Investigating for Dry Stack Tailings Facility Closure: Multidisciplinary Evaluation at the Pogo Mine, Alaska
* [email protected], Ph. +1-907-273-3604
Dan Neuffer and Christopher Stevens, SRK Consulting (U.S.) Inc., USARussell Staines, SRK Consulting (Australasia) Pty Ltd., AustraliaMakoto Umedera and Sally McLeod, Sumitomo Metal Mining Pogo LLC, USA
*
Sumitomo Metal Mining Pogo LLC (Pogo) is the operator of the Pogo underground gold mine, located near Delta Junction, Alaska. The mine has been in operation since 2006 and produces between 380,000 and 400,000 ounces of gold annually. Filtered tailings from the flotation circuit and waste rock from the mine are placed in the dry stack tailings facility (DSTF). Expansion of the DSTF from 7 M tonnes to 18 M metric tonnes has prompted further evaluation of the facility for closure planning. This paper presents data from several components of a multidisciplinary DSTF closure study, including geotechnical and geochemical test results, in situ temperature and pore pressure measurements, and estimated freezing characteristics of tailings samples. Results of this study indicate the DSTF is physically stable and is comprised of materials that are non-potentially acid-generating; these findings support operational material placement practices and elements of the DSTF closure plan. Geotechnical field and laboratory testing indicate that effective friction angles and dry densities of in situ DSTF materials are consistent with previous slope stability analyses. Geotechnical borehole drilling, thermal monitoring, and analysis indicate the presence of permafrost within the DSTF. Pore pressure measurements and drilling observations indicate a phreatic surface near the base of the DSTF. This study narrows the focus of data collection for future closure planning and provides an example of physical and chemical conditions within a dry stack tailings facility in a continental, subarctic climate. These findings are pertinent for planning, design, permitting, operation, and closure of dry stacks in similar climates.
Acid-generating Potential
Subsurface Investigation
The authors thank the following: Megan Miller, for geotechnical logging of drill core and installation of VWP sensors; Ben Green, for supporting design of the VWP program; and Ivan Clark and Dan Mackie for their discussion of hydrologic processes. Alaska Department of Natural Resources and Alaska Department of Environmental Conservation have provided and continue to provide valuable feedback for the DSTF closure study. The authors also thank Sumitomo Metal Mining Pogo LLC for supporting this study.
Summary
0
10
20
30
40
50
60
0 10 20 30 40 50 60
NP
(kg
CaC
O3/t
)
AP (kg CaCO3/t)
GP-1, n=25RR-1, n=24SB-1, n=16TailingsNP/AP = 1NP/AP = 2NP/AP = 3
7.0
7.5
8.0
8.5
9.0
9.5
0 10 20 30 40 50
Rin
se p
H
Depth (m bgs)
• Geochemical characteristics of DSTF materials affect water quality associated with the facility
• Rinse pH values were in the range of pH 7 – 9
• Nearly all samples were classified as non-potentially acid-generating (non-PAG) based on neutralization potential (NP) to acidification potential (AP) ratios greater than three (NP/AP > 3)
• Subsurface investigation to evaluate DSTF geotechnical, thermal, hydrogeological, and geochemical characteristics
• Major components of the facility include the starter and toe berms, shells, general placement area (GPA), and flow-through drain
• Three sonic boreholes were vertically drilled at SB-1, GP-1, and RR-1 (total depth of 107 m)
• Sonic core, standard penetration testing (SPT), and Shelby tube samples were collected
• Four Shelby tube, 38 modified California and 49 geotechnical grab samples
• Laboratory testing included grain size, Atterberg limits, natural moisture and density, consolidated-undrained triaxial shear, flexible wall permeability, and consolidation
• Vibrating wire piezometers (VWP) were installed at SB-1, GP-1, and RR-1 to measure ground temperature and pore pressure
Dry Stack Tailings and Waste Rock
Dry
Moist
Key Findings and Implications for DSTF Closure
Tailings Freezing Characteristics
Freezing characteristics of tailings estimated from grain size distribution Atterberg limits, estimatedspecific surface area
• Phase state of water in the DSTF affects physical stability, surface water runoff characteristics, permeability, and geochemical reactivity of materials in the stack
• Estimated unfrozen water content curves for DSTF tailings indicate liquid pore water is present at subfreezing temperatures • The unfrozen water content at −1°C is estimated to range between 7 and 11% of the dry mass
• Presence of unfrozen tailings can be inferred from the relatively warm ground temperatures, the fine-grained nature of the tailings, observational evidence from the drill core, and the estimated freezing characteristics
Summary of Measured DSTF Ground Temperature
• Geotechnical investigation, thermal monitoring, and analyses indicate the presence of both frozen and liquid water within the DSTF
• Pore pressure measurements and drilling observations indicate a phreatic surface near the base of the DSTF and isolated, perched saturated zones within the dry stack
• Density and shear strength of DSTF materials indicate static and pseudostatic slope stability of the stack and stability against liquefaction
• Geochemical analyses of DSTF materials show that acidic drainage from the stack is unlikely
• DSTF ground temperatures confirm the presence of permafrost, as thermally defined by temperatures at or below 0°C • DSTF can be expected to thermally evolve as additional material is added and in relation to changes in local climate and microclimatic effects
• Monitoring of VWP and evaluation of surface water and groundwater from and adjacent to the DSTF may further constrain components of the DSTF water balance, including runoff, seepage, and flow-through drain flux.
