It is important to evaluate the long-term stability of rock materials for the purpose of ensuring the long-term integrity of structures in a rock mass. In this study, we showed an evaluation method of the long-term strength of rock considering the change of the surrounding environmental conditions. It was shown that the long-term strength of rock decreased remarkably when the environmental condition changed from air to water condition and from low temperature to high temperature condition. This is due to the acceleration of subcritical crack growth in rock, because the relationship between the crack velocity and the stress intensity factor for subcritical crack growth is dependent on the temperature and existence of water. It is also recognized that the values of long-term strength of rock are affected significantly by the highest temperature and water if the rock can be surrounded by water. It is concluded that the long-term strength of rock under water environment with high temperature is important for ensuring the long-term integrity of structures in a rock mass.
Fracture processes of argillaceous schist and granite subjected to freeze-thaw cycles were investigated based on their deformation behaviors. It was found that argillaceous schist showed apparent anisotropy in deformation. Significant extensile strain was observed in the direction perpendicular to the cleavage plane around -3°C in freezing process on the water-saturated specimen, followed by extensile plastic strain at the end of the cycle. No apparent extension and anisotropy were found on the water-saturated specimen of granite, but decrease in contraction rate was seen at -3°C in freezing process with AE activity. The extensile plastic strain of both rocks gradually increased with the number of freeze-thaw cycles, but magnitude of the plastic strain on argillaceous schist was much greater than that of granite. These results indicate that there might be common mechanism in fracture process between both rocks, but significant differences in deterioration rate. Deterioration of argillaceous schist is much faster because fracture mainly develops by extension of well-developed cleavage in the rock. It was concluded that weak plane such as cleavage strongly affected deterioration of rocks by freeze-thaw cycle.
The precision stone, which is used for precision measurement and machining, is usually made from gabbro. Such a precision stone have to keep flat to a high accuracy, however, that flatness is changed with humidity. Here, we characterized the petrological and micro-structural properties of intact gabbro to use precision stone, and carried out deformation test under wet condition using three different samples in order to clarify the micro-deformation mechanism. As a result, the displacement caused by water is strongly related to nano-pore.
There are several reports that the static Young's modulus of rocks differs from the dynamic one. In this study, the relation between static Young's modulus and dynamic Young's modulus of rocks is examined. First, static Young's modulus was obtained by the uni-axial compression test, and dynamic Young's modulus was obtained by the ultrasonic transmission method. Secondly, their relationship was examined from the viewpoint of the strain level. In addition, the value of the Poisson's ratio for calculation of dynamic Young's modulus was examined. As a result, it was clarified that the dynamic Young's modulus becomes almost equal to the static one determined from the region of small strain if the Poisson's ratio used to calculate the dynamic Young's modulus is the appropriate value which is measured at the strain level without effect of crack closure.
The demand for the long-term safety utilization of rock structures has increased further in recent years. It has been reported that the repeated stress change induced the deterioration of rocks. However, there still remain many unknowns about the deterioration of rocks by repeated stress change. To obtain further insights into the deterioration of granite by repeated stress change, change in elastic property of specimen during the test was analyzed in detail. Cylindrical Westerly granite specimen with 20mm in height and 10mm in diameter was subjected to the cyclic loading with maximum 160 MPa (approximately 80% of the estimated uniaxial compressive strength) and minimum 0.5 MPa at room temperature. Axial and lateral strains, and the applied load were simultaneously measured at 100 points for one cycle, and Young's modulus and Poisson's ratio until failure were estimated applying these measured data. As a result, the characteristic changes in two elastic properties following the progress of the deterioration stage were revealed. At the initial degradation stage, the changes were very small. During the second steady stage, two properties were changed gradually and linearly. At the final accelerated stage, the changes were drastically. It was also found that these characteristic alterations were well accorded with the reported microcrack growth patterns.
Seafloor hydrothermal deposits have been found to be widespread in the waters off Okinawa Island. Since Japan currently depends heavily on foreign mineral resources, it is expected that such deposits will be developed in the near future. To assist with the design and manufacture of mining and crushing machinery and ore transportation equipment, in this study the mechanical properties of seafloor hydrothermal deposits were investigated. Tests were carried out on the strength, hardness and abrasiveness of boring cores and block samples of the deposits. The average uniaxial compressive strength was found to be between 20 and 60 MPa, the CERCHAR abrasiveness index was between 1 and 2, and the Mohs hardness was between 4 and 6. In uniaxial compression tests, in which a specimen was crushed up to 20% of its initial height, size of debris was a little smaller than tuff, andesite and sandstone. The specific energy calculated with the boring data was found to be closely related to the mechanical properties of the hydrothermal deposits, and its distribution beneath the seafloor was determined from some boring data. The results obtained in this study provide important guidelines for the development of mining techniques for seafloor hydrothermal deposits.
