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Tuesday, 11 August 2020

Attn: Tom Mehrtens

Senior Environment and Community Officer

Terramin Australia Ltd.

Unit 7/202-208 Glen Osmon Road

Fullarton South Australia 5063

2784_G\6330_Final

Dear Tom,

RE: BIRD-IN-HAND CEMENTED ROCK FILL CLARIFICATIONS FOR THE SOUTH

AUSTRALIAN GOVERNMENT

Mining One Consultants (Mining One) were engaged by Terramin Australia Limited (Terramin)

in response to further matters raised by the South Australian Government. Mining One’s scope

was to provide technical input relating to two main comments:

“Provide evidence to evaluate and determine the effectiveness of the backfill strategy to

protect worker safety”

And

“The potential for a hazard to be created as a result of water collecting on top of a cemented

backfill sill pillar should be investigated”

For context, “the backfill strategy” refers to Terramin’s intention of using cemented rock fill (CRF)

for all underhand cut and fill sill pillars at the Bird-in-Hand Gold Project.

Mining One’s technical commentary for the above quotations are is provided in proceeding

subsections of this letter report.

Yours sincerely,

Dr. Aidan Ford Senior Geotechnical Engineer

MINING ONE PTY LTD

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1 BACKFILL STRATEGY EFFECTIVENESS

Underhand exposure of consolidated backfill (i.e. paste, hydraulic fill, sandfill, post consolidated

rockfill, CRF and cemented aggregate fill (CAF)) in underground mines is considered common

mining practice. Typically, a mine will opt to use an underhand mining method for one or more

of the following reasons:

Mine seismicity issues (e.g. Raleigh, Kundana, Gwalia, Lucky Friday and Macassa);

Very poor ground conditions (e.g. Red Lake, Stillwater and Kencana); or

Recovery of sill pillars used to open multiple mining fronts (e.g. Pajingo and Kencana).

In all applications, the underhand exposure can be ‘man access’ or ‘unmanned access’,

depending on the application (e.g. cut-and-fill or open stoping). The degree of conservatism used

in the consolidated backfill design (e.g. thickness, strength, cure age, undercut span etc.) will

generally be higher for man access applications. In either case, the main consideration for

undercutting consolidated backfill is the backfill design and the QA/QC practices associated with

the fill placement. Put simply, if consolidated backfill is planned to be undercut, it should

be designed to be undercut and the insitu strength requirements should be demonstrable.

Mining One’s previous recommendation for using CAF (Bijelac & Roache, 2017) was based on

the increased compressive strength and the predictability of CAF compared to CRF. Mining One

has recommended CAF as a method of reducing the overall risks associated with man access

underhand mining. Mining One understand that Terramin’s preference for using CRF is based

on the overall project outcome when considering larger aspects of the Bird-in-Hand Gold Project

such as the overall cost, noise levels, air quality and the overall visual amenity of the operation.

As for which fill type is required, both types have successfully been used in underhand cut and

fill operations, meaning under the right conditions, either fill type may be suitable. Table 1

presents an empirical database of underhand cut and fill case studies where CRF was the

specified consolidated backfill.

Table 1 Example Underhand Cut and Fill Using Cemented Rock Fill (After Pakalnis, 2014,

Hughes, 2014)

Mine Percent

Cement

Fill compressive

strength Undercut Span

Sill

Thickness

Design

Comments

Anglo Gold

(1999 Site Visit)

Murray Mine

Queenstake (2004)

6.5% 5.50 MPa 7.6m 4.6m

Maximum size 2 inches for

fill material

Mined on Remotes

8.0% 6.90 MPa 9.1m 4.6m

8.0% 6.90 MPa 21.0m 4.6m

Eskay 7.0% 4.0 – 12.0 MPa 3.0m 3.0m -

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Mine Percent

Cement

Fill compressive

strength Undercut Span

Sill

Thickness

Design

Comments

Turquoise Ridge

9.0% 8.3 MPa 13.7m 4.0m

Stable since 2001

(Seymour et al. 2019)

