1. The Ground Response Curve, Pillar Loading and Pillar Failure G.S. (Essie) Esterhuizen Chris Mark Michael Murphy The findings and conclusions in this presentation are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health.

2. Background Review of retreat pillar mining Role of overburden response in loading and failure of pillars Gob

3. Overburden response Pillar loading Pillar failure

4. Overburden response

5. Stiff strong overburden Weak-soft overburden Overburden response can determine whether pillars fail or not Well developed pressure arch Poorly developed pressure arch

6. Weak Overburden behavior can be represented by a “ground response curve” Strong Load transferred to abutments by arching Load carried by internal supports Load transferred to abutments by arching

7. Ground response curves can be estimated using realistic numerical models Extraction panels Coal bed 3500 ft 2500 ft Strong beds

8. Ground response development conditions Ground response recorded at center of panel

9. 2,000 1,500 1,000 500 0 Stress (psi) Overburden stress Convergence (inches) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Width 80 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

10. 2,000 1,500 1,000 500 0 Stress (psi) Overburden stress Convergence (inches) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Width 80 ft Width 150 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

11. 2,000 1,500 1,000 500 0 Stress (psi) Overburden stress Convergence (inches) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Width 80 ft Width 150 ft Width 250 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

12. 2,000 1,500 1,000 500 0 Stress (psi) Overburden stress Convergence (inches) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Width 80 ft Width 150 ft Width 250 ft Width 500 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

13. 2,000 1,500 1,000 500 0 Stress (psi) Overburden stress Convergence (inches) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Width 1000 ft Width 80 ft Width 150 ft Width 250 ft Width 500 ft Effect of panel width on ground response Development conditions - Depth 1500 ft - Western US geology

14. Ground response at the pillar extraction line Ground response was recorded at center of extraction line Gob Extraction line

15. Ground response at the pillar extraction line Panel width 500 ft at 1500 ft depth – Western US geology 0.050.100.150.200.250.300.35 Stress (MPa) Convergence (m) At extraction line Development conditions 6,000 4,500 3,000 1,500 0 Stress (psi) 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Convergence (inches) Overburden stress

16. Pillar response

17. Pillar response curves Distributed stress over entire panel

18. Combined pillar and ground response curves Pillar W:H = 8 Overburden stress

19. Effect of panel width on pillar loading Pillar W:H = 8 Overburden stress

20. Effect of panel width on pillar loading Pillar W:H = 8

21. Effect of panel width on pillar loading Pillar W:H = 6 Overburden stress

22. Effect of panel width on pillar loading Yield pillar W:H = 3 Overburden stress 80 ft

23. Extraction line Development Pillar loading at the extraction line 500 ft wide panel at 1500 ft depth Western US geology

24. Extraction line Development

25. Pillar loading at the extraction line 500 ft wide panel at 1500 ft depth Western US geology Extraction line Development

26. Pillar loading at the extraction line 500 ft wide panel at 1500 ft depth Western US geology Extraction line Development

27. Pillar failure

28. Structural failure of pillars 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 0.0%2.0%4.0%6.0%8.0%10.0% Stress (MPa) Convergence W:H = 6 Structural strength = peak resistance Structural failure

29. Functional failure of pillars Functional failure Convergence

30. Conclusions Ground response and pressure arch formation are affected by: –Rock stiffness and strength –Panel width Ground response affects: –Pillar loading –Ultimate success/failure of pillars Ground response evaluation has demonstrated: –Importance of panel width on success of pillar mining –Structural failure of pillars does not necessarily imply functional failure –Functional failure appears to be related to ultimate pillar strain (%conv)