不同卸荷应力路径下煤样破坏特征实验研究

Study on failure characteristics of coal sample under different unloading stress paths

现有对煤样破坏特征的研究存在力学参数测试较单一、应力加载方向局限性较大等问题,在反演真实地质情况下数值模拟效果存在偏差,并且对煤岩动力灾害和冲击倾向性判定是基于现场实验进行的宏观研究,对于真三轴不同卸荷应力路径下煤样的破坏特性机理研究较少。针对上述问题,以陕西彬长胡家河煤矿工程地质为研究背景,利用高频振动采集及孔内成像三轴动静载实验系统设计了3种不同卸荷应力路径下煤样真三轴实验,对煤样破坏特征、峰值强度特性、声发射响应特征和分形规律进行研究。结果表明:①3种不同卸荷应力路径下煤样破坏模式均为拉-剪复合破坏,煤样宏观裂纹的起裂破坏大多发生在强度相对较低的煤样中;各煤样均为轴向应力不断增加,各水平应力在逐渐降低的过程中为煤样提供了拉应力,导致不同卸荷应力路径下煤样各表面破坏形态显著不同。②3种不同卸荷应力路径下,峰值破坏阶段的应力存在明显差异,标准差达4.35 MPa,占峰值强度平均值的29.25%,当应力载荷超出3种应力路径峰值强度平均值14.87 MPa时,煤样均发生破坏。③在高静载作用下,煤样初始受载后孔隙压密,内部结构较均匀,无裂隙扩展使得在初始阶段损伤变量为0;在损伤稳定发展阶段,煤样内部孔隙达到极限状态发生破裂形成微裂隙,损伤变量为0.04~0.17;在加载过程中微裂隙迅速发育、扩展并汇集成裂隙网,煤样出现宏观破坏,煤样承载能力迅速下降,在损伤加速发展阶段损伤变量呈先急剧增加后平稳的趋势,最大损伤变量达1.0。当煤样受力失稳发生拉-剪破坏后,声发射能量出现突增现象;当声发射能量与损伤变量曲线交汇时煤样开始破裂,声发射能量与煤样破坏具有良好的耦合性。④在不同卸荷应力路径下,煤样分形维数越大,破碎程度越高。

The existing research on the failure characteristics of coal samples has some problems,such as single mechanical parameter test and large limitation of stress loading direction.The numerical simulation effect has deviation in the inversion of real geological conditions,and the determination of coal and rock dynamic disaster and bursting liability is based on the macro-research of field experiments.There are few studies on the failure mechanism of coal samples under different unloading stress paths in true triaxial.In order to solve the above problems,taking the engineering geology of Hujiahe Coal Mine in Binchang,Shaanxi Province as the research background,the true triaxial test of coal samples under three different unloading stress paths is designed by using the triaxial dynamic and static load test system of high-frequency vibration acquisition and borehole imaging.And the failure characteristics,peak strength characteristics,acoustic emission response characteristics and fractal law of coal samples are studied.①The results show that the failure modes of coal samples under three different unloading stress paths are tensile-shear composite failure,and the initiation failure of macro-cracks mostly occurs in the coal samples with relatively low strength.The axial stress of each coal sample increases continuously,and each horizontal stress provides tensile stress in the process of gradually decreasing,resulting in significantly different surface failure forms of coal samples under different unloading stress paths.②There are obvious differences in the stress of the three different unloading stress paths at the peak failure stage,and the standard deviation reaches 4.35 MPa,accounting for 29.25%of the average peak strength.When the stress load exceeds the average peak strength of the three stress paths by 14.87 MPa,the coal samples are damaged.③Under the action of high static load,the pores of the coal samples are compacted after initial loading.The internal structure is relatively uniform,and no fracture expands.The damage variable value is 0 in the initial stage.In the stable development stage of damage,the internal pores of the coal samples reach the limit state and break to form micro-fracture,and the damage variable value is 0.04-0.17.During the loading process,the micro-fracture develop rapidly,expand and converge into the fracture network,and the coal sample is macroscopically damaged.The bearing capacity of the coal sample decreases rapidly,the damage variable value increases sharply first and then stabilizes,and the maximum damage variable value reaches 1.0 in the stage of accelerated damage development.The acoustic emission(AE)energy value increases suddenly when the coal sample is damaged by stress instability and tensile-shear failure.When the AE energy intersects with the damage variable curve,the coal sample begins to break,and the AE energy has a good coupling with the coal sample failure.④Under different unloading stress paths,the larger the fractal dimension of coal sample,the higher the degree of fragmentation.