The Quasi - Open Pit The Quasi - Open Pit MethodsMethodsMnge 315Mnge 315

©Dr. B. C. Paul spring 2003©Dr. B. C. Paul spring 2003

Open Pit Sequence Open Pit Sequence FlexibilityFlexibility

• We have just gone over classic open We have just gone over classic open pits than go down and push back in a pits than go down and push back in a variety of ways as it develops out to variety of ways as it develops out to the ultimate pit.the ultimate pit.

• Sometimes Site Specific constraints Sometimes Site Specific constraints don’t allow open pits to be developed don’t allow open pits to be developed in any direction that economics in any direction that economics chooseschooses

The Dipping Bed ExampleThe Dipping Bed Example

Even though this mine is tooSteep to backfill as mining goesDown-dip, this mine must stillSystematically move forwardAdvancing down-dip just likeYour strip mine in Homework #3

You can still use Learch Grossman to find an Ultimate PitBut you have little choice on the sequence

The Quarry – A Simple CaseThe Quarry – A Simple Case

Overburden Soil and Degraded Rock

Rock Layer #1

Rock Layer #2

Unique Geologic Characteristics – The ore zone is not a steeply dipping geometricObject, it is a bed. The ore spreads out in all directions. How far out you go isLikely to be determined by a property line, rather than the edge of an ore zone.

A Simple Midwestern QuarryA Simple Midwestern Quarry

Overburden Soil and Degraded Rock

Rock Layer #1

Rock Layer #2

Overburden is limited – One way to improve net present value is to put offStripping expenses. In a quarry there may be economies of scale in strippingAll at once – and little to be gained by putting off stripping.

Ore Does Not Really Have a Grade – Most of the Rock Bed has about the sameValue. Thus you optimize NPV by having more limited costs at first. Since rockClosest to surface usually has lower mining costs this suggests a top to bottomSequence.

High Rock QualityHigh Rock Quality

Overburden Soil and Degraded Rock

Rock Layer #1

Rock Layer #2

Hydrothermal alteration often deposits valuable minerals in rock but it also makesIt weak and fractured. Quarry rock is often strong and unfractured so it willStand and steep angles and needs only narrow berms to catch a rare break-awayRock.

May often useA bench heightThat matchesRock beds toSort productQuality andcharacteristics

Quarries can have Steep Quarries can have Steep dipping Bedsdipping Beds

• Can end up with a pit that advances Can end up with a pit that advances down dipdown dip

• Can have thicker overburdenCan have thicker overburden– May end up with something like a strip May end up with something like a strip

mine where overburden from oneside mine where overburden from oneside backfills the pit on the other.backfills the pit on the other.

Open Pit MinesOpen Pit Mines

• In some senses all forms of surface In some senses all forms of surface mines are open pitsmines are open pits

• Strip Mines tend to refer to coal Strip Mines tend to refer to coal operations with some very significant operations with some very significant differences from open pits in the metal/ differences from open pits in the metal/ industrial mineral senseindustrial mineral sense

• Quarries also tend to be unique subsetQuarries also tend to be unique subset• Placer and dredge operations also Placer and dredge operations also

unique subsetunique subset

Classic Open PitClassic Open Pit

• Characterized by a series of stair-step like Characterized by a series of stair-step like benches that each act as a working areabenches that each act as a working area

• Pit shapes tend to be more configured to Pit shapes tend to be more configured to geology of the deposit more than geology of the deposit more than equipment needs/convenienceequipment needs/convenience– Many pits are ovalsMany pits are ovals

• Fits the geometry of disseminated metal depositsFits the geometry of disseminated metal deposits

– Pits tend to be wider relative to lengthPits tend to be wider relative to length– Pits tend not to move like strip mine – pit Pits tend not to move like strip mine – pit

develops in placedevelops in place

Differences not Necessarily Differences not Necessarily SimpleSimple• Working Benches were encountered in Working Benches were encountered in

