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1 INTRODUCTION

In principal, daily draw control in a block caving operationshould be a simple process. An operator is required to seta tonnage target to be mined from each active draw point ona daily (or shift) basis. Draw points which are laggingbehind should have a higher tonnage order, while drawpoints which are overdrawn should have a lower (or zero)order. However, experience has shown that the fullimplementation of a daily draw order system is much morecomplex.

The objectives behind the draw control and draw orderprocess can be divided into two components, long term andshort term. The long term objectives are to mine in such amanner as to meet the long term mine plan, while the shortterm objective is largely focused on dealing withgeotechnical stability and with other practical operatingconsiderations.

This paper presents the methodology and challengeswhich were encountered during the development andimplementation of a daily draw order system. The system iscalled CMS and resides within Gemcom Software’s PC-BCsystem. CMS has been implemented and is operational atthe DOZ mine of PT Freeport Indonesia and is soon to becommissioned at the De Beers Finsch mine in South Africa.

The following overall aspects of the problem arediscussed below:• The planning loop. Long term, daily and shift planning• Data collection and storage• The Catch up concept• Draw control.• Reporting• Some lessons learned

2 LONG TERM, DAILY AND SHIFT TIME SCALES

The objectives of long term plans for block caving aresimilar in many respects to those for other mining methods.

Nevertheless the operational and geomechanicalparticularities of the mining method add several constraintsto the process of planning and scheduling the block cave. Afew relevant aspects of long term planning related to thispaper are the following::• Overall cave shape• Controlled opening and development of draw points• Maximizing NPV to the extent possible subject to the

various geotechnical and other constraints• Generating monthly plans which can be fed to the daily

draw control system

Key functions of the daily draw control are:• Meeting the monthly target.• Adapting the draw rapidly to compensate for localized

over or under-draw situations.• Being responsive to safety considerations (For example:

wet muck draw points).• Being responsive to geotechnical considerations such as

over-stressed draw points.• Being responsive to operational considerations such as

draw belling, tunnel capacities due to secondary blastingand maintaining even daily production.

• At a shift level, to enable dispatch of the LHDs to drawpoints in an orderly manner and to best achieve the dailydraw order, with respect to both draw compliance andtotal tonnage produced.

2.1. From long term to daily draw orderThe key data required for daily draw orders from the long

term plan are as follows:• Total tonnages for the month (separated into total,

production, undercut and development tons)• Tonnage targets for each draw point for the month• Maximum allowable draw rates per draw point per day

day which differs from the draw rate used in the long termschedules and includes availability and utilization of thedraw points

AbstractDaily draw control is an integral component of the cave management at an operating block cave mine. It requires theroutine gathering of actual production data such as tonnages and status per draw point as well as sampling information.This information is required to be stored and processed as well as being made available for long term productionscheduling.Some of the problems which have been encountered and overcome include the following: Allowances for time delaysin receiving daily actual data and the need to generate an order for the next day; the need to plan for the next monthbefore the current month is completed as well as the need for smooth data flow from one month to the next. Solereliance on Excel for such data manipulation is extremely error prone and risky.This paper describes an effective methodology which has been established in the PC-BC and CMS programs. A keycomponent of this system is the effective manipulation of the large amounts of data generated on a daily basis. Asecond important component is the assigning of responsibilities for the effective maintenance of this data in the longterm. The system is fully integrated into a SQL database for added data accessibility and security.The transition between the daily draw order cycle and the keeping track with the long range plans can then beestablished and reconciled. Benefits of the new methodology are discussed.

Combining long term scheduling and daily draw

control for block cave minesTony Diering, Principal Consultant, Gemcom Software International Inc.

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• Total tons mined per draw point to date• Draw point closure strategy which might be linked to a

NPV and/or a geomechanical sequence.• Total maximum tonnage per draw point (referred to as

Best HOD tons)

Note that there are no direct NPV or grade targets sentfrom long term to daily. If the daily draw meets the monthlytarget, then the overall grade and NPV goals of the longterm plan will be met.

The above statement highlights one of the overallstrategies in setting up the CMS system. That is to clearlyidentify and isolate the different responsibilities for theoverall process. We do not want daily operations personneltrying to second guess the financial strategies in the longterm plan, or for the daily draw order to presume to knowhow best to dispatch the LHDs to each draw point etc.

2.2. From daily draw order to shifts.The key data required for each shift is as follows:

• A list of active draw points• A tonnage target for each draw point. This is specified as

buckets• A tonnage tolerance for each draw point (called Extra

tons)• Overall draw point status (as available at the time the

order is generated)• deally, some indication as to the priority for the draw point

3 DATA COLLECTION AND STORAGE

In order to have an efficient system, we need readyaccess to the appropriate information. In order for thesystem to be robust, we need to be able to get by under anumber of conditions where we have insufficient, late orerroneous data. There is little point in telling a draw controloperator that he cannot continue because of a lack of data,when there is a deadline to generate an order. So, we haveto work around these types of exception as best possible.

The data required for routine generation of the daily draworder can be broken into categories shown in Table .

