International Journal of Rock Mechanics and Mining Sciences, 136, p.104538_1 - 104538_8, 2020/12
It is necessary to apply reliable numerical simulation techniques to assess the mechanical behaviour of repositories for the geological disposal of radioactive waste over a long period of time. Having a constitutive model that can describe the mechanical behaviour of bentonite is key to such numerical simulations. In this study, the applicability of linear elastic constitutive model with swelling term is examined focusing on the density heterogeneity by applying to the in situ Full-scale Engineered Barrier Experiment (FEBEX) in Grimsel. It was confirmed that this constitutive model underestimates the density change. Instead, mechanical behaviour was re-simulated by unsaturated elasto-plastic constitutive model using the changes in degree of saturation as input data. Although no feedback effects of mechanical behaviour on hydraulic and thermal behaviour were taken into account, two-dimensional stress and density could be reproduced.
Benbow, S. J.*; Kawama, Daisuke*; Takase, Hiroyasu*; Shimizu, Hiroyuki*; Oda, Chie; Hirano, Fumio; Takayama, Yusuke; Mihara, Morihiro; Honda, Akira
Crystals (Internet), 10(9), p.767_1 - 767_33, 2020/09
Details are presented of the development of a coupled modeling simulator for assessing the evolution in the near-field of a geological repository for radioactive waste disposal where concrete is used as a backfill. The simulator uses OpenMI, a standard for exchanging data between simulation software programs at run-time, to form a coupled chemical-mechanical-hydrogeological model of the system. The approach combines a tunnel scale stress analysis finite element model, a discrete element model for accurately modeling the patterns of emerging cracks in the concrete, and a finite element and finite volume model of the chemical processes and alteration in the porous matrix and cracks in the concrete, to produce a fully coupled model of the system. Combining existing detailed simulation software in this way with OpenMI has the benefit of not relying on simplifications that might be necessary to combine all of the modeled processes in a single piece of software.
Takayama, Yusuke; Kikuchi, Hirohito*
Genshiryoku Bakkuendo Kenkyu (CD-ROM), 27(1), p.12 - 21, 2020/06
In this study, an applicability of the modified Cam clay model to the buffer material under saltwater conditions was examined. First, consolidated-undrained triaxial test was conducted using NaCl solution and artificial seawater. Based on the consolidated-undrained triaxial compression test results and the existing consolidation test results, the difference in the mechanical behavior of the buffer material under distilled water and saltwater condition was clarified. In particular, there was a difference in the unloading behavior in the consolidation test. Through reproducibility analysis of these experimental data, it was confirmed that the mechanical behavior of the buffer material can be roughly reproduced by setting the swelling index according to the salt concentration.
Genshiryoku Bakkuendo Kenkyu (CD-ROM), 25(2), p.103 - 106, 2018/12
At the 34th Back-end Summer Seminar, I introduce the evaluation technique for long-term mechanical behavior of near field. This report summarizes the presentation at the seminar.
Takayama, Yusuke; Iizuka, Atsushi*; Kawai, Katsuyuki*
Environmental Geotechnics (Internet), 4(5), p.339 - 352, 2017/10
Due to its low permeability and excellent expansion characteristics, bentonite materials are considered to be the material of choice for engineered barriers in Trans-Uranium (TRU) disposal facilities. It is necessity to predict the mechanical behaviour of bentonite using numerical simulation to evaluate the long-term performance of a TRU disposal facility. In this paper, a comprehensive summary of the saturation process of bentonite and its mathematical modelling was explained. Subsequently, a series of numerical simulations were carried out. First, a one-dimensional swelling pressure test simulation was carried out in order to examine the swelling characteristics of buffer and backfill. Next, we simulated the saturation process in a TRU disposal facility from an initially unsaturated state. The mechanical behaviour of bentonite buffer and backfill in a TRU disposal facility during the saturation process was examined.