FOS estimated at 1.4 - 1.9

9.0% 8.3 MPa 3.7m 3.0m -

9.0% 8.3 MPa 7.3m 3.0m -

Midas 7.0% 3.4 MPa 2.7m 3.0m -

Deep Post 6.75% 4.8 MPa 4.9m 4.3m 28-day cure

Miekle Sth

Barrick

7.0% 5.5 MPa 4.6m – 6.1m 4.6m -

Gold Fields - AU 10% 4.45 MPa 5.0m 5.0m -

Cortez Hill 7.8% 6.0 MPa

6.0m – 11.0m

6m drives, 11m

intersections.

4.6m

28-day cure.

Maximum size 2 inches

With reference to Table 1, the design comments column relates to individual operational choices

that are not related to ‘standard’ design metrics (i.e. strength, sill dimensions and undercut span).

This column would not constitute general design requirements for but reflect site specific

practices.

Underhand exposure design charts for CRF (Stone, 1993) and more generally for consolidated

backfill (Pakalnis, 2014) imply that a design Factor of Safety (FOS) of 2 is common. The complete

underhand cut and fill database (Pakalnis, 2014) identifies three operations using paste or

hydraulic fill that use a FOS of 1.5.

Mining One adopted a FOS of 3 when calculating the design requirements for Terramin’s

consolidated backfill sill pillars. This higher than usual value was chosen based on using CRF,

as well as the man access requirement for underhand cut and fill mining. Mining One note that a

FOS of 3 may be considered conservative but can be modified to reflect and align with Terramin’s

acceptable level of risk or the site-specific consolidated backfill characteristics.

Underhand exposures (regardless of backfill selection) can be completed in instances

where the design, placement and testing results justify stable conditions in accordance

with Terramin’s operating requirements. If the consolidated backfill does not meet these

requirements, the underhand exposure must be modified to reduce the risks to personnel.

QA/QC practices will be a crucial consideration of the design, selection of fill material,

mixing, delivery and placement of backfill.

In order to provide an indication of the processes required to manage risks associated with

underhand cut and fill, a preliminary procedure is provided Appendix A. This is not an

implementable procedure but intended to outline the key steps and persons responsible for each

stage of the mining sill pillar design and exposure.

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2 WATER COLLECTING ON TOP OF A CEMENTED BACKFILL SILL PILLAR

Before addressing “The potential for a hazard to be created as a result of water collecting on top

of a cemented backfill sill pillar”, this notion carries with it some assumptions. For this hazard to

occur, it is implied that the consolidated backfill is impermeable barrier. It also implies that the

rock/consolidated fill interface and the surface support (e.g. shotcrete) are also impermeable.

While it is possible for mined development (e.g. sumps, dams, abandoned underground

workings) to accumulate and hold significant amounts of water, in these instances the

excavations are isolated and surrounded by undamaged rock. In other practical analogues,

excess water from paste or hydraulic backfill can leak through the country rock into nearby

development, implying that in some instances, the country rock has a capacity to discharge

accumulated water to nearby open excavations. In Mining One’s opinion, while the possibility of

water pooling on top of a cemented sill is plausible, given the specific application (underhand

exposure of a cemented sill pillar), the likelihood of this occurring to a significant capacity (i.e.

several meters of pooled water) would be considered possible, but low.

CRF (i.e. Terramin’s preferred consolidated backfill) can have high porosity, depending on the

fines content, grading curve and the degree of compaction. Porosity calculations by Shrestha et

al. (2008) report CRF porosity of between 17 and 29 percent from their calculations. If the insitu

CRF is highly porous, it would not be expected to prevent water flow. Some examples showing

the variable and porous nature of CRF are shown in Figure 1.

Figure 1 A) Insitu CRF (Lingga, 2018). B) Large scale CRF sample with moderate porosity.

