Influence of Roof/Floor Interface on Coal Pillar Performance

1. Influence of Roof/Floor Interface on Coal Pillar Performance Dr. Kyle A. Perry, P.E., Assistant Professor Dr. Kot F. Unrug, Emeritus Professor Kevin W. Harris, GRA Michael J. Raffaldi, GRA An Equal Opportunity University

2. Overview Background Purpose Modeling Parameters Important Results Pillar Strength Pillar Behavior Interfaces ARMPS SF vs. CMRR Design Recommendations An Equal Opportunity University

3. Background Empirical ARMPS AMSS ALPS Analytical Wilson Equation Numerical Finite Difference (FLAC3D) Boundary Element (LaModel) An Equal Opportunity University

4. Background Load Estimation Not Investigated Pillar Strength Focus Area In-situ Coal Strength Pillar Geometry Mark-Bieniawski (Mark, 1999) An Equal Opportunity University

5. Background Contributing Factors Coal Strength Pillar Geometry Roof/Floor Properties Strength Composition Integrity Pillar Contacts Behavior An Equal Opportunity University

6. Purpose Investigate Coal Pillar Strength Resultant Behavior Variable Interface Properties Numerical Study - FLAC3D W/H Ratios: 4, 7, and 10 Lithology: Massive Sandstone, Shale, Fireclay Fireclay Interface Constant An Equal Opportunity University

7. Model Geometry Incorporate Symmetry ¼ Pillar Section Increased Zone Density Better Accuracy Entry Width – 20’ Extraction Height – 6’ Model Boundaries Bottom – Fixed BC Sides – Roller BC An Equal Opportunity University

8. Modeling Parameters Initial Conditions 2:1 Vertical:Horizontal Stress 50% Mark-Bieniawski Strength Next Phase Applied Velocity -5 x 10-5 ft/step FISH Function Average Pillar Stress Average Pillar Strain An Equal Opportunity University

9. Table 1: Strata Material Properties (Lu et al, 2008, and Budhu, 2011) Material Properties An Equal Opportunity University

10. Rock Linear Elastic Mohr-Coulomb Model Coal Non-linear Elastic Strain-Softening Model Plastic Strain Dependency Table 2: Coal Seam Cohesion vs. Plastic Strain (Lu et al, 2008) Constitutive Models Table 3: Coal Seam Friction Angle vs. Plastic Strain (Itasca, 2009) An Equal Opportunity University

11. Interface Properties Non-linear Coulomb (Iannacchione, 1990) Reduce Strength Initiate Slip Bi-linear Function (Peng et al., 1983) = 20 = 10 = 500 PSI Cohesion 75 PSI 150 PSI 300 PSI An Equal Opportunity University

12. Interface Properties An Equal Opportunity University

13. Pillar Behavior and Strength Focus Areas Pillar Stress-Strain Pillar Strength Plastic Behavior Strain-Softening Perfectly Plastic Strain-Hardening Roof/Floor Strength Influence Interface Influence An Equal Opportunity University

14. W/H 4 Coal Pillar Sandstone Shale Fireclay Pillar Stress-Strain 75 PSI Mark-Bieniawski Pillar Strength 150 PSI Interface Cohesion 300 PSI An Equal Opportunity University

15. W/H 7 Coal Pillar Sandstone Shale Fireclay Pillar Stress-Strain 75 PSI Mark-Bieniawski Pillar Strength 150 PSI Interface Cohesion 300 PSI An Equal Opportunity University

16. W/H 10 Coal Pillar Sandstone Shale Fireclay Pillar Stress-Strain 75 PSI Mark-Bieniawski Pillar Strength 150 PSI Interface Cohesion 300 PSI An Equal Opportunity University

17. W/H 7 Pillar 75 PSI Interface Pillar Strength Results An Equal Opportunity University

18. Summary of Results W/H 4 Pillar Strain-Softening Mark-Bieniawski W/H 7 Pillar Perfectly Plastic (Transition) Mark-Bieniawski W/H 10 Pillar Strain-Hardening Functional Strain An Equal Opportunity University

19. Important Findings Roof/Floor Composition Mechanical Properties - Secondary Roof/Floor Pillar Contact Interface Properties - Dominant Explanation Strong Interface - High Confinement Weak Interface - Low Confinement An Equal Opportunity University

