Bench Blast Modeling

8/14/2019 Bench Blast Modeling

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ABSTRACT

The ability to accurately model the movement of gradeboundaries during a typical bench blast would be of significant benefit to mine operators. The movement of grade boundaries during the blasting process may havesignificant impact on grade recovery if not accounted for.The paper describes the preliminary stages of a researchproject which combines numerical blast models andconventional mine planning software to allow practicaluse of the results of a numerical blast simulation. A twodimensional bench blast is simulated using the UniversalDistinct Element Code (UDEC). The results of thesimulation are used to provide input to the Surpac mineplanning package which can be used to generate gradecontrol plans.

INTRODUCTION

Computer simulation of open pit bench blasting

continues to present a significant challenge to the miningindustry. Several models have been developed whichallow some visualization of burden movement as a resultof a typical open pit bench blast, (Chung et al. 1994;Minchinton and Lynch 1996; Scott et al. 1996; Jorgensonand Chung 1987). In general these codes are not widelyavailable and in some cases their distribution is limitedfor competitive reasons. Since 1993 the MiningEngineering Department at the Mackay School of Mineshas been investigating the use of off the shelf numericalmodeling software for use in bench blast modeling.Previous work (Gilbride 1995) demonstrates that it ispossible to model bench blast movement to some degree

using codes such as Itascas Universal Distinct ElementCode, UDEC. The blast model used in this paper islargely based on work by Gilbride. In associated work,the possibility of measuring and predicting grademovement in a bench blast has been investigated byseveral workers, (Zhang 1994; Harris 1997).

Grade Control

For many mining and explosives companies, theultimate goal of blast modeling research is the ability to

design a new blast and view the results in terms of burdenmovement and fragmentation without the need for fieldtrials. In addition to fragmentation and heave, the abilityto model blast movement will allow the engineer topredict the movement of grade boundaries and othergeologic features such as rock structure. For the majorityof open pit gold mines in the Western United States, thereis no geologic distinction between ore and waste rock.Blast hole sampling is used to determine grade boundariesprior to a given pattern being fired. As a result, gradecontrol practice includes attempts to minimize rock movement during the blast, allowing pre blast gradeboundaries to be used as dig limits on the resultingmuckpile. This paper describes a preliminary attempt touse a mine planning software suite, Surpac, in transferringblast model data into usable mine planning information.

UDEC MODEL

A blast model (Figure 1.) has been constructed usingUDEC. The model represents a single hole bench blast in

two dimensions and is based on blast designs typical tothe Nevada gold mining industry. Table 1 belowdescribes relevant blast geometry as applied in the model.

Property ValueBench Height 7 mBurden 3 mBlast Hole Diameter 170 mmStemming Length 2 mSub Drill 1 mTable 1. Blast geometry and rock properties.

Two joint sets are included in the model, the first

dipping at 15o

into the bench and the second dipping at45 o towards the free face. The joints are considered to becohesion less with a friction angle of 20 o, the blocks arenon deformable and the joint stiffness values are specifiedat 1000 GPa, in order to prevent block penetrations whichcause the model to crash.

The UDEC model is capable of modeling both theshock wave and gas pressure effects of the blast on therock mass. Incorporating gas pressurization and hence a

Bench Blast Modeling Using Numerical Simulation and Mine PlanningSoftware

I.R. FirthD.L. Taylor

Mackay School of Mines, University of Nevada, Reno. U.S.A.


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fluid flow algorithm in the code requires significantlylonger run times for the model.

Figure 1. Basic UDEC blast model.

For the purposes of the paper, an internal stressboundary was applied to the blast hole walls, with nosubsequent gas pressure loading. The boundary conditionmagnitude decays over time, approximating thedissipation of explosive energy throughout the rock massas the blast progresses.

Blasthole Pressure

A Blasthole pressure of 100 MPa has been used in themodel. Discussion of blast hole pressure in the literaturesuggests a wide range of values, often depending onindividual model requirements. Preece and Knudsen1991 suggest a value of 10 GPa as the maximum

explosion pressure for ANFO, while Potyondy et al.,1996, use a maximum pressure of 130 MPa.

