Method of recovering room-and-pillar coal pillar by using external replacement supports

A method for recovering room-type coal pillars by replacing with external supports, comprising the following steps:

• 1) casting a cement filling material wall around a room-type coal pillar by hanging bags on a single prop, and reserving a gap in the cement filling material wall;
• 2) mining the internal room-type coal pillar through the gap in the cement filling material wall, under a condition of supporting the overlaying strata with the cement filling material wall;
• 3) plugging the gap in the cement filling material wall and filling a cement filling material into the goaf area surrounded by the cement filling material wall, after the mining of the room-type coal pillar is completed;
• 4) recovering the single prop after the cement filling material is solidified and stabilized.