Uniaxial compression tests were carried out for the gypsum samples containing pre-existing flaws in non-overlapping geometry. The geometries of prepared samples were set to have same strength by the simple stress analysis derived only from the sample's geometries. Compared with the actual experimental strength and predicted strength, we discussed the failure mode governing the strength characteristic of the sample. In the case of lower uniaxial compressive strength than that of simple stress analysis, shear cracking so-called the secondary crack and crack coalescence were observed before peak strength. In these samples, a pre-existing flaw coalesced with another flaw by the shear secondary crack. The coalescence of flaws has possibilities to reduce the compressive strength. In the end, the stress analysis was improved by using the observed shear failure modes. The improved analysis was able to calculate the compressive strengths that agree with that of experiments. These results provide increased understanding of the fundamental property of rock material under uniaxial compression.

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