Triaxial tests were conducted under confining pressures of 115 MPa at 295 K using an ultra-compact triaxial cell. The samples were held for 24 h under the target consolidation pressure, and then constant strain-rate compression was applied measuring permeability. The structural changes were measured by thin section image analysis and micro-focus X-ray computed tomography. For the Shikotsu welded tuff, the permeability decreased monotonously with axial compression. The decrease ratio of permeability increased with confining pressure mainly due to pore collapse. An equation representing the post-compression permeability with confining pressure was proposed. For the Kimachi sandstone, the permeability first decreased with increasing axial stress, then began to increase when the total lateral strain recovered its value before the application of confining pressure, and then maintained an almost constant value in the post-peak region.The minimum and final permeability decreased with increasing confining pressure mainly due to compaction and large plastic deformation of clay cementing materials. The final permeability was larger under small confining pressures and smaller under high confining pressures than the permeability before axial compression. Based on these observations, new equations were proposed representing the minimum and stable permeability and the strain at the permeability values with confining pressure. For the Inada granite, the tendency of permeability change during axial compression was almost the same as for the Kimachi sandstone. A new equation representing the minimum permeability mainly due to elastic deformation as a function of the confining pressure was proposed. The final permeability was larger than that before compression, and the permeability decreased with smaller confining pressure mainly due to a decrease in the number and width of rupture planes and the absence of axial cracks from biotite. The permeability, however, increased under larger confining pressures mainly due to the formation of subrupture planes caused by the high stress concentration at the rough and stiff rupture plane under large confining pressure.

Triaxial compression tests were carried out at 295K and 353K under confining pressure of 1-15 MPa for Shikotsu welded tuff, Kimachi sandstone and Inada granite. The samples were kept for 24 hr consolidation and then axial compression was applied measuring permeability. Permeability decreased monotonously for Shikotsu welded tuff. The permeability decreased first, began to increase before peak load and showed almost constant value in the residual strength state for Kimachi sandstone and Inada granite. Permeability decreased by failure for Shikotsu welded tuff. It increased due to failure under low confining pressures but decreased under high confining pressures for Kimachi sandstone. It increased due to failure for Inada granite. The permeability at 353K was lower than that at 295K for all of the three types of rock. The mechanisms of the permeability decrease are also shown.