Blasting a Cavern Using Bulk Emulsion Explosives

S E Rodgers1

INTRODUCTION

The Elgas Underground Storage Facility at Port Botany, Sydney,is the first purpose-built cavern of its kind in Australia and allowslarge capacity, safe and environmentally benign storage ofLiquefied Petroleum Gas (LPG). The facility provides Elgas withthe flexibility to source product at competitive prices and willensure long-term security of supply. Construction has providedOrica Australia, formerly ICI Explosives, with a uniqueopportunity to provide a bulk explosive delivery systemspecifically designed to meet the exacting demands of the site(see Figure 1) and of the project.

THE CAVERNS

The storage facility, (see Figure 2) is located 130 m undergroundand will ultimately provide the capacity to store 65 000 tonnes ofLPG. Pressure exerted by ground water will contain the LPG inthe unlined rock cavern, with water saturation of the rock massbeing maintained through water curtain galleries 15 m above themain caverns. These galleries are 4 m wide by 3.5 m high, withthe gas storage caverns consisting of four 230 m long, 14 mwide, 11 m high galleries.

Construction for Elgas was being undertaken by the ConcreteConstruction Group and ran over three years, utilising threeshifts working around the clock. Including the two shafts, thetotal volume of rock excavated was 149 000 m3. Total explosivesusage was 600 tonnes.

PROJECT CONSTRAINTS

The unusual nature of the project was reflected in the tenderspecifications which placed considerable constraints onoperations. These included:

• A requirement for extremely smooth blasting which wasessential to the project.

• The rate of excavation required for the completion of thecaverns.

• The proximity to the site of three glass-lined cryogenicgas-storage tanks for which a maximum surface vibration of5 mm/s was acceptable. All blasts were designed to producevibration of less than 2 mm/s at nominated surface structures,allowing only a small margin for error. Highly accuratecharging was required as part of that blast design.

• The considerable importance of limiting overbreak.Perimeter holes were to be charged with a low energyproduct and blasting in accordance with the principles statedby Holmberg (Persson, Holmberg and Lee, 1993) applied tothe blast design of ‘buffer’ holes, (those adjacent to theperimeter) to limit blast damage.

• Wet conditions were expected in all areas, and reliabledetonation of course was required under these conditions.

• Transport and storage of explosives were very constrainedbecause the site is close to dense industrial and residentialdevelopment. Numerous deliveries of explosives per weekwould have been required if packaged explosives were to beused.

Explo ‘99 Kalgoorlie, WA, 7 - 11 November 1999 65

1. Senior Development Officer, Orica Explosives, PO Box 196, KurriKurri NSW 2327.

FIG 1 - An aerial view of the construction site, bounded by the container terminal on the right, Orica Chemicals’ glass-lined tanks, visible in theforeground and the Van Ommeron storage facility on the left. The airport runway is visible at the top of the photo.

THE BENEFITS OF BULK EMULSION SYSTEMS

ICI/Orica has been involved in the project since the tenderingprocess. The PowerbulkTM Drive bulk explosive system offeredby Orica was chosen for the project by the contractor to ensurethat all the project constraints listed above could be met. Thesystem offers the following advantages:• A minimal inventory of explosives maintained on-site. The

emulsion base, supplied by Orica via the Prospect QuarryServices plant, is a Class 5.1 oxidiser and is sensitised at theface during charging.

• The ability to vary energy in different regions of the blast.The use of different sensitising solutions allows products ofdifferent density to be manufactured at the face duringcharging.

• Tight control over the quantity loaded into each hole, throughthe use of a BlastrakTM controller which regulates pumpoperation.

• A wet-hole product which produces low levels ofpost-detonation fumes.

• Substantially lower levels of nitrate contamination ingroundwater compared to ANFO, with the ability to collectand quarantine product should there be spillage.

