Design of Back Forty Tailings Management Facility
BACK FORTY PROJECT, MICHIGAN
Presentation by: Kebreab Habte Ken Bocking David List
Prepared for: EGLE, Lansing October 16, 2019 2
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disposal facility facility disposal - co zoned a is TMF The
4.90M m 4.90M – tailings Total •
3
5.91M m 5.91M – rock waste Remaining •
3
remain on surface: on remain
The TMF is therefore designed to deposit the following waste streams that will will that streams waste following the deposit to designed therefore is TMF The
of waste rock rock waste of operation will require 19.05M m 19.05M require will operation
3
As per the mine permit commitment, backfilling the open pit at the end of of end the at pit open the backfilling commitment, permit mine the per As
48.81M t (24.96M m (24.96M t 48.81M – Rock Waste • ) ) yd 32.65M /
3 3
Tailings Tailings • 8.95M t (4.90M m (4.90M t 8.95M – ) ) yd 6.41M /
3 3
of - Life the over streams waste following the generate will mine The Mine Mine - Mine Waste Material Balance Balance Material Waste Mine 3
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Footprint area of TMF is 50.2 ha (124 acres) (124 ha 50.2 is TMF of area Footprint • TMF General Arrangement Plan Plan Arrangement General TMF 4
___ TMF General Arrangement Plan Cont.. Plan Arrangement General TMF 5
___ An overburden separator from the NWRF (10 m wide) m (10 NWRF the from separator overburden An •
) A waste rock crown at top of the facility (1.4M m (1.4M facility the of top at crown rock waste A •
3
A double liner system that extends under the perimeter wall perimeter the under extends that system liner double A •
) A free draining waste rock perimeter wall (4.5M m (4.5M wall perimeter rock waste draining free A •
3
A gently sloped base (~1%) base sloped gently A •
The TMF has: TMF The
= 4.9 Mm 4.9 =
3
4.5 Mm 4.5
3 1.4 Mm 1.4
3
Note: Cross Note: section has 10 times exaggeration on the vertical scale vertical the on exaggeration times 10 has section -
TMF Cross TMF Section - 6
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increase the factor of safety for stability for safety of factor the increase
The wide waste rock perimeter structure and lower phreatic surface significantly significantly surface phreatic lower and structure perimeter rock waste wide The •
The facility will have an emergency spillway spillway emergency an have will facility The •
phreatic surface will be only under the pond the under only be will surface phreatic
Most of the tailings will be unsaturated except just below the pond area, area, pond the below just except unsaturated be will tailings the of Most •
Supernatant water will be pumped out actively out pumped be will water Supernatant •
A granular drainage is provided under the entire footprint of the facility the of footprint entire the under provided is drainage granular A •
The perimeter dyke is competent frictional waste rock material material rock waste frictional competent is dyke perimeter The •
high, 36 m crest width, 3H:1V side slopes) side 3H:1V width, crest m 36 high,
Perimeter structure contain three waste rock dykes (each dyke is about 10 m m 10 about is dyke (each dykes rock waste three contain structure Perimeter •
76% solids content, is discharged in the facility facility the in discharged is content, solids 76% - 65 to dewatered Tailings, • The TMF is Designed to High Standard Standard High to Designed is TMF The 7
___ Base Grade Base
foundation soil foundation
strong strong – gravel and sand to sand
The subsurface soil is silty sand, sand, silty is soil subsurface The • Cut and fill and Cut
be prepared by cut and fill and cut by prepared be
The base grade of the facility will will facility the of grade base The •
gravity into an external sump external an into gravity
seepage and any leakage by by leakage any and seepage
to the northwest to convey convey to northwest the to
The base of the facility is graded graded is facility the of base The • TMF Base Grade Base TMF
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9.5 mm (0.37 in) overburden soil overburden in) (0.37 mm 9.5 < compacted in) (5.9 mm 150 ___ •
mil) HDPE secondary liner (textured below perimeter wall and below tailings) below and wall perimeter below (textured liner secondary HDPE mil) - (40 mm 1.0 •
leak detection and collection layer collection and detection leak Geocomposite •
kg/m 4.5 • ) Geosynthetic Clay Liner (GCL) Liner Clay Geosynthetic ) lb/ft (0.92
2 2
mil) HDPE primary liner (textured below perimeter wall and below tailings) below and wall perimeter below (textured liner primary HDPE mil) - (60 mm 1.5 •
