Temperature prediction in underground mine airways

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Numerical Modeling Procedures for Practical Coal Mine Design

A method is presented for creating realistic numerical models for practical coal mine ground control. The method includes procedures to collect the necessary mechanical input parameters from a geologic core log, procedures to set up a model and procedures to interpret calculation results. The input parameters come from a detailed geologic core log and extensive point load tests of estimate rock layer strength. A suite of material property input parameters is proposed which allow the user to go from core log to numerical model inputs. Rock bolt anchorage properties are also linked to the material properties of each geologic layer in the model. Following this procedure leads to very realistic calculations of the rock failure process and rock support system behavior. These calculations in turn enable realistic comparison of the effectiveness of alternative rock support systems. © 2006, ARMA, American Rock Mechanics Association

Catastrophic Collapse of Highwall Web Pillars and Preventative Design Measures

In highwall mining, once a sufficient number of web pillars are developed, they can fail catastrophically in a domino-type failure, posing a safety hazard to highwall miners and sterilizing mineable reserves. Three recent case histories of catastrophic web pillar failure in U.S. and Australian mines are documented and analyzed with two design approaches. The conventional design approach uses only a strength criterion, while the postfailure design approach uses both strength and stability criteria to insure an acceptable failure mode should strength failure occur. Catastrophic highwall web pillar failures can cost money through equipment losses, production losses, coal reserve sterilization and lost opportunity costs. Coal recovery may be higher. but the benefits of higher recovery must balance the potential costs of failure. Application of the conventional or postfailure design approach will decrease coal recovery and revenues, but the risk of potentially costly failure is much less

Using a Postfailure Stability Criterion in Pillar Design

Pillar design is the first line of defense against rock falls--the greatest single safety hazard faced by underground coal miners in the United States and abroad. To help advance the state of the art in this fundamental mining science, the National Institute for Occupational Safety and Health organized the Second International Workshop on Coal Pillar Mechanics and Design. The workshop was held in Vail, CO, on June 6, 1999, in association with the 37th U.S. Rock Mechanics Symposium. The proceedings include 15 papers from leading ground control specialists in the United States, Canada, Australia, the United Kingdom, and the Republic of South Africa. The papers address the entire range of issues associated with coal pillars and have a decidedly practical flavor. Topics include numerical modeling, empirical design formulas based on case histories, field measurements, and postfailure mechanic

Failure Mechanics in Multiple Seam Mining Interactions

Multiple seam mining interactions caused by full extraction mining, whether due to undermining or overmining, frequently involve tensile failure of the affected mine roof. The adverse ground control conditions may prevent mining for both safety and economic reasons. Prior researchers have identified the geometric, geologic and mining factors controlling multiple seam mining interactions. This numerical study examines the mechanics of these interactions using a modeling procedure that 1) incorporates the essential constitutive behavior of the rock such as strain-softening of the intact rock and shear and tensile failure along bedding planes and 2) captures the geologic variability of the rock especially the layering of weak and strong rocks and weak bedding planes. Specifically, the numerical study considered the effect of vertical stress, interburden thickness, and the immediate roof quality of the affected seam in both undermining and overmining situations. The models show that for overburden-to-interburden thickness (OB/IB) ratios of less than 5, interactions do not occur, and that for OB/IB more than 50, extreme interaction is a certainty. In between, the possibility of an interaction was found to depend on gob width-to-interburden thickness ratio, site specific geology and horizontal stress to rock strength ratio in addition to the OB/IB ratio. The models also showed that horizontal stress was profoundly altered well above or below a full extraction area and that these changes are likely to influence the success or failure of multiple seam mining. The role of horizontal stress in multiple seam mining interactions has received little attention in prior investigations

