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It is essential to investigate the influence of pores and fractures on physical and transport properties of rocks for understanding many key problems in seismology, volcanology and rock engineering. In rocks, water is primarily stored in and migrates through networks of pores and fractures at all scales. It is therefore important to know how fluid flow in such pores and fracture networks responds to the elevated pressures found at depth. Here, we report results from an investigation of changes in fluid permeability, and associated changes in P-wave and S-wave velocities, at elevated effective pressure for both intact and macro-fractured sandstone samples. Specifically, we used two types of sandstone; Clashach sandstone from Scotland and Shirahama sandstone from Japan, which have different porosities and pore size distributions. Wave velocities increased with increasing pressure for all cases. Additionally, wave velocities in macro-fractured samples were lower than those in intact samples. For Shirahama sandstone, permeability decreased with increasing pressure. By contrast, permeability of Clashach sandstone remained essentially constant over the whole pressure range. This lack of change is interpreted as being due to the presence of a well-connected network of large pores with high aspect ratios. It was also found that the difference in permeability between intact and macro-fractured sandstone samples was small. From these results, it is concluded that the networks of pores control the permeability in sandstone. © 2012 The Society of Materials Science, Japan.

Study of subcritical crack growth is important in estimating the long-term strength of rock and in ensuring the long-term stability of structures in a rock mass. The long-term strength is affected by the value of subcritical crack growth index (SCGI). However, the effect of environmental conditions on SCGI has not been clarified. In this study, we have estimated SCGI under various environmental conditions on a sample of Kumamoto andesite, using the Double-Torsion method. We show that the SCGI decreased when the temperature is higher in both air and water; the SCGI in water is smaller than that in air. When the relative humidity of the air increased, SCGI tended to be smaller and at 90 % relative humidity was close to the value in water. Under alkali conditions, the SCGI decreased slightly. Using the values of SCGI, the long-term strength was estimated and it is shown that the long-term strength decreased when SCGI was smaller. It is concluded that SCGI and the long-term strength is affected by the surrounding environment, specifically by the temperature and water. To ensure the long-term stability of rock, the control of the temperature and water migration is important.

High-strength and ultra low-permeability concrete (HSULPC) is a strong candidate for a radioactive waste package containing transuranic radionuclides (TRU waste) for geological disposal. The information and knowledge of the time-dependent fracturing of HSULPC and surrounding rock mass are essential to assess the long-term stability of such underground repositories. Here we measured crack velocity in andesite and HSULPC both in air and water to examine slow crack growth (subcritical crack growth) by Double-Torsion method. In air, the crack velocity in andesite increased when the temperature increased. On the other hand, the temperature had little effect on the crack velocity in HSULPC in air. In water, the crack velocity increased when the temperature was higher for both andesite and HSULPC. By using the experimental results of subcritical crack growth, the long-term strength was estimated based on the model of a single crack subjected to tension in an infinite plate. It was shown that the long-term strength of HSULPC was higher than that of andesite. When the temperature increased, the long-term strength of andesite both in air and water and that of HSULPC in water decreased. The long-term strength in water was smaller than that in air for both materials. It is concluded that water remarkably affects subcritical crack growth and the long-term strength in these materials.

The propagation of microcracks in granite was studied experimentally, focusing on the effects of the temperature and relative humidity on the properties of P-wave propagation. Since it is difficult to observe the propagation of a microcrack directly, P-wave propagation was investigated while controlling the temperature and relative humidity. The results showed that changes in the temperature and relative humidity of the surrounding environment affected the P-wave velocity and amplitude. An increase in the relative humidity affected the P-wave velocity if the temperature was high. On increasing the humidity at high temperature, the P-wave velocity decreased due to subcritical growth of microcracks arising from stress corrosion. We concluded that subcritical growth of microcracks arises from thermal stress and increased humidity.