Experimental Study on the Scale Effect Law of Shale Strength and Deformation under Different Loading Modes
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摘要: 为了认识加载方式对页岩强度与变形尺度效应的影响，将鄂尔多斯盆地长7段页岩露头切割成不同尺度岩样，进行了三轴压缩、单轴压缩和巴西劈裂试验，分析了不同加载方式下页岩强度与变形的尺度效应规律。三轴压缩试验中，页岩强度与变形的尺度效应在岩样高径比大于2和小于2时存在明显差异，岩样高径比为0.4~0.8时，端部摩擦效应和非均质性对页岩强度与变形的尺度效应起主导作用；页岩强度与变形参数的尺度效应，单轴压缩时最显著，巴西劈裂次之，三轴压缩最不明显；进行三轴和单轴加载时，页岩应力应变曲线的分段变形特征不存在尺度效应现象，但在巴西劈裂试验时，页岩应力应变曲线的分段变形特征多样，其与尺度的关系比较复杂；3种加载方式下，页岩的破坏模式均不存在尺度效应；非均质性、各向异性和端部摩擦效应是页岩在不同加载方式下强度与变形尺度效应程度不同的根本原因。上述研究结果可为室内选择合理的岩样加载方式、测试尺度和确定页岩力学参数等提供参考。Abstract: In order to explore the influence of different loading modes on the scale effect on shale strength and deformation, 7 shale outcrops of the Yanchang Formation in the Ordos Basin were taken and made into different sized core samples for uniaxial, triaxial compression and Brazilian splitting tests. The scale effect law of shale strength and deformation were analyzed under different loading modes. The results show that there are obvious differences in the scale effect on shale strength and deformation when core height/diameter ratio is larger or less than 2 in the triaxial compression test, and end friction effect and heterogeneity will play a decisive role when the ratio is between 0.4 and 0.8. In uniaxial compression tests, the scale effect of shale strength and deformation is the most significant, followed by Brazilian splitting, and triaxial compression. In uniaxial and triaxial loading tests of shale, there is no effect of scale on the sectional deformation characteristics of stress-strain curve of rock sample while it is various and the scale effect law is complex in Brazilian splitting test. Surprisingly, no scale effect is observed in shale failure modes under three loading modes. Heterogeneity, anisotropy and end friction effect are the fundamental causes in different effects of scale on strength and deformation under different loading modes. The experimental results can provide guidance for the selection of reasonable loading mode, testing scale and determination of rock mechanical parameters.
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Keywords:
- shale /
- loading modes /
- compressive strength /
- tensile strength /
- deformation /
- scale effect /
- laboratory testing
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图 1 三轴压缩试验得到的岩样强度、变形参数与尺度的关系
Figure 1. Relationship between scale and strength and deformation parameters of rock samples obtained from triaxial compression test
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图 2 三轴压缩试验得到的应力应变曲线
Figure 2. Stress-strain curve obtained from triaxial compression test
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图 3 三轴压缩试验得到的应力应变曲线的特征
Figure 3. Characteristics of stress-strain curves obtained from triaxial compression test
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图 4 三轴压缩试验中的岩样破坏形态
Figure 4. Failure forms of rock samples in triaxial compression test
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图 5 单轴压缩试验得到的岩样强度、变形参数与尺度的关系
Figure 5. Relationship between scale and strength and deformation parameters of rock samples obtained from uniaxialcompression test
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图 6 单轴压缩试验得到的应力应变曲线的特征
Figure 6. Characteristics of stress-strain curve obtained from uniaxial compression test
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图 7 单轴压缩试验得到的岩样破坏形态
Figure 7. Failure forms of rock samples obtained from uniaxialcompression test
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图 8 巴西劈裂试验得到的岩样强度、变形参数与尺度的关系
Figure 8. Relationship between scale and strength and deformation parameters of rock samples obtained from Braziliansplitting test
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图 9 巴西劈裂试验得到的应力应变曲线的特征
Figure 9. Characteristics of stress-strain curve obtained from Brazilian splitting test
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图 10 巴西劈裂试验得到的3种典型的应力应变曲线
Figure 10. Three typical stress-strain curves obtained from Brazilian splitting test
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图 11 巴西劈裂试验的岩样破坏形态
Figure 11. Failure forms of rock samples obtained from Brazilian splitting test
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图 12 页岩单/三轴压缩时内部缺陷示意
Figure 12. Schematic diagram of internal defects in shale under uniaxial/triaxial compression
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图 13 页岩巴西劈裂试验原理及外力加载特征
Figure 13. Principle of shale Brazilian splitting test and the characteristics of external force loading
表 1 尺度效应显著性对比结果
Table 1 Comparison on the significances of scale effect
加载方式 具有显著尺度效应 不具有显著尺度效应 尺度效应
显著程度强度与变形参数数目 决定系数均值 强度与变形参数数目 决定系数均值 单轴加载 7 0.801 1 1 0.307 0 最显著 巴西劈裂 4 0.742 2 4 0.165 7 较显著 三轴加载 4 0.684 7 4 0.101 8 不显著 
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