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Title: Blasting-induced damage in coal

Abstract

The paper is drawn from a project intended to explore a technique of prediction, control and optimization of fracture in coal induced by blasting. It evaluates the fines generated in coal submitted to dynamic loading stresses in an impact stamp mortar. The aim is to analyze a complex phenomenon of coal response to blast-generated stresses from a series of discrete simulations of shock and gas actions in controllable processes. It is learned that despite the nucleation of primary crushing and fractures to originate from the point of impact energy in coal, a secondary crushing appears to depart from within the burden progressing towards the free boundaries. The extension of the secondary crushing zone appears to be influenced by the magnitude of the breaking stresses generated and the coal burden distance. A strong dependence of fines on the coal`s innate discontinuities (strength) and the energy input is highlighted.

Authors:
 [1]
  1. Univ. of the Witwatersrand (South Africa)
Publication Date:
OSTI Identifier:
400808
Report Number(s):
CONF-950247-
TRN: IM9650%%214
Resource Type:
Conference
Resource Relation:
Conference: 21. annual conference on explosives and blasting techniques, Nashville, TN (United States), 5-9 Feb 1995; Other Information: PBD: 1995; Related Information: Is Part Of Proceedings of the twenty-first annual conference on explosives and blasting technique. Volume 1; PB: 371 p.
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; COAL SEAMS; EXPLOSIVE FRACTURING; FORMATION DAMAGE; COAL MINING; COAL FINES; SHOCK WAVES; STRESS ANALYSIS; PARTICLE SIZE; DISTRIBUTION

Citation Formats

Kabongo, K.K. Blasting-induced damage in coal. United States: N. p., 1995. Web.
Kabongo, K.K. Blasting-induced damage in coal. United States.
Kabongo, K.K. Sun . "Blasting-induced damage in coal". United States. doi:.
@article{osti_400808,
title = {Blasting-induced damage in coal},
author = {Kabongo, K.K.},
abstractNote = {The paper is drawn from a project intended to explore a technique of prediction, control and optimization of fracture in coal induced by blasting. It evaluates the fines generated in coal submitted to dynamic loading stresses in an impact stamp mortar. The aim is to analyze a complex phenomenon of coal response to blast-generated stresses from a series of discrete simulations of shock and gas actions in controllable processes. It is learned that despite the nucleation of primary crushing and fractures to originate from the point of impact energy in coal, a secondary crushing appears to depart from within the burden progressing towards the free boundaries. The extension of the secondary crushing zone appears to be influenced by the magnitude of the breaking stresses generated and the coal burden distance. A strong dependence of fines on the coal`s innate discontinuities (strength) and the energy input is highlighted.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Sun Dec 31 00:00:00 EST 1995},
month = {Sun Dec 31 00:00:00 EST 1995}
}

Conference:
Other availability
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  • A discrete element computer program named DMC_BLAST (Distinct Motion Code) has been under development since 1987 for modeling rock blasting (Preece & Taylor, 1989). This program employs explicit time integration and uses spherical or cylindrical elements that are represented as circles in two dimensions. DMC_BLAST calculations compare favorably with data from actual bench blasts (Preece et al, 1993). Coal seam chilling refers to the shattering of a significant portion of the coal leaving unusable fines. It is also refereed to as coal damage. Chilling is caused during a blast by a combination of explosive shock energy and movement of themore » adjacent rock. Chilling can be minimized by leaving a buffer zone between the bottom of the blastholes and the coal seam or by changing the blast design to decrease the powder factor or by a combination of both. Blast design in coal mine cast blasting is usually a compromise between coal damage and rock fragmentation and movement (heave). In this paper the damage to coal seams from rock movement is examined using the discrete element computer code DMC_BLAST. A rock material strength option has been incorporated into DMC_BLAST by placing bonds/links between the spherical particles used to model the rock. These bonds tie the particles together but can be broken when the tensile, compressive or shear stress in the bond exceeds the defined strength. This capability has been applied to predict coal seam damage, particularly at the toe of a cast blast where drag forces exerted by movement of the overlying rock can adversely effect the top of the coal at the bench face. A simulation of coal mine cast blasting has been performed with special attention being paid to the strength of the coal and its behavior at t he bench face during movement of the overlying material.« less
  • The Bureau of Mines has studied the problem of cracking in residential structure walls from vibrations produced by blasting in surface mines. Direct observations were made of blasting damage consisting primarily of cosmetic cracking.
  • Vibrations from thirty-seven blasts were monitored using three seismographs. Data analysis showed that square root, cube root, and site-specific scaling predicted ground motion with equal reliability. Imprecise prediction is caused by geologic and blast variability, particularly inaccurate delay times. Vibrations at some mines were consistently lower or higher than vibrations reported in the U.S. Bureau of Mines studies. Regulations based on USBM data could then be too conservative or lenient. Predominant vibration frequencies were generally less than 50 Hz, but several blasts had significant energy up to 125 Hz. In some cases energy was concentrated at frequencies corresponding to delaymore » intervals.« less
  • Control of the rock motion associated with blasting can have significant economic benefits. For example, surface coal mining can be made more efficient if the overburden material can be cast further with explosives, leaving less work for mechanical equipment. The final muck pile shape in very type of surface and underground blasting is controlled by the blasting induced motion of the rock. A theoretically sound method of predicting rock motion will be beneficial to understanding the blasting process. Discrete element methods have been used for some time to predict rock motion resulting from blasting. What all of these approaches hadmore » in common was the use of polygonal elements with corners and sides as well as aspect ratio. Reasonably good results were obtained but treatment of the interactions of the corners and sides of elements was a computationally intensive process that made long simulations with many elements expensive to perform. The use of spherical elements showed increased efficiency but lacked the mechanisms for treating the bulking of the rock mass. The computer program developed was converted from an explicit code to an event-driven code and some bulking mechanisms were added that allowed spherical elements to exert a torque on other spherical elements with which contact was made. The architecture of this program and its event-driven nature made it difficult to vectorize for efficient execution on vector processing machines. A new code called DMC (Distinct Motion Code) has been developed this past year. DMC was designed and written especially to take advantage of super computer vector processing capabilities. This paper will discuss the use of DMC to perform accurate rock motion calculations with very reasonable computation times. 9 refs., 7 figs., 3 tabs.« less
  • A method has been developed to simulate coupled gas flow and rock motion in a blasting situation. The method relies on gas flow calculations using a finite difference Flux-Corrected Transport (FCT) solver and sphere motion calculations using DMC (Distinct Motion Code). The coupling occurs when the porosity field used in the gas flow calculation is calculated using the current positions of the spheres and the flowing gas is used to calculate the loads on the spheres. The example calculation shows that the capability is in place and works reasonably well when compared with field experiments in terms of surface velocities,more » general rock motion and muck pile shape. Having this capability in place will allow explosive properties to be included in rock motion calculations. Rock motion simulations will be based on the actual physics that occurs in a blast instead of using force-time or velocity-time histories as has been done in the past. A capability that will be added in the near future will allow the viscosity terms to vary with pressure as actually occurs instead of remaining constant. Since very high pressures exist in the blastwell and in its' immediate vicinity this should have a significant effect on the gas flow. It will allow the calculations to be performed with a higher mean particle size which will reduce the heat transfer effects. 11 refs., 13 figs., 2 tabs.« less