It is noted that the current model in its described statewill not infact completely model the movement of abench blast to the point at which the rock blocks arecompletely at rest. The run time required for such amodel, when incorporating a gas flow algorithm, wouldbe measured in terms of days or weeks, even using amoderately powerful desktop PC. There are alsoconcerns over the numerical stability of UDEC whenblock separations become significant relative to the model(Gilbride 1995).

SURPAC INTEGRATION

General Approach

The next step in the process is to integrate the resultsfrom the UDEC model into Surpac, an integrated minedesign package produced by Surpac SoftwareInternational Pty Ltd. Redcliffe, Western Australia. Thegeneral approach is to take a file of the coordinates of theUDEC elements and generate closed strings in Surpac.

The strings can then be classified by reference to theirlocation in a block model database, allowing correlationwith properties such as rock type, grade, ownership etc.These properties can then be tracked as the blastprogresses.

The UDEC model is run for a specific number of timesteps, to allow significant block movements to occur. The

model is then halted and, using standard instructionsembedded in the UDEC command language, the locationof the centroid of each block in the model is written to atext file, the location of the corners of each block in themodel are written to a separate text file. This processcontinues, generating these files at predeterminedrepresentative time steps throughout the blast.

After the UDEC simulation has been completed, thecentroid and corner files are processed through an Excelspreadsheet using Visual Basic macros to produce commadelimited files for input to Surpac. Macros written forSurpac then process these files into Surpac string files,

with each block from the original simulation generating aclosed string. At this point the string files can beassociated with attributes from a block model of thedeposit, for visualization of the movement of criticalcomponents through the blast.

Results

For this study a simple classification was used tosimulate a stratified gold deposit, the strata planes runningparallel to the 15 o dipping joints. The deposit is separatedinto three zones: the upper zone represents a low grade,heap leach ore, the middle zone a high-grade, mill oreand the lowest zone is assumed to be waste. A typicalapproach to mining these two zones might be to use twoshort benches to avoid mixing of the ores and therebysignificantly increasing mining costs or, alternatively, theplan might be to just use an average grade and either shipdiluted ore to the mill or high grade to the heaps, eitherincreasing the processing costs or decreasing recovery.In any case, the two horizontal ore zones can causeproblems for the ore control engineer.

The following four figures show the movement of thebedded strata through the simulated blast. Figure 2.shows the in-situ material prior to detonation. Figure 5.shows the material after 10,000 cycles of the UDEC

model. While this last figure is not the final resting placeof the blasted material (due to time constraints in theUDEC model), it does indicate a horizontal segregation of the low-grade and high-grade ores in the resulting muck pile. If this were in fact verifiable, it would allow the twozones to be loaded separately for different processing andpossibly avoiding some of the problems inherent inhorizontal zones.

Figures such as these, can allow the mine planner totrack the location of specific portions of the rock mass


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through the blast to their final resting location. With thisinformation, accurate dig maps can be generated andgrade estimates of the loaded material can be made. Withthis information available, blasts can be designed on thebasis of efficiency of rock fragmentation, rather thanminimization of blast movement. This will allow forimproved overall fragmentation and hence lower total unitcost of mineral produced.

Figure 2. Blast model prior to detonation.

Figure 3. Initial burden movement after 2,000 cycles.

Figure 4. Continuing burden movement, 4,000 cycles.

Figure 5. Burden movement at 10,000 cycles.

DISCUSSION

It is noted that there exist various numerical blastmodels which may be more appropriate as the modelingplatform for this work. UDEC has been used in the firstinstance in order to demonstrate the concept. The use atwo dimensional blast model would not be appropriate forpractical application. Itasca produce two distinct elementcodes which operate in a three dimensional environment.Bearing in mind previous comments regarding the runtime required for accurate modeling using a twodimensional version of UDEC, it unlikely that the 3Dversion would improve this aspect of the work. Threedimensional codes are significantly more expensive thantheir 2D counterparts.

