wolverine mine tsf independent tailings review memorandum

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WOLVERINE MINE TSF INDEPENDENT TAILINGS REVIEW MEMORANDUM

DATE: October 26, 2018

TO: Dustin Meisburger, P. Eng.

Engineering Manager

Conuma Coal Resources Limited

Tumbler Ridge, BC

FROM: Independent Tailings Review Board

Dirk van Zyl, Ph.D., P.E., P.Eng., Review Consultant, Richmond, BC.

John J. Clague, Ph.D., P.Geo., Department of Earth Sciences, Simon Fraser University,

Burnaby, BC, Canada

Debora J. Miller, Ph.D., P.E., Miller Geotechnical Consultants, Inc., Fort Collins, CO,

USA

1. INTRODUCTION

In accordance with guidance provided in Part 10 of the Health Safety and Reclamation Code for Mines in

British Columbia that was published on July 20, 2016 (Guidance Document), Conuma Coal Resources

Limited (Conuma) convened an Independent Tailings Review Board (ITRB) to review the design,

construction, condition, and operation of the Wolverine Mine Tailings Storage Facility (TSF) near

Tumbler Ridge, B.C. The ITRB is made up of three independent subject matter experts who are not

currently involved in, or responsible for, the design, construction, or operation of the facility: Mr. Dirk

van Zyl, Ph.D. (geotechnical engineering), Mr. John J. Clague, Ph.D. (engineering geology), and Ms.

Debora J. Miller, Ph.D. (geotechnical engineering). The ITRB performed a site visit to the Wolverine

TSF on August 14 and 15, 2018. This report summarizes the ITRB’s responsibilities, activities,

observations, and non-binding comments and recommendations pertaining to the 2018 review of the

Wolverine TSF.

2. ITRB SCOPE AND RESPONSIBILITIES

The ITRB understands that its scope of work and responsibilities are as follows:

Provide an independent review of the construction, design, and condition of the TSF to Conuma

and regulators.

Provide engineers and operations personnel with practical guidance for operating the TSF, as well

as best practice from TSFs at other operations.

Review and comment on the design, operation, maintenance, surveillance by operations, as well

as site engineers and consulting engineers.

Provide non-binding advice to site engineers and the EoR.

3. ITRB ACTIVITIES

a. Review Background Information

Conuma and the Engineer of Record (EoR) provided relevant background information and reports on the

Wolverine TSF to the ITRB prior to and during the site visit. In addition to the project-specific

information, Conuma provided a list of web links to relevant regulatory guidance documents. The

following documents were provided for review by the ITRB:

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Design Reports and Drawings:

o Permit Level Geotechnical Designs for the Tailings Facility and Coarse Coal Reject

Pile, January 2005. This report contains the design basis to construct the TSF

embankment up to elevation 847 m.

o Mine Permit Amendment: Tailings and CCR Management Plan, April 2007. This

report provides additional foundation information and a revised design in support of

raising the dyke an additional 5 m to elevation 852 m.

o Wolverine Tailings Dam Raise – Stability Analysis Evaluation, July 2010. This

report presents stability analysis and recommendations for limiting the elevation of

the raised TSF embankment to elevation 850 m, prior to increasing the dam footprint

per the permit revision recommendations.

o Wolverine Tailings Dam – Design Revision Support Document, March 2012. This

report revises the design of the TSF embankment to accommodate coarse coal refuse

(CCR) by raising and extending an existing bench at elevation 847m such that it

eliminated an existing bench at elevation 842m. This resulted in a new bench at

elevation 848m with a continuous slope down to the existing toe of the embankment.

o Wolverine Mine TSF Toe Buttress Stability Assessment, December 2016. This report

presents an updated stability assessment of the TSF following construction of a rock

toe buttress that was built in 2014. The rock buttress was recommended based on

observed lateral deformations in the TSF foundation. Norwest recommended that a

toe ditch along the downstream slope of the TSF be filled with coarse rock to provide

an additional buttress support and increase the stability of the embankment slope.

