1. Challenging Geotechnical Doctrine with Datalogged Instrumentation

2. What is “Square of the wall”?

3. Smoky River Coal – 9G Mine Rocky Mountains, Alberta, Canada
200m wide wall
128m depth of cover
3m seam

4. Rear Pillar Stress Cells
200m wide wall
128m depth of cover
3m seam

5. 0m
95m
128m
200m
Rear Pillar Stress Cells
200m wide wall
128m depth of cover
3m seam
Geology?
Depth
Square
5m cell
10m cell
Wall Kickoff
15m cell
1.5m cell yielded immediately

6. Horizontal square (no change in stress cells)
Geology (stress peaked and dropped)
Depth of cover =wall width (vertical square, stress cells stabilised)

7. Theory
Start of wall loading peak is due to geology, wall width or is it depth dependent?
Rear abutment effect is depth or width dependent, whichever is smaller?

8. Longwall Abutment – How far does it redistribute? 15m? 50m? 9.3 x √h?

9. Phalen Colliery
Nova Scotia, Canada
Seam thickness 2.4m
Depth 550m

10. Principal Horizontal Stress Direction
Pillar Stress Cells
Phalen Colliery
Nova Scotia, Canada
Seam thickness 2.4m
Longwall 256m wide
Depth 550m

11. Not great for magnitude of stress
Difficult to install in coal
Excellent for change in stress
Extremely sensitive to small changes

12. This graph only shows immediate response to production startup
170m away however stress cells showed 500m away.

13. Longwall Abutment – How far does it redistribute. 9
Longwall Abutment – How far does it redistribute? 9.3 x √h = 120m Measured 500m!!!!! Kirsch equation = 2-3xD? (wall width 256m)

14. Kirsch Equations (2 x D)? Phalen Colliery Nova Scotia, Canada
Seam thickness 2.4m
Longwall 256m wide
Depth 550m
Kirsch Equations (2 x D)?

15. Longwall Vertical Stress Abutment Peaks at Gate End?

16. Pillar Instrumentation Site

19. USBM Chart recorders (windup spring powered)

20. Demonstration of peak just behind wall and then pillar yield (3m in, post failure loading and then second yield.

21. 8.8m into pillar still increasing load 120m behind the wall

22. No yield at the face except at MG corner

23. Load still increasing when the shearer cut the pressure cells out of the face and stainless steel tubing was ripped off these chart recorders, drawn through the boreholes and wrapped around the shearer drum

24. Resulting face and pillar stress contour allows real
observation of stress redistribution subtleties
MG Corner yield
Pillar offload
(upper roof cave?)

25. Longwall Stress Abutment Exponential
Longwall Stress Abutment Exponential? Rate of roof acceleration is dependent on face position?

26. Vertical stress increase in pillar as face approaches
Readings on 12hr intervals show linear stress buildup in pillar

27. Datalogged results show something very different
Massive rate (shearer cutting through)
Low or negative rate (stress redistributing across pillar)

28. Longwall Stress Abutment Rate of change in vertical stress (pillar load) is dependent on shearer cutting through at gate end Rate of change in horizontal stress (roof movement) is dependent on pulling gate end shields (goaf cave and redistribution of stress around new void) TARPs that are based on rate are extremely dependent on / sensitive to; reading time relative to shear (shield advance) cycle

29. Are there other things going on that we are not aware of?

30. Pillar stress cells (lower seam) being read by a datalogger during a time when no mining was in the vicinity

31. If read daily this would be the resulting data

32. If read 3 hourly this would be the resulting data

33. Read on 30 minute intervals

34. 40m
550m
Pillar was 510m below the ocean with a 1m tide

35. Interburden Ocean Seam Scale of tide relative to seam and stress cells

36. The government predicted high and low tide peaks plotted against data and discovered not only correlation in time and stress magnitude change (on all stress cells in the pillar) but also demonstrated when tide varied from prediction due to onshore and offshore winds.
Even shiftly readings would never have discovered this

37. Result Accurately measured the effects of 1
Result Accurately measured the effects of 1.2m of tide on a pillar 550m depth of cover (However did not alter mining process at high tide!!)

38. Same pillar stress cells underwent a strange offload / reload while longwall
was m away, over an 8 day period
(*Note the tidal effect still occuring)

39. Upper Seam Dry Workings
Flooded Workings
Longwall had recently undermined from a flooded workings to a dry workings
Upper Seam
Lower Seam
Upper Seam Dry Workings
Stress cells (lower workings)

40. Water draining out of overlying goaf
Water filling overlying goaf adjacent to cells
Water draining out of overlying goaf

41. Upper Seam Dry Workings
Flooded Workings
Lower Seam
Upper Seam Dry Workings

42. Water filling a goaf can accept a redistribution of vertical stress - off loading adjacent areas. (Flooding workings reduces load on pillars?)

43. Can you control subsidence (Smax, Strain) by rate of retreat?

44. Instrumentation site in sealed/reentered gateroad in overlying workings
Longwall in lower seam (2.4m thick, 138m below) undermines instrumentation site

45. 370m
1.8m seam
Gateroad
140m
2.4m seam

46. Automated Piezometer Instrumentation Setup
Constant Head
Water Tank
Continuous Water Hose
Survey Monument Piezometers Stations
Piezometers in hose but pinned to the floor, as subsidence occurred, hose and survey monuments subsided changing head relative to constant head tank thereby measuring subsidence on 30 minute intervals
Programmed Datalogger
Lower Seam

47. Rate and magnitude of subsidence (and strain were different when the rate of wall retreat was different (faster – more)

48. Undermining at 6m/day
Undermining at 4m/day

49. Subsidence response 140m above is to every 1m shear (geometrically) and almost immediate (< 1 hour) Smax greater when undermined at 6m per day than 4m per day (what would 30m per day do?) Could we actually plan to mine under rivers or infrastructure slower to control subsidence/water inflow risk?

50. Blast Monitoring for effect on Roof Movement Are daily or shiftly readings sufficient?

51. Shiftly (8hr) readings on roof exto in grunching section

52. 8hr readings over top of 30 minute intervals

53. Example 1
Readings on 30 minute intervals

54. Example 2
What geometry
led to these 2
fundamentally
different ground
responses?
Who blasted and didn’t put it on the shift report?

55. What could have been learned from 10 second intervals?
Blast effect is immediate on nearby roof movement but was stepped movement in some cases and led to time dependent stabilisation in others.
What could have been learned from 10 second intervals?
What could we learn from 10 second intervals on continuous miner cutouts
Cut out distances?
Time to bolt effect?
Importance of pretension?
Monitoring drilling of roofbolt holes?

56. Conclusion
What are we missing in the understanding of ground movement and stress redistribution by not continuously monitoring?
What could we learn from 10 second intervals on continuous miner cutouts
Cut out distances?
Time to bolt effect?
Importance of pretension?
Monitoring drilling of roofbolt holes?
Convergence on longwalls?
Pull tests?
On set of floor heave?

57. The Challenge
Don’t accept the excuse that we don’t have approved instrumentation – push for equipment approval, support suppliers in development, make use of recent developments in comms to CMs and Longwalls to bring it directly to surface
Think of what other geotechnical understanding may benefit from continuous monitoring