Good, bad & ugly lessons learned in the design and construction of heap leach pads Heap Leach Conference September 2013 Vancouver, B.C.

Transcription

1 Good, bad & ugly lessons learned in the design and construction of heap leach pads Heap Leach Conference September 2013 Vancouver, B.C., Canada Presented by: Allan Breitenbach, PE Principal Geotechnical Engineer Ausenco Denver Office 1

2 Admiral Perry in 1907: find a way or make a way Source: National Geographic Magazine 2

3 1972 potash evaporation ponds, Utah 3

4 PVC and CSPE (Hypalon) liners in 1970 s 4

5 1976 uranium tailings dam in Wyoming 5

6 Leach pads: highest lined fills in world 6

7 1979 gold heap leach pad in Montana 7

8 1986 copper heap leach pad in Arizona 8

9 Truck & dozer stack (15 m ore lift) 9

10 Rock size matters in ore recovery 10

11 Grasshopper conveyors to radial stack 11

12 Radial conveyor stack (7 m high ore lift) 12

13 Leach pad expansion (delay CapEx cost) 13

14 1995 dynamic (on/off) leach pads 14

15 Dynamic pad conveyors & excavators 15

16 Dynamic versus permanent leach pads 16

17 Interlift liners on copper & nickel heaps 17

18 Barren solution leach application 18

19 External double lined process ponds 19

20 Rain coat liners in high rainfall climates 20

21 Divert rainfall runoff from leach pad 21

22 Leach pad geomembrane liner issues Overly wet underliner composite clay soils Rock damage to exposed liner Blast fly rock Underliner/overliner fill placement rock sizes Wind damage to exposed liner Liner folds from temperature changes Stress cracking (less with PE spec change 1997) Low interface liner strength Flexible liners since 1970 s Texture for less flexible PE liners since

23 Increasing liner semi-crystallinity: Higher stiffness or hardness (less flexible) Higher modulus of elasticity (higher sheet tensile strength) Higher chemical & heat resistance Longer liner life (closure concern) Higher UV (sunlight) resistance Note: 1976 RCRA/EPA driven landfill liner design criteria items above red line versus items below red line (Breitenbach cookie jar theory: you can t have it all ) Lower liner sheet permeability (primary purpose of liner) Good Lower impact strength or puncture resistance (holes in liner) Lower stress crack resistance (especially folds/surface scratches) Lower 3D elongation (less settlement or ground adaptability) Higher expansion/contraction (less intimate clay contact/folds) Lower interface friction strength (more risk of slides) Bad to Ugly 23

24 Compacted low permeability soil fill 24

25 Composite liner intimate contact 25

26 Moisture / Density / Strength Interaction Good (maybe in lab but not in real world) Bad 26

27 Wet of optimum clays tend to dry & crack 27

28 Liner strength (moisture vs load time) Ugly Reference: Breitenbach & Swan 1999 Note: majority of heap leach pad slope failures occur in the first ore lift load (maximum change in stress) and not at ultimate heap height load (lowest stress change) 28

29 Underliner fill with no rain protection 29

30 Overly wet composite soil liner More Ugly 30

31 Geosynthetic clay liners (GCL) Better on dry subgrade surface Back to Good with textured liner contact with stitch-bond GCL sheet 31

32 GCL versus clay on steep slopes Best on steep valley walls 32

33 Temporary sand bags before fill cover Good: wind protection with sand bags 33

34 High winds #1 liner installation concern Bad: CQA impression of Clark Kent 34

35 Wind liner damage on less flexible liners Ugly: liner wind damage 35

36 Wind damage weakens PE liner at folds 36

37 Drain fill cover and liner fold concerns Good: wind protection 37 Bad: liner folds

38 High load liner stability in background 20 m near vertical cut on downhill side of existing 2% pad liner grade (could have been ugly) 38

39 Hail impact versus blast fly rock damage Good: no known hail damage to date 39

40 Mega-scale liner puncture testing Bad: liner stockpile near blast area 40

41 Worried CQA engineer after blast 41

42 Liner puncture thru HDPE 5 layers deep Ugly: deep holes in liner rolls 42

43 Drain fill cover: highest liner abuse Good: drain fill and drain pipe cover 43

44 1994 to 2005 drain pipe high load tests 12 to 24 diameter CPE drain pipe 44 Photo Source: Vector Engineering

45 Soil arching pipe load transfer to liner Bad to Ugly: load transfer to liner & less drainage area Photo Source: Vector Engineering 45

46 Lab mini-scale puncture testing Good: micro-dimple increase in strength Bad: pipe load transfer to liner up to 20% more Ugly: high load rock puncture thru liner 46

47 Size does matter (underliner/overliner) Note: So does the rock shape & gradation curve for liner interface strength and puncture resistance 47

48 1989 large scale pad liner tests Good: Smooth dry underliner & fine drain rock overliner fill to protect the geomembrane liner 48

49 Loaded haul truck passes on liner Note: flattened to 0.3 m thick fill after 10, 20 & 40 truck passes 49

50 D-8N dozer with 90 degree sharp turn 50

51 Hand excavation in bottom half of fill Bad: Engineer not realizing the amount of shovel work involved 51

52 Examining dozer liner tear damage Ugly: dozer sharp turns not acceptable 52

53 1985 to 1995 heap leach slope failures Bad: overly wet underliner fills Ugly: too many liner failures in first ore lift placement Good: not many high fill failures Foundation Low Fill High Fill 0 HEAP SLOPE FAILURES 53

54 Macro-Dimpling Non-Planar Surfaces Downhill & sidehill lined pad toe areas Non-Planar Berm Non-Planar Trench Steep Valley Walls Non-Planar Bench 54

55 Berm or trench stabilization Photo Source: Comanco Good: non-planar surface in pad toe area 55

56 Improve stability with non-planar berm or trench (placing from downhill toe inward) Source: Breitenbach and Athanassapoulos (Geosynthetics Conference 2013, Long Beach, California) 56

57 Steep & planar liner slope with bench Better: liner anchor bench on steep slopes 57

58 Non-planar benches on 3H:1V pad back slope Photo Source: Confidential Best: multiple non-planar benches on lower slopes 24 58

59 Non-planar stabilization stair-step benches Source: Breitenbach and Athanassapoulos (Geosynthetics Conference 2013, Long Beach, California) 59

60 High seismicity & liquefaction issues 60

61 M earthquake in Southern Peru 61

62 Cerro Verde heap toe slide above liner 62

63 Cajone heap at 10 m high near saturation 63

64 Cajone heap 2 m top interlift liquefied at recorded site PGA = 0.22g (no liner slide) 64

65 Failure is not an Option 65 Source: National Geographic Magazine

66 Six Good leach pad liner references: Breitenbach (1997), Overview of liner slope failures under high fill loads, Geosynthetics conference, Long Beach, California Breitenbach & Swan (1999), Influence of high load deformations on geomembrane liner interface strengths, Geosynthetics conference, Boston, Massachusetts Breitenbach (2004), Improvement in slope stability performance of lined heap leach pads from operation to closure, Geosynthetics Magazine Breitenbach & Thiel (2005), A tale of two conditions: landfill versus heap leach pad liner strengths, GRI-19 Las Vegas conference Breitenbach (2011), Old timer recalls history of geomembrane liner interface strength, Part 1 to 3, Geosynthetics Magazine Breitenbach & Athanassapoulos (2013), Improving the stability of high fill load structures built on low strength interfaces, Long Beach Conference 66

67 Any Heap Leach Liner Questions? 67