20. Fireclay Floor Results Strength Reduction Average 22% Higher W/H, More Effect W/H 10 Strength Reduced 26% Su & Hasenfus, 1997 Weak Roof/Floor Peak 30% Reduction Reason Poor Interface Strength An Equal Opportunity University

21. ARMPS SF vs. CMRR Database 645 Total Cases Limited Statistics (Mark, 2010) CMRR Classification 330 Definitive CMRR Ratings Weighted – High SF Diversity Logistic Regression An Equal Opportunity University

22. ARMPS SF vs. CMRR Results Minimal Correlation Mark & Barton, 1997 Supports Model Results CMRR Roof Quality – Excellent Support Requirements Pillar Strength – Poor Correlation – Interface Quality An Equal Opportunity University

23. Design Recommendations Purpose Site-Specific Parameters Importance Advantageous Increase Accuracy Pillar Strength Pillar Behavior Result In-situ Coal Strength Modification 900 PSI Constant An Equal Opportunity University

24. Design Recommendations Quantify Roof/Floor Material Properties Triaxial Testing Interface Properties Direct Shear Testing Case Studies Reasonable Thresholds Boundary Conditions Stress Field Horizontal Stress An Equal Opportunity University

25. Design Recommendations Solution Calculate Average Pillar Strength Back-Calculate In-Situ Coal Strength Representative of Conditions Correlation Mark-Bieniawski Equation Software ARMPS, ALPS, AMSS LaModel An Equal Opportunity University

26. Conclusions Roof/Floor Interface Significant Higher W/H Pillar Strength Pillar Mechanical Behavior Confinement Partings Roof/Floor Strength Secondary Supported by ARMPS SF vs. CMRR An Equal Opportunity University

27. Conclusions Design Recommendations In-situ Coal Strength Modification Implementation ARMPS, AMSS, ALPS, LaModel Limitations Single Pillar vs. Panel Span Mechanical Properties Roof/Floor Deformation Stress Distribution Interface Classification System An Equal Opportunity University

28. References Budhu, M. (2011). Soil Mechanics and Foundations. 3rd Ed. Hoboken, NJ: University of Arizona, p. 724. Heasley, K. (2007).“LaModel Calibration.” In: WVU College of Engineering and Mineral Resources Faculty, Staff, and Course Web Server. [http://web.cemr.wvu.edu/~kheasley/LaModelDownloads/Documents/Details/Calibration.pdf]. Iannacchione, A.T. (1990). “The effects of roof and floor interface slip on coal pillar behavior.” In: Proceedings of 31st U.S. Symposium on Rock Mechanics. Golden, CO: Colorado School of Mines, pp. 153–160. Itasca Consulting Group, Inc. (2009). FLAC3D Version 4.0 Example Applications. Minneapolis, MN, pp. 2-1–2-15. Lu, J., Ray, A., Morsy, K., Peng, S. (2008). “Effects of rock/coal interface property on coal pillar strength.” In: Proceedings of the 27th International Conference on Ground Control in Mining, pp. 262–267. Mark, C. (1999). “Empirical methods for coal pillar design.” In: Proceedings of the Second International Workshop on Coal Pillar Mechanics and Design, NIOSH IC 9448, pp. 145–154. Mark, C. (2010). “Pillar design for deep cover retreat mining: ARMPS Version 6 (2010).” In: 3rd International Workshop on Coal Pillar Mechanics and Design. ICGCM Pillar Design Workshop. Morgantown, WV, pp. 106–121. Mark, C., Barton, T.M. (1997). “Pillar design and coal strength.” In: Proceedings of the New Technology for Ground Control in Retreat Mining, NIOSH IC 9446, pp. 49–59. Peng, S.S. (2008). Coal Mine Ground Control. 3rd Ed. Morgantown, WV: West Virginia University, p. 78. Peng, S.S., Patrick, C.W., Khair, A.W. (1983). “Direct shear strength of Appalachian coals.” Geotechnical Testing Journal. 6(3): 144–150. Su, D.W.H., Hasenfus, G.J. (1996). “Coal pillar strength and practical coal pillar design considerations.” In: 15th Conference on Ground Control in Mining. Golden, CO: Colorado School of Mines, pp. 155–162. Su, D.W.H., Hasenfus, G.J. (1997). “Effects of in-seam and near-seam conditions and asymmetric pillar loading on coal pillar strength.” In: 16th Conference on Ground Control in Mining. Morgantown, WV: West Virginia University, pp. 329­–344. An Equal Opportunity University

29. Thanks! Questions??? An Equal Opportunity University