• Orica’s commitment to safety and quality. Being involvedearly in the project allowed the company’s knowledge of thehazards of handling bulk emulsion explosives to be availableto Concrete Constructions before work commenced on-site.As an ISO 9000 accredited company, every step in theproduct manufacture and delivery process is subject toproduct quality checks and QC records are kept for eachcharge-up.

POWERBULKTM DRIVE

The product supplied by Orica for the Elgas project isPowerbulkTM Drive, which was originally developed for theCheung Ching tunnel project in Hong Kong (Ngai et al, 1995;Kennedy, 1995). Some of the technical properties of the productare given in Table 1. Use of the product with the equipment

described below allowed the product to fulfil its potential andindeed extended the scope of its application to effectively meetthe project’s requirements.

PowerbulkTM Drive is manufactured from two non-explosiveraw materials. The primary component is an emulsion basealready widely used in surface operations, EP Gold DC. Theemulsion is manufactured at Liddell in the Hunter Valley bymixing a hot liquor of oxidising salts with fuel oil. Emulsifyingagents are added to stabilise the mixture, so that small droplets ofsolution are surrounded by an external fuel phase which gives theproduct its excellent water resistance.

An explosive product is formed when the emulsion is mixedwith a sensitising solution which reacts with components of theemulsion to form gas bubbles. These bubbles allow thepropagation of a detonation by heating rapidly when subjected toa shock wave, for example, from the detonation of a primer orbooster. The gas bubbles must be small and finely dispersedthrough the matrix for the product to be sensitised; this onlyoccurs at the end of the charging hose when the gasser solution isdistributed through the emulsion as it is pumped at speed throughspecially designed mixing orifices.

The sensitising solution also contains catalysts to speed thereaction and ensure that gassing times are acceptable and do notdelay blasting. As well as making the product sensitive todetonation, gas bubbles lower the density of the product andreduce the detonation velocity and the pressure generated bydetonation. A different sensitising solution is required for eachproduct density required but the emulsion base is common.

ORICA’S COMMITMENT

For this project, Orica designed and manufactured the followingspecialised equipment:• five Dangerous Goods Code approved 2 T containers;• BlastrakTM controller and battery packs;• a 650 kg 15 kg/minute ‘mini-pump’ unit (UG23; Figure 3);

and• a 2.5 T 30 kg/minute ‘maxi-pump’ unit, (UG24).

66 Kalgoorlie, WA, 7 - 11 November 1999 Explo ‘99

S E RODGERS

FIG 2 - The Elgas Underground Storage Facility.

Both pump units use mine (compressed) air to drive adouble-acting piston pump which delivers the emulsion. Theproduct pump is mechanically linked to the gasser pump whichdelivers a precise dosage of the gasser solution. The productpump’s design has been proven through extensive testing to beintrinsically safe under all failure conditions including externalfire, deadheading and dry running.

The diameter of the access shaft and the size of the opening ofthe stage in the operations shaft dictated the dimensions ofUG23, which was designed for the shorter advance and smallerexcavations in the water curtain galleries. The unit was raisedunder the kibble before every blast until there was sufficientroom underground to prevent damage.

UG24 is fitted with two maxi-pumps and three gasser solutiontanks, enabling three different density products to be delivered. Achange in density requires only selection of a different solution,although the quantity of product in the hose needs to beconsidered. Each product pump is capable of delivering up to 80kg/minute but the delivery rate was limited to enable smoothretraction of the loading hose in the small diameter holes. UG24is also fitted with a mini-pump for the flushing of the loadinghoses with water after charge-up to minimise product wastage.

CONTROL AND INFORMATION SYSTEM

BlastrakTM is an integrated control and information systemwithout which the pumping units cannot be operated. Poweredby a separate battery unit, the BlastrakTM ‘lunchbox’ records allpumping operations including hose priming, hole loading, QCchecks, calibration and hose flushing. Data can be downloaded toa PC to give a complete written record of the charge-up process,including the exact quantity loaded per hole and summary datasuch as the total pumping time and product used.