C) High quality CRF with low porosity (Warren et al. 2018). D-F large scale CRF

samples with varying porosity, G,H 240mm and 300mm diameter CRF samples

showing varying porosity (from Cordova, Saw & Villaescusa, 2016).

Given the preliminary nature of Bird-in-Hand Gold Project this project, it is difficult to define the

characteristics the CRF will have. Mining One understand that Terramin’s intent is to not use

crushed and screened run of mine waste. Given this understanding, Mining One would expect

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Terramin’s CRF to be porous and much less likely to be able to hold water. In Mining One’s

opinion, the “hazard to be created as a result of water collecting on top of a cemented backfill sill

pillar” should be separated into two distinctly different hazards:

Inundation of water into active working areas

This hazard refers to the potential for large amounts of pooled water suddenly entering active

areas. Mining One believe this hazard can be managed by Terramin being able to identify,

monitor and mitigate water pooling on top of sill pillars. Mining One has provided several practical

solutions of how this could be achieved:

‘Weep holes’ in sill pillars to dissipate pooled water;

Pumping and/or drainage systems for historical and backfilled areas;

Mine to minimise the possibility of water pooling in production areas (e.g. infrastructure

placement, development gradients);

Checking the water balances for older levels and sump systems to identify the location of

possibly pooled water; and

Ongoing Hydrogeological modelling to refine the estimated water make for active and inactive

areas of the mine.

The selection and practicality of these solutions could be detailed by Terramin in an inundation

management plan. This management plan could outline Terramin’s proposed processes for

identifying, mitigating, monitoring and managing the risks associated with water inundation.

Failure of sill pillars caused by unanticipated loading from water

Another hazard reduction method is to consider the added load of pooled water in the design

each sill pillar. This may be completed by estimating the percentage of voids in a given volume

of consolidated and unconsolidated backfill (i.e. the porosity) and assuming all voids are filled

with water. The sill pillar strength and dimension requirements can then be determined using

typical limit equilibrium equations. This will ensure the load capacity of the water is accounted for

as part of each sill pillar design.

Another possible inclusion of the effects of water pooling on top of a sill pillar is to consider the

pore pressure in the sill pillar as part of the design. When unconsolidated backfill is placed in a

void, the interlocking rock fragments and frictional forces tend to transfer vertical forces

horizontally. Within the backfill a stabilised rock arch forms which reduces the vertical loads

acting on the sill pillar (Caceres, 2005). By Ignoring this stabilising arching effects within the sill

pillar design, a justifiable appreciation of the effects of pore pressures within the unconsolidated

fill can be included. Alternatively, advanced numerical methods could be used to explicitly

consider the effects of water as well as other degrees of complexity not included in empirical or

analytical methods.

Empirical methods form the basis of the sill pillar design methodology and can account for water

loading. Empirical methods may be replaced with numerical methods, or site-specific design

methods during mine operations. As mentioned previously, Mining One have used a conservative

FOS for the design of consolidated backfill sill pillars. This high FOS can also accommodate

some degree of uncertainty associated with loads such as pooled water. Once empirical data for

the site specific CRF mix characteristics are known (e.g. the strength and the ability to hold

water), the FOS may be modified to reflect and align with Terramin’s acceptable level of risk or

the site-specific consolidated backfill characteristics.

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3 REFERENCES

Bijelac, M & Roache, B 2017. ‘Bird-in-Hang Gold Project Geotechnical Assessment’, Mining One

Consultants, Document Number 2241_G/4819 v4.

Caceres, CA 2005. ‘Effect of delayed backfill on open stope mining methods’, Masters Thesis,

University of British Columbia, Vancouver, Canada.

Cordova, M, Saw, H & Villaescusa, E 2016. ‘Laboratory Testing of Cemented Rock Fill for Open

Stope Support’, In Proceedings of 7th International Conference & Echibition on Mass Mining,

Sydney, New South Wales, Australia 9-11 May, 2016.

Hughes, PB 2014. ‘Design Guidelines: Underhand Cut and Fill Cemented Paste Backfill Sill

Beams’, PhD Thesis, The University of British Columbia, Vancouver, Canada.