“Strip Mines”“Strip Mines”– Advance benches using trucks and shovels developing Advance benches using trucks and shovels developing

ahead of draglineahead of dragline– Multiple coal seams may need a bench for eachMultiple coal seams may need a bench for each– Contour haulback operations often had benches lined up Contour haulback operations often had benches lined up

with layers of concernwith layers of concern– Mountain Top Removal can be developed as truck shovel Mountain Top Removal can be developed as truck shovel

benchesbenches

• Local and Industry Specific Word UsageLocal and Industry Specific Word Usage– Coal people like to “strip” mineCoal people like to “strip” mine– Metals/Industrial Minerals people like to “Open Pit”Metals/Industrial Minerals people like to “Open Pit”– Aggregate people like to QuarryAggregate people like to Quarry

Terrace Pit MiningTerrace Pit Mining

Note that all characterized by benches – often set up on truck shovel equipmentWorking heights. Can also be used with multi-seam coal mining

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Factors Controlling Terrace Factors Controlling Terrace PitPit• Bench Heights are often controlled by Bench Heights are often controlled by

the digging height of the shovelthe digging height of the shovel– In past drill accuracy limits have been In past drill accuracy limits have been

importantimportant

• Benches often give more gradual over-all Benches often give more gradual over-all slopeslope– Dragline highwall is mostly angle of reposeDragline highwall is mostly angle of repose– May need a specific bench width to provide May need a specific bench width to provide

working room – smaller over-all slope to working room – smaller over-all slope to highwallhighwall

More Terrace Pit ControlMore Terrace Pit Control

• Trucks and Shovels often control OB Trucks and Shovels often control OB movement rather than draglinesmovement rather than draglines– Dragline has limited boom length – favors Dragline has limited boom length – favors

narrow pitsnarrow pits– Trucks and Shovels need room for Trucks and Shovels need room for

equipment to drive across without getting equipment to drive across without getting tangled up or caught on steep slopetangled up or caught on steep slope•Tends to favor wider pitsTends to favor wider pits•Need for ramps tends to favor shorter pits rather Need for ramps tends to favor shorter pits rather

than having long moving range like draglinethan having long moving range like dragline

Dipping Thick Coal SeamsDipping Thick Coal Seams

• Common in Western part of U.S. for coalCommon in Western part of U.S. for coal

Mining on the Strike or the Mining on the Strike or the DipDip

• Area Strip minesArea Strip mines– Dragline moves down the strike with Dragline moves down the strike with

pickup and dump sequence moving pickup and dump sequence moving down dipdown dip

– Conspicuously advances down dipConspicuously advances down dip

Mining Down the StrikeMining Down the Strike

• Contour Haulback schemesContour Haulback schemes– Overburden movement is along the strikeOverburden movement is along the strike

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Coal Terrace PitsCoal Terrace Pits

• With relatively flat seam overburden With relatively flat seam overburden moves across the dipmoves across the dip

• With steeper dip overburden With steeper dip overburden movement tends to be down strikemovement tends to be down strike– Even possible to have full pit width Even possible to have full pit width

developed at once with contour mine developed at once with contour mine like advancelike advance

Evolving to True Open PitEvolving to True Open Pit

• Benches advance down the dipBenches advance down the dip• Pit length is limited for manageable Pit length is limited for manageable

network of truck haul roadsnetwork of truck haul roads• Overburden is not deposited in the Overburden is not deposited in the

same pitsame pit– Hauled to a distinct and separate pit Hauled to a distinct and separate pit

along the strikealong the strike– Phosphate Operations mine dipping Phosphate Operations mine dipping

phosphorous rich limestones that wayphosphorous rich limestones that way

Note the Distinct FeatureNote the Distinct Feature

• Pit is developed out to its stripping ratio Pit is developed out to its stripping ratio limit leaving the pit open the entire timelimit leaving the pit open the entire time– Only at end of the pit life is the pit backfilledOnly at end of the pit life is the pit backfilled

• Geometry and Equipment limitations in Geometry and Equipment limitations in hardrock and industrial minerals defies hardrock and industrial minerals defies a workable concurrent reclamation in a workable concurrent reclamation in the SMCRA sensethe SMCRA sense– One reasons the law was only applied to One reasons the law was only applied to

coalcoal