Good, bad & ugly lessons learned in the design and construction of heap leach pads

Presented by: Allan Breitenbach, PE

Principal Geotechnical Engineer

Ausenco Denver Office

Heap Leach Conference

September 2013

Vancouver, B.C., Canada

1

Source: National Geographic Magazine

Admiral Perry in 1907: “find a way or make a way”

2

1972 potash evaporation ponds, Utah

3

PVC and CSPE (Hypalon) liners in 1970’s

4

1976 uranium tailings dam in Wyoming

5

Leach pads: highest lined fills in world

6

1979 gold heap leach pad in Montana

7

1986 copper heap leach pad in Arizona

8

Truck & dozer stack (15 m ore lift)

9

Rock size matters in ore recovery

10

Grasshopper conveyors to radial stack

11

Radial conveyor stack (7 m high ore lift)

12

Leach pad expansion (delay CapEx cost)

13

1995 dynamic (on/off) leach pads

14

Dynamic pad conveyors & excavators

15

Dynamic versus permanent leach pads

16

Interlift liners on copper & nickel heaps

17

Barren solution leach application

18

External double lined process ponds

19

Rain coat liners in high rainfall climates

20

Divert rainfall runoff from leach pad

21

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 1988

22

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

• Lower liner sheet permeability (primary purpose of liner)

• 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)

Good

Bad to Ugly

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”)

23

Compacted low permeability soil fill

24

Composite liner intimate contact

25

Moisture / Density / Strength Interaction

Good

Bad

(maybe in lab but not in real world)

26

Wet of optimum clays tend to dry & crack

27

Liner strength (moisture vs load time)

Ugly

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)

Reference: Breitenbach & Swan 1999

28

Underliner fill with no rain protection

29

Overly wet composite soil liner

More Ugly

30

Geosynthetic clay liners (GCL)

Back to Good with textured liner contact with stitch-bond GCL sheet

Better on dry subgrade surface

31

GCL versus clay on steep slopes

Best on steep valley walls

32

Temporary sand bags before fill cover

Good: wind protection with sand bags

33

High winds #1 liner installation concern

Bad: CQA impression of Clark Kent

34

Wind liner damage on less flexible liners

Ugly: liner wind damage

35

Wind damage weakens PE liner at folds

36

Drain fill cover and liner fold concerns

Good: wind protection

Bad: liner folds

37

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

Hail impact versus blast fly rock damage

Good: no known hail damage to date

39

“Mega-scale” liner puncture testing

Bad: liner stockpile near blast area

40

Worried CQA engineer after blast

41

Liner puncture thru HDPE 5 layers deep

Ugly: deep holes in liner rolls

42

Drain fill cover: highest liner abuse

Good: drain fill and drain pipe cover

43

1994 to 2005 drain pipe high load tests

44

12” to 24” diameter CPE drain pipe

Photo Source: Vector Engineering

Soil arching pipe load transfer to liner

Bad to Ugly: load transfer to liner & less drainage area Photo Source: Vector Engineering

45

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

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

47

1989 large scale pad liner tests

Good: Smooth dry underliner & fine drain rock overliner fill to protect the geomembrane liner

48

Loaded haul truck passes on liner

Note: flattened to 0.3 m thick fill after 10, 20 & 40 truck passes

49

D-8N dozer with 90 degree sharp turn

50

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

51

Examining dozer liner tear damage

Ugly: dozer sharp turns not acceptable

52

0

2

4

6

8

10

12

14

16

18

20

HEAP SLOPE FAILURES

Foundation

Low Fill

High Fill

1985 to 1995 heap leach slope failures

Good: not many high fill failures

Bad: overly wet underliner fills

Ugly: too many liner failures in first ore lift placement

53

“Macro-Dimpling” Non-Planar Surfaces

Non-Planar Berm Non-Planar Trench

Non-Planar Bench

Steep Valley Walls

Downhill & sidehill lined pad toe areas

54

Berm or trench stabilization

Photo Source: Comanco

Good: non-planar surface in pad toe area

55

Improve stability with non-planar berm or trench (placing from downhill toe inward)

Source: Breitenbach and Athanassapoulos (Geosynthetics Conference 2013, Long Beach, California)

56

Steep & planar liner slope with bench

Better: liner anchor bench on steep slopes

57

24

Non-planar benches on 3H:1V pad back slope

Photo Source: Confidential

Best: multiple non-planar benches on lower slopes

58

Non-planar stabilization “stair-step” benches

Source: Breitenbach and Athanassapoulos (Geosynthetics Conference 2013, Long Beach, California)

59

High seismicity & liquefaction issues

60

2001 8.4M earthquake in Southern Peru

61

Cerro Verde heap toe slide above liner

62

Cajone heap at 10 m high near saturation

63

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

64

Source: National Geographic Magazine

Failure is not an Option

65

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

Any Heap Leach Liner Questions?

67