不同真三轴应力路径下钻孔煤体的力学特性及气体流动规律

Mechanical characteristics and gas flow laws of drilled coal under different true triaxial stress paths

为了研究钻孔煤体渐进破坏过程中的力学特性及气体流动规律,利用多功能真三轴流固耦合试验系统,开展真三轴钻孔煤体力学特性及气体流动规律试验,揭示不同真三轴应力路径下钻孔煤体的强度和破坏特征以及渗透率演化规律。试验结果表明:(1)不同钻孔方向煤体的强度存在差异,其中钻孔垂直层理面方向时强度最大,垂直端割理方向强度次之,垂直面割理方向强度最小;钻孔煤体在不同真三轴应力路径下的强度均符合线性Mogi-Coulomb强度准则和Nadai强度准则,但在原生割理影响显著时Nadai强度准则的拟合效果更好;(2)真三轴加载应力路径下钻孔煤体在垂直割理方向不同变形阶段的渗透率均显著小于真三轴加卸载应力路径下测得的渗透率;(3)在垂直层理面方向,钻孔周围产生的宏观裂隙主要为张拉裂隙;在垂直端割理方向,钻孔周围产生的宏观裂隙存在剪切、张拉2种类型;在垂直面割理方向,钻孔周围产生的宏观裂隙主要以剪切裂隙为主。此外,钻孔煤体在最大主应力加载,中间主应力和最小主应力同时卸载应力路径下的裂隙数目尤其是伴生裂隙数目明显增多。

To study the mechanical properties and gas flow laws during the progressive failure of the drilled coal,the multi-functional true triaxial fluid-solid coupling test system was used to carry out true triaxial tests.The strength and failure characteristics,and permeability evolution laws of the drilled coal under different true triaxial stress paths were obtained.The results show that:(1)There are differences in the drilled coal strength for different borehole directions,among which the strength perpendicular to the bedding plane direction is the largest,followed by the strength perpendicular to the butt cleat direction,and the smallest perpendicular to face cleat direction.Both the linear Mogi-Coulomb strength criterion and the Nadai strength criterion are well fitted with the strength data of the drilled coal under different true triaxial stress paths.The Nadai strength criterion has a better fitting effect when the pre-existing cleats are obvious.(2)The permeability of the drilled coal at different deformation stages,when subjected to the true triaxial loading stress path perpendicular to cleat directions,is significantly smaller than that measured under the true triaxial loading-unloading stress paths.(3)When perpendicular to the bedding plane,the macro-fractures around the borehole are mainly tension fractures.When perpendicular to the butt cleat,there are two types of shear and tension macro-fractures generated around the borehole.When perpendicular to the face cleat,the macro-fractures around the borehole were mainly shear cracks.Besides,the number of macro-fractures,especially those accompanied by macro-fractures for drilled coal tested under the stress path of increasing maximum principal stress and decreasing intermediate and minimum principal stresses increased significantly.