Stability control of narrow coal pillars in gob-side entry driving for the LTCC with unstable overlying strata: a case study

In underground coal mining systems, the subsidence of unstable overlying strata (UOS) has a key effect on the deformation of narrow coal pillars that are contiguous to the caved zone. This paper investigated the failure mechanism of coal pillars by combining the in situ testing and numerical modeling methods. Several striking findings can be drawn. (1) Six fracture patterns exist in the in situ pillar, and their distribution divides the coal pillar into three zones, namely, the fracture zone, stable zone, and collapsed zone. (2) The stable zone, which has a width that accounts for one third of the pillar, is decisive to pillar failures. (3) The deformation of the contiguous pillar can be divided into two stages. Prior to the excavation of mining entry under the UOS, the pillar moves towards the caved zone. After the excavation, a reversed area (0 to 1.8 m) of the pillar moves back to the current panel, while the remaining area (1.8 m to 3.0 m) moves continuously to the caved zone. Consequently, a pillar-reinforcement method was proposed. It is concluded that the method and experience successfully practiced in the Baodian coal mine can be applied in other coal mine systems.