Stress Evolutionary Laws of Retained Coal Pillar at Upper Layer in Forming and Recovering Stage

To ensure the efficient and safe recovering of retained coal pillar in upper layer, this paper studies the stress evolution laws of retained coal pillar during forming and recovering stage by numerical modeling and spot investigating. The results showed that the coal pillar appeared stress superposition because of working face mining in both sides of coal pillar, along the direction of working face tendency, the stress of coal rock under lower layer face appeared alternative form of original stress zone, stress-concentrated zone and distressed zone. The maximum stress is almost equal when the formation of coal pillar is 200 m and coal pillar recovers to 200 m, and the stress is 35 MPa and 35.05 MPa respectively; the location of the maximum is located at 100 m and 200 m along the direction of mining because of the influence of supporting. The stress evolutionary laws in forming stage can meet the safe requirements of recovering coal pillar.