stripped lower

In mining, stripping ratio or strip ratio refers to the ratio of the volume of overburden (or waste material) required to be handled in order to extract some tonnage of ore. For example, a 3:1 stripping ratio means that mining one tonne of ore will require mining three tonnes of waste rock. Stripping ratios are typically reduced to show the volume of waste removal required to extract one unit ton of ore, for example, 1.5:1 as opposed to 3:2.
When compared to surface mining, which requires overburden removal prior to ore extraction, underground mining operations tend to have lower stripping ratios due to increased selectivity.
All other factors being equal, mining at a higher stripping ratio is less profitable than mining at a lower stripping ratio because more waste must be moved (at a cost per unit volume) for an equivalent volume of revenue generating ore. If the ratio is too high given a particular price of ore and associated cost of mining then it may not be economical to conduct mining.

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