MULSIM/NL Application and Practitioner\u27s Manual

MULSIM/NL (multiple seams, nonlinear) is a new U.S. Bureau of Mines boundary-element-method (BEM) program for calculating stresses and displacements (i.e., convergence) in coal mines or thin, tabular metalliferous veins. This manual gives detailed operating instructions for MULSIM/NL and illustrates its use with several practical examples. While this manual concentrates on the practical aspects of actually running and using MULSIM/NL, another companion report titled MULSIM/NL-Theoretical and Programmer\u27s Manual provides mathematical and programming details to those engineers and programmers who need to fully understand the FORTRAN program or desire to alter and enhance it. MULSIM/NL analyzes one to four parallel seams that have any orientation with respect to the Earth\u27s surface. Three main features distinguish MULSIM/NL from its predecessors: (1) nonlinear material modes, (2) multiple mining steps, and (3) comprehensive energy release and strain energy computations. MULSIM/NL has six material models for the in-seam material including (1) linear elastic for coal, (2) strain softening, (3) elastic plastic, (4) bilinear hardening, (5) strain hardening, and (6) linear elastic for gob. The multiple mining step capability enables the user to simulate a changing mine geometry. Finally, MULSIM/NL performs comprehensive energy release rate calculations

Pillar Design to Prevent Collapse of Room-And-Pillar Mines

In some instances, extensive room-and-pillar workings can collapse with little warning and pose a serious risk to underground miners. Traditional strength-based pillar design methods applicable to coal or hard-rock mines use a factor of safety defined as pillar strength divided by pillar stress. Factor of stability, defined as local mine stiffness divided by post-failure pillar stiffness, may offer a way to design room-and-pillar mines and eliminate collapses. Three alternative design approaches to decreasing the risk of large-scale catastrophic collapses are described: the containment approach, the prevention approach, and the full-extraction approach. Until good data on the post-failure behavior of pillars become available, the containment and full-extraction options are the safest. The limitations in our ability to evaluate both the stability of old workings and the long-term performance of room-and-pillar mines are described

The Mechanics of Fine Fragment Formation in Coal

The following four step mechanism is proposed: (1) development of a crush zone, (2) macrocrack propagation, (3) shear movement along macrocracks and (4) additional fragmentation from shear. The cutting tool interacts with coal material and its inherent flaw structure, producing fine coal fragments near the tool grading into coarser fragments further away. Two fine fragment sources exist: the crush zone and the rough fracture surfaces. Fine particle creation is controlled in part by fracture toughness and also the inherent flaw geometry. Basis relationships are developed between the flaw geometry and fine fragment generation. Experimentation centers on extensive mixed mode fracture toughness testing. The system developed herein provides a record 117 measurements concluding that K[subscript Ic] is a reliable material property despite coal\u27s heterogeneity. Strain energy density theory adequately describes the onset and direction of crack growth under mixed mode loading conditions. Tentative relations exist between K{sub Ic} and fracture surface roughness and fracture velocity. Fracture toughness values are applied in crush zone size calculations with a boundary element program containing a failure criterion based on strain energy density theory. From postulates about the role of inherent flaws in fragmentation, the crushing extent is the locus of active 10 micron flaws. The calculations demonstrate the effect of attack angle and tool geometry on crushing extent and fine fragment formation. Future research should measure the fracture toughness of coal constituents at a microscopic scale and quantitatively describe their inherent flaw structure

Simulation of Cascading Pillar Failure in Room-And-Pillar Mines Using Boundary-Element-Method

A Cascading Pillar Failure can occur in certain room-and-pillar mines when one pillar in a mine layout fails which transfers its load to neighboring pillars causing them to fail, and so forth. Whether failure occurs in a stable, nonviolent manner or in an unstable violent manner is governed by the local mine stiffness stability criterion. To apply this stability criterion, a boundary-element-method computer program with a strain-softening material model calculates the local mine stiffness and the post-failure pillar stiffness. The behavior of computer simulations changes depending on whether the model satisfies or violates this stability criterion. Example analyses illustrate how the computer program can simulate stable and unstable failures in mines