Hydro-mechanical behavior of fractured sedimentary rock under stress axis rotation by crack tensor and permeability tensor

Tuesday, 16 December 2014
Takato Takemura, Nihon University, Department of Geosystem Sciences, Humanities and Sciences, Tokyo, Japan and Manabu Takahashi, AIST, Tsukuba, Ibaraki, Japan
Rotation of the principal stress axes and a changing stress distribution are generated by changing tectonic stress. The magnitude-9.0 subduction-zone earthquake of March 11, 2011, that occurred in the Japan Trench caused a change in the stress distribution in northern Japan due to the stress released. Such changes alter the mechanical properties of the rock mass, such as strength and deformability, and its hydraulic conductivities, and hence influence contaminant pathways.In this study, a true triaxial compression test was carried out under various stress conditions. The rock tested was a damaged sedimentary rock that had many fractures. Samples were subjected to various stress conditions, and permeability was measured after the required stress conditions had been reached. After the experiment, we determined the fracture geometry by using the crack tensor (Oda, 1983), and estimated the permeability tensor. As a result, measured permeability was different by a factor of 2 due to the change in the direction of the principal stress, even when the mean stress remained the same. In addition, the estimated permeability was in close agreement with the measured permeability, and it is suggested that we can estimate hydro-mechanical behavior by using fracture geometries obtained by observation. The directions of the fracture and principal stress axes are important factors for estimation of the long-term stability of the hydro-mechanical properties of a fractured rock mass.