Deformation mechanism of narrow coal pillar in the fully-mechanized gob-side entry with incompletely stable overlying strata

In order to understand the deformation mechanism of the narrow coal pillar in gob-side entry employing TCC under unstable overlying strata, borehole inspection camera has been utilized to observe the boreholes in the narrow coal pillar. Evaluation method of broken level based on the basic measuring scale has been proposed to undertake the statistical analysis of borehole fracture and the characteristic of fracture distribution has also been investigated. The results have shown that: 1) Transverse fractures and beddings are the two main fracture forms of the pillar; abnormal fracture zone is mainly located at both sides of the pillar; besides, there exist many transverse fractures with certain space intervals and small fractures in the middle of the coal pillar. 2) According to different broken levels, the pillar can be divided into three zones, viz., fracture zone A, stable zone B and fracture zone C; fracture zone A is near the roadway while fracture zone C is near the gob. Because of the asymmetry of the geometric distribution of these three zones, the stable zone is closer to the fracture zone C. 3) The key to control the deformation of the coal pillar is to control the rotating and slippage of the key block, and its coupling support control method has been proposed.