THE MYSTERY OF UNSATURATED SOIL MECHANICS – SOME MINING APPLICATIONS

1. THE MYSTERY OF UNSATURATED SOIL MECHANICS – SOME MINING APPLICATIONS Australian Geomechanics Society, SA & NT Chapter Monday 15 July 2013, Adeliade Professor David J Williams Email: D.Williams@uq.edu.au

2. Overview Unsaturated soil mechanics continues to play poor relation to saturated soil mechanics An unsaturated soil at a given density performs better than same soil in a saturated state • Some mining applications: • Wetting-up and drain down of waste rock dumps • Drain down, desiccation and rewetting of mine tailings • Drain down, desiccation and re-wetting of product coal • Bearing capacity and deformation of mine wastes • Shear strength and compressibility of clay-rich spoil • Performance of geo-covers placed on mine wastes

3. INTRODUCTION

4. Key Unsaturated Soil Mechanics Parameters INCREASED DECREASED DECREASED • Shear strength: • Or capacity of a soil to support load • Simplistically, shear strength can be tested under unsaturated conditions (adding “cohesion”) • Compressibility: • Or deformation of a soil under an applied load • Simplistically, compressibility can be tested under unsaturated conditions • Permeability (hydraulic conductivity): • Or rate of drainage under an applied load • Assessed by SWCC and ksat testing

5. Mining and Mineral Processing Wastes • Coarse-grained wastes (surface dumps): • Overburden (typically up to 200 mm) or waste rock (up to 1 m), particularly from surface mining (limited from underground mining) • Coarse reject (typically 50 mm) from coal processing • Slag or scats (typically 15 mm) from smelting • Spent heap leach material (typically 15 mm) • Fine-grained wastes (tailings storage facilities): • Crushed and ground tailings (typically silt-size) • Erosion sediments (fines)

6. Waste Rock and Coarse Processing Waste Dumps Waste rock dump Coarse reject dump Pumped co-disposal of coal washery wastes

7. Tailings Storage Facilities Metalliferous tailings Iron ore tailings Coal tailings Red mud Sand mining tailings In-pit coal tailings

8. WETTING UP AND DRAIN DOWN OF WASTE ROCK DUMPS

9. Impact of Waste Rock Wetting-Up Average annual rainfall ~600 mm Potential impact of waste rock dump wetting-up

10. Start-up 25 min ~30 mm 50 min ~60 mm 80 min ~100 mm Wetting-Up and Drain Down of Coarse-Grained Wastes

11. Moisture Profile after 100 mm 11 11

12. Surface Infiltration into Cadia’s Trial Waste Rock Dump

13. Changing Infiltration into Cadia’s Trial Waste Rock Dump with Time May 2006 November 2007 Note differential settlement- induced ponding 13

14. Average Base Seepage Beneath Top and Side Slopes of Cadia’s TWRD

15. Trigger Rainfall and Delay for Base Seepage Following Rainfall

16. Estimated Wetting-Up and Continuum Breakthrough Continuum breakthrough at: ~25% saturated for fresh WR ~60% saturated for weathered WR

17. Estimated Time to “Continuum Breakthrough” of Waste Rock Dumps Increasing rainfall Increasing dump height Increasing time Typical mine life ~20 years, during which time WRD is likely to be uncovered

18. DRAIN DOWN, DESICCATION AND REWETTING OF MINE TAILINGS

19. Impact of Tailings Slurry Deposition Average annual rainfall ~600 mm Assuming 0.5 m/year rate of rise at 25% solids or 1 m/year at 50% solids

20. Mt Keith Tailings Laboratory Column Test

21. Comparison of Field and Laboratory SWCC Data for Mt Keith Tailings

22. Comparison Between Field and Laboratory Hydraulic Conductivities

23. Matric suction sensor Piezometer TDR moisture sensor Sensors on Towers inTrial TSF Cell 23

24. Deposition in Trial TSF Cell 24

25. Matric Suction vs. Time 25

26. Matric Suction vs. Depth 26

27. DRAIN-DOWN, DESICCATION AND RE-WETTING OF PRODUCT COAL

28. Dewatering and Atmospheric Effects on Product Coal Coarse Fine Ultra-Fine 11.5 m 38o Field stockpile

29. SWCCs of Product Coal Size Fractions 29

30. Duration of Applied Pressure – Centrifuging of Fine Product Coal

31. Duration of Applied Pressure – Vacuum Filtration of Ultra-Fine Coal 31

32. Vacuum filtration vs. Briquetting of Ultra-Fine Product Coal 32

33. Sampling Field Coal Stockpile for PSD and Moisture Content 33

34. Wetting-Up of Model Coal Stockpile 34

35. Stockpile Total Moisture Content Distribution 35

36. BEARING CAPACITY AND DEFORMATION OF MINE WASTES

37. Tailings Bearing Capacity Issues Too thin a cover over soft tailings 4-5 m “Bow-waving” of crusted tailings “Bow-wave” 1.5 m

38. Covering Wet Tailings –Victorian Gold Mine Spreading thin cover D6 dozer 2-3 m surcharge 1 m cover, avoiding “bow wave” failure Ponded water due to drainage of excess pore water pressure Surcharging edge of tailings

39. Shear Strength Profiles of Coal Tailings

40. Safe Height of Fill on Coal Tailings H = Nc.sv/F. ~5.14.sv/(3 x 18) ~0.095 sv

41. SHEAR STRENGTH AND COMPRESSIBILITY OF CLAY-RICH SPOIL

42. Jeebropilly Spoil – Relative Unscalped PSDs CLAY Weathered Rock Unweathered coarse-grained Rock Unweathered fine-grained Rock

43. Mt Arthur Spoil – Relative Unscalped PSDs

44. Gravimetric Moisture Content vs. Matric Suction Jeebropilly rocky spoil is less dense and wetter than Hunter Valley spoil, although similar degree of saturation, and hence matric suction

45. Jeebropilly Weathered Rock PSDs

46. Comparison of Direct Shear Friction Angles (-2.36 mm) Note substantial softening on wetting up of clay-rich Jeebropilly Spoil

47. UPC 150 mm, 10 MPa Consolidometer

48. 76 & 150 mm Consolidometer Testing of Jeebropilly Weathered Rock Note low loose density and substantial consolidation on loading and wetting of clay-rich Jeebropilly Spoil

49. 76 & 150 mm Consolidometer Testing of 3-Month Old Mt Arthur Sandstone Note high loose density and limited consolidation on loading and wetting of Mt Arthur Sandstone

50. Large Texture Changes on Weathering of Jeebropilly Weathered Rock (-19 mm) 0 days 8 days (32.6 mm rain) 15 days (+23.2 mm rain) 21 days (+6.8 mm rain)