As-Built Construction Reports:

o 2006 Tailings Facility Starter Dyke As-Built Report

o 2010 Tailings Facility Downstream Toe Drain As-Built Report

o 2016 Downstream Toe Buttress As-Built Report

o 2017 Tailing Facility Raise As-Built Report

Instrumentation and Annual Dam Safety Inspection Reports:

o Annual Dam Safety Inspection Reports 2007 – 2013

o Annual Dam Safety Inspection Report 2015

o 2015 Recommended Monitoring Frequency during Care and Maintenance


o 2016 Instrumentation Plan and QPOs. This letter report recommended installation

of additional slope inclinometer instruments on the TSF.

o 2017 Instrumentation Installation Report. This report documents the drill logs and

installation of new vibrating wire piezometers and slope inclinometer instruments.


Supplemental Documents:

o 2014 Dam Breach Inundation Study

o 2015 Dam Safety Review by Tetra Tech

Regulatory Documents

o Health, Safety and Reclamation Code (HSRC) for Mines in British Columbia (June

2017 edition, https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-

industry/mineral-exploration-mining/documents/health-and-safety/code-

review/health_safety_and_reclamation_code_2017_rev.pdf)

https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/mineral-exploration-mining/documents/health-and-safety/code-review/health_safety_and_reclamation_code_2017_rev.pdf





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o Guidance Document: Health, Safety and Reclamation Code for Mines in British

Columbia, Version 1.0 (July 2016, https://www2.gov.bc.ca/assets/gov/farming-

natural-resources-and-industry/mineral-exploration-mining/documents/health-and-

safety/part_10_guidance_doc_10_20july_2016.pdf)

o Canadian Dam Association Technical Bulletin, Application of Dam Safety

Guidelines to Mining Dams (2014)

o Mining Association of Canada:

Guide to the Management of Tailings Facilities (2017)

Developing an Operations, Maintenance and Surveillance Manual for

Tailings and Water Management Facilities (2013)

b. Site Inspection and Review Presentations by Engineer of Record and Site Personnel

The ITRB were given a site safety briefing and then escorted on a field inspection of the Wolverine TSF

on Tuesday, August 14, 2018. The board members were accompanied by Conuma Personnel (including

Dustin Meisburger, Engineering Manager and TSF Qualified Person) and Mike Allen of Stantec

representing the Engineer of Record (Dr. Richard Dawson of Stantec who was unable to attend in person).

Following the field tour, the ITRB convened in an on-site meeting room on August 14 and 15, 2018, with

representatives from Conuma and the EoR. Dr. Richard Dawson (the EoR) joined the meetings via

teleconference. The following PowerPoint presentations were delivered to the ITRB:

Technical Overview by Dr. Dawson, Covering the Following Subjects:

o Overview

o Foundation Conditions

o Design and Construction History

o Stability Update

o Instrumentation Review

o Pore Water Pressure Study

TSF Abutment Sequence

TSF Erosion

TSF OMS

South Dump

South Dump Spillage

c. ITRB Deliberation and Presentation to Mine Managers

Following the presentations on August 14 and 15, 2018, the ITRB members met privately to discuss the

information provided and to develop a preliminary summary of our observations, comments, and

recommendations. A PowerPoint presentation was developed and the findings presented to

representatives of Conuma and the Stantec (Norwest) design engineering team.

4. OBSERVATIONS

a. General Observations from Field Inspection

General observations from the field inspection are as follows:

At the time of the ITRB field inspection, tailings were being deposited into the impoundment area

from spigots on the north end of the facility. The tailings delivery pipeline runs along a bench on

the upstream slope of the embankment and tailings slurry is delivered from evenly-spaced spigots

in a controlled manner. Ponded water around the decant tower was being maintained well away

from the dam embankment, with a broad, shallow-gradient tailings beach area formed upstream

https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/mineral-exploration-mining/documents/health-and-safety/part_10_guidance_doc_10_20july_2016.pdf





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from the dyke. The decanted pond water is pumped up from the tailings pond and delivered back

to the plant area by another pipeline that runs along the access road on the west side of the TSF.

