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Hata, Koji*; Niunoya, Sumio*; Aoyagi, Kazuhei; Miyara, Nobukatsu*
Journal of Rock Mechanics and Geotechnical Engineering, 16(2), p.365 - 378, 2024/02
Excavation of underground caverns, such as mountain tunnels and energy-storage caverns, may cause the damages to the surrounding rock as a result of the stress redistribution. In this influenced zone, new cracks and discontinuities are created or propagate in the rock mass. Therefore, it is effective to measure and evaluate the acoustic emission (AE) events generated by the rocks, which is a small elastic vibration, and permeability change. The authors have developed a long-term measurement device that incorporates an optical AE (O-AE) sensor, an optical pore pressure sensor, and an optical temperature sensor in a single multi-optical measurement probe (MOP). Japan Atomic Energy Agency has been conducting R&D activities to enhance the reliability of high-level radioactive waste (HLW) deep geological disposal technology. In a high-level radioactive disposal project, one of the challenges is the development of methods for long-term monitoring of rock mass behavior. Therefore, in January 2014, the long-term measurements of the hydro-mechanical behavior of the rock mass were launched using the developed MOP in the vicinity of 350 m below the surface at the Horonobe Underground Research Center. The measurement results show that AEs occur frequently up to 1.5 m from the wall during excavation. In addition, hydraulic conductivity increased by 2 to 4 orders of magnitude. Elastoplastic analysis revealed that the hydraulic behavior of the rock mass affected the pore pressure fluctuations and caused micro-fractures. Based on this, a conceptual model is developed to represent the excavation damaged zone (EDZ), which contributes to the safe geological disposal of radioactive waste.
Aoyagi, Kazuhei; Ishii, Eiichi; Chen, Y.*; Ishida, Tsuyoshi*
Rock Mechanics and Rock Engineering, 55(4), p.1855 - 1869, 2022/04
Times Cited Count：4 Percentile：58.09(Engineering, Geological)Safety assessments related to the long-term migration of radionuclides in high-level radioactive waste disposal repositories need to consider the evolution of permeability associated with excavation-damaged-zone (EDZ) fractures. We observed EDZ fractures preserved by resin injection around a gallery in the Horonobe Underground Research Laboratory in Japan with the aim of assessing the sensitivity of aperture to shear displacement (i.e., dilation angle) in EDZ fractures. To date, shear displacement along EDZ fractures has not been quantified despite its importance for estimating the evolution of fracture aperture around excavations after the repository is backfilled. Enlarged photographs of EDZ fractures fixed by resin were examined to obtain reliable and accurate measurements of the shear displacement and aperture of EDZ fractures without additional disturbance. Measured shear displacement and fracture aperture are poorly correlated, meaning that fracture aperture is insensitive to shear displacement after fracture formation. This insensitivity is closely reproduced by a previously reported empirical relationship and is attributed to the relatively high levels of normal stress acting on fracture surfaces at 350 m depth, which suppress shear-induced dilatation. Considering this insensitivity between fracture aperture and shear displacement, shear-induced dilation of EDZ fractures is estimated to be negligible even if the fractures are shear-displaced after the repository is backfilled, so long as the normal stress acting on fracture surfaces does not reduce substantially. The applied resin injection and fracture observations constitute a useful tool for helping to assess the likelihood of shear-induced dilation after the repository is backfilled.
Kojima, Satoru*; Kagami, Saya; Yokoyama, Tatsunori; Kariya, Yoshihiko*; Katayama, Yoshikazu*; Nishio, Gaku*
Proceedings of 5th International Workshop on Rock Mechanics and Engineering Geology in Volcanic Fields (RMEGV 2021) (Internet), 6 Pages, 2021/09
A local tephra embedded in a hand-auger boring core drilled at near-shore of Kinugasanoike Pond about 4.6 km NE of Mt.Yakedake, one of the most active volcanoes in central Japan, is composed mainly of several kinds of volcanic glass shards (microlite-bearing, blocky, fluted and micro-vesicular types) with minor amounts of crystal minerals including quartz, plagioclase, hornblende, biotite, and pyroxene. Plant remains recovered from the horizon 10 cm below the tephra layer yield 
C ages of 2,331-2,295 (19.2% probability distribution) and 2,270-2,155 (76.2% probability distribution) cal yrs BP. We measured major element compositions of 241 individual glass shards using Electron Probe Micro Analyzer. They are plotted on the SiO
-KO, SiO-NaO+KO, and FeO
-KO diagrams in a region different from those of major regional tephras distributed in central Japan. The clast and chemical compositions coincide with those of a tephra embedded in the Nakao pyroclastic flow deposits distributed about 2 km NNW of Mt. Yakedake dated as around 2,300 cal yrs BP. The tephra could be used as a local marker of 2,300 cal yrs BP in the southern part of Northern Japan Alps.
