Marvel Loch – Core and Shell Mass Blast

Marvel Loch – Core and Shell Mass Blast Joseph Hoang, St Barbara Ltd, Michael B Lovitt, Orica Mining Services, Services, Scott Wishart, St Barbara Ltd & Nick Reed, St Barbara Ltd ABSTRACT Marvel Loch Underground forms part of St Barbara Limited’s Southern Cross Operations which is located 360km (224 miles) east east of Perth. Southern Cross Operations produces approximately 120,000 ounces of gold gold each year. Mining occurs in over 8 lodes using a variety of stoping methods including Uphole Benching, Long Hole Open Stoping, and Core and Shell. The core and shell mining method allows the Marvel Loch mine to recover the pillars left after initial stopes have been mined with open stoping powder factors and thereby minimising the mining costs. This method has been employed on the Exhibition Pipe lode, the focus of this paper, and has been in use since it was first implemented in December 2008. This paper reviews the blast design and charging technical challenges that were solved to bring about the successful firing that occurred on 8 December 2010, which utilised ~121t (133 ton) of Ammonium Nitrate Emulsion and 1,857 i-kon detonators to break ~390,000t (430,000 ton) in-situ material. These include 80m long blast holes, special blasting techniques used for commencement of the critical crown blast area and changes to void availability requiring re-sequencing of the blast initiation design. The software used to model this blast was pivotal in the ability to control the vibration levels achieved and being able to quickly re-sequence the blast and output logging sheets to facilitate the electronic initiation. The Orica operations team enabled the project charging task with minimal disruption and the dual charging units allowed peak charging placement of 23 tonnes (25.4 ton) of explosives placement in a 24 hour period.

INTRODUCTION St Barbara is an Australian gold producer and mineral explorer listed on the Australian Stock Exchange. St Barbara's key assets include its Leonora and Southern Cross Operations, both of which are located in Western Australia. It has Mineral Resources of 7.6 million ounces of gold including Ore Reserves of 2.8 million ounces of gold as at 30 June 2011, as well as an extensive landholding comprising granted tenements and tenement applications of approximately 4,600 square kilometres (1776 sq miles).

Copyright © 2012 International Society of Explosives Engineers 2012G - Marvel Loch - Core and Shell Mass Blast

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The Southern Cross Operations comprise the Marvel Loch 2.2 Mtpa (2.43 M ton pa) processing facility, the Marvel Loch Underground mine and a number of development opportunities. The Southern Cross Operations have produced a minimum of 120,000 ounces for each of the last four years. The Marvel Loch underground mine is the foundation of St Barbara's Southern Cross operations with gold mineralisation extending over 1.3 km strike length and has been identified to depths of over 700 th metres (2,300 feet) below the surface. On the 8 December 2010, ~121t (133 ton) of Ammonium Nitrate Emulsion and 1,857 electronic detonators was charged to break ~390,000t (430,000 ton) in-situ material in the Exhibition pipe. Marvel Loch has never attempted a mass blast to this magnitude, since it was first trialed in 2008 with the 465-558 Exhibition Pipe stope. Since 2008, this has accepted has been the normal method used at the culmination of the stoping cycle in certain mining lodes.

Figure 1 Marvel Loch position inside Australia


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Exhibition Pipe Mining Method The Exhibition Pipe lode employs a ‘Core and Shell’ mining method; where a mixture of Up Hole Benching and Sub Level Open Stoping mining (core) is utilized, until a mass firing recovers the remaining sill and rib pillars of ore (shell). Dry rock fill is introduced from above to support the ore cavity left behind and so the ore fired in the mass blast process is extracted underneath this rock fill. Geotechnical issues associated with large open stopes are minimized and 100% of the ore is extracted using open stoping powder factors, which are the lowest with drill and blast mining methodology. The potential problems that this method presents are the stability of the pillars of the Shell after the “Cores” have been extracted, the risks associated with the mass blast of the shell and the dilution / recovery of the shell after the mass blast has been fired. This method has been used at many mines before and references have been included. The process commences at the 365 level with Panel A drilled and fired as the initial undercut. Rings drilled both up and down on the 417 level are fired into the undercut void below. This core panel is mined from selected drawpoints at the 365 level. The process repeats for Panels B and C in this order towards the start of the ore drive.

465 FWDR

4

417 ODR

4

Panel C

3

4

Panel B

2

4

Panel A

1

365 ODR

365 FWDR

365 DP8

365 DP1

Figure 2 Schematic of sequencing order, annotating the production sequence: #1 Panel A, #2 Panel B, #3 Panel C and # 4 Rib and Sill pillar


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With the core panels mined (blue in figure 2), the remaining sill and rib pillars (red in figure 2) are drilled from the 465 level and at selected drawpoints at the 365 level. Firing the rib and sill pillars is achieved in a single firing with the end result a Sub Level Cave (SLC) draw method from all eight drawpoints. The backfill and some of the unrecoverable ore from the above (Exhibition 465-558) stope will form an overlying Blended ore/waste blanket and be drawn through to the 365 draw points during the bogging of the stope.

