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AusRock 2014: Third Australasian Ground Control in Mining Conference

Paper Number: 29

Pillar Recovery Adjacent to Stabilised Rockfill at the Ballarat Gold Project

B L Sainsbury1, D P Sainsbury2, J Western3, P E Petrie4 and V Mutton5

1. Senior Geotechnical Engineer Castlemaine Goldfields Limited 10 Woolshed Gully Drive, Mt Clear, Victoria, Australia. bsainsbury@cgt.net.au 2. Principal Geotechnical Engineer Mining One Consultants Level 9, 50 Market Street, Melbourne, Victoria, Australia. dsainsbury@miningone.com.au 3. Underground Project Co-ordinator Castlemaine Goldfields Limited 10 Woolshed Gully Drive, Mt Clear, Victoria, Australia. jwestern@cgt.net.au 4. Senior Mining Engineer Castlemaine Goldfields Limited 10 Woolshed Gully Drive, Mt Clear, Victoria, Australia. ppetrie@cgt.net.au 5. Senior Technical Services Engineer Orica PO Box 554, Nowra, NSW, Australia. verne.mutton@orica.com

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ABSTRACT

Remnant rib pillars at Castlemaine Goldfields Ltd.’s Ballarat Gold Project have been sterilised by the

placement of loose rockfill in production stopes. The application of selectively injecting low viscosity grout

into the backfilled stopes to form a stabilised material has enabled recovery of the previously sterilised

pillars. This paper outlines the engineering design, quality control and quality assurance program

implemented to ensure the safe and efficient recovery of the rib pillars. The successful program included

laboratory testing, numerical exposure stability modelling and in situ monitoring.

INTRODUCTION

At the Ballarat Gold Project, resources have been sterilised within the Llanberris Compartment due to the

placement of loose rockfill adjacent to rib pillars. Potential exists for the recovery of these high grade pillars

through injecting low viscosity cement grout into the loose rockfill to form artificial Stabilised Rockfill (SRF)

adjacent to the rib pillars. A similar pillar recovery technique has been used to recover sill pillars at the

Crusader Mine in Western Australia (Sainsbury et. al., 2007) and Cracow Mine in Queensland (Potts et. al.,

2012).

BACKGROUND

The Ballarat Gold Project is located in the city of Ballarat, approximately 100 km due west of Melbourne,

Victoria, Australia. A combination of stoping methods are used to recover narrow vein gold mineralisation

(generally 2-5m width) with an annual production target of 50,000 oz. The mining operation extends

approximately 4.7 km north of the portal reaching a maximum depth of 695 metres beneath the city of

Ballarat, as illustrated in Figure 1.

FIG 1 Ballarat Gold Project – existing underground workings beneath existing surface infrastructure

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Typical ore zones consist of a stockwork of quartz veining or massive quartz lobes hosted within inter-

bedded sandstone, siltstone and shale sediments. The quartz can be highly fractured, particularly

surrounding major geological structures, and can resemble sugar cubes in intensely fractured zones. Gold

resources have been defined within the Victoria, Llanberris, Sovereign and Britannia compartments and

these will form the principal mining fronts through 2014/2015.

Development of the Llanberris Decline commenced in 2011 at approximately the 550m Level. The LLB638

and LLB648 Levels were first accessed during 2012. Initial stopes on the LLB648 Level were designed

using a modified Avoca (or continuous fill) stoping method. Empirical estimates based on Mathews Stability

Chart (Mathews, 1981) determined a 21m panel length for the initial void, with subsequent firings of

approximately 6m. However, during development of the initial void, when the open span was 11m, significant

hangingwall failure occurred to within 2m of a permanent vent drive access (LLB638VD) – as shown in

Figure 2. The stope was immediately backfilled with loose rockfill and mining continued along strike with a

modified span length of 13m and installation of cablebolts in the hangingwall. Vertical rib pillars

(approximately 3-5m width) were left in the stoping horizon to provide hangingwall stability.

FIG 2 Llanberris as-built mining geometry on the LLB638 and LLB48 Levels

Reconciliation of the grades and tonnes post mining of the stoping panel suggested that a significant

resource was sterilised within the rib pillars. As a result, a conceptual plan was developed to recover the

pillars by injecting low viscosity cement grout into the loose rockfill to form artificial SRF pillars that would

provide support to the loose rockfill during extraction of the pillar material.