g/m 542 • woven cushion geotextile geotextile cushion woven - Non ) oz/yd (16
2 2
m (1.0 ft) thick 9.5 mm (0.37 in) aggregate (MDOT aggregate in) (0.37 mm 9.5 thick ft) (1.0 m 0.3 • 29A) leachate collection layer collection leachate 29A) -
g/m 330 • woven filter geotextile geotextile filter woven - Non ) oz/yd (10
2 2
m (1.0 ft) thick random overburden soil protection layer protection soil overburden random thick ft) (1.0 m 0.3 •
The double liner system of the facility includes the following, from top to bottom: bottom: to top from following, the includes facility the of system liner double The • TMF Base Liner System Liner Base TMF
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___ contamination groundwater to risk
The drainage layer will limit the head of leachate over the liner, reducing reducing liner, the over leachate of head the limit will layer drainage The •
0.3 m drainage layer and 0.3 m overburden protection layer protection overburden m 0.3 and layer drainage m 0.3
Top of the liner is approximately 0.6 m above the base grade, containing containing grade, base the above m 0.6 approximately is liner the of Top •
Top of protection layer protection of Top TMF Top of Liner Grade Liner of Top TMF 10
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pipelines, and one perimeter pipeline perimeter one and pipelines,
The Leak Collection System includes a geocomposite, two interior perforated perforated interior two geocomposite, a includes System Collection Leak The •
interior perforated pipelines, and two perimeter pipelines pipelines perimeter two and pipelines, perforated interior
The Leachate Collection System includes a blanket of coarse aggregate, two two aggregate, coarse of blanket a includes System Collection Leachate The • Leachate and Leak Collection Systems Systems Collection Leak and Leachate
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The pipelines that penetrate the liner system will be insulated insulated be will system liner the penetrate that pipelines The
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pipes penetrate the liner system to convey flow to the external sump sump external the to flow convey to system liner the penetrate pipes
Pipe boots will be provided where the perimeter seepage and leak collection collection leak and seepage perimeter the where provided be will boots Pipe •
pipelines against freezing against pipelines
A thermal cap is provided to protect the perimeter seepage and leak collection collection leak and seepage perimeter the protect to provided is cap thermal A •
The ditch will discharge into the external sump external the into discharge will ditch The •
hour storm event event storm hour - 24 year, - 100 the convey to designed is ditch perimeter The •
system of the facility and also for creating a perimeter ditch ditch perimeter a creating for also and facility the of system
A 1 m (3.3 ft) high perimeter berm is proposed for anchoring the base liner liner base the anchoring for proposed is berm perimeter high ft) (3.3 m 1 A • TMF Perimeter Liner Anchor Berm Berm Anchor Liner Perimeter TMF 12
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The external sump will have a double liner system liner double a have will sump external The •
discharging into the external sump external the into discharging
Thermal cover is provided over the seepage and leak collection pipelines pipelines collection leak and seepage the over provided is cover Thermal • Leachate and Leak Collection System System Collection Leak and Leachate
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up dyke, the perimeter wall will be raised in 2 m lifts m 2 in raised be will wall perimeter the dyke, up - start high m 10 the After
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VWPs will be installed to monitor tailings foundation performance performance foundation tailings monitor to installed be will VWPs •
situ tested (e.g. CPTu) prior to dyke raise dyke to prior CPTu) (e.g. tested situ - in be will foundation Tailings •
raises
woven geotextile will be used as a filter between the the between filter a as used be will geotextile woven - non ) oz/yd (16 g/m 542 A •
2 2
Upstream face of the wall will have crushed rock transition and overburden filter filter overburden and transition rock crushed have will wall the of face Upstream •
The wall will have about 36 m (118 ft) wide crest and 3H:1V side slopes side 3H:1V and crest wide ft) (118 m 36 about have will wall The •
262 m (859.6 ft), maximum height of about 35 m (115 ft) (115 m 35 about of height maximum ft), (859.6 m 262