Lingga, BA 2018. ‘Investigation of Cemented Rockfill Properties Used at a Canadian Diamond

Mine’, Masters Thesis, University of Alberta, Alberta, Canada.

Pakalnis, R 2014. ‘Empirical Design Methods – Update (2014). In Proceedings of 1st

International Conference on Applied Empirical Design Methods in Mining. Lima, Peru June, 2014.

Seymour, JB, Martin, LA, Raffaldi, MJ, Warren, SN & Sandbak, LA 2019. ‘Long-Term Stability of

a 13.7 x 30.5m (45 x 100-ft) Undercut Span Beneath Cemented Rockfill at the Turquoise Ridge

Mine, Nevada’, Rock Mechanics and Rock Engineering, Vol. 52, pp. 4907-4923.

https://doi.org/10.1007/s00603-019-01802-y.

Shrestha, BK, Tannant, DD, Proskin, S, Renison, J & Greer, S 2008. ‘Properties of cemented

rockfill used in an open pit mine’. Proceedings of GeoEdmonton 2008: The 61st Canadian

geotechnical conference and 9. joint CGS/IAH-CNC groundwater conference: a heritage of

innovation. Canada: N. p., 2008.

Stone, DMR 1993. ‘The Optimization of Mix Designs for Cemented Rockfill’, In Proceedings of

Minefill 93, The Southern African Institute of Mining and Metallurgy, Johannesburg, pp. 249-253.

Warren, SN, Sandbak, LA, Seymour, J & Raffaldi, M 2018. ‘Estimating the Unconfined

Compressive Strength (UCS) of Emplaced Cemented Rockfill (CRF) from QA/QC Cylinder

Strengths’, In Proceedings of Society for Mining and Metallurgy and Exploration Engineering

(SME) Annual Meeting, Minneapolis, Minnesota, United States of America 25-28 February, 2018.

Yu, TR 1996. Consolidated Rockfill, Course notes presented at Cheng-Kung University, 257 pp.

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APPENDIX A

Processes for Cemented Rock Fill Design and Placement

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This document describes the expected stages throughout the design, placement and underhand

extraction of a cemented rock fill (CRF) sill pillar. Important QA/QC practices are also presented.

Stage One – Design and Signoff

The first stage of the CRF construction is the sill pillar design and signoff. Prior to all further

activities commencing, the relevant engineer (mining or geotechnical engineer) will calculate and

document the planned CRF sill pillar design. At a minimum, the sill pillar thickness, target

strength, curing time, shear support, mix recipe, minimum testing requirements and special

requirements such as blast mats, mesh floors or post grouting tubes should be documented. The

CRF sill pillar design should consider the actual mined geometry, the practical limits of the CRF

construction method and planned underhand exposure span.

Given the preliminary nature of the Bird-in-Hand Gold Project, empirical methods will most likely

form the basis of the sill pillar design methodology. Empirical methods may be replaced with

numerical methods, or site-specific design methods during mine operations.

The design method must sufficiently address the known failure mechanisms and have a

justifiable methodology. A suitably conservative Factor of Safety should be applied to the design

to ensure the safety of all personnel during underhand exposure. The Factor of Safety should be

specified by the operating company, Mine Manager or Site Senior Executive to reflect the

acceptable level of risk. Review and final signoff of the CRF pillar should be completed by a

suitably senior member of staff (i.e. senior engineer or manager) for each sill pillar design.

A fill instruction will then be generated by a suitable technical person (e.g. mining engineer or

surveyor) in order to relay the relevant design information, task sequences and testing

requirements to the mine’s operations department.

