. The presence of such discontinuities complicates the designs of rock blasting in open pit mines and makes backbreak control difficult. Many studies have contributed to the improvement of rock blasting in these types of rock formations. A number of empirical methods have been proposed and applied in backbreak control in jointed rock masses. However, an in-depth understanding of the mechanism is still lacking. This study employs finite element method to investigate the mechanism of rock breakage in jointed rock masses, attempting to provide an insight into the behavior of jointed rock mass under the dynamic loading of explosive. A nonlinear, explicit, dynamic, three-dimensional finite element code, DYNA3D, is employed for the modeling of bench blasting. The computer code has a large number of material models including a high explosive material model, which facilitate the studies of rock mass behavior under the load of explosive detonation. The flexible contact interface incorporated in the program makes it possible to describe rock blocks and their movements in jointed rock masses. The dynamic behaviors of rock blocks can, therefore, be readily simulated. The computer simulation covers the entire bench blasting process including rock deformation, failure, movement, fragmentation pattern, and throwing trend of rock fragments. A number of case studies are conducted, which include bench blasting in intact rock mass and in jointed rock mass with various joint patterns, as well as presplitting blasting with various delay intervals. For the jointed rock mass, the joint planes with different orientations separate rock mass into rock blocks. Based on the orientations of joint planes, the following cases simulating bench blasting in various jointed rock masses are studied: (1) horizontal joints (IX = 0°); (2) joints inclined at -35° from the horizontal plane, dipping away from front free surface (IX = - 35°); (3) joints inclined at 35° from the horizontal plane, dipping towards front free surface (IX = 35°); (4) joints parallel to front free surface (IX = 90°); (5) strikes of joint perpendicular to front surface, inclined at -17.4° from horizontal plane (f3 = - 17.4°). Presplit blasting is also studied as an approach to control backbreak in jointed rock masses. Based on various delay intervals, three cases are studied: (1) simultaneous initiation model (,1t = 0 ms); (2) short delay initiation model (,1t = 0.22 ms); and (3) 25-ms delay initiation model (,1t = 25 ms). In the above simulation studies, special evaluation indexes are developed in order to objectively describe rock mass failure, ground vibration and relative movements of rock blocks. The patterns of fragmentation and rock block movements in blast region are compared among the simulated cases. As a result of this study, the following conclusions can be drawn regarding the bench blasting with geological discontinuities: (1) the fragmentation pattern both in the burden and back area may be significantly affected by the rock mass discontinuity pattern; (2) a rock joint with a dip angle greater than the frictional angle of the joints and dipping towards free surface, may be prone to causing backbreak and inducing slope instability; (3) changing the orientation of the free surface to a certain favorable direction may reduce backbreak and improve the slope stability; (4) presplitting method is one of the effective approaches to control backbreak and maintain slope stability. Based on the simulation results, the pattern with simultaneous initiation presplit method has the best effect on the control of backbreak, and such conclusion agrees with blasting practices and experimental analysis. The current studies have provided important information regarding bench blasting in jointed rock masses, but further refinement of the simulation models will enhance the capability of the models and permit more in-depth studies. The following future studies are suggested: (1) more realistic rock masses can be simulated by introducing complex rock discontinuity systems; (2) a damage model of continuum fracture can be reasonably employed to simulate the dynamic fracture of brittle rock. Key words-DYNA3D, finite element method, computer simulation, bench blasting, jointed rock mass, rock fragmentation, backbreak control
tDepartment of Mining and Geological Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775-5800, USA. tConference Reference: USA-826-2 476
Pergamon
PH:
Int. J. Rock Mech. Min. Sci. Vol. 35, No. 4/5, p. 476, Paper No. 121, 1998 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0148-9062(98)00137-5 ISBN: 0080433332 ISSN: 0148-9062/98 $19.00 + 0.00
Computer Simulation of Open Pit Bench Blasting in Jointed Rock Mass z. JIAt G.CHENt s. HUANGt
Paper No. 12It Full paper on enclosed CD-ROM Geological discontinuities, such as bedding planes and joint planes, have significant effects on rock blasting. The presence of such discontinuities complicates the designs of rock blasting in open pit mines and makes backbreak control difficult. Many studies have contributed to the improvement of rock blasting in these types of rock formations. A number of empirical methods have been proposed and applied in backbreak control in jointed rock masses. However, an in-depth understanding of the mechanism is still lacking. This study employs finite element method to investigate the mechanism of rock breakage in jointed rock masses, attempting to provide an insight into the behavior of jointed rock mass under the dynamic loading of explosive. A nonlinear, explicit, dynamic, three-dimensional finite element code, DYNA3D, is employed for the modeling of bench blasting. The computer code has a large number of material models including a high explosive material model, which facilitate the studies of rock mass behavior under the load of explosive detonation. The flexible contact interface incorporated in the program makes it possible to describe rock blocks and their movements in jointed rock masses. The dynamic behaviors of rock blocks can, therefore, be readily simulated. The computer simulation covers the entire bench blasting process including rock deformation, failure, movement, fragmentation pattern, and throwing trend of rock fragments. A number of case studies are conducted, which include bench blasting in intact rock mass and in jointed rock mass with various joint patterns, as well as presplitting blasting with various delay intervals. For the jointed rock mass, the joint planes with different orientations separate rock mass into rock blocks. Based on the orientations of joint planes, the following cases simulating bench blasting in various jointed rock masses are studied: (1) horizontal joints (IX = 0°); (2) joints inclined at -35° from the horizontal plane, dipping away from front free surface (IX = - 35°); (3) joints inclined at 35° from the horizontal plane, dipping towards front free surface (IX = 35°); (4) joints parallel to front free surface (IX = 90°); (5) strikes of joint perpendicular to front surface, inclined at -17.4° from horizontal plane (f3 = - 17.4°). Presplit blasting is also studied as an approach to control backbreak in jointed rock masses. Based on various delay intervals, three cases are studied: (1) simultaneous initiation model (,1t = 0 ms); (2) short delay initiation model (,1t = 0.22 ms); and (3) 25-ms delay initiation model (,1t = 25 ms). In the above simulation studies, special evaluation indexes are developed in order to objectively describe rock mass failure, ground vibration and relative movements of rock blocks. The patterns of fragmentation and rock block movements in blast region are compared among the simulated cases. As a result of this study, the following conclusions can be drawn regarding the bench blasting with geological discontinuities: (1) the fragmentation pattern both in the burden and back area may be significantly affected by the rock mass discontinuity pattern; (2) a rock joint with a dip angle greater than the frictional angle of the joints and dipping towards free surface, may be prone to causing backbreak and inducing slope instability; (3) changing the orientation of the free surface to a certain favorable direction may reduce backbreak and improve the slope stability; (4) presplitting method is one of the effective approaches to control backbreak and maintain slope stability. Based on the simulation results, the pattern with simultaneous initiation presplit method has the best effect on the control of backbreak, and such conclusion agrees with blasting practices and experimental analysis. The current studies have provided important information regarding bench blasting in jointed rock masses, but further refinement of the simulation models will enhance the capability of the models and permit more in-depth studies. The following future studies are suggested: (1) more realistic rock masses can be simulated by introducing complex rock discontinuity systems; (2) a damage model of continuum fracture can be reasonably employed to simulate the dynamic fracture of brittle rock. Key words-DYNA3D, finite element method, computer simulation, bench blasting, jointed rock mass, rock fragmentation, backbreak control
tDepartment of Mining and Geological Engineering, University of Alaska Fairbanks, Fairbanks, AK 99775-5800, USA. tConference Reference: USA-826-2 476