Takayasu, Kentaro; Onuki, Kenji*; Kawamoto, Koji*; Takayama, Yusuke; Mikake, Shinichiro; Sato, Toshinori; Onoe, Hironori; Takeuchi, Ryuji
JAEA-Technology 2017-011, 61 Pages, 2017/06
The Groundwater REcovery Experiment in Tunnel (GREET) was put into effect as development of drift backfilling technologies. This test was conducted by making the Closure Test Drift (CTD) recovered with water after carrying out a plug around 40m distance from northern edge face of horizontal tunnel of depth 500m, for the purpose of investigation of recovering process of rock mass and groundwater under the influence of excavation of tunnel. This report presents the efforts of backfilling investigation using bentonite composite soil and execution of backfilling into borehole pits excavated in the CTD which were carried out in fiscal 2014 as a part of GREET, and succeeding observation results inside pits from September 2014 to March 2016.
Mihara, Morihiro; Hirano, Fumio; Takayama, Yusuke; Kyokawa, Hiroyuki*; Ono, Shintaro*
Genshiryoku Bakkuendo Kenkyu (CD-ROM), 24(1), p.15 - 25, 2017/06
A computer program MACBECE has been developed to provide rigorous calculations of the long-term mechanical behavior of a TRU waste geological repository. Consideration is given to the expected chemical alteration of repository components, including cementitious materials and bentonite, and the mechanical interactions between repository and host rock. The long-term mechanical behavior of a TRU waste repository was evaluated in a deep soft rock site, where creep deformation is likely to occur from the initial construction phase to 10 years after repository closure. It was found that the stress didn't fall into a singularity of yield surface of the EC model applied to the mechanical behavior model of bentonite. The calculated displacement of the inner diameter of the repository, considering mechanical interaction between repository and host rock, was about half that of a result in 2nd progress report on R&D for TRU waste disposal in Japan.
Takayama, Yusuke; Tachibana, Shinya*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Kobayashi, Ichizo*
Soils and Foundations, 57(1), p.80 - 91, 2017/02
Bentonite has remarkable swelling characteristics and low permeability that makes it a suitable buffer material in a purpose built repository for the geological disposal of radioactive waste. It is necessary to use reliable numerical simulation techniques to demonstrate that the repository is safe and mechanically stable for tens of thousands of years. Constitutive model that describes the mechanical behavior of bentonite is a key technique in such numerical simulations. The current paper proposes a constitutive model for bentonite, which treats bentonite as an unsaturated elasto-plastic material that changes its mechanical properties as it becomes saturated. In the proposed model, the swelling index and an expression formula for negative dilatancy are treated as functions of degree-of-saturation. Montmorillonite content is used as an input parameter in the proposed model and so is applicable to a variety of bentonite based materials of varying montmorillonite content. Experimental results from swelling volume and swelling pressure tests taken from the literature are shown to be satisfactorily predicted by the proposed model.
Ichikawa, Yasuaki*; Kimoto, Kazushi*; Sato, Toshinori; Kuwabara, Kazumichi; Takayama, Yusuke
JAEA-Research 2015-025, 31 Pages, 2016/03
It is important to evaluate the stability of a repository for high-level radioactive waste not only during the design, construction and operation phases, but also during the post-closure period, for time frames likely exceeding several millennia or longer. The rock mass around the tunnels could be deformed through time in response to time dependent behavior. On the other hand, it was revealed that the chemical reaction of groundwater in a rock had an influence on the long-term behavior. An evaluation of the microcracks to have an influence on these mechanical and chemical coupled phenomena should be worked on chiefly. In fiscal year 2014, this study performed numerical analysis to examine the supersonic scattering attenuation decrement behavior in the crystalline rock and a measurement sequentially last year. The measurement of the head and surface waves were carried out. As a result, group speed was provided. On the other hand, the spread scattering analysis of the elastic wave by the FDTD (Finite Difference Time-Domain) method made a numerical analysis. However, a laboratory finding is different from expectation of the simulation, and crystal anisotropic influence of a microcrack and rock-forming minerals is thought about as a cause of this estrangement. Therefore it was revealed that it was necessary to examine these two points of influence more in future.
