Underground mining method for unexploited coal in boundary open-pit mine

An underground mining method for unexploited coal in a boundary open-pit mine, comprising the following steps:

• step 1:
• determination of a shaft construction platform, comprising:
• arranging the shaft construction platform at one rock step to two rock steps above a coal seam;
• step 2:
• construction of intermediate bridges, comprising:
• laying a mining area material starting from a pit bottom to form the intermediate bridges communicating a working slope with a non-working slope, wherein the intermediate bridges are arranged at intervals, and a distance between two adjacent ones of the intermediate bridges is 10 times a height of each of the intermediate bridges;
• step 3:
• sealing of slopes, comprising:
• laying mining area clay on the working slope where no intermediate bridge is built and on a side slope with an outcrop of the coal seam as a sealing layer to seal the slopes;
• step 4:
• digging of auxiliary vertical shafts and main inclined shafts, comprising:
• firstly, constructing a first main inclined shaft extending into the coal seam in the slopes in an inclined or pseudo-inclined manner at a position of the shaft construction platform where an end slope and the working slope intersect;
• then vertically constructing an auxiliary vertical shaft at a position of the shaft construction platform where a first intermediate bridge and the working slope intersect, and constructing a second main inclined shaft at a position of the shaft construction platform where a second intermediate bridge and the working slope intersect, wherein a bottom of the auxiliary vertical shaft is even with a bottom of the first main inclined shaft and a bottom of the second main inclined shaft;
• and continuing to dig the auxiliary vertical shafts and the main inclined shafts starting from a third intermediate bridge in a pattern of every two main inclined shafts sandwiching one auxiliary vertical shaft;
• step 5:
• construction of a water-sealed pit bottom, comprising:
• digging the pit bottom downward to form a digging space on a side close to the working slope between two adjacent ones of the intermediate bridges, and filling clay into the digging space to form an artificial water barrier layer, wherein a boundary of the artificial water barrier layer is connected to a boundary of the sealing layer;
• step 6:
• mining of the unexploited coal, comprising:
• constructing a roadway communicating the main inclined shafts and the auxiliary vertical shafts and arranging an underground stope face for mining of the unexploited coal at the end slope of the open-pit mine, wherein every two main inclined shafts and one auxiliary vertical shaft constitute an unexploited coal mining unit;
• and
• step 7:
• laying of water purification slopes, comprising:
• building the water purification slopes extending to the artificial water barrier layer on two sides of a slope surface of a respective one of the intermediate bridges corresponding to each of the auxiliary vertical shafts.