The embankment has been constructed of compacted coarse coal refuse, is about 20 m high, and

has a wide crest (about 30 m). The embankment crest is long (approximately 900 m) in the north-

south direction (parallel to the valley orientation), with shorter (approximately 250 m) east-west

segments on the north and south ends that tie back into the abutments.

The embankment crest was reported to be at its final design elevation (elev. 852 m) everywhere

except near the tie-ends at both the north and south abutments. The ITRB was told that the

abutment tie-ins remain to be completed because the main haul road embankment encroaches in

the areas that need to be excavated to construct the final tie-ins with seepage cutoffs. (The haul

road was designed and built for a project based on the original (2007) TSF design crest elevation

of 847 m). Design concepts for these abutment tie-ins have been developed, and the design team

discussed these concepts with the ITRB in the field and during the follow-on presentation

briefings.

Some rilling and gullying erosion was observed on the upstream slope of the embankment,

reportedly due to a recent (July 19-22, 2018) heavy rainfall event. That event and its effects were

discussed in more detail during the presentation briefings. The downstream slope of the

embankment is well vegetated and appeared to be stable with little evidence of surface erosion

except for a small area near the downstream toe of the slope on the north end which is un-

vegetated.

There is a coarse coal refuse stockpile located within the south portion of the impoundment area.

The ITRB was told that the elevation of this stockpile is maintained at operational levels below

the crest of the dyke (elev. 852 m).

The rock toe buttress materials appeared to be generally clean. Seepage was observed emerging

from the rock backfill in the area of one of the exposed culverts that extends from the buttress

under the forest service road and railroad embankments to drain towards the river.

b. Design and Construction

Figure 1 shows a typical cross-section of the Wolverine TSF embankment. The embankment is designed

as an engineered fill, consisting of coarse coal refuse compacted in controlled, thin horizontal layers (lifts)

under specified moisture and density controls. The embankment fill raises were designed to be advanced

in stages, from the original 4 to 6 m high starter dyke at elev. 839.5 m, progressing in a downstream

method of construction to an approximately 20 m high embankment at a final crest elevation of 852 m.

The final embankment section has a low, wide profile, with a 3H:1V upstream slope, approximately 30 m

wide crest and 2H:1V downstream slope with a 10 m wide bench at elevation 842. In 2016, at the

recommendation of the EoR, the open downstream toe drain ditch was backfilled with large, clean rock to

provide a free-draining buttress to enhance stability. The rock backfill was placed in a manner to keep the

ends of four existing culverts exposed for inspections.

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Figure 1. Embankment Cross-Section at Sta 7+00

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Figure 2 shows the geologic cross-section of the Wolverine TSF. The facility is founded on a layered

sequence of sediments, described from top to bottom as follows (refer to Figure 2): Peat (up to 1 m

thick), Alluvial Fan (0-20 m thick); Upper Clay (2 m thick); Boulder Gravel (10 m thick); Sand/silt (2 m

thick); Lower Clay (50-130 m thick); and Glacial Till and Colluvium. The sediments lie on Cretaceous

sedimentary rocks, which include conglomerate, sandstone, and shale.

The site is geotechnically complex, largely due to the difficulty in predicting potentially adverse behavior

on loading of the thick, saturated Lower Clay layer in the TSF foundation (Figure 2). This geologic

material is characterized as near normally to lightly over-consolidated, sensitive glaciolacustrine silty

clay. The design recognizes that there exists the potential for large deformations in the Lower Clay zone

that could lead to vertical and lateral differential settlements and cracking in the TSF embankment, or

slope instability involving deep foundation undrained shear failure through the Lower Clay zone. To

address these conditions, the design included:

A surveillance program with extensive instrumentation to monitor performance and confirm

geotechnical design parameters.

Slope stability factors of safety ≥ 1.3 for undrained loading of the Upper and Lower Clays by the

main embankment.

