Experimental Study on the Scale Effect Law of Shale Strength and Deformation under Different Loading Modes

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.