Stage Two – Material Approval, Grading and Stockpiling

A stockpile of appropriate backfilling material should be available whenever backfilling is

required. This process is to ensure a constant backfill material supply is available in order to limit

downtime which may result in cold joints in the sill pillar. The definition of ‘appropriate backfilling

material’ will depend of the site-specific mix but will likely be run-of-mine waste material with a

specified grading requirement (i.e. no oversize and a specified fines content). The grading

requirements and managing processes will depend on the site-specific preferences and mix

characteristics, but as a minimum, could utilise a visual inspection from a suitably trained and

competent person to identify appropriate and inappropriate fill.

If material within a stockpile has been deemed inappropriate, it should be clearly marked as

inappropriate and removed from the stockpile. The size of the stockpiled material will depend on

the daily backfill rate and cement availability. Fill lines or delineators can be used within the

material stockpile bay to give a visual indication of the amount of backfill material stockpiled, and

if more is required. Managing backfill stockpiles and fill appropriateness should be the

responsibility of the underground operation’s department, but may be delegated to the shift

supervisor, foreman or other suitably senior staff member. Significant issues for example,

insufficient material or concerns with material quality should be elevated to the technical services

department and relevant operations managers as required.

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Stage Three – Survey Mark-up

The mine surveyor will mark out the planned CRF sill pillar design underground. Survey mark-

up will depend on the CRF sill pillar particulars, but as a minimum will be the sill pillar fill height.

The location of shear support or blast mat heights may also be marked by the mine surveyor.

Stage Four – Preconditioning and Site Preparation

This stage captures the placement of any design elements that are required prior to backfilling.

This stage may include the installation of a mesh floor, shear pins, shear paddles, blast mats (i.e.

a layer of uncemented rock, sand, gravel etc. to reduce blast damage) or post grouting

infrastructure. The required site preparation and install sequence will be specified within the fill

instruction generated during the sill pillar design and sign off. At the completion of this Stage

Four, a technical services or operations representative should then sign off that all requirements

have been completed, and record any variations observed from the original design. Re-work may

be required if the install quality is deemed inappropriate.

Stage Five – Batch Mixing

A loader operator will initially place some of the required backfill material into the mixing bay. The

operator will then place a bund at the entrance to avoid spillage of the cement slurry. The cement

slurry will be mixed at an external or internal batch plant. The slurry will then be delivered to the

underground mixing bay by an agi-truck or transmixer. The agi-truck or transmixer will then be

reversed into the mixing bay and discharge its load. The amount and type of cement slurry

delivered will depend on the mix design specified on the fill instruction and the mixing bay

capacity. A loader will then remove the bund wall and mix the slurry and waste rock together.

The objective for the loader operator is then to coat all the backfill material with the cement slurry.

The time it takes to mix will depend on the operator’s experience, the size of the batch, the size

of the loader and the mix design. Depending on how the mix is progressing additional water may

be required. The maximum amount of additional water that can be added will be specified as part

of the fill instruction.

Stage Six – Fill Placement

Once the CRF has been mixed, fill placement will then proceed using a loader. The backfill will

be placed by tipping up to the required fill height, and then progressively working out over the fill.

Compaction of the CRF will naturally occur from the weight of the loader working on top of the

fill. This method of fill placement is essentially the same methodology as backfilling a stope,

meaning the procedure, relevant safety precautions and key responsibilities will be similar. The

fill placement should be performed continuously as to limit the formation of cold joints. The backfill

operator should report any issues and their location identified during the fill placement. This

information should be elevated to the relevant site engineer and or technical services

department.

Stage Seven – Reporting and Reconciliation

At the completion of each shift, reporting of the shift activities is required. This reporting will be

the number of loads placed, and the total tonnes of cement consumed for the shift. End of month

totals concerning the rock consumption should then be cross checked with cement consumables

to give an indication of the bulk cement consumption and likely bulk insitu cement content.

Significant discrepancies must be identified and followed up on. The sill pillar thickness may be

picked up by the mine surveyor in order to capture the as-built sill pillar thickness.

