Please teach the open pit coal mine earth stripping project familiar friends, or Gansu Jingtai County friends ~ urgent!

Very true ah, in the open pit mine, stripping hundreds of millions of cubic meters is a very common thing, the most commonly used indicators of open pit mines is the stripping ratio, simply digging a ton of coal has to dig a few cubic meters of soil, a coalfield reserves on hundreds of millions of tons of more than one, the stripping ratio of the general smaller also have to be 4, 5, like coking coal worth of coal, the state provides for the economic rationality of stripping ratio of 15, in practice, sometimes stripping ratio of 20 is also making money, you estimate yourself! How much of the earth is stripped.