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Journal Articles

Paleohydrogeology of the Horonobe area, Northern Hokkaido, Japan; Groundwater flow conditions during glacial and postglacial periods estimated from chemical and isotopic data for fracture and pore water

Mochizuki, Akihito; Ishii, Eiichi

Applied Geochemistry, 155, p.105737_1 - 105737_15, 2023/08

 Times Cited Count:0 Percentile:0.01

Understanding the difference in groundwater flow between glacial and interglacial periods is crucial for predicting the impact of future climate changes on groundwater movement. This study assesses the difference in groundwater flow between the last glacial period (LGP) and the postglacial period (PGP) in fractured mudstones of the Horonobe area, Japan, by combining the data for stable isotopes ($$delta$$D and $$delta$$$$^{18}$$O) and Cl$$^{-}$$ concentration of fracture and pore waters with radiocarbon ($$^{14}$$C) age. The isotopic compositions of fractures and pore waters indicate that groundwater at 28$$sim$$250 m deep in a borehole closest to the recharge area comprises meteoric water, recharged under the same climates as the present. The fracture water has isotopic compositions more similar to meteoric water than the matrix pore water near the fracture. The $$^{14}$$C age of fracture water suggests meteoric water recharge during the PGP. At greater depths in the borehole and sampling points in other boreholes, the isotopic compositions indicate the mixing of glacial meteoric and altered connate water, with the fracture water having comparable isotopic compositions with the matrix pore water. The recharge timing of meteoric water is inferred to be the LGP or before based on $$^{14}$$C dating. These results suggest that the meteoric water recharged during the PGP flows at a shallow depth, whereas the meteoric water recharged during the LGP intruded to greater depths. This result is consistent with previous inferences from surface geophysical and geological surveys that the depths of local valleys during the LGP were greater by $$<$$50 m than the present ones and enhanced the downward hydraulic gradient. Combining the chemical and isotopic compositions of groundwater with $$^{14}$$C age helps assess the groundwater flow during the LGP and PGP in fractured rocks.

Journal Articles

Effects of flow dimension in faulted or fractured rock on natural reductions of inflow during excavation; A Case study of the Horonobe Underground Research Laboratory site, Japan

Ishii, Eiichi

Hydrogeology Journal, 31(4), p.893 - 911, 2023/06

 Times Cited Count:0 Percentile:0(Geosciences, Multidisciplinary)

Major inflows can occur during tunnel/shaft excavations in faulted/fractured rock masses even if pre-excavation grouted. In such cases, post-excavation grouting may be required for reducing the inflows. However, following the diffusion equation for fluid pressure, inflows can naturally reduce to half to one-tenth within the first several days to several weeks when the flow dimensions of the surrounding rock masses are closed to one, while the inflows hardly reduce when the flow dimensions are as high as three. Thus, if the flow dimensions are closed to one, the natural reductions in inflow may be also effectively available for the countermeasures. Nevertheless, the usability of relationship between changes in inflow and flow dimensions has been poorly verified at actual tunnel/shaft-excavation sites. Actual inflows may be strongly affected by different factors including grouting, degassing, excavation processes, and far-field boundary conditions. The author analyzed changes in inflow at six locations where major inflows occurred during tunnel/shaft excavations in faulted/fractured siliceous rocks and investigated flow dimensions in the rocks by packer tests in boreholes. The results confirmed that the inflows for several days to several weeks immediately after the inflows occurred change consistently with the flow dimensions estimated from the packer tests. Thus, even if major inflows were encountered during excavations, the subsequent changes in inflow are predictable based on the flow dimensions, and the flow dimensions can be estimated by preliminary packer tests or from the changes in inflow at the inflow locations for the first several days.

Journal Articles

Discrimination of the current level of activity of groundwater flow in deep underground by using stable water isotopes

Mochizuki, Akihito; Ishii, Eiichi

Isotope News, (784), p.23 - 27, 2022/12

no abstracts in English

Journal Articles

Constant-head step injection tests to quantify the stress dependence of fracture transmissivity in an excavation damaged zone; A Case study from the Horonobe Underground Research Laboratory

Ishii, Eiichi

International Journal of Rock Mechanics and Mining Sciences, 159, p.105229_1 - 105229_17, 2022/11

 Times Cited Count:1 Percentile:25.67(Engineering, Geological)