Seepage controls including a pervious blanket underdrain below the downstream portion of the

embankment, a rock toe drain (buttress), and management of tailings deposition to maintain a

minimum 100 m wide beach at an approximately 1% grade sloping away from the embankment

to maintain a long seepage path from the impounded water

Design was done from initial permitting through construction to current operations by a single

engineering firm (Norwest, now Stantec). The presence of the thick, sensitive glaciolacustrine clay in the

deep foundation of the TSF is a particular challenge that was recognized by the designers (Norwest) from

the outset of the project, and still requires a high level of attention. The ITRB compliments the mine

owner for maintaining the continuity of the engineer-of-record for this challenging project during the

transition of ownership. Engineering and monitoring has been of high quality from the start of this

project, and Conuma has continued to support the ongoing engineering efforts required in investigating

and monitoring of the Wolverine TSF.

Construction of the embankment in phases to a final elevation of 852 m using compacted CCR is in

accordance with appropriate standards of care. The compacted CCR material is very good material for

embankment construction and appears to be tolerant of foundation deformations, based on lack of evident

cracking.

The ITRB believes the embankment design is appropriate for the site conditions. The proposed design for

completion of the abutment tie-ins was reviewed, and ITRB concurs with the approach presented.

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Figure 2 . Geologic Cross-Section of Wolverine Tailings Storage Facility

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c. Operations

The updated OMS manual for the tailings facility was presented to the ITRB. This document now

includes quantifiable performance objectives (QPOs) and trigger action response plans (TARPs). The

ITRB supports periodic updating of the OMS manual.

The tailings beach is maintained at a target width of 100 m upstream from the embankment. There are 13

spigots operated one at a time to control the beach width. The tailings surface rises at a rate of

approximately 0.5 m per year or less. Under present conditions, the required freeboard is easily

maintained. The ITRB acknowledges the diligence of the TSF-Qualified Person (Mr. Dustin Meisburger)

and operating personnel in maintaining these conditions.

d. Surveillance and Monitoring

Routine visual inspections of the dam and conveyance pipeline are conducted on a regular basis. The

ITRB considers the surveillance schedule to be appropriate.

Instrumentation consists of five arrays of nested vibrating wire piezometers and slope inclinometers.

Crest survey monuments will be installed in 2018. The piezometers and inclinometers are read manually.

The current reading schedule is once per month for the piezometers and every six weeks for the

inclinometers. The ITRB considers the type, number, and locations of the present instrumentation to be

appropriate, and supports the planned installation of crest monuments to allow tracking of vertical

settlement and differential settlement of the embankment.

e. Maintenance

In mid-July 2018, just prior to the ITRB site inspection, the Wolverine TSF experienced a heavy rain

event (106 mm in 3 days). This event caused erosion and gullying of the upstream slope of the

embankment, some rilling erosion on the downstream toe especially on the north portion of the

embankment, and partial blockage of the spigot service road. The road was cleared and the upstream

slope gullies were repaired promptly.

5. ITRB DISCUSSION

Instrumentation records show that the foundation has responded to staged construction of the TSF.

Excess pore pressures continue to dissipate and lateral deformation is ongoing in the Lower Clay layer.

Figure 3 shows representative pore pressure dissipation patterns, and Figure 4 is a graph of the rate of

lateral deformations as measured by inclinometers in the upper portion of the Lower Clay. Excess pore

pressure dissipation rates (Figure 3) appear to be leveling off with time, whereas rates of lateral

deformation (Figure 4) in the upper 30 m of the Lower Clay appear to be constant with time and are

continuing. These measurements taken together indicate that the upper portion of the Lower Clay may be

undergoing a phenomenon called undrained creep. Continued undrained creep deformations could result

over time in post-construction stress relaxation and loss of undrained shear strength within the Lower

Clay zone. In contrast, the anticipated strength gain that would accompany drained consolidation may be

occurring at a diminishing rate, as indicated by the excess pore pressure graphs (Figure 3).

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Figure 3. Excess Pore Pressure Dissipation in Wolverine TSF Foundation

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Figure 4. Lower Clay Lateral Displacement

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11/18/2010 4/1/2012 8/14/2013 12/27/2014 5/10/2016 9/22/2017 2/4/2019

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Lower Clay Displacement in A Direction

TF-SI-002

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The stability of the TSF embankment has been assessed using conventional limit equilibrium methods.

