| The strength properties of coal are the most commonly used mechanical parameters in coal mining design,pillar design and roadway stability evaluation.The characteristics of uniaxial compressive strength of bituminous coal under different saturation conditions,the tensile strength characteristics of bituminous coal disk of different bedding directions and the evolution of triaxial compressive strength characteristics of coal under different confining pressure conditions are studied.At the same time,in order to overcome the difficult nature in coal sampling at mine sites,the feasibility of application of the portable needle penetration instrument in evaluating the uniaxial compressive strength of bituminous coal is discussed for the first time.In this study,the following main conclusions are drawn:(1)For the first time,needle penetration test was applied to evaluate the uniaxial compressive strength of bituminous coal.Through the experimental study on the uniaxial compression of dry coal samples and saturated coal samples,it is found that the needle penetration instrument has a high reliability in evaluating the uniaxial compression strength of dry or unsaturated coal samples.(2)After soaking the coal in a hydrostatically pressurized chamber,it is found that the uniaxial compressive strength of bituminous coal samples increased with the soaking time.However,the soaking in hydrostatically pressurized chamber has no significant effect on the curve of axial stress-axial strain coal samples.However the radial elasticity modulus increased with the soaking time.(3)Indirect tensile strength tests show that an asymmetric "V-shaped" relationship existed between the bedding dip angle and the tensile strength of bituminous coal.Here,the angle α is defined as the inclination angle of the bedding to the horizon.The maximum tensile strength of the coal is achieved when the bedding dip angle α=0°.With the increase of α,the tensile strength of the coal samples decreased approximately following a linearly,and the minimum value was obtained when bedding dip angle α=60°.When the bedding dip angle α>60,the descending trend of the tensile strength changed to be an increasing trend.At α=60°,the maximum value after the asymmetrical “V” valley bottom was achieved,but the tensile strength at this moment was lower than the tensile strength when α=0°.(4)The bituminous coal bedding dip has a significant influence on the final cracking mode of Brazilian tests.The crack deviated significantly from the center line of the specimen with the α increased from 0° to 60°,forming a “bow” shape.Then,the crack gradually converged towards the centerline when α increased beyond 60°.The typical Brazilian split crack appeared at ? =60°.Due to existence of the bedding,the secondary cracks were generated on the bedding of specimen.When α increased to around 60°,secondary cracks became a part of the primary cracks,resulting in a substantial decrease in the tensile strength of the coal.(5)With the increase of lateral confining pressure,the triaxial compressive strength of bituminous coal sample increased significantly,together with the cumulative axial strain at the peak stress.However,the change of the Young’s modulus was negligible.Compared with the significant increase in peak compressive strength,the crack initiation stress of coal sample was only slightly increased.With the increase of lateral confining pressure,the coal sample presents three types of failure modes: splitting failure under the low confining pressure,failure of splitting and shearing under the medium confining pressure and shear failure under the high confining pressure. |