Fukui, Katsunori*; Hashiba, Kimihiro*; Sato, Toshinori; Kuwabara, Kazumichi; Takayama, Yusuke
JAEA-Research 2015-015, 61 Pages, 2015/11
With respect to high-level radioactive waste disposal, knowledge of the long-term mechanical stability of shafts and galleries excavated in rock is required, not only during construction and operation but also over a period of thousands of years after closure. On the other hand, it is known that rock and the rock mass surrounding the disposal gallery shows time dependent behavior such as creep or the stress-relaxation. It becomes the issue in the stability evaluation of the disposal gallery to grasp the behavior. About this issue, we pushed forward research development. In the fiscal year of 2014, the creep test was continuously conducted and the total testing time exceeded 17 years. The testing equipment and procedure were examined to investigate the deformation, failure and time-dependency of rock under wet conditions and between room temperature and 100C. The long-term strength of rock under triaxial stress state was researched with the aid of laboratory testing results and in situ stress measurement.
Kuwabara, Kazumichi; Sato, Toshinori; Sanada, Hiroyuki; Takayama, Yusuke
JAEA-Research 2015-005, 378 Pages, 2015/07
This report presents the results of following rock mechanical investigations conducted at the -500m Stage. (1) Laboratory tests using cores and block samples obtained at the -500m Stage. (2) In-situ stress measurement using Compact Conical-ended Borehole Overcoring (CCBO) method at the -500m Stage. (3) In-situ stress measurements using Differential Strain Curve Analysis(DSCA) method at the -500m Stage. (4) Development of rock mechanical model.
Takayama, Yusuke; Sato, Toshinori; Sanada, Hiroyuki; Tada, Hiroyuki*; Kumasaka, Hiroo*; Fukuda, Tsuyoshi*; Kobayashi, Shinji*
JAEA-Research 2015-003, 102 Pages, 2015/07
In the rock mechanical study in the Mizunami Underground Research Laboratory Project, the development of the evaluation method for the excavation damaged zone due to excavation of shafts and research galleries is one of the important issues. In this study, three-dimensional excavation analysis is conducted. Crack tensor used in these analysis were measured using the data collected in wall surface observation in study tunnels such as the horizontal tunnels to a depth of 500 m. The results of analysis were compared with existing measurements at a depth of 500 m obtained using in-situ strain meters. The excavation analysis using crack tensor and initial stress obtained from the Phase I and Phase II were carried out to examine the influence of crack tensor and initial stress on the results of analysis.
Takayama, Yusuke; Sato, Toshinori; Onoe, Hironori; Iwatsuki, Teruki; Saegusa, Hiromitsu; Onuki, Kenji
Dai-43-Kai Gamban Rikigaku Ni Kansuru Shimpojiumu Koenshu (CD-ROM), p.313 - 318, 2015/01
In the Mizunami Underground Research Laboratory, groundwater recovery experiment is being conducted to construct the method to understand the transition of geological environment due to groundwater recovery at the -500m access and research gallery-north. As a part of this experiment, backfill test is planned using drilling pits filled with artificial materials (clay and concrete) to evaluate the influence on the surrounding rock mass due to the interaction of rock and artificial materials. In this study, numerical simulation of the backfill test has been carried out to predict the qualitative hydro-mechanical behavior.
Takayama, Yusuke*; Ikuta, Yuki*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Taki, Tomihiro; Sakao, Ryota; Ichikawa, Yasuaki*
Unsaturated Soils; Research & Applications, p.659 - 665, 2014/06
In this paper, a multiple unsaturated soil barrier consisting of gravel, sand and bentonite-mixed soil layers is considered as a hardly permeable covering to prevent rainwater from infiltrating into waste deposits. Not only the quite low permeability of bentonite-mixed soil layer but also a function of capillary barrier between granular layers having different water-retention capabilities is expected to prevent water infiltration. A series of numerical simulation is carried out to evaluate performance of the multiple soil barrier using saturated/unsaturated soil/water coupled FE computation technique. The multiple unsaturated soil barrier system considered here is 10m long and 1.5 m thick consisting of sandy soil surface, sand filter, gravel layer, sand filter and bentonite-mixed clayey layer. And the soil barrier system is assumed to be placed as a covering with a 5% inclination on the waste deposit. The performance of the multiple soil barrier system is shown as a management chart to maintain impermeable ability associated with possible rainfall intensity. Particularly, in this paper, actual rainfall record in Okayama area is employed to determine the rainfall intensity which is imposed as a boundary condition in the computation. According to the simulation results, it is found that the capillary barrier is a key issue in evaluating the long term performance of the barrier system. And, it is confirmed that the barrier system considered in this paper can work well against the maximum intensity of the rainfall in the past record.