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STABILISED ROCKFILL

FB200 High Yield Grout

Stabilised rockfill is produced by injecting a low viscosity cementitous material into waste rock to produce an

exposable backfill material. FB200 has been developed by Orica (formerly Minova Australia Pty. Ltd. and

Fosroc Chemfix Pty. Ltd.) for the backfilling of voids around underground structures and the consolidation of

broken strata. It is primarily used in the coal mining industry for ventilation control. FB200 cement grout is

based on a calcium sulphoaluminate cement which forms the mineral ettringite (CaO.Al2O3.SO3.32H2O) on

reaction with water. The material can be placed at a water / powder ratio of between 2:1 and 4:1 to produce

a dimensionally stable cement grout, unlike General Purpose (GP) Portland cement which would exhibit

excessive shrinkage at the same ratios.

Sainsbury et al (2003) have previously used FB200 to stabilise rockfill to enable the extraction of crown

pillars at the Crusader Mine, Australia. Figure 3 illustrates a stable 5m wide horizontal (overhead) exposure

of SRF after sill pillar recovery.

FIG 3 View from beneath stabilised rockfill sill pillar at the Crusader Mine (after Sainsbury et al 2003)

At the Ballarat Gold Project, vertical rather than horizontal SRF exposures are planned during extraction of

the remnant rib pillars.

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Laboratory Characterisation of Stabilised Rockfill

To minimise the technical risks associated with the construction of artificial FB200 stabilised rockfill rib pillars,

a basic series of unconfined compressive strength (UCS) laboratory tests were conducted to investigate the

strength and behaviour of the stabilised rockfill matrix. Test results completed for the Ballarat Gold Project

were compared to previous laboratory testing completed by Sainsbury, Cai and Thompson (2001) whereby

the influence of sample size and shape, particle size distribution, curing time, water quality, cement : water

ratio and confinement were considered.

The laboratory results for stabilised rockfill at the Ballarat Gold Project are consistent with results previously

published by Sainsbury et al (2001), as illustrated in Figure 4a. The optimum UCS obtained in the laboratory

using FB200 cement grout is 4.45 MPa with a strength of 2 MPa achieved after 14 days curing. The full

stress-strain response of Ballarat Gold Project SRF is illustrated in Figure 4b. During the early stages of

loading, cracking of the cement bonds was clearly audible. With increased load, the sample entered the

brittle/ductile transition phase where the rockfill particles were observed to rotate and interlock. In the post-

peak phase, the interlocking particles were observed to begin crumbling in a controlled manner while the

sample maintained approximately 80% of its peak load.

FIG 4 (a) Compiled strength testing of FB200 stabilised rockfill (b) Stress-Strain behavior of Ballarat Gold Project stabilised rockfill

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NUMERICAL ANALYSIS OF STABILISED ROCKFILL EXPSOURE STABILITY

In order to determine the minimum required width of the artificial stabilised rockfill pillars, development of a

numerical modelling methodology was essential in understanding the behaviour and failure mechanisms

associated with the vertical exposures after extraction of the remnant rib pillars. The artificial stabilised

rockfill pillars were required to be of sufficient strength to minimise dilution to the adjacent pillars and prevent

the inrush of uncemented backfill and water in previously mined stopes. They must also be strong enough to

provide acceptable ground conditions for driving over during backfilling operations.

3D Polyhedral Distinct Element Method (3DEC)

A three-dimensional polyhedral distinct element modelling method (3DEC) was used to represent the in situ

behaviour of the stabilised rockfill exposures at the Ballarat Gold Project. This modelling approach allowed a

detailed representation of the filling process resulting in the most accurate initial stress distribution. In

addition, based on the ability to represent varying particle shapes and sizes, an accurate representation of

the initial rockfill porosity can be achieved, along with the resulting friction, interlocking and dilatational

response when mobilised. The surrounding rockmass can also be simulated as a strain-softening material

that conforms to as-built wireframes.

FIG 5 (a) Typical size and fragmentation of waste rockfill at Ballarat Gold Project. (b) Simulated rockfill material in 3DEC

A series of simulated UCS and triaxial loading experiments were conducted in 3DEC to calibrate the block

and contact material properties to match the stress-strain response recorded in the laboratory. Figure 6

illustrates the calibrated (blue) and laboratory (green) stress-strain response for the 2 MPa Ballarat Gold

Project SRF material.

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FIG 6 Calibrated and observed SRF material response

The material properties used to calibrate the SRF response are presented in Table 1. In order to simulate the

inherent variability caused by the simple sump mixing system, the contact cohesion and tensile strength has

been simulated with a normal distribution.

