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Direct tension tests are indispensable for obtaining an accurate understanding of the deformation and failure characteristics of rocks in tension. For this technical note, the authors collected original papers published in international journals and surveyed the previously reported methods and results of direct tension tests on rocks. Some important factors affecting the strength and stress–strain curve are discussed, such as the shape, size, anisotropy, and water content of the specimen; the loading rate; and the confining pressure. The relationship between the results of the direct tension tests and those of other tests also is discussed. Although various types of testing apparatuses have been adopted in direct tension tests on rocks, the dependence of test results on the apparatuses has not been investigated. In this study, direct tension tests were conducted using various testing apparatuses in seven laboratories. The results of the direct tension tests showed that direct tensile strengths were not dependent on the type of test apparatus used or whether a flexible linkage system was used. The results of the tuff in this study showed that the coefficient of variation of the direct tensile strengths is smaller than that of the Brazilian tensile strengths. Moreover, the results revealed that the coefficient of variation of the direct tensile strengths is comparable to that of the uniaxial compressive strengths of the tuff. The authors noted that to obtain the stress–strain curve from the pre-failure region to the post-failure region, it is necessary to conduct a test with a high-stiffness machine and without a flexible linkage system. This technical note summarized the results of the standardization activity by the authors in the Japanese Geotechnical Society.

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We investigated experimentally the change in permeability in three different types of soft rocks including mudstone, sandstone, and tuff from the Fureoi and Takinoue formations, which are representative of the CCS demonstration site at Tomakomai in Japan, and another type of mudstone from the Besho Fm. (B-M). Permeability during deformation, shear fracturing, and post-failure slipping is estimated from measured flow rate and differential pore pressure. In addition, the morphologies of the shear fracture zones were examined using the X-ray CT scanning technique. All the samples exhibit typical brittle-fracturing behaviors, except for the B-M sample which does not show a major shear fracture. During the fracturing process, the permeability increased by one to three orders of magnitude. Further changes in the post-failure slipping and stress relaxation regimes show strong dependence on the type of rocks. Observed changes in permeability can be interpreted as results of fracturing creation, shear zone smoothing, closure and reactivation of fractures under different stress regimes.

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