In an underground repository for high-level radioactive waste disposal, fracture transmissivity in an excavation damaged zone (EDZ) along tunnels or deposition holes can decrease during the post-closure period via processes such as self-sealing by clay-swelling at the EDZ's fracture surface or an increase in effective normal stress acting on the fractures owing to swelling of backfilling or buffer materials. Hydromechanical coupling models for the stress-dependence of fracture transmissivity are helpful to estimate the change in an EDZ's fracture transmissivity after closure. The applicability of the applied models should be confirmed by in situ tests at the given site; this appears to be facilitated by using constant-head step injection tests. However, injection testing is rarely applied to EDZ fractures. To investigate the applicability of injection tests, the present study performed them on single, tensile EDZ fractures in the Horonobe Underground Research Laboratory hosted by poorly swelling mudstone. Furthermore, the Barton-Bandis normal stress-dependent fracture-closure model quantified the stress-dependence of EDZ facture transmissivity. The fracture's hydraulic aperture increased gradually during injection, and its variation was well reproduced by fitting the model. Although the model requires the normal stress, this parameter was reasonably estimated by the fitting analyses. Constant-head step injection tests coupled with the Barton-Bandis model is believed to be a convenient method for preliminarily quantifying or verifying the stress dependence of EDZ fracture transmissivity, at least for poorly self-sealed, tensile EDZ-fractures.

Journal Articles

Effect of fault activation on the hydraulic connectivity of faults in mudstone

Ono, Hirokazu; Ishii, Eiichi

Geomechanics for Energy and the Environment, 31, p.100317_1 - 100317_9, 2022/09

 Times Cited Count:4 Percentile:63.67(Energy & Fuels)

Journal Articles

Variation in fault hydraulic connectivity with depth in mudstone; An Analysis of poroelastic hydraulic response to excavation in the Horonobe URL

Ozaki, Yusuke; Ishii, Eiichi; Sugawara, Kentaro*

Geomechanics for Energy and the Environment, 31, p.100311_1 - 100311_13, 2022/09

 Times Cited Count:3 Percentile:52.76(Energy & Fuels)

This study analyzed the long-term hydraulic pressure data during the excavation of Horonobe URL to estimate the variation of effective-hydraulic-conductivity. We performed the numerical simulation with the poroelastic effect for the estimation because the observed hydraulic pressure is highly affected by the Mandel-Cryer effect. The evaluation of the observed data based on our simulation results showed that the effective-hydraulic-conductivity gradually decreases from 400 m to 500 m in depth and is as low as the intact rock at depths greater than 500 m. Not only the analysis based on our simulation results but also the analysis based on analytical solution indicate the domain with different hydraulic properties in the Wakkanai Formation. These results suggest that the fracture-hydraulic-connectivity changes not abruptly but gradually over several tens of meters around the predicted boundary.

JAEA Reports

Long-term monitoring of pore pressure/groundwater level using deep/shallow boreholes in the Horonobe Underground Research Laboratory Project

Honda, Norihisa; Dei, Shuntaro; Ishii, Eiichi

JAEA-Data/Code 2022-002, 37 Pages, 2022/06

JAEA-Data-Code-2022-002.pdf:2.85MB
JAEA-Data-Code-2022-002-appendix(CD-ROM).zip:5.68MB

Long-term monitoring of pore pressure/groundwater level has been performed at the deep boreholes HDB-1-11 and PB-V01 and seven shallow boreholes in the Horonobe Underground Research Laboratory Project. This report summarizes the results obtained from the starts of monitoring to March 2021.

Journal Articles

Assessment of the level of activity of advective transport through fractures and faults in marine deposits by comparison between stable isotope compositions of fracture and pore waters

Mochizuki, Akihito; Ishii, Eiichi

Hydrogeology Journal, 30(3), p.813 - 827, 2022/05

 Times Cited Count:3 Percentile:63.67(Geosciences, Multidisciplinary)

Assessment of the level of activity of advective transport through faults and fractures is essential for guiding the geological disposal of radioactive waste. In this study, the advective flow (active, inactive) of meteoric water through fractures is assessed by comparing stable isotopes ($$delta$$D and $$delta$$$$^{18}$$O) between fracture and pore waters obtained from four boreholes in marine deposits in the Horonobe area, Japan. At 27-83-m depth in one borehole and 28-250 m in another, the isotopic compositions of pore and fracture water reflect mixing with meteoric water, with stronger meteoric-water signatures being observed in the fracture water than in pore water of the rock matrix. At greater depths in these boreholes and at all sampling depths in the other two studied boreholes, the isotopic compositions of fracture and pore waters are comparable. These results suggest that the advective flow of meteoric water is active at shallow depths where fossil seawater is highly diluted in the two boreholes. This interpretation is compatible with the occurrence of present or paleo meteoric waters and tritium, whereby present meteoric water and tritium are limited to those depths in the two boreholes. This difference in the level of activity of advective flow is probably because of the glacial-interglacial difference in hydraulic gradients resulting from sea-level change. Although fractures are hydraulically connected to the surface through the sedimentary rock, advective flow through them is inferred to remain inactive so long as sea level does not fall substantially.