• Evaluation of the long-term metals-leaching potential of DSTF materials through laboratory testing, comparison to operational monitoring data, and water quality modelling may inform selection and design of a closure cover system
Ground Temperature and Pore Pressure
Site Sensor description
Sensor depth 1
(m b.g.s.)
Elevation2 (m a.m.s.l.)
Temperature (°C)
Measurement date/period
SB-1 Shallow 7.6 726.3 −0.1 to −0.7 10/3/2012 to 10/22/2013
SB-1 Deep 31.9 702.0 −0.7 10/22/2013
GP-1 Shallow 19.2 738.5 −0.7 10/22/2013
GP-1 Deep 41.8 716.0 −0.4 10/22/2013
RR-1 Shallow 0.6 764.1 9.1 to −6.2 10/1/2012 to 10/22/2013
RR-1 Mid 18.6 746.2 −0.8 10/22/2013
RR-1 Deep 28.7 736.1 −0.6 10/22/2013
Free
zing
poi
nt d
epre
ssio
n
-10-8-6-4-200
5
10
15
20
25
30
Unf
roze
n w
ater
, % d
ry m
ass
108231085910804b
1081910849b1085310858
SSA - 21 m2 g-1
Temperature (oC)
Sample ID
Ice lens from 5.1 m b.g.s
Acknowledgements
SB-1 (14.3 to 15.8 m)
SB-1 (23.5 to 25 m)
This study contributes to further understanding of the geotechnical, thermal, hydrologic, and geochemical conditions of the Pogo Mine DSTF to support closure planning. Specifically, this study confirms specific aspects of the DSTF operational construction and closure plans while narrowing the focus of data collection for future closure planning.
20132012Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov.
20132012Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov.
Pres
sure
hea
d (k
pa)
Tem
pera
ture
(o C
)
-10
-8
-6
-4
-2
0
2
4
6
8
10
-60
-50
-40
-30
-20
-10
0
10
RR-1 Shallow (764.1 m amsl)
RR-1 Deep (736.1 amsl)
RR-1 Thermistor
RR-1 VWP
RR-1 Mid (746.2 m amsl)
Water height above sensor (m
)
-6
-5
-4
-3
-2
-1
0
1
RR-1
Sonic drill rig used for recovery of core and installation of VWP at boreholes SB-1, GP-1, and RR-1
1. Sensor depth is based on the depth below ground surface (b.g.s.) at the time of sensor installation (October 2012).2. Elevations are presented as metres above mean sea level (m a.m.s.l.).
Ele
vatio
n (m
)
Ele
vatio
n (m
)
Distance in Metres
LEGEND
Compacted Tailings
GPA Materials
Rock Shell
Flow-through Drain
Starter Berm and Toe Berm
Sand
Silt
Clay
Gravel
Cobble
Cobble & Gravel
Silt & Gravel
Gravel & Sand
Lithology Units:
Vibrating Wire Piezometer / Thermistor Location
Permeability
Inferred Water Height Above Sensor
Sample Testing Types:
Consolidation
Triaxial
Cross-section Units
NW SE
640
680
720
760
800
840
640
680
720
760
800
840
Flow-through drain
Composite Shell (Shell 2)
Composite Shell (Shell 3) Native Ground
Rockfill Shell (Shell 1)
GPA Materials
0+000 0+100 0+200 0+300 0+400 0+500 0+600 0+700 0+800 0+900 1+000 1+100
2012 DSTF Surface
SB-1GP-1 RR-1
18M Ton DSTF Surface
VWP: 8.7 kpaTemp: -0.1 to -0.7oC
VWP: 41.7 kpaTemp: -0.7oC
VWP: 23.6 kpaTemp: -0.7oC
VWP: 10.1 kpaTemp: -0.4oC
VWP: -3.7 kpaTemp: 9.1 to -6.2oC
VWP: -29 to -40 kpaTemp: -0.8oC
VWP: 5.2 kpaTemp: -0.6oC
DSTF Cross-sectionA A’
20132012Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov.