Perimeter wall is free draining constructed of waste rock waste of constructed draining free is wall Perimeter • to a maximum elevation of of elevation maximum a to
(CPTu) test
Piezocone penetration penetration Piezocone
Vibrating wire piezometer (VWP) piezometer wire Vibrating TMF Perimeter Dyke Dyke Perimeter TMF 14
___ Emergency spillway includes riprap lined channels and pipe culverts pipe and channels lined riprap includes spillway Emergency •
The emergency spillway will eliminate the risk of overtopping overtopping of risk the eliminate will spillway emergency The •
the open pit via LLCS 1 LLCS via pit open the
Emergency spillway is provided to convey storm events up to PMP to to PMP to up events storm convey to provided is spillway Emergency • TMF Emergency Spillway Emergency TMF 15
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top of the facility facility the of top
Floating pump barge will be used to pump the water accumulated at the the at accumulated water the pump to used be will barge pump Floating •
50 m (164 ft) apart apart ft) (164 m 50
Tailings will be discharged from perimeter spigot points, located about about located points, spigot perimeter from discharged be will Tailings • TMF Distribution and Decant System System Decant and Distribution TMF 16
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of water from coming to the TMF the to coming from water of 3.04M m 3.04M
3
76% solids content in the mill will eliminate about about eliminate will mill the in content solids 76% - 65 to thickening Tailings •
perimeter wall, bottom leachate collection system and floating pump barge pump floating and system collection leachate bottom wall, perimeter
Supernatant and consolidation tailings water will be drained through the the through drained be will water tailings consolidation and Supernatant • TMF Water Management Management Water TMF 17
___ TMF Stage Developmental Plan Plan Developmental Stage TMF 18
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Cont Plan Developmental Stage TMF … 19
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Cont Plan Developmental Stage TMF … 20
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Cont Plan Developmental Stage TMF … 21
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backfilling the open pit open the backfilling
The remaining material on the NWRF and the SWRF will be used for for used be will SWRF the and NWRF the on material remaining The •
to form a stable post stable a form to off runoff water runoff off - shed easily will that landform closure -
Waste rock will be removed from the NWRF and placed on top of the TMF TMF the of top on placed and NWRF the from removed be will rock Waste • TMF Closure TMF 22
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GCL GCL
Closure cover on side slopes (3H:1V) will include all the multilayers above except for for except above multilayers the all include will (3H:1V) slopes side on cover Closure •
mm (5.9 in) topsoil topsoil in) (5.9 mm 150 •
450 mm (17.7 in) growth layer growth in) (17.7 mm 450 •
g/m 330 • woven filter geotextile geotextile filter woven - non ) oz/yd (10
2 2
m (1.0 ft) thick 9.5 mm (0.37 in) aggregate (MDOT aggregate in) (0.37 mm 9.5 thick ft) (1.0 m 0.3 • 29A) drainage layer layer drainage 29A) -
mil) LLDPE liner LLDPE mil) - (60 mm 1.5 •
kg/m 3.5 • ) Geosynthetic Clay Liner (GCL) Liner Clay Geosynthetic ) lb/ft (0.92
2 2
0.3 m liner bedding bedding liner m 0.3 •
0.6 m transition layer layer transition m 0.6 •
liner system containing the following, from top to bottom: bottom: to top from following, the containing system liner
Closure cover on benches and crown of the TMF will include a multilayer composite composite multilayer a include will TMF the of crown and benches on cover Closure • TMF Closure Cont... Closure TMF 23
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post closure drainage closure post
Chutes and drainage ditches (designed for PMP) will be provided to manage the the manage to provided be will PMP) for (designed ditches drainage and Chutes • TMF Closure Cont… Closure TMF
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Seepage analysis (SEEP/W) analysis Seepage
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Consolidation analysis (CONDES0) analysis Consolidation •
Seismic (SHAKE2000 and Boulanger and Idriss 2014) Idriss and Boulanger and (SHAKE2000 Seismic -
Static (Sadrekarimi 2014 and 2016) and 2014 (Sadrekarimi Static -
Liquefaction analyses Liquefaction •
Filter compatibility analysis (NRCS 2017) (NRCS analysis compatibility Filter •
Thermal analysis (TEMP/W) analysis Thermal •
Staged tailings deposition plan (Goldtail and AutoCAD Civil 3D) Civil AutoCAD and (Goldtail plan deposition tailings Staged •