Nara, Yoshitaka*; Kato, Masaji*; Sato, Tsutomu*; Kono, Masanori*; Sato, Toshinori
Proceedings of 5th ISRM Young Scholars' Symposium on Rock Mechanics and International Symposium on Rock Engineering for Innovative Future (YSRM 2019 and REIF 2019) (USB Flash Drive), 6 Pages, 2019/12
It is important to understand the long-term migration of radionuclides considering carious rock engineering projects such as the geological disposal of radioactive wastes. The network of fractures and pores in a rock mass can play important roles as the pathway of the fluid flow of rock. Usually groundwater contains fine-grained minerals such as clays. It is probable that the accumulation of the fine-grained minerals occurs in a fracture if the groundwater flows in a fracture in a rock. In this study, we have conducted the permeability measurement using water including clays. Specifically, we used a macro-fractured granite as a rock sample, and investigated the change of the permeability under the flow of the water including clays. It was shown that the hydraulic conductivity decreased with elapsed time.
Aoyagi, Kazuhei; Chen, Y.*; Ishii, Eiichi; Sakurai, Akitaka; Ishida, Tsuyoshi*
Proceedings of 5th ISRM Young Scholars' Symposium on Rock Mechanics and International Symposium on Rock Engineering for Innovative Future (YSRM 2019 and REIF 2019) (USB Flash Drive), 6 Pages, 2019/12
In the excavation of a repository for high-level radioactive waste (HLW) disposal, it is important to understand the hydro-mechanical characteristics of the Excavation Damaged Zone (EDZ) induced around the gallery because EDZ can lead to the migration pathway of radionuclides. Thus, we performed the resin injection experiment at the 350 m gallery of Horonobe Underground Research Laboratory in Japan to investigate the characteristics of fractures induced around the gallery wall in excavation. In the experiment, we developed a low viscosity resin mixed with a fluorescent substance and injected to the borehole drilled about 1 m in length. After the experiment, we overcored around the injection borehole. The observation on the cut surface of the overcore under ultraviolet light revealed that the fractures were distributed within 0.8 m from the gallery wall. Fractures are interconnected each other in particular within 0.25 m from the niche wall. Furthermore, fractures with large aperture (about 1.0 mm) were developed in that region. These observed results will be fundamental information for understanding of the fracturing process in the EDZ.
Okazaki, Yasuyuki*; Hayashi, Hisashi*; Aoyagi, Kazuhei; Morimoto, Shingo*; Shinji, Masato*
Proceedings of 5th ISRM Young Scholars' Symposium on Rock Mechanics and International Symposium on Rock Engineering for Innovative Future (YSRM 2019 and REIF 2019) (USB Flash Drive), 6 Pages, 2019/12
In the design of tunnel support, the behavior of the rock mass around a tunnel and the stress acting on the tunnel support may be predicted using a numerical analysis. However, in such a numerical analysis, it is common to assume that each stratum comprises a homogeneous material, ignoring the heterogeneity of the geomechanical properties inherent to the rock mass. For this reason, it is not unusual for the results of the numerical analysis to differ from the actual behavior. We performed a tunnel excavation analysis considering the heterogeneity of the geomechanical properties in the rock mass to investigate the local increase in the tunnel support stress obtained in the 350 m gallery at the Horonobe Underground Research Laboratory. The results revealed that, in order to predict the locally increased support stress in advance, it is necessary to carry out a tunneling excavation analysis considering the heterogeneity of the geomechanical properties. It was also revealed that the scale at which the geomechanical properties fluctuate is an important factor.