In the first implementation of the CMS system at Freeport,much of the information was stored in Excel sheets. Thesehad the advantage of allowing for easy data entry andvisualization, but obviously lacked the data security of aSQL database. In a project with De Beers Finsch mine, theCMS system was upgraded to utilize the GEMS database(from Gemcom) running on the SQL Server platform.

The current system is much more effective in producingdaily orders as well as management of the transition fromone period to another.

4 CATCH UP PROCESS

The overall strategy is that if the daily draw order canreasonably meet the monthly targets, that the monthlytarget, in turn, will drive the cave shape towards the longterm plan. Usually, this will not be achievable in a single

Table 1: Data requirements for daily draw controlmanagement.

Totals / Other By draw point

Setup Shift details Draw point areasPriority info Report positions

Strategy "rules"User status codesReport format info

Occasional Min buckets/shift Best HODAve. dpt availability Best HOD tons

Panel status

Monthly Total tons targets Target tonsDevelopment tons Past tonsProduction tons PRC limitsNew draw points

Daily Target tons Order tonsControl dates Actual tons

Latest assaysLatest statusDpt priorities

Special order dpts

Storage GEMS Profiles SQL workspace

Figure 1: Example of progress with ten draw pointsillustrating the process of catch up.

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month, but may require several months, depending on theextent of deviation from plan.

The process of steering the cave shape back to plan iscalled "Catch up" This is shown schematically in a 10 drawpoint example representing a cross section (Figure 1). Ingeneral, the longer term constraints on minimum andmaximum draw point tonnages will limit how quickly drawshape can be restored.

Figure 1 illustrates how the initial irregular tonnage profileis smoothed out as quickly as possible as permitted by themaximum tonnage per period constraints. If the maximumtonnage per draw point is reduced (say from 300 per periodto 250 per period), then it takes longer for the smoothingeffect to be completed. This is shown in Figure 2 below.

Figure 2: With reduced maximum tonnages, the speed atwhich catch can occur is reduced.

5 DRAW CONTROL

A methodology has been devised to define the dailytonnage that is to be taken from each draw point, in order tomeet the monthly target. If an objective function can bedefined, then it is possible to use Linear Programmingprincipals to derive a solution.

A rating system has been set up for each draw point."Points" can be allocated to each draw point based on thefollowing:• Extent of over- or under-draw relative to the monthly

target• Number of days idle • The forecast grade of the draw point• Other factors such as whether the draw point is in

drawbell status• Safety or other special operational requirements

By using a points system, it is possible for each operationto assign relative degrees of importance to the abovecomponents. Once the points have been allocated, we canalso assign the minimum and maximum tonnages allowableper draw point in a shift. The maximum tonnage is derivedfrom the overall PRC (Production Rate Curve) for each drawpoint. The minimum tons (if non-zero) is set as a minimumnumber of buckets per shift which is considered reasonablefor each operation.

The maximum tonnage per draw point per day alsohas to be adjusted for the expected monthlyavailability for the draw point, as well as some furtheradjustments for draw point which are heavily over- orunder-drawn.

Then, a draw order can be assigned to each draw point,starting with those draw points with the highest "points"rating. Further adjustments are required to ensure that thedraw order can achieve the daily production target set by thedraw control officer.

This is described in more detail by Samosir, Brannon andDiering (2000).

6 REPORTING

If the right data is stored in an efficient database, then theflexibility to generate useful production and managementreports increases dramatically. The CMS system currentlygenerates the following reports on a daily basis:• Detailed draw order by draw point (an example is shown

in Table 1)• Summary of draw order with tunnel and draw point type

summaries (an example is shown in Table).• Comparison of Actual vs Order for previous day.• Compliance summary (Refer to the example in Table 2).

Summary of draw point by "condition" as follows:- Overpull- Underpull- Hang up too long- Over 100% drawn, but still active- Other : For example, no monthly target set in LTP- Inactive too long. (ie Draw point is active status, but tons

are not being taken from the draw point)- Hang up or temporary closed for too long.- Low grade warnings

• Month to date summaries for Order and Actual tons anddraw point Status.

• Month to date summaries for draw point assay samples.(E.g. Cu and Au)

• Detailed report by draw point for internal use by Cavemanagement department.

• Summary report for use by cave managememtdepartment.

• Draw point closure analysis considering the following:- % draw (vs Best HOD tons)- HOD for neighbours and this draw point- Grades, # low during last 30 days, latest and PC-BC

slice file (reserve)- Current metal prices and shut-off grade- Rock type

• Draw point sampling recommendation. Draw point maybe sampled for any of the following:- Tons since last sample- Days since last sample- Check up unusually high or low grade samples- Check up on unexpectedly large change in grade.

7 SOME LESSONS LEARNED

7.1. Draw point priorityDetermination of draw point priority is an interesting

challenge. There are various alternatives and each is likelyto have different appeal to different mines.

The draw point priority rating system in CMS tends toassign priorities to draw points in the following order:1. Safety draw points. Usually wet muck draw point which

are required to be mined regularly to avoid moisture buildup.

