The deformation behaviour and wave velocity variation of Tage Tuff, a transversely isotropic sedimentary rock, during desiccation in controlled environment was investigated in order to determine its deformation characteristics and elastic parameters. Strain as well as compressional (P) and shear (S) wave velocities were measured in a set of cylindrical Tage Tuff specimens cored across and along the bedding plane. Then, five independent dynamic engineering constants of transversely isotropic Tage tuff were calculated using the measured wave velocities and density during drying.Strain, P wave velocity and dewatering rates were sensitive to the degree of saturation as well as measured direction with respect to the bedding plane. Rapid increase in shrinkage as well as P and S wave velocities in below 35% saturation level occur due to the desiccation-driven hardening of Tage tuff. In relation to the calculated Young's and shear moduli variation with desaturation, we can conclude that the drying process strengthen the Tage tuff. Moreover, sensitivity of P wave velocity variation pattern with saturation according to the bedding direction gives an insight into the understanding of the water/air distribution within a transversely isotropic rock during desiccation.
This article is the summary of the published paper (Yasuhara et al. 2015). In this study, a series of long-term, intermittent permeability experiments utilizing Berea sandstone and Horonobe mudstone samples, with and without a single artificial fracture, is conducted for more than 1000 days to examine the evolution of rock permeability under relatively high temperature and confining pressure conditions. Effluent element concentrations are also measured throughout the experiments. Before and after flow-through experiments, rock samples are prepared for X-ray diffraction, X-ray fluorescence, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy to examine the mineralogical changes between pre- and post-experimental samples, and also for micro-focus X-ray CT to evaluate the alteration of the microstructure. Although there are exceptions, the observed, qualitative evolution of permeability is found to be generally consistent in both the intact and the fractured rock samples – the permeability in the intact rock samples increases with time after experiencing no significant changes in permeability for the first several hundred days, while that in the fractured rock samples decreases with time. An evaluation of the Damkohler number and of the net dissolution, using the measured element concentrations, reveals that the increase in permeability can most likely be attributed to the relative dominance of the mineral dissolution in the pore spaces, while the decrease can most likely be attributed to the mineral dissolution/crushing at the propping asperities within the fracture.
Long-term monitoring of distorted railway tunnels, such as that of inner space displacement, is normally carried out based on manual measurement. As the measurement work has to be carried out within limited timeframes, mostly in the middle of the night, it is difficult to obtain measurement data at short intervals. On the other hand, the introduction of automatic monitoring methods, which is effective to solve the problem described above, necessitate a huge amount of work and expense for installing and maintaining cables as well as costly equipment, especially in the tunnels which are usually long in the longitudinal direction.Therefore, this research developed a monitoring method using a wireless sensor network in order to lighten the data acquisition workload and reduce the costs for monitoring railway tunnels. The end devices of wireless sensor system are compact and light with power saving and can be used with batteries. A small data logger unified with coordinator, which operate with batteries, was developed to apply this system to the site where it is difficult to prepare electricity. A relay method was also proposed for transmitting the data over a long distance. The performance of the monitoring system was verified by means of field monitoring in two actual railway tunnels.
Gold mines in South Africa have mining depths of up to ~4 km. Mining in such depths under high-stress conditions induces many earthquakes, which pose serious hazard to mine workers. To mitigate seismic risks and to understand the role ofmicroearthquakes in earthquake generation processes, we have conducted ultra-sensitive seismic observations, which allow acoustic emissions (AEs) to be detected down to MW −5 (typical rupture size ~ several centimeters), at two South African gold mines. We have found out various modes of AE activity previously unknown from conventional seismic networks (detectability only down to MW ~ −1): AE activity before and after an MW 2.2 earthquake, AE activity likely reflecting damage accumulation processes ahead of an advancing mining front, and AEs occurring steadily along geological faults. This paper summarizes these findings, which received the fiscal 2016 frontier award of the Japanese Society for Rock Mechanics (JSRM).
This article is the summary for the paper (Liu et al. 2015) receiving the best research paper award from the Japanese Society for Rock Mechanics (JSRM) in the fiscal year of 2015. A fractal model to represent the geometrical characteristics of rock fracture networks is proposed to link the fractal characteristics with the equivalent permeability of fracture networks. Fractal dimension DT is utilized for representing the tortuosity of fluid flow and fractal dimension Df is utilized for representing the geometrical distribution of fractures in the networks. The results indicate that the equivalent permeability of fracture networks can be significantly influenced by the tortuosity of fluid flow, the aperture of fractures and a random number for generating the fractal length distribution of fractures in networks. Using the proposed fractal model, a mathematical expression between equivalent permeability K and fractal dimension Df is proposed for the models with large Df. The difference of calculated flow volume between the models considering and without considering the influences of tortuosity of fluid flow could be as high as 17.64% ~19.51%, emphasizing that the effects of tortuosity should not be neglected and should be included in the fractal model for accurately estimating the hydraulic behavior of fracture networks.
Evaluating geological conditions ahead of a tunnel face accurately and quantitatively is essential for a safe and reasonable tunnel excavation. Recently, it has also become important to ensure that geological surveys do not interrupt the daily tunnel excavation cycle. In this research, the authors have developed a new seismic tomography technique between a tunnel and ground surface. In order to conduct an accurate seismic tomography analysis, the source and receiver systems must be synchronized with negligible errors. Furthermore, utilizing the tunnel blasting vibrations as survey sources and the newly developed automatic data acquisition system made it possible to conduct the survey without interrupting the tunnel excavation. The results of applying this new method to the tunnel construction site show that the predicted values were in good agreement with actual geological conditions around the tunnel face.