Orica Explosives, in conjunction with Sandia NationalLaboratories, has developed a 2D distinct element code

called DMC (Distinct Motion Code). It is reported thatthis code is capable of running a full blast simulation inminutes using standard desktop computer hardware.

A common theme of both UDEC, DMC and othercodes is their ability to produce visual results. If theresults in terms of rock block locations are to be used asinput to other software, such as Surpac, this capability isin many ways redundant. The visual results of thesimulation, while interesting, are irrelevant when theblock centroid locations can simply be used as data to befed into a block model.

Assuming that a three dimensional blast model can bedeveloped which will give accurate results in reasonabletime, there will be the need to validate the model in someway. This raises the question of blast movementmeasurement, as distinct from modeling. There iscurrently no proven method for accurately measuring themovement of ore boundaries, specific geologic units orthe intermixing of burden from multiple row shots.Several workers have attempted use of bags, pipes, chalk,dyes and other similar methods. In general, thesemethods provide limited information and can be highly


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disruptive to the drilling and blasting operations on amine. The Mackay School of Mines has conductedseveral studies in this area (Zhang 1994, Harris 1997).

Current blasting practices in mines at which gradecontrol is considered an overriding consideration tend tobe at odds with conventional theory in terms of maximizing diggability and fragmentation, while

minimizing the total cost per ounce. Choke or bufferblasting, short inter-row timing and short benches arepossible in easy breaking conditions, such ashydrothermally altered gold deposits, but these methodscan cause significant operational difficulty in morecompetent rock masses.

CONCLUSIONS

The use of a three dimensional numerical model andintegration with as drilled assay data would allowaccurate grade boundaries to be determined from blastmodeling data. Blasting practices may then be altered to

allow for more efficient use of explosive energy, whilemaintaining accurate grade control during the loadingcycle.

REFERENCES

Chung, S. McGill, M. Preece, DS. (1994) Computer CastBlast Modeling. Proceedings of the 20 th AnnualConference on Explosives and Blasting Technique.Society of Explosives Engineers, Austin, Texas. Preprint.10 pages.

Gilbride, LJ. (1995) Blast Induced Rock MovementModeling for Bench Blasting in Nevada Open Pit Mines.Master of Science Thesis. Mackay School of Mines,University of Nevada, Reno. 285 pages.

Harris, GW. (1997) Measurement of Blast Induced Rock Movement in Surface Mines Using Magnetic Geophysics.Master of Science Thesis. Mackay School of Mines,University of Nevada, Reno. 251 pages.

Jorgenson, GK. Chung, SH. (1987) Blast SimulationSurface and Underground with the SABREX model. CIMBulletin, Vol. 80, No. 904, August, 37-41.

Minchinton, A. Lynch, PM. (1996) Fragmentation and

Heave Modeling using a Coupled Discrete Element GasFlow Code. Proceedings of the 5 th InternationalSymposium on Rock Fragmentation by Blasting.Mohanty, B (Ed) Montreal, Quebec, Canada. 71-80.

Potyondy, DO. Cundall, PA. Sarracino, RS. (1996)Modeling of Shock and Gas Driven Fractures Induced byBlasting Using Bonded Assemblies of Spherical Particles.Proceedings of the 5 th International Symposium on Rock

Fragmentation by Blasting. Mohanty, B (Ed) Montreal,Quebec, Canada. 55-62.

Preece, DS. Knudsen, SD. (1991) Blast Induced Rock Motion Modeling, Including Gas Pressure Effects.Proceedings of the 24 th U.S. Oil Shale Symposium,Colorado School of Mines Quarterly. Vol. 83. No. 4. 13-19.

Scott, A. (Ed) Cocker, A. Djordjevic, N. Higgins, M. LaRosa, D. Sarma, KS. Wedmair, R. (1996). Open Pit BlastDesign-Analysis and Optimization. Julius KruttschnittMineral Research Center, University of Queensland. 338pages.

Zhang, SZ. (1994) Rock Movement due to Blasting andits Impact on Ore Grade Control in Nevada Open Pit GoldMines. Master of Science Thesis. Mackay School of Mines, University of Nevada, Reno. 155 pages.

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