Miyakawa, Kazuya; Aoyagi, Kazuhei; Sasamoto, Hiroshi; Akaki, Toshifumi*; Yamamoto, Hajime*
Proceedings of 5th ISRM Young Scholars' Symposium on Rock Mechanics and International Symposium on Rock Engineering for Innovative Future (YSRM 2019 and REIF 2019) (USB Flash Drive), 6 Pages, 2019/12
The construction and operation of geological repositories require excavation and ventilation of galleries, with significant groundwater drainage. Desaturation of rock around galleries is unavoidable and may affect hydraulic properties and redox conditions. This study used numerical modeling to assess the influence of dissolved gas on the degree of saturation of rock surrounding excavated galleries, focusing on siliceous mudstone rock in the 140 m, 250 m, and 350-m-deep galleries of the Horonobe Underground Research Laboratory, Japan. Based on previous 
electrical survey, the degree of saturation in the 250 m gallery was higher than that in the 140 m and 350 m galleries. In the Horonobe area, deep groundwater contains high concentrations of dissolved methane, and exsolution of this methane from pore water can affect desaturation. Simple numerical modeling, including simulation of multiphase flows, was undertaken for each gallery to confirm the effect of dissolved gas and rock permeability on desaturation. A sensitivity analysis was performed by varying dissolved gas contents and permeability. Results indicate that the dissolved gas content affects both the degree of saturation and its spatial extent, whereas rock permeability affects only the latter. Higher dissolved gas concentrations result in lower degrees of saturation with a greater spatial extent of desaturation, and higher permeability leads to greater extents of desaturation. It is therefore likely that gas content, rather than rock permeability, caused the observed variations in the saturation degree.
Aoyagi, Kazuhei; Ishii, Eiichi
Rock Mechanics and Rock Engineering, 52(2), p.385 - 401, 2019/02
Times Cited Count：16 Percentile：66.66(Engineering, Geological)Excavation of deep underground openings induces permeable fractures around the opening due to stress redistribution. Such a zone is called excavation damaged zone (EDZ). In a high-level radioactive waste disposal project, the EDZ might provide pathways for the migration of radionuclides around the facility. Thus, this study focused on the development of a method for estimating the highest potential hydraulic conductivity in the EDZ around a gallery in the Horonobe Underground Research Laboratory, Japan. Borehole televiewer surveys, rock core observations, and hydraulic tests were undertaken to investigate the extent and magnitude of hydraulic conductivity in the EDZ around the gallery. The observed extent of the EDZ shows good agreement with the EDZ estimated from hydro-mechanical coupling analysis. The measured hydraulic conductivities of the EDZ are within the range of those based on the Mean Stress Index (MSI), which is defined as the ratio of the effective mean stress derived from numerical analyses to the tensile strength of intact rock. Given that the rock mass is relatively homogeneous and artificial damage (e.g., blasting-induced damage) can be neglected, as in the Horonobe Underground Research Laboratory, the MSI model is likely to be applicable in estimating the highest potential hydraulic conductivity in the EDZ.
Ishida, Tsuyoshi*; Fujito, Wataru*; Yamashita, Hiroto*; Naoi, Makoto*; Fujii, Hirokazu*; Suzuki, Kenichiro*; Matsui, Hiroya
Rock Mechanics and Rock Engineering, 52(2), p.543 - 553, 2019/02
Times Cited Count：16 Percentile：66.66(Engineering, Geological)We pressurized and injected water in a hole drilled downward from a floor of the 500 m level gallery in MIU, central Japan. Acoustic emissions (AEs) monitored with 16 sensors in four boreholes located 1 m away from the HF hole exhibited two-dimensional distributions, which likely delineate a crack induced by the fracturing. Expansions of the regions in which AEs occurred were observed only immediately after the first and second BDs. Many AE events in other periods were distributed within the regions where AE events had already occurred. The initial motion polarities of P-waves indicate that tensile-dominant AE events occurred when the regions expanded and they were distributed primarily on the frontiers of the regions where AE events had already occurred. The experimental results suggest that increasing the injection flow rate is effective for generating new cracks in the refracturing, with the new crack expansions being induced by tensile fracturing.
Aoyagi, Kazuhei; Tokiwa, Tetsuya*; Fujita, Tomoo
Rock Mechanics and Rock Engineering; From the Past to the Future (EUROCK 2016), p.1023 - 1028, 2016/07
Tokiwa, Tetsuya*; Aoyagi, Kazuhei; Fujita, Tomoo
Rock Mechanics and Rock Engineering; From the Past to the Future (EUROCK 2016), p.901 - 906, 2016/07
Aoyagi, Kazuhei; Tsusaka, Kimikazu; Kondo, Keiji; Inagaki, Daisuke
Rock Engineering and Rock Mechanics; Structures in and on Rock Masses, p.487 - 492, 2014/05
In a high-level radioactive waste (HLW) disposal project, it will be important to evaluate an Excavation Damaged Zone (EDZ). In particular, in an EDZ, new fractures are expected to develop in response to excavation. These fractures can cause increase in permeability of rock mass around the gallery. In order to investigate density of these fractures in an EDZ, the authors integrated information of fracture mapping of a gallery into a result of seismic tomography survey conducted in the Horonobe underground Research Laboratory. As a result, seismic velocity decreased almost linearly as the density of fracture increased. Also, it was found that density of fracture in an EDZ is expected to be evaluated by the trend which is calculated from simple numeric model. This analysis provides useful data for HLW disposal from a viewpoint that one can evaluate condition of fractures in an EDZ using the result of seismic tomography survey.
