A field investigation for overlying strata behaviour study during protective seam longwall overmining

Abstract

Field experiments were conducted in a protected coal seam using three borehole methods: multi-point displacement measuring, double end seal with water injection method and borehole television photographing. These methods were used to study the movement law of overburden strata in the Sihe no. 2 Coal Mine, China. Results show that, parallel to the retreating direction, the overlying strata of the #9 coal seam can be divided into four sections: the coal wall support zone (0∼8.1 m), the roof separation zone (8.1∼23.0 m), the caving compaction zone (23.0∼43.0 m) and the analogous in situ zone (>43.0 m). In the vertical perpendicular retreating direction, the height of the boundary between the fracturing and caving zones is 4.3∼5.7 m, while the height between the fracturing and deformation zones is 27.5∼30.5 m, with an overburden strata bulking factor of 1.36∼1.54. After the lower protective seam (#9 coal seam) is mined, the overlying strata failure mainly develops in the weak rock layers of the fracturing and caving zones, rather than in the deformation zone. The number of mine-induced fractures is greatly increased, the fractures are broadened and mine interactivity fractures are mostly oriented horizontally.