A considerable amount of coal reserves are located in protection pillars that lie under built-up region in active mining areas at the Czech part of the Upper Silesian Coal Basin. The commonly used controlled caving longwall mining method is not applicable in these areas because significant deformation of the surface is not permitted. For this reason the room and pillar method with stable coal pillars has been tested in order to minimise subsidence of surface.

Stress-deformation monitoring was essential as this was the first application of the conventional room and pillar mining method within the Upper Silesian Coal Basin mines. More than six kilometres of roadways were driven within two panels during last three years. To determine pillar stability, vertical stress and horizontal displacement of coal pillars were measured in coal pillars which are located within a row of pillars forming the panels. Two monitored pillars diamond in shape and slightly irregular sides have been observed into the first mined panel "V" and three monitored pillars have been observed into the second panel "II". To measure the increase in vertical stress due to mining, hydraulic stress cells were installed in each coal pillar. The 5-level multipoint rib extensometers measured displacements of all sides within each monitored pillar. The results of stress-deformation monitoring allowed pillar loading and yielding characteristics to be described.

1 INTRODUCTION

The pilot project of the mining modified room and pillar method with stable pillars has been running since 2014. The method was tested within the shaft protective pillar located in CSM-North Mine (Karvina coal sub-basin in the Czech part of the Upper Silesian Coal Basin—USCB) coal seam No. 30, where the risk of rockbursts was low and roof conditions were acceptable for bolting reinforcing. However, the variable geology and several faults of regional importance complicated the mining conditions.

The project includes the detailed geomechanical monitoring of stress and deformation in the driven roadways and the surrounding rock mass. Monitored pillars with 3.5m in high and different sizes were selected to determine stress-deformation characteristics under different geotechnical conditions. Two monitored pillars diamond in shape and slightly irregular sides were approximately 860 m2 and 1200 m2 in size into the first mined panel “V” (locality A) and three monitored pillars were approximately 590 m2, 590 m2 and 730 2 in size into the second panel “II” (locally B). Mining depth of room and pillar trial ranged from 700 to 900m. being prhaps the deepest room and pillar mining in the world coal mines.

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