The brittle fracture is the most studied process in rock mechanics fields, especially the post-failure behavior of rock is one of the key issues for rock mechanical problems. However, even at present, it is still difficult to obtain complete stress-strain curve of brittle rocks in the laboratory experiments. Therefore, a new Distinct Element Method (DEM) code for the uniaxial compression tests with radial strain control was developed and Class II behavior of rock was simulated. The simulation results show good agreement with the complete stress-strain curve obtained from the laboratory experiment with radial strain control. These imply that the DEM simulation may be a strong tool for the analyses of rock failure mechanisms such as class II behavior. The post-failure behavior of rock was discussed in detail by using a newly developed DEM code. The simulation results suggest that the loading control methods strongly affect the failure mechanisms and processes of rocks under uniaxial compression (Class I and II). Moreover, it is also found that the key to understand the Class II behavior of rock is the localized deformation, such as shear band. A clear shear band appears in the axial strain controlled uniaxial compression test, and a significant increase of the axial strain occurs with the crack surface slips along the shear band. As a result, complete stress-strain curve of the entire rock shows the Class I behavior. On the other hand, clear shear band does not appear in the radial strain controlled uniaxial compression tests, because formation of shear band does not grow due to the unloading in a critical state. Since clear shear band is not formed, most local small parts of the rock still keep elastic behavior, and as a result, the complete stress-strain curve of the entire rock shows Class II behavior.
The effect of the humidity on the fracture toughness 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 fracture toughness to compare the results to those of subcritical crack growth. All measurements 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 fracture toughness. The fracture toughness decreased with the increase of the humidity. The decrease of the fracture toughness was more remarkable when the content of clays in rock was higher. It is considered that the amount of clays had a significant effect on the change of the fracture toughness of sandstone. Degradation of clays with increasing the water content is considered to affect the decrease of the fracture toughness. It was shown that the decrease of the fracture toughness with increasing the humidity was less than the decrease of the stress intensity factor for subcritical crack growth. This was caused because stress corrosion had less effect on the fracture toughness. It is concluded that the crack growth in sandstone is affected by the humidity and the content of clays.
Information concerning size and shape of TBM debris is essential for designing the efficient loading, hauling and dumping system in TBM excavation. The information is also very important to decide how to recycle the TBM debris. However, only very limited information is available at present, and then theoretical or calculation method to estimate size and shape of TBM debris is still open to discussion. In this paper, size and shape of TBM debris are estimated by the well-known Nishimatsu's equation that is usually applied to a roadheader or a shield type machine with chisel bits. In this study, the equation was applied to TBM excavation with disc cutters in which shear failure or plane extends from a new groove to an adjacent one. Estimated size and shape of TBM debris were found to be consistent with the measured results. Side force applied to a disc cutter and the resultant stress on the disc cutter were discussed. The maximum debris size encountered in tunnel excavation was also discussed assuming that it followed the Gumbel distribution. It can be said that the proposed approach based on the Nishimatsu's equation indicated a promising results for future extension.
The understanding of source and flow path of the groundwater provides important strategy for the environmental management of the mine area. Then, in order to clarify a source and flow paths of groundwater around a closed mine, stable isotopes of hydrogen and oxygen and water quality of river water and groundwater sampled from the mining shafts and levels in the Hosokura mine were studied. The hydrogen and oxygen stable isotopes indicating altitude effect for river water and the chemical character of sampled water suggest groundwater flows mainly from the north area of Hosokura mine to the ore bodies under the center of the mine area. The distributions of total head and darcy velocity calculated by the numerical simulation analysis show down streamlines from highland to the ore bodies and correspond well with the flow system estimated from stable isotopes and water quality of the sampled water.
We have investigated crystallization behavior of cordierite-based oxynitride glass made by melting procedure of the powder mixture of cordierite and silicon nitride. The results clearly indicated that two polymorphs exist, a stable α form that has significant low coefficient of thermal expansion and a metastable μ form. TTT diagram of the crystallization of α form shows a typical C-shape curve that has a nose around 1420 K, above 1373 K, the α form can be directly obtained by isothermal crystallization of the oxynitride glass and the μ form inverts to the α form at temperatures below 1273 K. The rate of the crystallization of cordierite-based oxynitride glass is found to be smaller in comparison with that of the oxide system, which can be explained with the rate-controlling step of the formation of the α-cordierite grain and the diffusivity in the oxynitride glass. We also have fabricated α-cordierite ceramics that have the nitrogen content of 1.55mass% (2.2at%) which is revealed to be much smaller than that of the batch composition (5mass% (7.0at%) ) , which is due mainly to the decomposition reaction of nitrogen in the oxynitride melts and glasses during heat-treatments. However, the fabricated nitrogen-containing α-cordierite ceramics had the coefficient of thermal expansion of 10-7 K-1, which is comparable with the oxide α-cordierite ceramics. Moreover, the bending strength and the Young's modulus of the α-cordierite ceramics were found to be significantly improved with adding nitrogen, which would be attributed to the incorporation of nitrogen into the microstructure of α-cordierite.