Calculated factors of safety for stability are largely governed by the undrained shear strength that is

assumed for the Lower Clay layer. The design (2007 Mine Permit Amendment) is currently based on in-

situ testing in boreholes using corrected CPT values and down-hole pressure meter tests performed in

2006. The undrained shear strength of the Lower Clay layer is assumed to vary linearly with depth

according to the relationship:

𝑆𝑢 = 0.2 𝜎𝑣′

where Su = undrained shear strength (kPa), and 𝜎𝑣′ = effective vertical stress (kPa).

This relationship is consistent with published values for sensitive clays. The ITRB considers this to be an

appropriately conservative estimate for the peak undrained strength for the limit equilibrium stability

analysis based on the data presented.

However, from the behavior of pore pressure response and deformation rates, it is apparent that the Lower

Clay may be undergoing undrained creep deformation. This can lead to a loss of shearing resistance due

to stress release, or strain softening. In some cases this mechanism results in loss of mobilized shear

strength along localized shear bands, which can lead to a post-loading progressive failure mode involving

the Lower Clay.

Vane shear testing in boreholes done for the original design in 2004, and in 2006 in combination with the

CPT and pressure meter testing, consistently indicated lower undrained strengths than had been assumed

for the currently permitted (2007) design, which is based on the CPT and pressure meter data. The

original (2004) basis of design proposed the following relationship from the vane shear data:

𝑆𝑢 = 0.14 𝜎𝑣′

It may be reasonable to consider the vane shear relationship as a possible lower-bound (i.e., residual)

undrained shear strength for purposes of slope stability sensitivity analysis.

6. RECOMMENDATIONS

Considerations for Potential Failure Modes Analysis

1. ITRB recommends conducting a Potential Failure Modes Analysis (PFMA) for the TSF. The

PFMA workshop should include appropriate subject matter experts (including persons with

expertise on sensitive lacustrine clay behavior), design personnel, and site operations personnel.

The purpose of a PFMA is to broaden the scope of the TSF evaluation to include potential failure

modes that may have been overlooked, such as progressive foundation failure due to strain

softening of the Lower Clay under sustained undrained creep deformations. Other potential

failure modes related to earthquake and flood loading may be developed more fully to understand

the likelihood of their occurrence and consequences of the possible release of tailings materials.

This will facilitate identification of potential failure modes and allow an assessment of the

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adequacy of the current monitoring program and development of appropriate quantifiable

performance objectives (QPOs) and trigger-action response plans (TARPs).

Instrumentation

2. Consider a review of existing LIDAR data for historic surface deformation along the north and

south arms of the TSF embankment. Those areas are most vulnerable to differential settlement

because of the transitioning thickness of the compressible Lower Clay under the embankment.

Large differential settlements may lead to cracking of the embankment.

3. Investigate the availability of InSAR data that could be used for determining historic surface

deformation and for future monitoring of surface deformations.

4. Consider additional crest settlement instruments on the north and south arms of the embankment

(in addition to planned monuments at the existing instrument arrays).

5. Investigate the possibility of installing shape arrays in one or more of the existing inclinometer

casings to provide a continuous automated collection of deformation in the Lower Clay.

Geotechnical Assessment

7. ITRB supports a site-specific seismic assessment to select seismic parameters for potential re-

analysis of seismic stability and liquefaction potential.

8. ITRB supports proposed additional geotechnical analyses and modeling to better understand

long-term effects of undrained creep deformations within the Lower Clay. These might include:

a. Sensitivity analysis using limit equilibrium methods.

b. Additional 2D modeling to understand the potential for undrained creep rupture or

progressive failure using FLAC or other appropriate software.

c. Depending on the outcome of 2D modeling in 8b, consider collecting additional deeper

pore pressure or other data as needed to confirm or calibrate the models.

OMS Manual

9. ITRB supports the continued development of quantifiable performance objectives (QPOs) and

trigger action response plans (TARPs).

10. ITRB recommends including visual inspection and instrument readings following significant

seismic events.

11. ITRB supports training of responsible personnel (especially new personnel).