The Quasi - Open Pit Methods
Mnge 315
©Dr. B. C. Paul spring 2003
Open Pit Sequence Flexibility
• We have just gone over classic open pits
than go down and push back in a variety
of ways as it develops out to the ultimate
pit.
• Sometimes Site Specific constraints don’t
allow open pits to be developed in any
direction that economics chooses
The Dipping Bed Example
Even though this mine is too
Steep to backfill as mining goes
Down-dip, this mine must still
Systematically move forward
Advancing down-dip just like
Your strip mine in Homework #3
You can still use Learch Grossman to find an Ultimate Pit
But you have little choice on the sequence
The 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 geometric
Object, it is a bed. The ore spreads out in all directions. How far out you go is
Likely to be determined by a property line, rather than the edge of an ore zone.
A Simple Midwestern Quarry
Overburden is limited – One way to improve net present value is to put off
Stripping expenses. In a quarry there may be economies of scale in stripping
All at once – and little to be gained by putting off stripping.
Overburden Soil and Degraded Rock
Rock Layer #1
Rock Layer #2
Ore Does Not Really Have a Grade – Most of the Rock Bed has about the same
Value. Thus you optimize NPV by having more limited costs at first. Since rock
Closest to surface usually has lower mining costs this suggests a top to bottom
Sequence.
High Rock Quality
Overburden Soil and Degraded Rock
Rock Layer #1
Rock Layer #2
May often use
A bench height
That matches
Rock beds to
Sort product
Quality and
characteristics
Hydrothermal alteration often deposits valuable minerals in rock but it also makes
It weak and fractured. Quarry rock is often strong and unfractured so it will
Stand and steep angles and needs only narrow berms to catch a rare break-away
Rock.
Quarries can have Steep dipping
Beds
• Can end up with a pit that advances down
dip
• Can have thicker overburden
– May end up with something like a strip mine
where overburden from oneside backfills the
pit on the other.
Open Pit Mines
• In some senses all forms of surface mines
are open pits
• Strip Mines tend to refer to coal operations
with some very significant differences
from open pits in the metal/ industrial
mineral sense
• Quarries also tend to be unique subset
• Placer and dredge operations also unique
subset
Classic Open Pit
• Characterized by a series of stair-step like
•
benches that each act as a working area
Pit shapes tend to be more configured to
geology of the deposit more than equipment
needs/convenience
– Many pits are ovals
• Fits the geometry of disseminated metal deposits
– Pits tend to be wider relative to length
– Pits tend not to move like strip mine – pit develops in
place
Differences not Necessarily Simple
• Working Benches were encountered in “Strip
Mines”
– Advance benches using trucks and shovels developing ahead of
dragline
– Multiple coal seams may need a bench for each
– Contour haulback operations often had benches lined up with
layers of concern
– Mountain Top Removal can be developed as truck shovel
benches
• Local and Industry Specific Word Usage
– Coal people like to “strip” mine
– Metals/Industrial Minerals people like to “Open Pit”
– Aggregate people like to Quarry
Terrace Pit Mining
Add Picture
Note that all characterized by benches – often set up on truck shovel equipment
Working heights. Can also be used with multi-seam coal mining
Factors Controlling Terrace Pit
• Bench Heights are often controlled by the
digging height of the shovel
– In past drill accuracy limits have been
important
• Benches often give more gradual over-all
slope
– Dragline highwall is mostly angle of repose
– May need a specific bench width to provide
working room – smaller over-all slope to
highwall
More Terrace Pit Control
• Trucks and Shovels often control OB
movement rather than draglines
– Dragline has limited boom length – favors
narrow pits
– Trucks and Shovels need room for equipment
to drive across without getting tangled up or
caught on steep slope
• Tends to favor wider pits
• Need for ramps tends to favor shorter pits rather
than having long moving range like dragline
Dipping Thick Coal Seams
• Common in Western part of U.S. for coal
Mining on the Strike or the Dip
• Area Strip mines
– Dragline moves down the strike with pickup
and dump sequence moving down dip
– Conspicuously advances down dip
Mining Down the Strike
• Contour Haulback schemes
– Overburden movement is along the strike
Add picture
Coal Terrace Pits
• With relatively flat seam overburden
moves across the dip
• With steeper dip overburden movement
tends to be down strike
– Even possible to have full pit width developed
at once with contour mine like advance
Evolving to True Open Pit
• Benches advance down the dip
• Pit length is limited for manageable
network of truck haul roads
• Overburden is not deposited in the same
pit
– Hauled to a distinct and separate pit along the
strike
– Phosphate Operations mine dipping
phosphorous rich limestones that way
Note the Distinct Feature
• Pit is developed out to its stripping ratio
limit leaving the pit open the entire time
– Only at end of the pit life is the pit backfilled
• Geometry and Equipment limitations in
hardrock and industrial minerals defies a
workable concurrent reclamation in the
SMCRA sense
– One reasons the law was only applied to coal