Study on the strength and failure characteristics of layered sandwiched coal-rock assemblages
[Objective]This study aims to investigate the causes of roadway roof disasters.These disasters primarily occur owing to the failure of different layered composite rock structures under specific geo-stress or mining-induced stress,leading to overall instability and collapse.In a particular mine,the layered coal-bearing roof of the roadway represents what is known as a"composite defect roof"in terms of both rock assembly structure and mechanical properties.This composite defect is characterized by weak coal seams within the rock assembly,which reduces the overall mechanical strength and increases the risk of roof falls.Therefore,it is crucial to examine the mechanical characteristics and failure features of these coal-rock composites to understand and control the mechanism of roadway roof falls.[Methods]The study employed uniaxial compression tests on composite specimens comprising different structural combinations.These combinations included sandy mudstone,medium sandstone,and coal layers of varying thicknesses(20,30,40 mm).The samples for these tests were obtained from core drilling and borehole inspection in the mine.[Results]The results indicate that the compressive strength of the composite coal-rock assemblages is generally lower than that of the individual rock counterparts.For instance,the compressive strength of medium sandstone with a 30 mm coal layer reaches 85.95%of the strength of an individual medium sandstone,decreasing as the coal layer gets thinner.By contrast,sandy mudstone with a 20 mm coal layer has a strength of only 32.5%of the individual rock,which increases with coal thickness but not significantly beyond 30 mm.The failure patterns of the composites vary with the type of rock assembly,including X-shaped conjugate oblique shear,single oblique shear,and partial tensile failures.[Conclusions]The research concludes that the uniaxial compressive strength of composite rocks is consistently lower than that of individual rocks.The weakest compressive strength is observed when the middle coal layer is thin,even though there is higher strength at both ends of the SMS composite.In the case of SMZ and ZMZ combinations,if one end has high compressive strength or both ends have comparable strengths,the coal strength,which is close to that of medium sandstone,transfers stress to the sandstone,thereby significantly increasing the overall strength.The experimental data show that coal,having a marginally weaker capacity for elastic deformation,tends to crack first.These cracks then spread toward the contact surface,eventually leading to a complete failure.This sequence confirms the generation of radial tensile stress at the contact surfaces of layered composite rock assemblages.
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