Journal Articles

Resin-injection testing and measurement of the shear displacement and aperture of excavation-damaged-zone fractures; A Case study of mudstone at the Horonobe Underground Research Laboratory, Japan

Aoyagi, Kazuhei; Ishii, Eiichi; Chen, Y.*; Ishida, Tsuyoshi*

Rock Mechanics and Rock Engineering, 55(4), p.1855 - 1869, 2022/04

 Times Cited Count:3 Percentile:52.76(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.

Journal Articles

Study in the sedimentary processes of organic matter in the Pliocene to Miocene siliceous rocks in the Horonobe district, northern Hokkaido, Japan

Muraoka, Ami*; Chiyonobu, Shun*; Arato, Hiroyuki*; Martizzi, P.*; Ishii, Eiichi

Sekiyu Gijutsu Kyokai-shi (CD-ROM), 87(1), p.86 - 88, 2022/00

no abstracts in English

Journal Articles

The Highest potential transmissivities of fractures in fault zones; Reference values based on laboratory and in situ hydro-mechanical experimental data

Ishii, Eiichi

Engineering Geology, 294, p.106369_1 - 106369_12, 2021/12

 Times Cited Count:8 Percentile:64.58(Engineering, Geological)

The transmissivities (T) of fractures can be related to the fracture roughness ($$JRC_{rm 0}$$), initial aperture ($$E_{rm 0}$$), effective normal stress ($$sigma$$'$$_{rm n}$$), and the tensile strength ($$sigma$$'$$_{rm t}$$) of the intact rock, based on the Barton-Bandis model and their data, and the T (or $$E_{rm 0}$$) can increase by shear-induced dilation. Previous studies revealed that the T of fractures in fault zones, detected as flow anomalies by borehole investigations at six sites, uniformly decreases with the increasing effective mean stress normalized to the $$sigma$$'$$_{rm t}$$. If this uniform change in T can be explained by $$sigma$$'$$_{rm n}$$-dependent fracture-normal-displacement following the Baron-Bandis model, the T represents the highest potential T of fractures in fault zones which can increase by shear-induced dilation. To verify this possibility, this study estimated the $$E_{rm 0}$$ of the fractures using the T, $$sigma$$'$$_{rm t}$$, and possible $$JRC$$ and $$sigma$$'$$_{rm n}$$. Then, using this estimated $$E_{rm 0}$$, the changes in T were simulated, varying $$sigma$$'$$_{rm n}$$. The results well reproduced the observed uniform change in T.

JAEA Reports

Update on the regional-scale 3D geological model in the Horonobe Underground Research Laboratory Project

Sakai, Toshihiro; Ishii, Eiichi

JAEA-Data/Code 2021-009, 13 Pages, 2021/08

JAEA-Data-Code-2021-009.pdf:1.9MB
JAEA-Data-Code-2021-009-appendix(CD-ROM).zip:42.79MB

Japan Atomic Energy Agency is performing the Horonobe Underground Research Laboratory Project, which includes a scientific study of the deep geological environment as a basis of research and development for the geological disposal of high level radioactive wastes, in order to establish comprehensive techniques for the investigation, analysis and assessment of the deep geological environment in the sedimentary rock. The numerical data of 3D geological model in regional-scale was compiled in 2019 as JAEA-Data/Code 2019-007, and then this report updates a part of the numerical data of 3D geological model around the underground facilities.

Journal Articles

Does fault activation affect the hydraulic disconnectivity of faults in mudstone?