The entire system is housed in a rugged yellow case and has alarge backlit graphical LCD display which provides criticalcharging and pumping information for the pump technician. Amembrane keypad provides ‘one-touch’ control of manyfunctions with simple menu driven operation. In addition, asimplified radio remote control device has been developed withlarge GO and STOP buttons. In regular operation, the quantity ofproduct to be loaded per hole for a particular hole type (forexample buffer hole or lifter) is input by an Orica technician asspecified by the blast design. When operated, either by thehose-handler with the remote or using the ‘lunchbox’, theBlastrakTM unit simply counts down until that quantity is reachedand then stops the pump.

CHARGING

Both the maxi- and mini-pump units require only one Oricatechnician who oversees pumping and product quality, takingsamples to ensure that the gassing process is achieving thecorrect product density in an acceptable time. Each pump’soperation is controlled separately by a different member of theCCG charge-crew at the face via the remote start/stop unit(Figure 4). As the hole is filled with product, the hose is gentlypushed out of the hole, ensuring the hole is completely charged,there being no discontinuities in the product and good contactbetween the product and the primer. The pump will stop wellbefore the hole is filled, as allowance has been made for thevolume the product will expand with gassing and also for thedesired collar. The CCG charge-crew is also responsible forpreparing the face, priming the holes and firing.

During the course of the project, the charging cycle has beencharacterised when PowerbulkTM Drive has been used. Table 2shows the activities associated with charging a cavern face ofapproximately 100 holes, (3 m advance and 480 kg of emulsion).

BLASTING

As excavation of the water galleries progressed, concerns as tosurface vibration were allayed as the recorded levels were farlower than anticipated. However, the desire for perimeter controlremained, requiring a still lower charge density in the perimeter.

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BLASTING A CAVERN USING BULK EMULSION EXPLOSIVES

Density1 (g/cm3) 0.80 0.90 1.00 1.10 1.20

REE2 - Weight Strength (Relative to ANFO = 100% @ 0.8 g/cm3) 69% 76% 83% 89% 96%

REE2 - Bulk Strength (Relative to ANFO = 100% @ 0.8 g/cm3) 69% 86% 103% 123% 143%

REE2 - Bulk Strength (Relative to ANFO = 100% @ 0.95 g/cm3) 53% 65% 78% 93% 109%

Calculated VOD3 (km/s) 4.5 4.9 5.3 5.7 6.1

Unconfined VoD in 45 mm diameter (km/s) 3.8 3.9 4.2 4.3 4.4

1. Nominal Density only.

2. The ‘Relative Effective Energy’ (REE) of an explosive is the energy calculated to be available to do effective blasting work and is calculated using theOrica IDeX computer code.

3. VOD depends on conditions of use including confinement and blasthole diameter.

TABLE 1The technical properties of PowerbulkTM Drive.

FIG 3 - The mini-pump unit, UG23, including the radio remote, theBlastrak ‘lunchbox’ and the battery pack.

The gasser tanks are visible at left.

This was met by Orica in a very simple way - the charging withemulsion of small diameter plastic tubing placed inside the blasthole. This introduces an air gap between the product and theblasthole and eliminates the effect on the detonation ofconfinement provided by the rock mass.

Loading of decoupled charges combines the advantages ofbulk products, including speed of charging, competitive price andease of use, with the advantages of a low energy packagedproduct designed for cautious blasting such as Powergel

Trimex. Technical data for decoupled bulk charges for varioushole and diameters are given in Table 3.

