1. Calculating Stripping Ratios for Area Strip Mines Mnge 315
©Dr. B. C. Paul spring 2003 revised 2010
Note – These notes contain information considered common knowledge to those familiar with Strip Mining. Books such as
“Elements of Practical Coal Mining” and SME’s “Surface Mining” were specifically referenced

2. Stripping Ratio as a Fundamental
Ability to surface mine depends on the value of the ore being high enough to offset the cost of Overburden Removal
Stripping Ratio determines relative amounts of each to be moved
If coal generates a $5/ton earning and overburden costs 45 cents per cubic yard to move – what is the maximum economic stripping ratio
$5.00/ 0.45 = just over 11 cubic yards per ton
Called a “Break Even Stripping Ratio”

3. Complexities Warning
Units of Stripping Ratio depend on convenience of the industry
Coal overburden is normally light and machines are limited by volume in cubic yards
Coal is sold by the ton
Cubic Yards/ton makes sense
Hard Rock Metal
Both ore and overburden are trucked
Both are heavy and trucks are usually weight limited
Tons/ton makes sense (unit-less)

4. More Warnings
Geometry problems usually measure volumes which convert to tons only if density is the same for all materials
Material may be rehandled
Get operating and geologic stripping ratios
Don’t use 40 cents per cubic yard to figure stripping ratio off of geometry if you have 10% rehandle

5. Fixed and Variable Costs
Example of the Car
If you have a car
Insurance
Licensing Fees
Storage or Parking Space
These Costs constant per unit of time regardless of use of car
Variable Costs
Gas
Tires
Oil Changes
Largely Dependent on mileage you drive

6. Fixed and Variable Costs in Mining
Some mining costs are fixed (many administrative features for example)
Others are variable – direct materials handling cost
What goes into Break Even Stripping Ratio
Direct materials costs go into Break Even
Fixed Costs are charged to the average stripping ratio you mine over the time

7. Simple Stripping Ratios
Bottom of a
Strip Mine
Top of a Strip Mine
For a pit 1 mile long and
160 ft across, how significantly
Different is the length at the top
And bottom of the pit?
Volumes are Length * Width * Height
Ratio is A/B
What happens if the length term in
Both A and B is the same number
It cancels – Long strip pits can be
Analyzed as 2D problems!!!
Life is Good

8. The 2D Cross Section Area = Base * Height Interest Item – Slope Angle
Overburden
Interest Item – Slope Angle
Has no effect
The problem is 1D
Only depends on
Thickness and density
Base for both coal and the overburden is the same
That means the base will cancel!

9. The Coal Area Strip Mine Formula
Strip Ratio = OB Thickness (in ft) * 11.11/ Coal thickness in inches
Assumes coal is 80 lbs/ft^3 (good for bituminous)
Area Strip Mine Geometry
Geologic Stripping Ratio – no accounting for rehandle or less than 100% coal recovery

10. Your Turn
Use What You Have Learned About Calculating Stripping Ratios for Strip Mines and Break Even Limits to Determine the Size of a Coal Reserve
Do Assignment #3

11. A Little Explanation Very seldom is a coal seam the same
Distance from the surface always
Less overburden means
More profit.
Project worth is increased if you take the
Biggest profits first – thus strip mines start
Shallow and work into deeper stuff.
Is it possible that eventually the coal
Will get deep enough to exceed the break-even
Stripping ratio – of course!
In your homework you will look at a coal reserve – start shallow and work until it is
Deep enough you can’t afford it anymore. Then you’ll tell me how far you could mine.