| Water is one of the most active factors in rock engineering,and it a dual influence in coal mining.On the one hand,water injection is an effective method for mining thick coal seam,mitigating and preventing coal bumps,and coal and gas outburst disasters,reducing dust.On the other hand,water may result in large area roof fall,water bursting in mines.The seepage of water can induce the change of geological structure,resulting in the occurrence of earthquakes.Therefore,it has important theoretical and practical values for studying the mechanical properties and change laws of coal with different water content,preventing the occurrences of disasters and providing an optimal design of water injection and hydraulic fracturing.Currently,there are few studies on the fracture behaviours of coal with water content.To this end,in this paper,the fracture failure of coal with water content under static or dynamic load is taken as engineering background,and the theory analysis,experimental testing in the lab and numerical simulation technique are used to study the effect of water content on mechanical properties of coal.The mechanical characteristics of wet coals with water contents of 0,1.8%and 3.6% under uniaxial compression,conventional compression,and Brazilian splitting were investigated.The static complex fracture behaviors of colas with different water contents were studied.The influence of water content on the dynamic fracture toughness and crack propagation velocity was revealed.The effect of micro-structure on the macro-mechanical properties of coal was discussed.The study results can provide a reference for the inhibiting the occurrence of the disasters during coal mining,and also investigating the facture mechanism of coal with water content.The main achievements are as follows:(1)The effect of water content on the tensile and compression mechanical properties of coal was investigated.The servo-controlled RMT150-C system was used to conduct the uniaxial and triaxial compression experiments.The relations between the water content and the tensile strength,uniaxial compressive strength and elastic modulus were obtained.The change laws of the triaxial compressive strengths and elastic moduli of coals under different confining pressure with the change of water content.The influence of water content on stress-strain curves and failure modes was analyzed.The loss rates of the internal friction angles and the cohesive forces of coals with different water contents were obtained.(2)A series of fracture tests were performed using the asymmetric semi-circular bend(ASCB)specimen made of granite.The influence of water content on the fracture,crackinitiation angle and failure modes were analyzed in detail combined with several mixed mode fracture criteria.The loss rates of mode I/II fracture toughness of coal specimens with water contents of 3.6% and 1.8% were obtained.The values of KIIC / KIC of coal specimens with different water contents were determined.A comparison between the fracture toughness ratios predicted by the general maximum hoop stresses criteria and experimental results was investigated.The relation of experimentally determined crack initiation angles and curves of the maximum hoop stresses criteria was obtained.The shortcomings of the ASCB specimen and NSCB specimen for fracture toughness determination of rock materials were discussed.(3)For semi-disc specimen with water content and other low strength materials,the problems of weak transmission wave by using the conventional split Hopkinson pressure bar(SHPB)was analyzed,and the corresponding modified measures were provided.The incident bar and transmission bar made of aluminium were used during the test to reach the impedance matching.Meanwhile,the transmission bar with two supports was used to reduce the attenuation of transmission wave.In addition,the cone strike was used to obtain the repeat the sine shaped waves,also eliminated the vibration.Ultimately,the strong transmission wave can be obtained.(4)Based on the modified SHPB testing system,the dynamic fracture tests on the coal with different water contents were conducted.The crack propagation gauge(CPG)and high speed camera were used to record the crack propagation and failure behavior.The effect of water content on the dynamic fracture toughness,crack propagation velocity and failure mode was investigated.The relation between the dynamic fracture toughness and loading rate was obtained.The loss rates of crack propagation velocities of coal with different water contents were determined.The stain characteristics and failure behavior of coal during the test were obtained.(5)The scanning electron microscope and the ultrasonic detector were used to investigate the microscopic structure and P-wave velocity of coal with different water contents,respectively.The microscopic structure and P-wave velocity after different immersion times were obtained.Based on the nuclear paramagnetic resonance technique,the porosity,the amplitude of T2 spectrum,the spectrum area and the proportion of the secondary spectrum under different water contents were investigated.Combined the study results at home and abroad,the influence of water on the softening and reinforcement mechanisms was discussed.There are 97 pictures,11 tables,and 158 references in the dissertation. |
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