Ore Draw Control The production rings inside all drawpoints (both core and shell) at the 365 level are drilled and fired strategically to create a tapered draw profile creating the undercut. The toes of each hole are given a selected charge length and fired with the rib and sill pillars. After the mass blast the extraction of the ore is controlled to minimize the mixing, and therefore the dilution, of the waste rock following the ore down. Once all draw points are declared waste or ‘closed’, the next remnant production ring in the drawpoint is fired and bogged in accordance to a SLC grade sampling procedure. Firing one ring at a time, allows optimal recovery of the draw bin ore material and the overlying ore that could not be drawn through to the initial drawpoint brow locations. Figure 3 illustrates the profile of material in the SLC arrangement and potential material flow characteristics.

465mRL 465 FWD

465 FWD

465 FWD

465 FWD

417mRL

445 RL

365mRL

Figure 3 SLC Material flow through to 365 drawpoints after firing the pillars


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Drilling Metres Down Under (MDU) were employed to complete all drilling for the sill and pillars and V30 raisebores used in the slots of each core panel. Two ‘In-The-Hole-Hammer’ (ITH) Cubex drill rigs were utilized at the 465 footwall drive adjacent to the crown level. Hole diameter ranged from 115mm and 1 140mm with top holes on each ring drilled at 140mm (5 / 2 inch) to create a “crater” effect when fired. Toe distance between holes were drilled from 3.5m (11.5 feet) to 5.5m (18 feet), with burden distance between rings at 3.0m (9.8 feet) for rib pillars and 3.5m (11.5 feet) for the sill pillars. MDU began th nd drilling on the 12 July and finished by 2 December to complete 27,060m (88,800 feet) of drilling, on an average daily rate of 140m (459 feet) for both rigs combined. Core panels were drilled with a single Tamrock Solo Drill, with all hole diameters drilled at 89mm (3 ½ inch). Toe distance between holes were spaced at 3.0m (11.5 feet) with burden between rings at 2.4m (7.8 feet). There was approximately 32,200m (105,643 feet) in the core panels and 14,300m (46,916 feet) in the rib pillars drilled, on an average daily rate of 280m (919 feet). Exh 465 Level

Exh 465 Level

445 RL

445 RL

Exh 417 Level

Exh 417 Level

401 RL

401 RL

Exh 365 FWD

Exh 365 FWD Exh 365 Level

Exh 365 Level

Figure 4 Drilling between levels using both ITH and Solo Drill. Left d iagram shows rings design for a sill and right diagram show ring design for rib pillar

Problems Encountered With Drilling The drilling process took several months to complete. Whilst drilling, failures occurred at the crown pillar above Panel A and impacted on the void area available to move the blasted rock into. The


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continuing concern with failures of the remaining crown pillars made the scheduled firing date even more critical. Close monitoring of the remaining crown pillars was needed to ensure that sufficient void would be available to allow for a 30% swell factor of rock once fragmented. Failure to allow enough void could leave the extremities of the blast frozen, thereby increasing collateral damage and limiting extraction. Another problem experienced was drill deviation within the rib pillar holes as they extended past 40m (131 feet). Some holes in the ribs extended to almost 80m (262 feet) which the in-the-hole hammer rigs were quite capable of. Holes were surveyed using downhole surveying tools and this showed holes deviating but in a controlled way trending towards the right and dipped down. This created the issue of certain holes prematurely breaking into the 417 level or panel stopes. Similarly, due to the deviation, parts of the pillar could not be drilled with sufficient hole density and were left unfired. This would increase chances of oversize material reporting to the drawpoints and requiring secondary breakage.

Charging The charge design for the entire rib and sill pillars was planned by Orica, with specialized cratering charges needed to blast the sill pillars. The initial holes to break the crown pillars are at a non ideal angle to break the base of the pillar effectively and so cratering charges utilizing 140mm (5 ½ inch) holes were used in a Vertical Crater Retreat style to enable future crown holes to blast effectively. The charging event utilized 2 Hypercharge units with 8 member operations team and support from the Orica product technical specialists.

Figure 5 Orica Operators charging holes


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The entire rib and sill pillars had taken six days to charge, using approximately 121 tonnes (133 ton) of ammonium nitrate emulsion and 1,857 electronic detonators. Scheduling of the charging units to ensure that they could charge in the shortest time was carried out to minimize the loading time. The firing of the blast at the earliest time was critical to the stability of the region and for the ongoing production of the mine. Most activities during the charging operation were centered around this blast and minimising collateral damage from air blast and so many tasks were around securing ventilation doors and services.

