Damage Characteristics and Stress Distribution Law of Overburden Rock in Multiple Mining of Close Coal Seams

ABSTRACT

Multiple mining of close coal seams alters the initial development position, growth height, and fracture morphology of overlying strata, which brings severe challenges to water inrush prevention and gas leakage control in close‐seam coal mines. Taking Working Face 10104 of a contiguous coal seam mine in Shanxi Province as the engineering background, theoretical analysis, numerical simulation, and field measurement are adopted to investigate the failure characteristics of the “three vertical zones” in overlying strata and stress transfer laws under three mining scenarios: single upper‐seam mining, single lower‐seam mining, and combined multi‐seam mining. The main conclusions are drawn as follows: (1) For close coal seams, after single upper‐seam mining, the heights of the caved zone and fractured zone are 2.42–6.82 m and 15.17–26.37 m, respectively. After combined mining of upper and lower seams, the caved zone height increases to 8.68–13.08 m, and the fractured zone height rises to 38.39–49.59 m. (2) After single lower‐seam mining in close coal seams, the caved zone height ranges from 8.68 to 13.08 m, and the fractured zone height ranges from 35.19 to 46.39 m. (3) Under single upper‐seam mining, single lower‐seam mining, and combined multi‐seam mining conditions in close coal seams, the failure characteristics of the “three vertical zones” and stress transfer laws can be summarized as follows: after single lower‐seam mining, the caved zone height is 8.68–13.08 m, and the fractured zone height is 35.19–46.39 m. Affected by previous upper‐seam exploitation, both heights are approximately twice those under single upper‐seam mining. (4) Upper‐seam mining forms a pressure‐relief zone within the overlying gob. During subsequent lower‐seam mining, the induced stress peak increases obviously; however, with continuous advance of the lower seam working face, the stress peak decreases greatly. (5) Field borehole water injection tests reveal the overburden failure rules: after upper‐seam mining, the caved zone height is about 12 m and the critical height of the fractured zone is around 18 m; after lower‐seam mining, the caved zone height reaches approximately 22 m, and the critical fractured zone height is about 36 m.