Stripping Ratio in Surface Mining

Calculating the Stripping Ratio

The stripping ratio (SR) is defined as:

SR = Volume (or weight) of waste removed / Volume (or weight) of ore mined

Units depend on the application:

• Volume-based: m³ waste / m³ ore, or yd³ waste / yd³ ore (commonly called bank cubic meters or bank cubic yards, BCM or BCY)
• Weight-based: tonnes waste / tonne ore, or tons waste / ton ore
• Mixed: m³ waste / tonne ore (common in coal mining, where ore is measured in tonnes but waste in volume)

The stripping ratio can be calculated for the overall mine life (overall stripping ratio, also called life-of-mine SR) or for a specific time period or mining phase (instantaneous stripping ratio or periodic SR). The instantaneous SR varies as the mine progresses, typically starting low in early mining phases (shallow ore, little overburden) and increases as the pit deepens.

In coal strip mining, the ratio is often expressed as cubic yards of overburden per ton of coal. A 10:1 ratio means 10 cubic yards of overburden must be removed for every ton of coal extracted. In metalliferous open pit mining, the ratio is usually BCM waste per BCM ore or tonnes per tonne.

Breakeven Stripping Ratio

The breakeven stripping ratio is the maximum stripping ratio at which mining remains profitable. Above this ratio, the cost of waste removal exceeds the value of the ore recovered. It is calculated as:

SR_breakeven = (Revenue per unit ore − Mining & processing cost per unit ore) / Cost of waste removal per unit waste

Or equivalently:

SR_breakeven = (Ore value − Ore cost) / Waste cost

For example, if ore in place is worth $25/tonne (after accounting for grade, recovery, and commodity price), mining and processing costs are $8/tonne of ore, and waste removal costs $3/tonne: SR_breakeven = (25 − 8) / 3 = 5.67:1. Any block of ore with a stripping ratio below 5.67:1 is profitable to mine; above that ratio, it should be left in place (or mined underground if feasible).

The breakeven stripping ratio changes with commodity price. When metal prices rise, higher stripping ratios become economic, the pit gets deeper and wider, and more ore is classified as mineable reserves. When prices fall, the breakeven SR drops, the economic pit shrinks, and reserves decrease. This is why mining reserve estimates change with commodity price cycles.

Stripping Ratio in Mine Planning and Pit Optimization

The stripping ratio drives several critical mine planning decisions:

Pit limit determination: The ultimate pit limit is the point where the incremental stripping ratio on the pit walls equals the breakeven stripping ratio. Modern pit optimization uses the Lerchs-Grossmann algorithm or floating cone methods to find the optimal pit shell that maximizes NPV. Every additional bench of depth has a higher incremental stripping ratio because the pit walls must be pushed back to maintain safe slope angles.

Pit slope angles: Steeper pit slopes reduce the stripping ratio because less waste is excavated for the same ore exposure. However, steeper slopes increase the risk of slope failure. The economic optimum balances the cost savings of steeper slopes against the probability-weighted cost of slope failures. A 5-degree increase in overall slope angle can reduce the stripping ratio by 15–30% in a deep pit.

Mining sequence and scheduling: The instantaneous stripping ratio varies year by year. Mine plans are designed to balance the stripping ratio over time, avoiding years with very high ratios (cash flow negative) by pre-stripping in earlier years. Deferred stripping strategies push waste removal to later periods but require careful financial analysis because the NPV of future waste removal costs is lower than present costs.

Surface vs. underground transition: As a surface mine deepens, the stripping ratio increases until it exceeds the breakeven value. At that point, the remaining ore may be economic to mine underground. The transition depth depends on the ore body geometry, underground mining costs, and the breakeven stripping ratio. Some operations run both surface and underground mines simultaneously on the same deposit.