The effect of the humidity on subcritical crack growth in sandstone in air was investigated experimentally. Rocks used in this study were Berea sandstone, Shirahama sandstone and sandstone obtained at Kushiro coalmine (Kushiro sandstone). Double-Torsion method was used in this study to measure the crack velocity and the stress intensity factor. All experiments were conducted under the same temperature and different relative humidity to investigate the effect of the humidity. It was shown that the humidity affected the crack velocity strongly. The crack velocity increased with the increase of the humidity. The increases of the crack velocity were much larger than that estimated from the classical stress corrosion theory. The increases of the crack velocity in Shirahama sandstone and Kushiro sandstone were larger than that in Berea sandstone by several orders of magnitude. This difference may be due to the difference of the rock fabric. Shirahama sandstone and Kushiro sandstone include clay minerals much more than Berea sandstone. It is probable that the fracture resistance of clay decreased due to the increase of the humidity. Consequently, large increases of the crack velocity occurred in Shirahama sandstone and Kushiro sandstone. It is concluded that the humidity has the strong effect on subcritical crack growth in sandstone and the control of the humidity is important to prevent the weakening of rock.
The advection and dispersion of contaminated materials in a rock mass are one of the important factors to evaluate the characteristics of the rock mass as a natural barrier function for a radioactive waste disposal project. In this study, X-ray CT method was applied to evaluate the advection and dispersion of solute in the crack which exists in rock mass. In order to visualize advection and dispersion phenomena, the tracer migration test have been conducted and tracer migration process was visualized by X-ray CT scanner. Here, an indicator, coefficient of tracer density increment, was newly introduced to analyze tracer migration. This is the indicator that represents the density of the tracer in the crack, and the evaluation of the density distribution of the tracer becomes possible. In this study, the influences of the crack aperture to the tracer migration process are discussed.
The time-dependent behavior of methane hydrate sediments is of great significance in the long-term mechanical stability of methane hydrate reservoirs and also in the evaluation of gas productivity from methane hydrate reservoirs. However, almost no information is available concerning the time-dependent behavior of methane hydrate sediments. In this study, firstly, drained triaxial compression creep tests were conducted for synthetic methane hydrate mixed with Toyoura sand under the conditions of a confining pressure of 9 MPa, pore water pressure of 8 MPa and temperature of 278 K. Secondly, experimental data on creep strain and strain rate were analyzed. Finally, the results obtained in creep tests were discussed and compared with the strain-rate dependence of strength in constant-strain-rate tests. The findings can be summarized as follows. (1) Creep strain monotonically increased with elapsed time. Strain rate decreased gradually at first, then took the minimum value and finally began to increase. (2) At the beginning of creep test, strain rate was inversely proportional to the m-th power of elapsed time. Just before failure of the specimen, strain rate was inversely proportional to the m-th power of residual time. (3) The lower the creep stress level, the longer the creep life. (4) Creep behavior was closely related to the strain-rate dependence of strength in constant-strain-rate tests.
This paper described that relationship between Acidithiobacillus feroooxidans T23-3 and fungus TEF1-1 in an enrichment culture. In order to evaluate the relationship, some culture experiments using cell-free extracts from T23-3 and TEF1-1 were carried out. Chemolitotrophic iron-oxidizing bacterium, A. ferrooxidans T23-3, was able to grow using inorganic compounds in cell-free extract of fungus TEF1-1 or A. ferrooxidans T23-3. Since the fungus TEF1-1 was able to grow using cell-free extract of the both microorganisms at pH 1.5, the fungus used some organic compounds in the cell-free extract as energy and carbon sources. When the cell-free extract of TEF1-1 used for the cultivation of A. ferrooxidans T23-3, lag-phase was observed because of lack of magnesium ion. Minimum concentration of magnesium ion for complete growth of A. ferrooxidans T23-3 was found to be 2.0 x 10-5 mol/dm3 from the experiments using synthetic medium. Also the cell-free extract contained enough nutritious components except magnesium ion for the growth of A. ferrooxidans T23-3. It was suggested that fungi grew using organics and inorganic ions released from the cell of chemolithotrophic iron-oxidizing bacteria by bacteriolysis and/or metabolisum and that the bacteria grew using inorganic ions from the lysis of fungi cells in the enrichment culture of iron-oxidizing bacteria. In addition, chemolithotrophic iron-oxidizing bacterium fixes carbon dioxide to convert some organic compounds in the culture. Therefore, it was evaluated that the relationship between chemolithotrophic iron-oxidizing bacteria and fungi was mutualistic symbiosis in the enrichment culture.