Ikuta, Yuki*; Takayama, Yusuke*; Tsurumi, Shuhei*; Ichikawa, Yasuaki*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Taki, Tomihiro; Sakao, Ryota
no journal, ,
As a part of remediation of closed uranium mine, "Yotsugi Mill Tailings Pond" has been remediated in Ningyo-toge Environmental Engineering Center, Japan Atomic Energy Agency. The aim of remediation is to take measures to reduce the load of water treatment and the radon exposure to humans in future, and to prevent the occurrence of relevant environmental contamination. So far, the upstream of the Pond has been covered by the multi-layered capping in which layers are set to use gravel, sand and decomposed granite soil to drain penetrated rainwater to the neighboring river. This capping system needs to be monitored on a long-term basis and to be evaluated to work as expected in advance, therefore seepage analysis and settlement analysis has carried out. As a result, Settlement of analysis was well accorded with measured value and marked change of saturation accompanied by rain was not observed in the seepage analysis.
Sasaki, Yosuke*; Takayama, Yusuke*; Ikuta, Yuki*; Iizuka, Atsushi*; Kawai, Katsuyuki*; Sakao, Ryota; Taki, Tomihiro
no journal, ,
As part of environmental remediation of Ningyo-toge Uranium Mine, multi-layered capping is constructed to reduce rainwater infiltration at the upstream of the Mill Tailings Pond. Settlement is expected due to consolidation characteristics of mill tailings and clayish sediments which exist below capping. Numerical simulation is carried out to expect the settlement amount during and after the capping construction. Settlement amount has been measured periodically, and the simulation result and measurement data are compared to evaluate the simulation method.
Kuwabara, Kazumichi; Takayama, Yusuke; Sanada, Hiroyuki; Sato, Toshinori; Tanno, Takeo*; Itamoto, Masaharu*; Kato, Harumi*
no journal, ,
In-situ stress measurements using CCBO (Compact Conical-ended Borehole Overcoring technique) were performem with two boreholes (13MI34, 13MI35) at depth of 500m in Mizunami Underground Research Laboratory. As a results, the Maximum stress in the horizontal plane was 15 MPa, and its azimuth were nearly N-S direction.
Takayama, Yusuke; Sato, Toshinori; Kuwabara, Kazumichi; Sanada, Hiroyuki
no journal, ,
no abstracts in English
Sato, Toshinori; Kuwabara, Kazumichi; Takayama, Yusuke; Tanno, Takeo*; Itamoto, Masaharu*; Kato, Harumi*
no journal, ,
This paper shows the results of in-situ stress measurement, uniaxial load and unload test, P-wave velocity measurement at the Mizunami Underground Research Laboratory. As the results of these measurements, microcrack distributed in Granite at deep depth influences physical and mechanical properties of rock and in-situ stress state.
Kuwabara, Kazumichi; Sato, Toshinori; Takayama, Yusuke; Tanno, Takeo*; Kato, Harumi*; Itamoto, Masaharu*
no journal, ,
It is very important to understand an initial stress state of the rock mass in doing a design, the construction of large-scale facilities under the ground. Groundwater recovery experiment will be conducted at the MIU GL.-500m closure test drift. This report described what stress measurement were performed by the CCBO (Compact Conical-ended Borehole Overcoring technique) to understand an initial stress state of intact rock and a stress state of the -500m gallery wall surface. As a result, the magnitude of principal stresses were 16.8 MPa, 10.2 MPa and 7.5 MPa. The maximum principal stress level was approximately same at the GL.-500m level. The direction of maximum principal stress is approximately 60 from a horizontal plane. The width of stress redistributed zone was about 2m from the wall of research galleries. Numerical analysis was carried out to examine an outbreak condition of core disking.