TABLE 1 Calibrated large-scale 3DEC block and contact properties

Simulated UCS (kPa)

Block Properties Contact Properties

E (GPa) kn (GPa/m) ks (GPa/m) c (kPa) (deg.) t (kPa)

2020 50 0.2 80 40 1500 +/- 100 25 750 +/- 25

A series of large-scale (pillar-size) numerical investigations were completed to determine (a) the minimum

required stabilised pillar width to provide stability during extraction of the pillars and (b) provide guidance

regarding the engineering risks associated with product penetration issues.

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Figure 7 provides a view of the single pillar model that was constructed. Filling of the two adjacent stopes

was simulated by placing the loose rockfill in 2m layers to ensure representative internal stress distribution

within the fill mass. To simulate the delayed injection and hydration of FB200, the contact cohesion and

tensile strength was set to zero during the filling process and later modified to the calibrated strength

response.

FIG 7 3DEC Model Geometry

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Figure 8 presents the response of a series of analyses conducted to assess the performance of the vertical

SRF exposures after remnant rib pillar recovery. As expected, without any SRF, inrush of the loose rockfill is

expected to occur during extraction of the pillar. A 3m skin of SRF material on either side of the pillar is

sufficient to retain the loose rockfill and remain stable after pillar recovery. Due to the low viscosity nature of

the FB200 grout it is not possible to create the SRF in such a precise manner and simulations of poor FB200

penetration were also conducted (not shown here). The as-built SRF pillar geometry (discussed in

subsequent sections) were also simulated and were shown to retain the loose rockfill and remain stable after

pillar recovery.

FIG 8 Simulated response of SRF vertical exposure stability during remnant rib pillar extraction

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IMPLEMENTATION OF ARTIFICIAL STABILISED ROCKFILL PILLARS

Development

The engineering design for the pillar extraction included the development of a parallel drive on the lower

LLB648 extraction level and crosscutting over to the two pillar locations. Development of the cross-cuts

required break-through into and advance through the original LLB648NOD that had been filled with loose

rock fill. A total of 90m of waste development; 49m in the parallel extraction drive and 41m between three

cross-cut drives was completed over approximately one month. The underground development completed

as part of the pillar recovery project is presented in Figure 9.

Upon breakthrough to the existing lower LLB648NOD, the waste rockfill material was left in place to minimise

disturbance to the hangingwall and subsidence up on the LLB638 Level. This material would be stabilised

and later bogged from the drive prior to the pillar production blast. The hangingwall was cable bolted from

the lower LLB648 crosscut extraction drive and LLB638 VD to provide additional hangingwall stability during

bogging and backfilling operations. A plan and section view of the development completed is provided in

Figure 9.

FIG 9 Development of the parallel drive and cross-cuts on the LLB648 Level

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Drilling and Placement of Injection Holes

Injection holes for the placement of FB200 product were designed from the upper LLB638VD. Assuming a

lateral flow of 1m for the grout, an injection hole ring spacing of 2m was determined. Rings for each of the

stopes were offset by 1m from the edge of the remnant rib pillars. A total of two injection hole rings were

designed on each side of the rib pillars. The injection hole ring location and typical injection section plan is

provided in Figure 10. Each of holes in the ring was staggered to ensure any blockages would not cause

major penetration issues.

FIG 10 Plan and typical section view through stope/s showing the injection hole placement

In addition to the FB200 injection holes, monitoring holes were also designed (as shown in Figure 10) to

provide additional information regarding the penetration of product within the stope. These would also

provide additional fill points during the pumping operations if required.

Track-mounted diamond drill rigs were used to drill 76mm (NQ2) holes through the hangingwall and loose

rockfill material in the stopes. In order to maintain the stability of the hole prior to placement of the injection

pipe, the hole was drilled without the inner tube. Once the hole reached the required depth the bit was

cleared using a spear attached to a wireline. The injection pipe was fed inside the drill string, and the drill

string then retracted over it, leaving the injection pipe in the hole. 1.5m stickup was left on each of the

injection pipes to couple to the outlet hose from the pump/hopper set up. A total of 900m was drilled

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between each of the panels to be stabilised at an average rate of 18m/shift. Figure 11 provides some

photographs of the installation process of the injection holes.

FIG 11 Injection pipe installation

The injection pipe consisted of 40mm outside diameter polypropylene pipe with holes drilled at 250mm

intervals. The holes formed the exit points for the grout. A modified drill press was used to streamline the

pipe perforation process. Barbs were cut into the end of the pipe to hold it in place while the drill rods were

retracted. Due to the large variance in the final as-built geometry of the stopes, each of the injection holes

and pipes were individually designed and labelled.