Monthly water balance for various climatic conditions climatic various for balance water Monthly •
Material balance balance Material •
design of the TMF: the of design
The following analyses have been completed to support the the support to completed been have analyses following The Design Analyses Completed Completed Analyses Design
drainage)
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due to poor poor to due - level water high
(conservatively assumed assumed (conservatively ___
Static Liquefaction Liquefaction Static
drainage)
due to poor poor to due - level water high
(conservatively assumed assumed (conservatively
Static Slope Stability Stability Slope Static
(good drainage) (good
Static Slope Stability Stability Slope Static
- pseudo and (static Stability • static) analysis (SLOPE/W): e.g.: (SLOPE/W): analysis static) Design Analyses Completed Cont… Cont… Completed Analyses Design 26
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Hydraulic analysis of chutes and spillway spillway and chutes of analysis Hydraulic - PMP -
closure drawdown seepage (SEEP/W transient) (SEEP/W seepage drawdown closure - Post -
Cover infiltration (HELP model) (HELP infiltration Cover -
pressure)
static unsaturated, and static unsaturated & low ground ground low & unsaturated static and unsaturated, static - pseudo
Veneer stability analyses (static unsaturated, static saturated, saturated, static unsaturated, (static analyses stability Veneer -
Closure Analyses Closure •
Geomembrane hydrostatic puncture test (ASTM D5514) (ASTM test puncture hydrostatic Geomembrane -
Loss, ASTM D5891) D5891) ASTM Loss,
GCL chemical compatibility (Swell Index, ASTM D5890, and Fluid Fluid and D5890, ASTM Index, (Swell compatibility chemical GCL -
Liner tests tests Liner • Design Analyses Completed Cont... Cont... Completed Analyses Design 27
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Instrumentation and monitoring plan monitoring and Instrumentation •
Operations, Maintenance and Surveillance (OMS) Manual Manual (OMS) Surveillance and Maintenance Operations, •
CQC/CQA plan CQC/CQA •
Technical specifications specifications Technical •
IFC drawings drawings IFC •
Detailed design design Detailed •
The following will be completed per the conditions in the Part 632 Permit: 632 Part the in conditions the per completed be will following The Detailed Design Detailed Traditional Upstream Tailings Management Facilities vs Zoned Tailings and Waste Rock Co-Disposal Facilities
CONCEPTS AND EXAMPLES
October 16, 2019
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___ raises dam subsequent
Coarse tailings excavated from tailings beach are used to construct the the construct to used are beach tailings from excavated tailings Coarse •
location
Fine tailings and low specific gravity slimes settle away from discharge discharge from away settle slimes gravity specific low and tailings Fine •
Coarse tailings and high specific gravity tailings settle near discharge location location discharge near settle tailings gravity specific high and tailings Coarse •
Tailings segregate during deposition during segregate Tailings •
Tailings discharged at around 30% solids content by weight weight by content solids 30% around at discharged Tailings •
Starts with a free draining low starter dyke starter low draining free a with Starts •
The following are characteristics of typical upstream TMFs: upstream typical of characteristics are following The
Traditional Upstream Raised TMFs Raised Upstream Traditional
disposal vs Traditional Upstream Raised TMF TMF Raised Upstream Traditional vs disposal - Co Zoned
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___ complete is raise dyke upstream After
Coarse starter dyke starter Coarse Tailings excavation during upstream dyke raise dyke upstream during excavation Tailings
The following photos show how a typical upstream TMF is constructed: is TMF upstream typical a how show photos following The
Zoned Co Zoned disposal vs Traditional Upstream Raised TMF Cont.. Cont.. TMF Raised Upstream Traditional vs disposal -
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mill reducing 3M m 3M reducing mill of water from coming to the TMF the to coming from water of
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76% solids content in the the in content solids 76% - 65 to thickened be will Tailings - content solids Tailings •
tailings facility which is graded for gravity drainage gravity for graded is which facility tailings
A granular drainage layer beneath the entire base of the the of base entire the beneath layer drainage granular A - system Underdrain •
