Provide evidence to evaluate and determine the ...
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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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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.
P:\2784_G\6330_Final.docx
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