Seal Placement

Due to the low viscosity of the cement grout required for the penetration of the rockfill void spaces, a two

stage grout curtain was designed to prevent the flow of product beyond the backfilled stope. First, fibrecrete

was sprayed over the exposed waste backfill exposed in the LLB648NOD crosscuts. Tekseal (a product from

Orica) was then pumped into the lowest injection pipes in each injection ring to provide an initial

seal/retardant at the base of the intended grout curtain. Some product leakage was observed through the

fibrecrete during the pumping process however this was minimised through the use of fast setting grout that

was applied to rags and held over the leakage points for approximately 30 seconds. It was found that the

FB200 set as long as its movement was retarded for a short period of time.

Pumping of FB200

A 3 tonne capacity steel hopper was used to feed 1.2 tonne bulka-bags of FB200 product to the twin mixing-

pumping units. A Volvo L60 wheel loader with a jib was used to position the bag above the hopper and an

internal spike within the hopper perforated the bags reducing dust and the requirement for operators to work

at heights. Two electric powered placer pumps were used to mix and pump the grout via 40mm polypipe to

the injection holes. FB200 requires a short specific interval between mixing and placement to allow complete

hydration and mixing. Consistency in the time period was achieved by pumping the grout through 100m of

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40mm polypipe prior to placement. Pumping rates averaged 10-11 bulk-bags/shift with one pump operating

and 15-18 bulka-bags/shift with two pumps operating simultaneously.

FIG 12 FB200 pump set-up

Quality Assurance / Quality Control

FB200 quantities were calculated for each of the injection holes based on as-built CMS pick-ups of each of

the stopes and an estimated average porosity of 30% within the backfilled stopes. For each ring and hole,

the number of bulka bags to be injected was specified along with the expected height of the FB200 after the

completion of pumping. Two adjacent rings were pumped alternately with a maximum of 3 bags being

placed into an injection hole at any one time. By limiting the amount of product that was pumped into

individual holes without a break, layers of FB200 could build up within the stope through a short-term curing

process. The filling strategy also included pumping into the lowest hole on each ring and gradually working

up the ring – filling up the stope. If the holes filled prior to the full bag limit being placed, these left-over bags

were added to the target amount in the overlying hole. During the filling activities each of the injection holes

and monitoring holes were dipped to ensure that product placement was being achieved at the

right/expected locations. Based on these measurements, a FB200 fill profile could be established. At the

completion of each shift, the fill profile was reassessed and the fill priority re-established.

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Development Engineering Controls

Significant investment in the underground development commitment and purchase of product required an

aggressive schedule of works to be completed. Approximately 3 months were scheduled to complete the

development, stabilisation measures (grout injection) and production drilling. Some of the engineering

controls put in place to minimise the engineering risk associated with each of these elements are

summarised in Table 1.

TABLE 1 Engineering Risk Matrix

Risk Engineering Control

Development of the parallel extraction drive and cross-cuts through poor quality rock mass.

Geological logging around the Llanberris 638/648 Levels provides RQD’s 10-20.

Development designed at minimum size 4.5m x 4.5m and stepped out west to take advantage of

slightly better ground conditions.

Instability of permanent mine infrastructure and over-break / dilution during pillar extraction.

Previous stoping in adjacent panels resulted in significant hangingwall over-break.

Hangingwall cables installed from top and bottom accesses.

Numerical modelling completed to understand the final extraction subsidence profile.

Inrush of loose rockfill at the LLB648 drawpoint from the adjacent stopes if the stabilisation

measures don’t work.

Remote operations used to bog the pillar and during pillar backfilling operations.

FB200 penetration through the stope backfill. A level of redundancy built into the injection hole design.

QA/QC program implemented. Frequent monitoring of FB200 fill levels in all injection and

monitoring holes.

Potential for water in adjacent stopes causing inrush during bogging, restricting penetration of

FB200 product and/or increasing water content/decreasing stabilised rockfill strength.

Dewatering activities completed from both the LLB638 and LLB648 Levels to minimize water

accumulating in the stopes.

Remote operations used to bog the extraction level.

Accurate and effective placement of grout curtain in the base of the stope to restrict flow of FB200.

A fibrecrete seal placed on the backfill rill pile at each of the cross cuts to provide secondary backup

to the primary Tekseal seal placement.

Keeping the injection holes open for grout tube placement. Previous production drilling in the area proved to be challenging with cave-ins common.

Diamond drill rigs (rather than percussion) used to restrict damage to hangingwall rockmass.

Drill casing retracted over the top of the grout injection tube that was placed down the centre after

hole reached target depth.