2. Geotechnical considerations. Any draw point showingsigns of stress or high convergence gets high priority.

3. Draw bell status. It is important to give these drawpoints high priority to avoid delays to the developmentand undercut process.

4. Under-drawn draw points.5. "Normal" draw points.6. Over-drawn draw points (in various categories).

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7.2. Hang up / secondary breakage prioritiesIt turns out that the same drivers for the draw point

priority for a daily draw order can also be applied toassign hang up clearance or secondary breakagepriorities. If a draw point has been idle (hung up) for toolong, then the priority to clear the hang up increases. Ofcourse, when these priorities are sent to the shift level,additional shift based considerations may need to beapplied, such as how many draw points in a panel arehang up and the overall crew availability etc.

7.3. Adaptive vs shared shift ordersThere are two basic strategies which can be followed at

the shift level:• The daily order is divided equally amongst each shift.

This is probably preferable in a multi-shift environmentwhich is not fully automated. Doing this provides eachshift with a more or less equal chance of meeting theirtargets. However, the second and subsequent shifts arenot responsive to over or under-draw from the previousshift.

• The second (or third) shift must be adaptive to the resultsof the first shift to best try to meet the daily order. This isthe system which will be used at Finsch and it will behighly automated. This seems to be a better approach,but there is an acknowledged draw back which needs tobe managed. The issue is that there may be a tendencyto mine the easier draw points in the earlier shifts and toleave the harder ones for later on. This leads to therebeing a higher risk of not making daily total tonnagetargets. Another factor is that if there is a large number ofdraw points active, it may not be practical to try to mineevery draw point every shift.

7.4. LHD Dispatch sequenceGiven that a draw point priority or rating system is already

included in the CMS draw order generation routine, it makessense to use that same rating system to try to dispatch theLHDs in an optimal manner. This was tried at Freeport DOZwith some success.

It makes sense to try to mine from those draw pointswhich are furthest behind, so that they have the best chanceto catch up. However, if production continually starts fromthe "hardest" draw points, the risk that the total draw order(in terms of tons) will not be met increases. At Freeport,during the initial tonnage build up, when daily tonnage wasimportant, tons were generally scheduled from one side of apanel to the other. The starting side was the morefavourable ground. Once full production was achieved, LHDdispatch on the basis of a priority rating was successfullyimplemented.

There is a assumption in the above that the LHD dispatchsystem is sufficiently advanced to be able to facilitate thistype of optimization.

Table 1: Example of a detailed draw order

Panel 15

Draw Point P15-01W P15-02W P15-03W P15-04W P15-05W P15-06WStatus CP CP CP CP CP CPTo Date 125189 139245 0 0 0 0Order 0 0 0 0 0 0Extra 0 0 0 0 0 0

Panel 16

Draw Point P15-01W P15-02W P15-03W P15-04W P15-05W P15-06WStatus CP CP CP CP A ATo Date 180162 0 0 195409 173457 155869Order 0 0 0 0 0 3Extra 0 0 0 0 0 0

Panel 17

Draw Point P15-01W P15-02W P15-03W P15-04W P15-05W P15-06WStatus A CP CP A A ATo Date 155334 168611 179781 173907 156208 147859Order 0 0 0 0 0 0Extra 0 0 0 0 0 0

Panel 18

Draw Point P15-01W P15-02W P15-03W P15-04W P15-05W P15-06WStatus A SP SP A A ATo Date 152146 160769 163615 170552 152393 147669Order 0 0 0 0 0 0Extra 0 0 0 0 0 0

Table 2: Example Compíance Report.

Table: 3 Example of a draw order with tunnel anddraw point type summaries

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7.5. Keep it simplePerhaps the most important lesson learned in

implementing CMS was to keep things simple whereverpossible. This is achieved in part by keeping long termplanning considerations out of the daily draw order as far aspossible and only sending the information necessary togenerate the daily order. The CMS system has a number ofscaling parameters which allow the relative importance ofvarious parameters to be adjusted to suit each site.

8 CONCLUSIONS

The CMS system has been successfully developed andimplemented at Freeport DOZ block cave. It is currentlybeing implemented at De Beers Finsch Mine for productionstart up during 2005.

Initial development was done on site at Freeport. Thiswas very useful in allowing the day to day exceptions to bestudied and accommodated in the system.

At the same time, the interface between the daily draworder and the monthly and longer term planning wasstreamlined and is now operating smoothly at Freeport.

Although hard to measure directly, there seems littledoubt that the current system has been proven to beeffective in facilitating a smoother operation with obviousbenefits to the bottom line.

ACKNOWLEDGEMENTS

The author is grateful to PT Freeport Indonesia and DeBeers Finsch Mine for their assistance in the developmentof the CMS system. Thanks are also due to GemcomSoftware International Inc. for time and funding to completethe work and submit this paper.

REFERENCES

• Diering, T, 2000. PC-BC: A block cave design and drawcontrol system, Proceedings MassMin 2000, Brisbane,pp. 469-484.

• Samosir, E, Brannon C and Diering, T, 2000.Implementation of Cave Management System (CMS)Tools at the Freeport DOZ Mine. Proceedings MassMin2004, Santiago, Chile.