We calculated quality factor from seismic reflection data in order to classify the lithology around the Median Tectonic Line (MTL). Since seismic velocity around the MTL was not accurately estimated, it was difficult to classify the lithology and characterize the MTL. To calculate quality factor from seismic data, we used two methods: (1) spectral ratio method and (2) peak frequency method. In this study, we applied these two methods for the multi-channel seismic reflection data as well as zero-offset Vertical Seismic Profiling (VSP) data acquired at MTL in Ehime Prefecture, Japan. The result shows that quality factor is significantly changed at the lithological boundary beneath the MTL. From the estimated quality factor, we interpret that lithological boundary beneath the MTL is dipping to north. This study further shows that quality factor can be useful seismic attribute for lithology classification, even when seismic velocity cannot be estimated.
It is well-know that unforeseeable geological condition in underground construction projects causes cost overrun and delay of construction period. The reason why unforeseeable geological condition occurs is the uncertainty of geological condition estimated by geological surveys. Authors have estimated construction cost in the case of optimistic, likelihood, and pessimistic scenarios and compared the estimation with an actual one in order to evaluate the risk of construction cost variation in mountain tunnel projects. However, it is also important to evaluate the case in which unforeseeable geological condition, which is seldom occurred, causes high cost overrun in risk management. From such a viewpoint, this study proposes the methodology to evaluate probability of each rock classification by modeling the results calculated by external drift kriging, which is one of Geo-Statistics theory, as random valuable. In addition, the methodology to evaluate geological condition with core evaluation point, which this study uses, depends on data obtained in limited areas because the methodology uses borehole data. Therefore, the result of the estimation changes drastically if there is non-typical data. From such a viewpoint, this study proposes the methodology to eliminate non-typical data through evaluating the relation between each evaluation item of core evaluation point and physical property and using data in other tunnel construction project.
Pore pressure of the rock mass is successfully monitored at higher frequencies (0.1 Hz ∼ 2 Hz) using borehole. The frequency response of monitoring of pore pressure is significantly improved by closing the well, which reduces a high-cut response caused by wellbore storage effect in open wells. The response of pore pressure for stress or strain change is represented by linear poroelastic theory that describe mechanical coupling between rock mass and pore fluid. We revealed that even in crystalline rock the pore pressure change induced by deformation such as barometric pressure change, earth tides, and seismic waves is in agreement with theoretical model derived from the linear poroelastic theory. Pore pressure can be a proxy of stress and/or strain as studies on such as crustal deformation that have been made. It should be noted, however, that the applicable range of frequency is usually limited and absolute value of stress is difficult to be determined from pore pressure monitoring. Since amplitude and phase of pore pressure change induced by deformation depends on frequency, the sensitivity of pore pressure as a proxy of stress differs with frequency. The sensitivity may vary with time during a cycle of earthquake. The response to seismic waves follows prediction from poroelastic theory for both large and intermediate earthquakes. The sensitivity is robust at least for the amplitude of deformation induced by seismic waves. The response of pore pressure to tidal strain varies before and after large earthquake is observed, which may be attributed to apparent change of the sensitivity caused by change in diffusivity.
Recently, the frequency of rainfall induced landslide has drastically been increasing in Asian countries due to climate change. Also in Japan, shallow landslide caused by torrential rainfall, which was rarely observed in the past, has frequently occurred. Therefore, it is important to clarify the mechanism of rainfall induced slope failure in order to mitigate landslide risks. From such a viewpoint, authors have conducted field monitoring in Thailand, considering the similarity of precipitation characteristic between squalls in Thailand and torrential rainfall in Japan. Based on the results of field monitoring, this study proposes the methodology to investigate numerically the effect of rainfall infiltration into subsoil on slope stability, by focusing on the decrease of shear strength of the unsaturated soil due to increase of volumetric water content at relatively shallow area. The results show that the decrease of shear strength due to rainfall infiltration at the toe portion may become the trigger of shallow landslide.
The effects of tensile speed on the fracture strength of notched plates made from virgin and recycled materials were investigated. The virgin material was short-glass-fiber-reinforced polypropylene (GFPP). GFPP plates, each containing 30% E-glass fiber by weight, were fabricated by injection molding. In addition, pellets of recycled GFPP were kneaded and an injection-molded plate, referred to as the recycled GFPP (R-GFPP) plate, was fabricated from these pellets. The mean fiber lengths of GFPP and R-GFPP were 3.5 and 0.4mm, respectively. The notch-root radii of both the GFPP and the R-GFPP plates were 0.5, 1, and 2mm, while their notch depths ranged from 2 to 5mm. Tensile tests were performed at tensile speeds of 103, 102, 10, 1, and 8.33 × 10-5mm/s at a temperature of 23°C. It was found that all the notched specimens (GFPP and R-GFPP) failed in a brittle manner at the maximum load. The maximum elastic stress at fracture was determined from the notch-root radius and the time to fracture but was independent of the notch depth. The obtained results could be explained of the basis of the severity of the stress fields near the notch roots of the specimens.