Poly pipe modified on end to grip into backfill and resist movement during casing retraction.

Durability of the poly pipe (i.e., crushing and deformation of the pipe during placement).

High strength PN 12.5 40mm polypipe used.

Production Schedule Slip A specific underground project co-ordinator was enlisted to co-ordinate tech services and operations and maintain control of the schedule and logistics.

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PILLAR EXTRACTION

Post pumping and stabilisation, the fibrecrete and backfill rill piles in the LLB648 NOD / Crosscut

intersections were bogged revealing the first exposures of SRF. Inspection of the vertical stabilised rockfill

exposures within the LLB648 crosscut provided good confidence in the performance of the SRF prior to the

pillar production blast. Within the exposure of the stabilised rockfill, the FB200 could be seen in layers and

minimal dilution was observed during production drilling. A view of the initial vertical SRF exposure in the

cross cut immediately in front of Pillar 1 is provided in Figure 13.

FIG 13 Initial exposure of SRF within the original LLB648 Level during bogging of the cross-cuts and exposure of the pillars

A cavity survey of the remnant pillar void conducted after mucking of the ore confirmed minor hangingwall

overbreak but no yielding of the SRF exposures. The pillar hangingwall overbreak was of a similar extent to

the original stopes on either side of the remnant pillars. Figure 14 provides a view of the vertical SRF

exposure after extraction of Pillar 1A from the LLB638 Level.

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FIG 14 SRF exposure after extraction of Pillar 1A

After the cavity survey was completed the void and associated crosscut were immediately filled with loose

rockfill prior to extraction of next pillar.

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RECONCILIATION

Reconciliation of the grade and tonnes recovered through each of the pillars concluded that significant

economic benefit was gained by implementation of stabilised rockfill at the Ballarat Gold Project. In addition

to the 100% recovery of the planned ounces, the hangingwall over-break contained estimated mineralisation

that was only identified during drilling of the grout injection holes. A summary of the design and as-built

tonnes and grade for each of the remnant rib pillars are provided in Table 2.

TABLE 2 Design versus reconciled grade and tonnes for each of the pillars extracted

grade g/t tonnes ounces

Pillar 1A Design 37.01 1011 1320

Pillar 1A Reconciled 20.97 2210 1635

Pillar 1 Design 13.54 900 430

Pillar 1 Reconciled 8.7 2347 720

CONCLUSION

Two remnant rib pillars were formed between backfilled stopes in the Llanberris LLB648 Level. The

extraction of such rib pillars has traditionally resulted in significant dilution and loss of ore. Investigation of

the geomechanical criteria for the safe and efficient extraction adjacent to stabilised rockfill has led to the

successful extraction of two rib pillars. Reconciliation of both of the pillars provides recovery of 125% and

170% for Pillar 1A and Pillar 1 respectively. Laboratory testing of the strength and deformation

characteristics of stabilised rockfill material and the numerical investigation of the failure mechanisms and

behaviour of exposures have solved the technical risks associated with the proposed extraction method.

ACKNOWLEDGEMENT

The authors of this paper would like to thank the personnel at the Ballarat Gold Project; Darren Watkins

(Mining Manager) and Lance Faulkner (General Manager) for their support during the execution of the pillar

recovery project, the technical team for their input and assistance and most importantly the underground

personnel who from the outset were engaged and interested in a successful project outcome.

REFERENCES

Mathews, K.E, Hoek, E., Wyllie, D.C and Stewart, S. 1981. Prediction of stable excavation spans for mining

at depths below 1000m in hard rock. Report No. DSS Serial No.OSQ80-00081, DSS File No.

17SQ.233440-0-9020, 43 p.

Potts, L, Mutton, V and Joyce, D, 2012. Pillar recovery at Cracow gold mine using grouted consolidated

mullock, in Proceedings Narrow Vein Mining 2012 , pp 179-184 (The Australasian Institute of Mining

and Metallurgy: Melbourne).

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Sainsbury, D., Cai, Y. and Thompson, B. 2001. Investigation of the Geomechanical Properties of Stabilised

Rockfill. Minefill 2001: Proc. 7th International Symposium on Mining with Backfill. SME Colorado.

Sainsbury, D., Cai, Y., Hebblewhite, B., Finn, D., Whitford, J. and Berry, M, 2003. Investigation of the

geomechanical criteria for safe and efficient crown pillar extraction beneath stabilised rockfill at the

Crusader Mine. ISRM 2003–Technology roadmap for rock mechanics, South African Institute of

Mining and Metallurgy, 2003.