不同围压下花岗岩破裂机制及形状效应的离散元研究

Study on the macro-micro failure mechanism of granite and its geometry effect under the different conditions of confining pressure by discrete element

  • 摘要: 在花岗岩地层中开挖隧道时会引起围岩的变形破坏,多表现为岩爆、板裂、塌方等形式,通过室内单轴和三轴压缩试验能得到花岗岩的宏观力学参数及其渐进破坏机制。室内岩石试验可以从宏观角度分析花岗岩的破坏本质,而通过PFC2D离散元软件模拟室内单轴及三轴试验,则可以从微观方向研究分析花岗岩的破坏过程。本文以港珠澳大桥连接线南湾隧道工程为背景,综合采用室内岩石力学试验和离散元数值模拟方法,从宏观、微观两种角度对不同围压条件下花岗岩的宏细观力学参数、破裂机制及其形状效应进行了对比分析,全面的研究分析了花岗岩的变形破坏本质。研究结果表明:① 数值模拟与室内试验所得的结果,无论是宏观力学参数还是最终破坏形态均较为接近;随着围压的增大,岩石的峰值强度增加、弹性模量基本不变;随着试件长径比L/D增大,岩石峰值强度减小、弹性模量增大。② 采用基于相对轴向应变和单位面积裂隙数量的统计方法,能更加合理分析岩石微观渐进破裂机制;随着试件长径比L/D增大,岩石最终破坏形态逐渐从张拉破坏转变为剪切破坏;当L/D=1.0时单位面积内最终裂纹数量最大,当L/D=2.0时剪切裂纹所占比例最大。③ 随着岩石试件长径比L/D的增大,其峰值强度有所减小且受围压影响明显,弹性模量也明显的增大但与围压的关系不显著。④ 通过分析裂隙数量与应变关系可知,在不同围压条件下,岩石内部裂隙数量随着轴向变形的增加呈现“S”型曲线增长,当轴向变形接近峰值应变时裂隙出现突变增长,且仅当围压较小时最终裂隙数量趋于收敛。⑤ 随着试件长径比L/D的增大,岩石破坏时单位面积内裂隙数量逐渐减少,且减小速度增快。总的来说,岩石试样的形状改变对花岗岩的整体峰值强度和弹性模量有着明显的影响,因此,室内试验中应合理设计岩石试样的形状,以获取准确的岩石强度和力学参数。

     

    Abstract: When the tunnel is excavated in the granite stratum,the surrounding rock will deform and fail,and the fail- ure modes of the surrounding rock are mostly rockburst,slab cracking,landslide,etc. The macroscopic mechanical pa- rameters of the granite and the progressive failure model can be obtained by indoor uniaxial and triaxial compression tests. The indoor compression test can analyze the failure nature of granite on macroscopic scale, while the failure process of granite under uniaxial and triaxial compression on microscopic scale can be simulated by PFC2D soft codes. By taking the Nanwan tunnel engineering along the Hong Kong-Zhuhai-Macao bridge connecting line as the study background,the macro-microscopic mechanical parameters,fracture mechanism and their shape effect of granite sample were analyzed by employing both of laboratory test and numerical simulation of discrete element. Research results show that: ① The results of numerical simulation are similar as the results form laboratory test in not only the macro scale mechanical parameters but also the final failure pattern. With the increase of confining pressure,the peak strength of rock increases accordingly,while the elastic modulus varies little. With the increase of specimen ratio between length and diameter L / D,the peak strength of rock decreases accordingly,while the elastic modulus increases. ② The statisti- cal method based on the comparative strain and crack number per some area is more effective to analysis the micro scale progressive failure mechanism with the increasing of specimen ratio between length and diameter L / D,the final failure pattern changes from shear to tensile progressively. The final crack number per some area will be largest when the L / D is equal to 1. 0. The percent of shear cracks will be largest when L / D is equal to 2. 0. ③ With the increase of the aspect ratio of rock specimens L / D,the peak strength decreased and the influence of confining pressure was obvi- ous. The elastic modulus also increased obviously but the relationship was not significant with confining pressure. ④ By statistically comparing the relationship between the number of fractures and the strain,the number of cracks inside the rock increases with the increase of axial deformation like the “S” curve under different confining pressure condi- tions. When the axial deformation is close to the peak strain,the crack grows abruptly,and the number of final cracks tends to converge only when the confining pressure is small. ⑤ With the increase of the length-to-diameter ratio of the specimen L / D,the number of cracks per unit area decreases gradually when the rock fractures,and the speed of de- creasing will increase. In general,the shape change of rock sample has significant influence on the whole peak strength and elastic modulus of granite. Therefore,the shape of the granite sample should be rationally designed in order to get the real parameters and strengths of rock during indoor test.

     

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