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Stage Eight – Underhand Exposure Design Signoff

Post filling assessment is required to ensure that the insitu CRF sill pillar is appropriate for the

planned underhand exposure. The underhand exposure requires final review to ensure that the

design requirements have been met. The underhand exposure design methodology should

mirror the initial design but should treated more as justifying that the requirements for safe

underhand exposure are present in the sill pillar. If the insitu sill pillar conditions cannot be

sufficiently justified, the underhand exposure design will need to be modified or abandoned to

reduce the risks to personnel. A similar signoff from a senior member of staff should be required

for all underhand exposures.

Stage Nine – QA/QC and Laboratory Testing

While specified as Stage Nine, QA/QC processes will be completed throughout the entire backfill

process. Due to the large size of aggregate, the massive volume of fill materials, and the specific

placement sites in underground openings, the quality control for consolidated rockfill relies a

great deal on the attentiveness of the operators. Quality control and evaluation of consolidated

rockfill can be performed at each stage of handling:

Stage One – Design and Signoff

Internal peer review and sign off procedures;

Design guidelines and routine assessment methodologies;

External or internal review of design practices;

Iterative design philosophy based on site specific performance and case histories.

Stage Two – Material Approval, Grading and Stockpiling

Ensuring appropriate material grading;

Ensuring the cleanliness of backfill material;

Monitoring moisture content of the backfill material;

Accounting for attrition of aggregate from re-handling (mainly associated with passes);

Stockpile auditing and discarding of inferior backfilling material.

Stage Three – Survey Mark-up

Fill instruction based on signoff process;

Inspections by trained and competent person;

Stage Four – Preconditioning and Site Preparation

Fill instruction based on signoff process;

Inspections by trained and competent person;

Re-work as required;

Signoff upon completion.

Stage Five – Batch Mixing

Slurry sample testing;

Water testing / water appropriateness testing;

Slump testing for workability;

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CRF laboratory testing;

Regular supervision by technical representative;

Operator feedback;

End of shift reporting for material physicals with monthly reconciliation.

Stage Six – Fill Placement

CRF Laboratory testing;

Insitu sampling;

Regular supervision by technical representative;

Operator feedback on placement activities;

End of shift reporting for material physicals with monthly reconciliation.

Stage Seven – Reporting and Reconciliation

Daily reporting, monthly reconciliation of physicals;

Reviewing significant deviations and discrepancies.

Stage Eight – Underhand Exposure Signoff

Internal peer review and sign off procedures;

Design guidelines and routine assessment methodologies;

External or internal review of design practices;

Re-evaluation based on reconciled data and laboratory testing;

Post consolidation (if required and economic);

Design modifications as required; and

Abandonment of lift if deemed appropriate.

Laboratory testing may be obtained from either specifically prepared samples, constructed during

the mixing and placing stages. Alternatively, after the CRF has been placed, core samples may

be taken to measure the insitu response considering compaction. Alternative methods such as

using a pressuremeter may aid in estimating the insitu strength by calculating the CRF stiffness,

which can then be related to the compressive strength.

Depending on the laboratory testing sample dimension, considerations of the scale effects need

to be included. Based on the information presented by Yu (1995), the mean insitu fill strength

should be about 66% of a 150mm diameter laboratory sample, 90% of a 300mm diameter sample

and approximately the same as a 460mm diameter sample.

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DOCUMENT INFORMATION

Status Final

Version Final

Print Date 11/08/2020

Author(s) Aidan Ford

Reviewed By Ben Roache

Pathname P:\2784_G Terramin Cemented Rock Fill Clarifications for SA

Gov\WPO\6330_Final.docx

File Name 6330_Final2

Job No 2784_G

Distribution PDF emailed to client for comment

DOCUMENT CHANGE CONTROL

Version Description of changes/amendments Recipient Author (s) Date

1 Draft version for comment T. Mehrtens A. Ford 29/07/20

2 Minor clarifications requested by client

added

T. Mehrtens A. Ford 03/08/20

DOCUMENT REVIEW AND SIGN OFF

Version Reviewer Position Signature Date

1 B. Roache Geotechnical Manager

28/07/2020