perimeter wall perimeter
drain out of the facility while eliminating the risk of tailings migration into the the into migration tailings of risk the eliminating while facility the of out drain
Transition and filter zones of dyke dyke of zones filter and Transition • Allow tailings consolidation water to easily easily to water consolidation tailings Allow -
liquefiable and erosion resistant) erosion and liquefiable - non draining,
Perimeter dyke dyke Perimeter • Constructed of waste rock 36 m wide crest (strong, free free (strong, crest wide m 36 rock waste of Constructed -
The Back Forty TMF design mitigated the known risks of traditional upstream TMFs: upstream traditional of risks known the mitigated design TMF Forty Back The
- Co Zoned Forty Back disposal TMF TMF disposal
BOTTOM DRAINAGE DRAINAGE BOTTOM
PHREATIC SURFACE WITH WITH SURFACE PHREATIC
NO BOTTOM DRAINAGE DRAINAGE BOTTOM NO
Zoned Co Zoned disposal vs Traditional Upstream Raised TMF Cont… Cont… TMF Raised Upstream Traditional vs disposal - 32
___ and operations to ensure that the design intent is being satisfied being is intent design the that ensure to operations and
The facility will be monitored closely during construction construction during closely monitored be will facility The - review Performance •
carried out assuming elevated water table confirmed the stability of the facility the of stability the confirmed table water elevated assuming out carried
Conservative seismic and static liquefaction analyses analyses liquefaction static and seismic Conservative - analysis Liquefaction •
increase the factor of safety for stability stability for safety of factor the increase
of the facility that lowers the phreatic surface within the facility significantly significantly facility the within surface phreatic the lowers that facility the of
material as perimeter structure and placement of a drainage layer at the bottom bottom the at layer drainage a of placement and structure perimeter as material
Placement of very wide competent frictional waste rock rock waste frictional competent wide very of Placement - analysis stability Slope •
Probable Maximum Precipitation, thus preventing overtopping preventing thus Precipitation, Maximum Probable
Emergency spillway spillway Emergency • It will safely convey extreme storm events up to the the to up events storm extreme convey safely will It -
strength consolidated thickened tailings thickened consolidated strength
Dyke raise foundation foundation raise Dyke • The dyke will be raised over high density and high high and density high over raised be will dyke The -
pumping of bleed water from decant area will result a very low phreatic surface phreatic low very a result will area decant from water bleed of pumping
Phreatic surface surface Phreatic • Free draining underdrain and perimeter wall as well as as well as wall perimeter and underdrain draining Free -
Zoned Co Zoned disposal vs Traditional Upstream Raised TMF Cont… Cont… TMF Raised Upstream Traditional vs disposal - 33
___ 68% solids content solids 68%
Tailings are deposited at 60% to to 60% at deposited are Tailings •
method
berms are raised by the upstream upstream the by raised are berms
The perimeter waste rock fill fill rock waste perimeter The •
Open pit gold mine gold pit Open •
TMF
Open Pit Open
1. Canadian Malartic Mine, Quebec Mine, Malartic Canadian 1. Operating TMFs with Similar Features to Back Forty TMF TMF Forty Back to Features Similar with TMFs Operating
34
Sand and gravel over thickened tailings thickened over gravel and Sand ___
Perimeter wall is sand and gravel gravel and sand is wall Perimeter •
tailings facility tailings
content) deposited over previously slurry slurry previously over deposited content)
Thickened tailings at 63% to 68% solids solids 68% to 63% at tailings Thickened •
Underground gold mine gold Underground •
Sand and gravel over soft tailings soft over gravel and Sand
TMF
2. Musselwhite Mine, Ontario Mine, Musselwhite 2.
Operating TMFs with Similar Features to Back Forty TMF TMF Forty Back to Features Similar with TMFs Operating … Cont 35
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using waste rock berms berms rock waste using
Interior of the facility partitioned partitioned facility the of Interior •
subaqueous slurry tailings tailings slurry subaqueous
Thickened tailings disposed over over disposed tailings Thickened •
Underground copper and zinc mine mine zinc and copper Underground •
TMF
3. Neves Corvo Mine, Portugal Mine, Corvo Neves 3.
Operating TMFs with Similar Features to Back Forty TMF TMF Forty Back to Features Similar with TMFs Operating … Cont