Ono, Hirokazu; Takeda, Masaki; Ishii, Eiichi

Extended abstract of International Conference on Coupled Processes in Fractured Geological Media; Observation, Modeling, and Application (CouFrac 2020) (Internet), 4 Pages, 2020/11

Journal Articles

Poroelastic hydraulic-response of fractured mudstone to excavation in the Horonobe URL; As an indicator of fracture hydraulic-disconnectivity

Ozaki, Yusuke; Ishii, Eiichi; Sugawara, Kentaro*

Extended abstract of International Conference on Coupled Processes in Fractured Geological Media; Observation, Modeling, and Application (CouFrac 2020) (Internet), 4 Pages, 2020/11

We perform the numerical simulation of the response of hydraulic head observed in HDB-6 during the excavation of the Horonobe URL to verify the existence of low effective permeable domain in the subsurface. The low permeable domain as an intact rock due to the low hydraulic fracture connectivity is estimated to exist in the deep domain while the permeability of the shallow domain is relatively high due to the hydraulic fracture connectivity there. Our simulation shows that the observed hydraulic head is affected by the Mandel-Cryer effect due to the hydrogeological structure and the effect for the duration of over years requires the low permeability as an intact rock in the deep domain. These results verify the existence of the low effective permeable domain in the deep subsurface estimated by the previous study.

Journal Articles

A Conventional straddle-sliding-packer system as a borehole extensometer; Monitoring shear displacement of a fault during an injection test

Ishii, Eiichi

Engineering Geology, 275, p.105748_1 - 105748_12, 2020/09

AA2019-0169.pdf:3.99MB

 Times Cited Count:6 Percentile:44.79(Engineering, Geological)

A constant-head step injection test using a conventional straddle-packer system was performed for a normal fault in siliceous mudstone. The test applied a new method whereby axial displacements of isolated test sections in a borehole during injection are monitored by measuring the pressures of sliding packers and the pore pressure in the test section. The measured pressures and axial displacement, and the injection flow rate, were used to estimate the hydraulic aperture, shear displacement, normal compliance, normal stress, shear stiffness and hydraulic dilation angle of the fault during the test. The injection successfully yielded a large shear displacement during normal faulting of up to 13.3-49.5 mm (including the estimation error), which left residual shear displacement of 2.8-10.4 mm after a remarkable shear-slip event. The shear stiffness during faulting is estimated to be 2.3 $$times$$ 10$$^{7}$$ to 8.4 $$times$$ 10$$^{7}$$ Pa m$$^{-1}$$ (considering the estimation error), which is consistent with empirically predicted values based on previous studies. The hydraulic dilation angle was inferred to be effectively zero as the residual shear displacement did not leave any increase in hydraulic aperture. The experimental method applied here does not require specialized equipment and could aid in the investigation of the hydromechanical behavior of subsurface fractures or aquifers.

JAEA Reports

Poro-elastic parameter acquisition test using siliceous mudstone (Wakkanai formation)

Aoki, Tomoyuki*; Tani, Takuya*; Sakai, Kazuo*; Koga, Yoshihisa*; Aoyagi, Kazuhei; Ishii, Eiichi

JAEA-Research 2020-002, 83 Pages, 2020/06

JAEA-Research-2020-002.pdf:8.25MB
JAEA-Research-2020-002-appendix(CD-ROM).zip:6.63MB

The Japan Atomic Energy Agency (JAEA) has conducted with the Horonobe Underground Research Project in Horonobe, Teshio-gun, Hokkaido for the purpose of research and development related to geological disposal technology for high-level radioactive wastes in sedimentary soft rocks. The geology around the Horonobe Underground Research Laboratory (HURL) is composed of the Koetoi diatomaceous mudstone layer and the Wakkanai siliceous layer, both of which contain a large amount of diatom fossils. Since these rocks exhibit relatively high porosity but low permeability, it is important to investigate the poro-elastic characteristics of the rock mass. For this objective, it is necessary to measure parameters based on the poro-elastic theory. However, there are few measurement results of the poro-elastic parameters for the geology around HURL, and the characteristics such as dependence on confining pressure are not clearly understood. One of the reasons is that the rocks show low permeability and the pressure control during testing is difficult. Therefore, a poro-elastic parameter measurement test was conducted on the siliceous mudstone of the Wakkanai formation to accumulate measurement results on the poro-elastic parameters and to examine the dependence of the parameters on confining pressure. As a result, some dependency of the poro-elastic parameters on confining pressure was observed. Among the measured or calculated poro-elastic parameters, the drained bulk modulus increased, while the Skempton's pressure coefficient, and the Biot-Wills coefficient in the elastic region decreased with the increase in confining pressure. The measurement results also inferred that the foliation observed in the rock specimens might impact a degree of dependency of those parameters on confining pressure.

JAEA Reports

Synthesis report on the R&D for the Horonobe Underground Research Laboratory; Project carried out during fiscal years 2015-2019

Nakayama, Masashi; Saiga, Atsushi; Kimura, Shun; Mochizuki, Akihito; Aoyagi, Kazuhei; Ono, Hirokazu; Miyakawa, Kazuya; Takeda, Masaki; Hayano, Akira; Matsuoka, Toshiyuki; et al.

