ICGCM Papers:
Numerical Modeling Developments and Applications
 
 
Calibrating a Caving Model for Sedimentary Deposits—estimation of Load Distribution Between Gob and Abutment
35th International Conference on Ground Control in Mining
Calibrating a Caving Model for Sedimentary Deposits—estimation of Load Distribution Between Gob and Abutment
by
Mark K Larson, NIOSH, Office of Mine Safety and Health Research, Spokane, United StatesThierry Lavoie, Itasca Consulting Group, Inc. 111 Third Avenue South, Suite 450, Minneapolis, United States
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[Conference] 35th International Conference on Ground Control in Mining
[Price] Free  [Comments] 0
[Topical Area] Numerical Modeling Developments and Applications
[Author] Mark K Larson, NIOSH, Office of Mine Safety and Health Research, Spokane, United StatesThierry Lavoie, Itasca Consulting Group, Inc. 111 Third Avenue South, Suite 450, Minneapolis, United States
[Abstract] 
Key Conclusions:
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• The sedimentary caving model appears to simulate caving so that the model can calibrate to a wide range of possible surface subsidence profiles. • A sufficient number of interfaces and weak to moderate interface properties are required to simulate the settling process, which competes with the bulking process of the caved zone until equilibrium is achieved. • The model that most closely matched the expected surface subsidence profile was used to calculate the gob loading, and the angle β was calculated to be 16.1°. Such information is useful for calibrating numerical models that might be used to evaluate mine layout design and help reduce risk of injury and death to miners.
Key Findings:
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Key findings are as follows: • Two competing mechanisms are involved in the caving process. The bulking process proceeds as material caves. Above the cave zone, strata settles. The rate of bulking versus the rate of settling determines the final profile. To facilitate settling, interfaces that represent joints must be included in the model at regular intervals and weak-to-moderate strength properties must be given to the interfaces to allow some slip and settlement. • Bulking is controlled by the ubiquitous joint properties of each overburden member. For example, in the following figure, the interface properties were set at 10 psi for cohesion and 15° for friction. The ubiquitous joint properties are represented in the figure as fractions of the corresponding intact rock property. The results show that adjustment of the ubiquitous joint properties can produce a range of maximum subsidence for the case of this 9-ft-thick coal seam. Ubiquitous joint tensile strength may play the most significant role in determining the extent of bulking. The extent of the surface subsidence trough is slightly affected by the amount of settling achieved, which was controlled by the bulking (i.e., the ubiquitous joint properties). • In this case, the surface topography varied such that the overburden height was actually about 750 ft near the middle of the headgate, and similar or less over the entire panel. Overburden depth in the model was 1100 ft, which corresponds to depth in the headgate of a third panel for simplicity. Actually, the greater overburden height in the model would decrease the amount of subsidence. Target maximum subsidence was 7.34 ft, but maximum subsidence over the third panel suggests that a target maximum subsidence of about 6.2 ft would be more appropriate for the model. The orange curve is slightly short of that amount. Figure 1. Surface subsidence of sedimentary caving model with various ubiquitous joint parameters. • The effect of variation of interface properties affects extent of the trough, to some degree, but requires adjustment of ubiquitous joint properties to attain the same maximum subsidence. • A very weak ubiquitous joint can cause unrealistic deformation that results in surface heave at caving break points at the surface. The model may not simulate reality well in cases where bulking is the only mechanism. • In this study, all ubiquitous joint properties were kept constant. Where more is known about the characteristics of the overburden, it is possible to vary properties for the purpose, for example, of limiting bulking for a strong, massive unit. • In this case, with the model using ubiquitous joint cohesion and tension set at 0.45 times that of the corresponding intact properties, the angle β was calculated to be 16.1°.
Objective of the Paper:
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The purpose of this paper is to report on a parametric study that explores whether the sedimentary caving model gives realistic results; the model can be adjusted to calibrate to a case of known subsidence; and if the model can be adjusted to simulate a wide range of subsidence behavior.
Problem Statement:
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Heasley (2008) outlined three steps to calibrating a numerical model that might be used to evaluate mine layout design—that is calibrating: (1) rock mass stiffness, (2) gob stiffness, and (3) coal strength. While progress has been made in the first and third steps, the gob stiffness, which controls the distribution of overpanel weight between the gob and the abutment, is usually just calibrated to the empirical assumption (Mark, 1987), i.e., the volume of overpanel weight that goes to the abutment can be described by the angle β, which is assumed to be 21°. The only attempt to measure gob loading of a specific case was performed by Maleki, Hustrulid and Johnson (1984). They obtained several point measurements of stress in the gob with the progression of mining. The ability to reasonably estimate gob loading on a case-by-case basis has been lacking until now. Board and Pierce (2009) developed a constitutive model to simulate block caving of porphyry deposits. The National Institute for Occupational Safety and Health (NIOSH) had Itasca Consulting Group develop a similar model that is appropriate for sedimentary deposits. Accordingly, Pierce and Board (2010) developed a similar model for sedimentary deposits, which they called the Longwall Modeling Environment, but herein is referred to as the sedimentary caving model. The model relies on bulking of material in the caving process, through use of ubiquitous joints. It was thought that gob loading estimation could be achieved by comparing load on the floor below the seam before and after mining, if the model were calibrated to match the subsidence profile. Such estimation would improve numerical models that are used to evaluate mine layout design.