For rock tunnelling projects, it was difficult to predict geological conditions in an excavation cycle. As a result, it would cause serious accidents such as face collapses. Furthermore, it is only the face observation where the geological condition can be confirmed directly during the tunnel excavation, and it is difficult to completely grasp without overlooking the face condition, and as a result it would cause rock fall disaster. In order to prevent such geological problems, we developed a system, "Smart Face Watcher" that evaluates and visualizes the geological conditions in real time on site. This system consists of two analysis methods: "3D real time geology forecasting system by drilling data" and "Rock fall risk assessment system by image analysis". The system makes it possible to make decisions to choose the appropriate auxiliary method depending on the geological conditions in front of the face and around the tunnel and to quickly determine the face shotcrete thickness based on the rock fall risk from the face. In addition, by using the cloud server, it is possible to share ground evaluation results in real time with the headquarters, which enables geologists to support sites nationwide.

View full abstract

This article is the summary of the published paper (Sakaguchi et al. 2017). In this study, to examine the change in in-situ stress between before and after the 2011 Tohoku-oki earthquake, we performed stress measurements after the earthquake in the Kamaishi mine. The in-situ stress measurement period was from 1991 to 2016. The results showed that the magnitudes of the three-dimensional principal stresses and the vertical stress drastically increased during the mainshock and, at one year after the earthquake, were more than double those before the earthquake. The principal stress magnitudes then decreased with time, and returned almost to the pre-earthquake levels at about three years after the earthquake. The increasing and decreasing trends in stress in the Kamaishi mine can be interpreted in terms of the effects of coseismic rupture behavior of the Tohoku-oki earthquake mainshock and the occurrence of aftershocks in the Sanriku-oki low-seismicity region (SLSR), where the Kamaishi mine is located. The drastic increase in stress suggests that the SLSR may act as a barrier to further rupture propagation. In addition, the consistency between the change in measured stress and the change in seismicity in the Kamaishi regions suggests that the results of stress measurements, even those at a much shallower depth than the earthquake source fault, can be useful for understanding rupture propagation behavior.

View full abstract

In deep underground mines, stress re-distribution induced by mining activities could cause fault-slip. In general, it is difficult to estimate fault-slip-induced ground motion in the vicinity of mine openings because of the complexity of the dynamic response of faults and the presence of geological structures. In this paper, a case study is conducted for a Canadian underground mine, herein called “Mine-A”, which is known for its seismic activities. Using a microseismic database collected from the mine, a back analysis of fault-slip is carried out with mine-wide 3-dimensional numerical modelling. The back analysis is conducted to estimate the physical and mechanical properties of the causative fracture or shear zones. One large seismic event has been selected for the back analysis to detect a fault-slip related seismic event. In the back analysis, the shear zone properties are estimated with respect to moment magnitude of the seismic event and peak particle velocity (PPV) recorded by a strong ground motion sensor. The estimated properties are then validated through comparison with peak ground acceleration recorded by accelerometers. Lastly, ground motion in active mining areas is estimated by conducting dynamic analysis with the estimated values. The present study implies that it would be possible to estimate the magnitude of seismic events that might occur in the near future by applying the estimated properties to the numerical model. Although the case study is conducted for a specific mine, the developed methodology can be equally applied to other mines suffering from fault-slip related seismic events.

View full abstract

This article is the summary for the paper receiving the frontier award from the Japanese Society for Rock Mechanic (JSRM) in the fiscal year of 2018. In order to evaluate the fracture toughness of rock, until now, the proposed test methods assumes that the rock specimen is isotropic homogeneous material. Also, the specimen size of several centimeters is recommended. However, rock is a natural material and is composed of millimeter-sized mineral particles, ground mass and substrates, etc. In addition, rocks are said as heterogeneous at microscopic viewpoints because it includes weak planes such as pores and preexisting micro cracks. Numerous studies have been conducted on the effect of microstructure characteristics on the fracture toughness of rocks. It is discussed that the fracture toughness of granite, which is a crystalline rock, is influenced by micro cracks, orientation of mineral grains, kind of mineral grains within the rock and grain boundary characteristics. In this study, a micro material testing device for rock was developed based on the microscopic strength test method used in the field of metal material mechanics, and the microscopic fracture toughness test method was constructed at the same time. In addition, microscopic fracture toughness of mineral grains within granite was evaluated using a micro material testing device.

View full abstract

Knowing how pore pressure migrates at the fluid injection is essential to understand various phenomena in the reservoir, such as induced seismicity and fluid flow behavior. We took the privilege of our study, Basel Switzerland, where principal stress magnitude and orientation are available. We estimated the pore pressure required to cause shear slip using Coulomb failure criteria from stress information, the geometry of the fault planes of microseismic events, and a constant coefficient of friction. We performed a time series analysis of pore pressure distribution, and the results indicated that lower pore pressure migrated faster and further during the stimulation. Increasing injection pressure change the pressure gradient from the injection well to the edge of the stimulated zone. Higher pore pressure stared migration according to the increase of injection pressure, which slowed the migration of lower pore pressure. Our observations suggest the best sufficient injection pressure for each reservoir, and it can stimulate the existing fractures effectively and safely. We also found pore pressure migrated to the far field even after shut-in and redistribution of pore pressure at shut-in brought sufficient pore pressure increase to induce seismicity in the peripheral region. After the shut-in, the pore pressure gradient away from the well lessened, and eventually pressure became almost uniform. These observations suggest that pore pressure redistribution due to shut-in caused uniformly distributed pore pressure at the edge of the seismic zone. Shut-in pressure destabilized a large part of the fault existed in those zones, resulting in the outbreak of larger induced seismicity at the shut-in phase.

View full abstract

This article is the summary for the thesis (Huang, 2018) receiving the best doctoral thesis award from the Japanese Society for Rock Mechanics (JSRM) in the year of 2018. Firstly, the effects of fracture surface roughness, normal stress, shear displacement and shear directivity on permeability of 3D self-affine rough fractures are studied, and a predictive model is proposed to estimate the permeability of rough fractures during shear. Secondly, a numerical analysis of the shear effect on the hydraulic response of 3D fracture intersection model is presented. Simulation results reveal the development and variation of preferential flow paths through the model during the shear. Finally, a numerical procedure is originally developed to address flow problem through 3D rock discrete fracture networks (DFNs). In this method, fractures are modeled as circular discs with arbitrary size, orientation and location. Fracture networks are established with fractures following statistical distributions, after which the model is triangulated and fluid flow is calculated by solving the Reynolds equation using Galerkin method. The results show that the permeability of 2D DFN models underestimates the permeability of 3D DFNs by approximately 10.45~80.92%. A multi-variable regression function is proposed for predicting 3D fracture network permeability. The proposed model provides a simple method to approximate permeability of 3D fracture networks using parameters that can be easily obtained from analysis on outcrop trace maps of fractured rock masses.

View full abstract

Currently, the high demands and needs to improve productivity and labor-saving in construction industries leads to many automation and mechanization. This report aims to address the shortage of skilled worker in the future as well as improvement of safety, productivity and quality. Comprehensive collection and utilization of information were carried out and with target to eliminate the over reliance of experience worker in mountain tunneling, “Auto-mated Angle Control System” technology was developed to reduce the overbreak of tunnel during excavation. The concept of this method is to ensure that the productivity of drilling for blasting operation will not be influenced by the worker skills. This system was tested in Shin-Tomei Expressway Takatoriyama west tunnel construction project and results confirmed that the system successfully minimized the over-break of tunnel during excavation. We hope that this report will contribute to the improvement of safety and productivity.

View full abstract