Mechanical behaviors of deep pillar sandwiched between strong and weak layers

A variety of coal room and pillar mining methods have been efficiently practiced at depths of up to 500 m with least strata mechanics issues.However,for the first time,this method was trialled at depths of 850-900 m in CSM mine of Czech Republic.The rhomboid-shaped coal pillars with acute corners of 70°,surrounded with 5.2 m wide and 3.5-4.5 m high mine roadways,were used.Pillars were developed in a staggered manner with their size variation in the Panel Ⅱ from 83 m×25 m to 24 m×20 m(corner to corner)and Panel Ⅴ from 35 m×30 m to 26 m×16 m.Coal seam inclined at 12° was affected by the unusual slippery slickenside roof bands and sometimes in the floor levels with high vertical stress below strong and massive sandstone roof.In order to ensure safety,pillars in both the panels were continuously monitored using various geotechnical instruments measuring the induced stresses,side spalling and roof sagging.Both panels suffered high amounts of mining induced stress and pillar failure with side-spalling up to 5 m from all sides.Heavy fracturing of coal pillar sides was controlled by fully encapsulated steel bolts.Mining induced stress kept increasing with the progress of development of pillars and galleries.Instruments installed in the pillar failed to monitor actual induced stress due to fracturing of coal mass around it which created an apprehension of pillar failure up to its core due to high vertical mining induced stress.This risk was reduced by carrying out scientific studies including the three-dimensional numerical models calibrated with data from the instrumented pillar.An attempt has been made to study the behavior of coal pillars and their yielding characteristics at deeper cover based on field and simu-lation results.