Garitte, B.*; Gens, A.*; Liu, Q.*; Liu, X.*; Millard, A.*; Bond, A.*; McDermott, C.*; Fujita, Tomoo; Nakama, Shigeo
Rock Mechanics in Civil and Environmental Engineering, p.767 - 770, 2010/00
In this paper, a benchmark exercise from the DECOVALEX-2011 project is presented. Experimental results from a laboratory drying test in Opalinus Clay are used. First, the DECOVALEX project is introduced and the experimental setup of the laboratory drying test is described. Then the theoretical formulations and the parameters determination are explained. Finally, the modelling results obtained by the five different teams are compared to the measurements.
Matsui, Hiroya; Takeuchi, Shinji; Sugihara, Kozo; Yui, Mikazu
Proceedings of International Symposium on Rock Mechanics "Rock Characterization, Modelling and Engineering Design Methods" (SINOROCK 2009) (CD-ROM), p.953 - 957, 2009/05
Grouting has practical importance for the reduction of groundwater inflow for the construction of underground facilities. Considering the performance assessment aspects of a radioactive waste repository, the performance of engineered barrier system could be adversely affected by high pH plume generated from grout using material such as ordinary Portland cement. Therefore, a quantitative estimation of the effectiveness of grouting and grout material is essential. Study of grouting performance was carried out in Mizunami URL excavated in crystalline rock. The target volume for the investigation is the rock surrounding a refuge niche where the pre-excavation grouting was performed. Ten investigation boreholes were drilled and several kinds of investigations were carried out. The groundwater flow analysis was performed to estimate the effect of grouting quantitatively. The results suggested that the existing grouting technology is effective to reduce groundwater inflow into drift and the hydraulic conductivity in surrounding rock may decrease more than one order of magnitude before grouting.
Sanada, Hiroyuki; Matsui, Hiroya; Fujii, Yoshiaki*
Proceedings of International Symposium on Rock Mechanics "Rock Characterization, Modelling and Engineering Design Methods" (SINOROCK 2009) (CD-ROM), 5 Pages, 2009/05
This paper shows the results obtained from triaxial and isotropic consolidation test to select mechanical behavior of Horonobe siliceous rocks. The consolidated undrained and drained triaxial compression tests showed different behaviors of strain-softening, pore pressure and dilatancy between diatomaceous and siliceous mudstones. Stress-strain behavior changed from strain-softening to ductile behavior under high confining pressure and pore pressure increased gradually even after peak strength for diatomaceous mudstone. Diatomaceous mudstone yielded under hydrostatic pressure of 10 MPa in isotropic consolidation test. This yielding was regarded as pore collapse based on the variation of hydraulic conductivity which was estimated from variation of volumetric strain in isotropic consolidation test, effective porosity and microscopic observation before and after the yielding.
Sugita, Yutaka; Sanada, Hiroyuki; Matsui, Hiroya
Proceedings of International Symposium on Rock Mechanics "Rock Characterization, Modelling and Engineering Design Methods" (SINOROCK 2009) (CD-ROM), 5 Pages, 2009/05
Japan Atomic Energy Agency (JAEA) has been conducting the underground research laboratory (URL) projects in Neogene diatomaceous rock at Horonobe-cho, Hokkaido, Japan. This paper summarizes development in surface-based investigations of a geomechanical conceptual model and estimation of physical and mechanical parameters for prediction of mechanical behavior of the rock mass in response to excavation of the facility. From the results of the surface investigations on rock mechanics, it is clarified that rock mass is divided into three zones, and that rock mass memorizes the mechanical properties depend on the original geological conditions at the sedimentary period, though present geological structure is complex due to folding and faulting. Hence the three-dimensional distribution of rock properties can be estimated and developed taking the geological structures (stratigraphy, displacement due to folding and faulting) into account.