20132012
Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov.
Pres
sure
hea
d (k
pa)
-20
-10
0
10
20
30
40
50
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0GP-1 Shallow (738.5 m amsl)GP-1 Deep (716.0 m amsl)
GP-1 Thermistor
GP-1 VWP
-2
-1
0
1
2
3
4
5
Water height above sensor (m
)
GP-1
Tem
pera
ture
(o C
)
Compacted Tailings (typical)
Non-mineralized Waste Rock (typical)
GPA - Tailings
2012 Dry Stack Tailings Facility (DSTF)General Placement Area(GPA) - Waste Rock
South Diversion Ditch
North Diversion Ditch
1816
000
E
1818
000
E
1816
000
E
1818
000
E
3818000 N
3820000 N
3818000 N
3820000 N
800m
775m
750m
725m
675m
700m
775m
750m
800m
825m
850m 875m
925m
825m
850m
875m
900m
800m
825m
850m
875m
900m
925m
975m
950m
850m
A'
A
Old North Diversion Ditch (closed 2013)
Old South Diversion Ditch (closed 2013)
Flow ThroughDrain
18M Metric Tonne DSTF Extent
Anchorage
Fairbanks Pogo Mine
Alaska
Recycle Tailings Pond (RTP)
Shell 3
Shell 2
Shell 1
2012 DSTF Extent
New North Diversion Ditch for DSTF Expansion
Pogo Mine is an underground gold mine operated by Sumitomo Metal Mining Pogo LLC (Pogo), located approximately 60 km northeast of Delta Junction, Alaska.
The site is characterised by a continental climate with relatively low annual precipitation (~356 mm), relatively cool summers (mean of 10°C), and cold winters (mean of −13°C). Warm (>-2 °C) discontinuous permafrost is present beneath naturally vegetated terrain.
DSTF History
• DSTF has been in operation since February 2006. Placement of approximately 5.3 M metric tonnes of tailings and waste rock through 2011
• Original design capacity of approximately 6.8 M metric tonnes
• 2011 - DSTF material balance confirmed the need for expansion of the facility by 2013
• 2012 - Increase of the permitted DSTF storage capacity from 6.8 M tonnes to 18 M metric tonnes was approved
• 2012 - Update of DSTF closure study was initiated
• 2013 - Pogo completed expansion of the surface water diversion ditches and haul road
Pogo Dry Stack Tailings Facility (DSTF)
Ice lens Waste rock Unfrozen tailings
• Silt-size tailings were described in the field to be moist, with most intervals unfrozen (gravimetric moisture content from 13 to 22%)
• Sand and gravel waste rock were described in the field to be dry (gravimetric moisture content from 2 to 10%)
• Dry densities of tailings ranged from 1.68 to 1.97 g/cm3
• SPT blow counts corrected to (N1)60 in tailings averaged 13 for 10 samples (range from 2 to 21)
• Effective friction angles of tailings ranged from 34−35°
Temperature
• Ground temperatures thermally equilibrated over a two- to three-month period following installation
• Shallow ground temperatures ranged from 9.1°C to −6.2°C in response to surface heating and cooling (VWP Sensor RR-1 Shallow)
• Deep ground temperatures below the depth of seasonal variation ranged from −0.4°C to −0.8°C
• Permafrost, as thermally defined, is present within the DSTF
Pressure
• VWP recorded both positive and negative pore pressures
• Positive pore pressures recorded at the site may be caused by a phreatic water surface above the sensor
• Negative pressures may be caused by either soil matrix suction in unsaturated materials or cryosuction under freezing conditions
• SB-1 Deep sensor - maximum pressure of 41.7 kPa, equivalent to a water height of 4.3 m above the sensor
New South Diversion Ditch for DSTF Expansion
SB-1 Shallow (726.3 m amsl)
SB-1 Deep (702.0 m amsl)
20132012
-20
-10
0
10
20
30
40
50
Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov.
20132012Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov.
Pres
sure
hea
d (k
pa)
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0SB-1 Thermistor
SB-1 VWP
-2
-1
0
1
2
3
4
5
Water height above sensor (m
)
SB-1
4.3 m of water over sensor
Tem
pera
ture
(o C
)