高霞,王楠楠,秦程,等. 柔性边界三轴压缩条件下含瓦斯水合物煤体宏细观力学性质[J]. 煤炭学报,2024,49(6):2691−2710. doi: 10.13225/j.cnki.jccs.2023.0672
引用本文: 高霞,王楠楠,秦程,等. 柔性边界三轴压缩条件下含瓦斯水合物煤体宏细观力学性质[J]. 煤炭学报,2024,49(6):2691−2710. doi: 10.13225/j.cnki.jccs.2023.0672
GAO Xia,WANG Nannan,QIN Cheng,et al. Macro-meso mechanical properties of gas hydrate bearing coal under triaxial compression with flexible boundary condition[J]. Journal of China Coal Society,2024,49(6):2691−2710. doi: 10.13225/j.cnki.jccs.2023.0672
Citation: GAO Xia,WANG Nannan,QIN Cheng,et al. Macro-meso mechanical properties of gas hydrate bearing coal under triaxial compression with flexible boundary condition[J]. Journal of China Coal Society,2024,49(6):2691−2710. doi: 10.13225/j.cnki.jccs.2023.0672

柔性边界三轴压缩条件下含瓦斯水合物煤体宏细观力学性质

Macro-meso mechanical properties of gas hydrate bearing coal under triaxial compression with flexible boundary condition

  • 摘要: 为探究不同边界条件下围压对含瓦斯水合物煤体的宏细观力学性质影响规律,对饱和度80%的煤样进行围压12、16、20 MPa下双轴离散元试验。首先,考虑颗粒形状效应、水合物胶结作用以及热缩管的影响,选取抗滚动阻力线性模型和平行黏结模型,分别构建了刚性边界和柔性边界的含瓦斯水合物煤体双轴数值模型。其次,通过与室内试验结果(偏应力−轴向应变曲线、体积应变−轴向应变曲线、内摩擦角、黏聚力以及试样破坏模式)对比,验证了数值模型的可靠性,并发现柔性边界能更好地反映试样轴向应变−偏应力特征、剪胀特性以及强度特性。基于所建数值模型,从试样内部位移场、力学平均配位数、平均孔隙率、接触力链以及水合物黏结破坏角度揭示围压和边界条件对含瓦斯水合物煤体宏细观力学性质的影响规律。结果表明:① 随着围压的增大,刚性边界的破坏型式多表现为单斜面剪切破坏;柔性边界的破坏型式由单叉型剪切破坏转变为单斜面剪切破坏。② 随着围压的增加,2种边界的力学平均配位数均呈增大趋势,平均孔隙率均呈减少趋势,试样更密实,试样强度增加。③ 随着围压的增加,颗粒间法向接触力继续增加,试样强度增加,分布在轴向附近的法向接触力随之增大,而在水平向附近的法向接触力变化较小,竖向与水平向法向接触力差异越明显,各向异性更突出;在峰值强度处,随着围压从12 MPa增加到20 MPa,柔性边界法向接触力增加了54.50%,刚性边界法向接触力增加了45.70%。④ 在不同围压和边界条件下,试样产生张拉和剪切2种不同破坏形式,试样内部起裂主要以水合物和煤间剪切裂纹为主导;随着围压的增大,2种边界内部剪切裂纹数量呈逐渐减小趋势。研究结果从细观尺度上揭示围压对含瓦斯水合物煤体强度变形破坏等宏观力学特性的影响机制。

     

    Abstract: To explore the influence of confining pressure on the macro-meso mechanical characteristics of gas hydrate bearing coal (GHBC) under different boundary conditions, the biaxial discrete element tests were carried out subjected to the confining pressures of 12, 16 and 20 MPa for GHBC with saturation of 80%. Firstly, the biaxial numerical models of GHBC were established for flexible and rigid boundaries, using the linear model of the rolling resistance and the parallel bonding model. These numerical models incorporated the influences of particle shape effect, the hydrate cementation and the heat-shrinkable pipe. Then, the reliability of the numerical model was verified, by comparing with the indoor test results (stress-strain curves, bulk strain curves, internal friction angle cohesion and specimen failure modes). It is found that the flexible boundary can better reflect the deviatoric, stress-axial strain, the shear expansion and the strength characteristics of the sample. Based on the established numerical model, the roles of the confining pressure and the boundary condition on the macro-meso mechanical properties of GHBC were clarified from the perspectives of the internal displacement field, the mean mechanical coordination number, the mean porosity, the contact force chain and the hydrate bond failure. The results show that: ① with the increase of confining pressure, the numerical sample with the rigid boundary mostly exhibits the single inclined plane shear failure, while that with the flexible boundary varies from the single fork shear failure to the single inclined plane shear failure. ② With the increase of confining pressure, for the two boundaries, the mean mechanical coordination numbers increase and the mean porosity decrease, leading to a denser and higher strength of the sample. ③ With the increase of confining pressure, the normal contact force between particles continues to increase, and the sample strength increases. The normal contact force distributed near the axial direction increases, while that near the horizontal direction varies weakly. The higher the confining pressure is, the more difference between the vertical and the horizontal normal contact forces have, the more prominent the anisotropy is. The normal contact force increases by 54.50%, at the flexible boundary, and increases by 45.70%, at the peak strength point, with the confining pressure increasing from 12 MPa to 20 MPa. ④ Under different confining pressures and boundary conditions, the samples fail with two different failure modes, tensile and shear. The samples mainly crack from the shear between the hydrate and coal. The research results reveal the mechanism of the influence of confining pressure on the strength deformation and failure of GHBC on the mesoscale.

     

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