Figure 6 Blast site immediately prior to firing showing hookup condition

Initiation Timing This blast utilized i-kon™ electronic blasting system to fire the mass blast. The electronic detonator system provided high accuracy delay timings and a large degree of flexibility. The blast was timed in such a way that rib pillars A, B & C would be fired in two separate directions directly into the existing voids. The timing was complex as slots had to be developed at the base of the ribs to allow the draw points to be established for the successful extraction of the blasted tonnes from this culmination of the blasting events for a mining block. Once the draw points had been established the base of the ribs were fired into the stope voids. Then the crown pillar had to be breached above panel B. This is where the cratering charges were located. Once the crown has been breached this allows an avenue for the waste from above the crown to access the void and so the remainder of the crown and ribs have to be fired as quickly as possible to fill the void before the waste can move into it. It was quite likely that the last parts of this blast would be over confined due to the lack of void effect. Panel A crown failed prematurely, filling the void required to blast rib pillar A. This resulted in the need adjust initiation sequence; this was a simple matter using underground software, Shotplus™ U/G. The Management team was confident there was sufficient void space remaining for firing to go ahead. Pillar A was timed to fire into the Panel B void, instead of Panel A (the original plan). Rib pillars B and C are


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fired into the Panel B and C respectively and remaining sill or crown pillars are timed to fire in a ‘lateral downward’ direction after the rib pillars are fired. This computer aided design re-sequenced the blast quickly and the program allows for optimisation of the charge versus time window envelope. This minimizes the blast duration and thereby blast hole dislocation, while maximising the fragmentation effect without incurring high vibration.

Figure 7 Blast Optimisation process using Shotplus TM-UG

Result Blast vibration monitoring was carried out in two locations on the surface. Vibration levels were well within environmental limits with a maximum detected Peak Particle Velocity (PPV) of 3.61mm/s (0.142 inches/s). The vibration record shows good correlation to the charge design with minimal quiet periods and fairly even vibration throughout the blast. The high charge weights in the middle of the blast relate to increasing the blasting intensity at the culmination of the central rib pillar. It can be seen that as these charges have minimal connection to the stope walls at detonation the corresponding vibration does not increase. This allows for better fragmentation and reduced oversize. The increase in vibration in the latter stages of the blast would be due to the lack void and increased intensity of the blasting, but


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generally it is well controlled. More detailed review of blasting took place but there were not significant findings to include here.

Figure 8 Vibration Record (blue) versus Charge Design kg per 15ms window(dotted red)

St Barbara and Orica personnel inspected the levels underground (after a 12hr re-entry delay) and encountered little damage to the 465 level and the 365 extraction level. The time required time for rehabilitation works was minimal and production bogging commenced soon after the blast. Initial bogging comments from operators were positive as fine blasted material at all drawpoints was reported. To this date, the underground operation continues to mine ore from the Exhibition Pipe with 2,000 2500 tonnes (2200 - 2750 ton) hauled daily.

CONCLUSIONS The combined efforts of St Barbara, Barminco, MDU and Orica Technical Services ensured the blast was successfully fired on schedule on the 8th December 2010. The high level of planning from the start of the project and the continuous commitment between client and contractor was crucial in making the mass pillar blast a success. These mass blasts are a critical part of the Marvel Loch production schedule and failure in the successful firing and extraction of this mass blast would compromise the viability of this operation. The electronic blasting systems available now allow precision blast design. Computer aided design is the key to being able to unlock the benefits and allow very fast redesign of initiation sequencing. Present


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software allows the optimisation of the initiation sequencing to minimize both high intensity blasting and lulls in the blast. Both of these have the potential for reducing the effectiveness of the design. The first increases peak vibration levels which can manifest itself in damage and therefore dilution or addition cost of returning to work. The second allows dislocation of future firing blast holes, lowering the frequency of vibrations, extending the blasting event and allowing rock to fall off prior to being blasted.

Figure 9 One of the many draw points after the blast showing minimal damage

This project also shows the viability of short term project teams, available through sub-contractors, to achieve a large goal whilst not adding to the capital and ongoing production costs of the project.

ACKNOWLEDGEMENTS Special thanks to St Barbara for approval of this paper and the technical services teams for their support on the project to achieving a great result. To Barminco who provided labour and assistance wherever it was needed. To MDU on the tremendous effort in completing the drilling on time, safely. Finally, an immense appreciation and gratitude to Orica for their technical consultation of the project from start to finish.

REFERENCES 1.

ECKERMANN, D (2011) ‘MARVEL LOCH UNDERGROUND MASS BLAST’, CASE STUDY

2.

GOOGLE MAPS 2011 FOR FIGURE 1

3.

HULME ET AL, IMPROVING BLASTING PRACTICES AT MT CHARLOTTE MINE THROUGH VIBRATION MEASUREMENT AND ANALYSIS, 2ND INTERNATIONAL SYMPOSIUM ON ROCK FRAGMENTATION BY

4.

BLASTING, KEYSTONE 1987

LOVITT, M, MASS BLASTING AT KCGM PTY LTD MT CHARLOTTE OPERATIONS BLASTING, UNDERGROUND BLASTING WORKSHOP, 1992

5.

WWW.STBARBARA.COM.AU


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Marvel Loch – Core and Shell Mass Blast

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