The Global Experience with Coal Bursts Rob Thomas 26.11.14

1. The Global Experience with Coal BurstsRob Thomas26.11.14

2. Content • What is a coal burst? • Causative factors • Soft Controls • Hard Controls • Adopted guidelines • Conclusions

3. What is a Coal Burst?

4. What is a Coal Burst? • The sudden and catastrophic failure of a coal rib as a result of an excessive build-up in vertical stress around a mine opening • Sometimes associated with floor heave and less frequently, roof falls • Commonly regarded as “random events insofar as their occurrence is not calculable”

5. Two Types of Coal Burst • Static coal bursts - related to the gradual build-up in vertical stress around a mine opening • Dynamic coal bursts - related to a sudden increase stress due to either mining or geologically induced seismic events and unlike the above, these are often independent of the mining process and as such, can occur several 10’s or even 100’s of metres away from the working face

6. What is a Coal Burst? • Can occur during roadway development and in existing roadways subjected to an increase in stress from another roadway or longwall face • Can also occur during longwall retreat, both on the longwall face and in the neighbouring gate roads

7. The German Experience

8. The German Experience

9. The German Experience

10. The Polish Experience

11. The US Experience

12. Basic Mechanics • Related to the ability of coal to concentrate vertical stress around the edge of the opening where the coal is in a largely unconfined state

13. Basic Mechanics • Conversely, if the coal and / or surrounding strata softens, the load is redistributed over a larger area which lowers and pushes the stress peak further away from the opening into an area where the coal is in a more confined state

14. Some Stats • Between 1994 and 2013 in the US, 41% of the reported coal bursts occurred on the longwall face, 20% in the tailgate, 13% in the maingate and 12% during pillar extraction • In Germany, between 1981 and 2004 there were 37 reported coal bursts, 17 occurred in roadways and 20 on the longwall face • See table for Polish stats

15. Causative Factors • (i) >400m Depth of Cover

16. Causative Factors

17. Causative Factors (ii) Competent Strata in Immediate Roof and / or Floor

18. Causative Factors • Germany – 5m thick in first 15m of roof or 5m thick in first 10m of floor and 60 to 130 MPa • Poland and Czech – 15m thick and 90 to 130 MPa • US – 10m thick and >100 MPa • Russia – 8m thick and >70 MPa • India – 8m thick and 50 to 75 MPa • China – 10 to 40m thick and up to 130 MPa

19. Causative Factors • (iii) Interactions

20. Causative Factors • (iv) Critical Pillars

22. European Critical Pillars

23. Causative Factors • (v) Coal Type • Several studies suggest that given the appropriate levels of stress and confinement, most coals will burst • The stronger the coal the greater the ability of the rib to store stress • Of particular concern is the presence of dull coal, poorly defined cleat and stone bands in the seam • Germany works on the basis that all coals with >10% of volatile matter are conducive to bursting • Polish guidelines suggest that the risk of a coal burst increases if the UCS of the coal is >16 MPa

24. Causative Factors • (vi) Geological Structure • Concentration in stress between workings and zone of softened strata • The weakening influence of the structure on the coal and / or floor strata • Block-type loading

25. Causative Factors • (vii) Dynamic Loading • Caving of competent unitsin the upper roof

26. Causative Factors • Fault reactivation

27. Causative Factors • (viii) In-seam Drainage • As a by-product of gas drainage, water is drained from the coal seam • On average, a 1 to 5% increase in moisture content, reduces the UCS of coal by 30 to 40% and the elasticity of the coal by between 30 and 60%

28. Causative Factors

29. Soft Controls • Test Drilling

30. Test Drilling

31. Depth of Critical Zones • Holes to be drilled to a minimum length of 3M or 3M+A in the case of a longwall face or roadway development • M = total volume of material that can be extruded into the roadway • A = anticipated advance rate per shift or day

32. Test Drilling

33. Influence of Rock Stress on Drill Cuttings in a 46mm Test Hole

34. Soft Controls (ii) Drehrohsystem

35. Drehrohsystem

36. Soft Controls (iii) Seismic Monitoring

37. Soft Controls (iv) Computer Modelling

38. Soft Controls (v) Rock Testing • Polish testing • The German Solid Rock Diameter (SRD) core testing and logging procedure incorporates RQD, bed spacing, rebound value, sound velocity, the Dynamic Young’s Modulus and point-load data

39. Hard Controls • Sequence of Extraction

40. (ii) Pillar Design

41. Hard Controls (iii) Stress Relief Drilling

42. Stress Relief Drilling

43. Hard Controls

44. Hard Controls (iv) Blasting and Hydraulic Fracturing

45. Hard Controls (v) Long-term Water Infusion • Typically infusion starts several months to more than 1 year prior to mining • Infusion in each hole can last for several weeks or months and requires the use of low pressures and flow rates of around 5 litres per minute • The typical volume of absorbable water is 10 to 50 litres per cubic metre of coal and in some cases only a 1 to 2% increase in moisture content is sufficient to eliminate coal bursts

46. Water Infusion in the US

47. Hard Controls (vi) Support?

48. Control System Adopted in German Mines

49. German Guidelines • Q1 - is there a history of coal bursts? • Q2 – is there a >5m thick sandstone in first 10m of roof • If yes, are the following relevant? - Start-up of longwall - Neighbouring workings located within 20 to 40m - Overlying workings - Nearby structure - Changing seam thickness - Sandstone located within 5m of floor • If the answer to Q1 or Q2 is yes, the area is classed as coal burst prone

50. Polish Guidelines