MEASUREMENT

Initial blasting on-site with bulk products was accompanied byintensive assessment of the process by Orica technical personnelin addition to the standard QC measures conducted by Orica’soperators. This yielded several important results, including:• a difference in the actual hole volume, which when measured

directly, was found to be larger than the nominal drill bitdiameter would suggest;

• that cup density samples obtained from the end of theproduct hose are consistently lower in density than productdensities in-hole measured by collar lengths. This had beenpredicted from measurements of gassed bulk products usedin stoping but was able to be confirmed during developmentcharging at Botany; and

• that the process controlled delivery quantity and final in-holedensity extremely well, with the standard deviation in finalcollars being less than 20 cm.

These were important results in meeting Orica’s commitmentto quality and also in adding to our understanding of the gassingprocess and the product/blasthole interaction.

CONCLUSION

The Elgas Underground Storage Facility is a challenging,exciting and innovative project which will significantly improvethe state of supply of LPG in Australia and which has requiredmuch of those involved with its design and construction. Theproject has also shown the advantages of all parties workingclosely with each other and external authorities at all stages fromtendering through implementation.

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S E RODGERS

Activity Time (minutes)

Unit moved to face and air lines connected 10

Product hose primed 2

Initial cup sample taken to ensure process withinspecification

5

Loading face (40 holes) 15

Cup sample taken 3

Gasser changeover 2

Cup sample taken1 3

Loading completed (remaining 60 holes chargedand product hose flushed)

25

Unit moved to safe location for blasting 10

Gassing sufficiently complete to allowing firing2 0-20

Total ~80

1. A cup sample is required to be taken for each new gasserconcentration used, as well as at the beginning of the charge-up.

2. The time required for the product to gas out depends on producttemperature. As a guide, gassing will be complete in less than 30minutes at 30°C. Product temperatures were maintained above thislevel by rapid turnover of emulsion or through the use of heat tracingon the hoppers of the pump units.

TABLE 2The activities associated with charging.

Density (g/cc) Decoupled ChargeDiameter (mm)

REE in-hole ofdecoupled charge inminimum diameter

(relative to fully coupledANFO@.95 g/cc) in 45 mm

0.80 28 18%

0.80 35 29%

1.00 25 20%

1.00 28 25%

1.00 35 40%

cf Trimex

1.10 19 19%

Note: At the time of writing this article, the use of PowerbulkTM Drive inPVC tubes as well as Powergel Trimex has been discontinued andShearcordTM detonating cord is used in the perimeter.

TABLE 3The energy output of decoupled charges.

FIG 4 - Loading perimeter holes in the lower half of a cavern with UG24;(a water hose is visible to the left of the operator).

Although the Sydney LPG Cavern is unique in Australia in itsstringent requirements for perimeter control achieved throughcontrolled product delivery and performance, the benefits oflimiting blast damage would be significant for many othermining projects when the total cost of D&B, including groundsupport, re-work and explosive costs is considered. Havingestablished a successful loading system for this projectdemonstrates that Orica can deliver this and other benefits ofbulk emulsion technology to mine development and civilengineering tunnelling applications.

ACKNOWLEDGEMENTS

Orica Explosives would like to thank Concrete ConstructionsGroup and Elgas for their permission to publish this article. Thegenerous assistance of Paul Kohout of Jpak Blast Consult PtyLtd, is gratefully appreciated by the author. Thanks are also dueto colleagues at Orica who have been involved in this project.

Powerbulk and BLASTRAK are trademarks of Orica AustraliaPty Ltd. Powergel and Trimex are registered trademarks of OricaAustralia Pty Ltd.

REFERENCES

Persson, P, Holmberg, R and Lee, Jaimin, 1993. Rock Blasting andExplosive Engineering, (CRC: London).

Ngai, Weyman, Scovira, C T, Scott, D and Kennedy, B J, 1997.Chemically Sensitised Bulk Emsulsions and Pump Systems used inHong Kong Underground Civil Works, paper presented at ISEE 23rdAnnual Conference on Explosives and Blasting Technique, LasVegas, 2-5 February.

Kennedy, Brian, 1995. New Explosive Mini-Pump System for HongKong, Explosives Notes, Issue No 10, November.

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