JAEA-Research 2019-013, 276 Pages, 2020/03

JAEA-Research-2019-013.pdf:18.72MB

The Horonobe Underground Research Laboratory (URL) Project is being pursued by the Japan Atomic Energy Agency (JAEA) to enhance the reliability of relevant disposal technologies for geological disposal of High-level Radioactive Waste through investigations of the deep geological environment within the host sedimentary rock at Horonobe Town in Hokkaido, north Japan. The investigations will be conducted in three phases, namely "Phase 1: Surface based investigations", "Phase 2: Construction phase" (investigations during construction of the underground facilities) and "Phase 3: Operation phase" (research in the underground facilities). According to the research plan described in the 3rd Mid- and Long- term Plan of JAEA, "Near-field performance study", "Demonstration of repository design option", and "Verification of crustal-movement buffering capacity of sedimentary rocks" are important issues of the Horonobe URL Project, and schedule of future research and backfill plans of the project will be decided by the end of 2019 Fiscal Year. The present report summarizes the research and development activities of these 3 important issues carried out during 3rd Medium to Long-term Research Phase.

JAEA Reports

Visualization of fractures in an excavation damaged zone in the Horonobe Underground Research Laboratory, 2 (Joint research)

Aoyagi, Kazuhei; Chen, Y.*; Ishii, Eiichi; Sakurai, Akitaka; Miyara, Nobukatsu; Ishida, Tsuyoshi*

JAEA-Research 2019-011, 50 Pages, 2020/03

JAEA-Research-2019-011.pdf:3.48MB

In this research, we performed the resin injection experiment at the 350 m Gallery of Horonobe Underground Research Laboratory in order to identify the distribution of fractures induced around the gallery owing to excavation. We also observed the rock cores obtained around the resin injection borehole under ultraviolet light. As a result, the extent of the development of EDZ fracture was 0.9 m from the gallery wall. In the depth within 0.4 m from the gallery wall, the density of the EDZ fracture is higher than the depth more than 0.4 m from the gallery wall. As a result of the analysis on the fracture aperture by image processing, the fractures with a large aperture (1.02 mm in maximum) were observed within 0.3 m from the gallery wall, while the maximum aperture was 0.19 mm in the depth more than 0.3 m from the gallery wall.

JAEA Reports

Hydraulic tests for the excavation damaged zone around the 350m niches in the Horonobe Underground Research Project, 2

Samata, Yoichi; Ishii, Eiichi

JAEA-Data/Code 2019-020, 69 Pages, 2020/03

JAEA-Data-Code-2019-020.pdf:4.0MB
JAEA-Data-Code-2019-020-appendix1(DVD-ROM).zip:86.89MB
JAEA-Data-Code-2019-020-appendix2(DVD-ROM).zip:232.04MB
JAEA-Data-Code-2019-020-appendix3(DVD-ROM).zip:369.23MB

In Horonobe Underground Research Laboratory Project, hydraulic tests for the excavation damaged zone have been performed in order to characterize the hydrological properties of the zone. This report summarized the results of the hydraulic tests and pore-pressure monitoring which have been done from April 2016 to March 2019.

Journal Articles

Mudstone redox conditions at the Horonobe Underground Research Laboratory, Hokkaido, Japan; Effects of drift excavation

Mochizuki, Akihito; Ishii, Eiichi; Miyakawa, Kazuya; Sasamoto, Hiroshi

Engineering Geology, 267, p.105496_1 - 105496_11, 2020/03

 Times Cited Count:4 Percentile:27.35(Engineering, Geological)

The mechanical and hydraulic properties of rocks around mine drifts change significantly during the construction and operation of a radioactive-waste repository, with air intrusion causing the oxidation of rock and groundwater in excavation-damaged zones (EDZ). Redox conditions in such zones associated with niches excavated in mudstone at the Horonobe Underground Research Laboratory (URL), which is believed to be generally representative of conditions that could exist in the EDZ of a repository, were studied with the aim of improving our understanding of factors that control redox conditions in such rock-groundwater systems. Groundwater Eh values around the niches have reducing values of less than -150 mV. The SO$$_{4}$$$$^{2-}$$ concentration, regarded as an oxidation indicator, is consistently as low as 1 $$mu$$mol L$$^{-1}$$. Gas occupies more than 50% of zone volumes, including CH$$_{4}$$ and CO$$_{2}$$ with traces of N$$_{2}$$ and O$$_{2}$$. Cores drilled from host rock around a URL gallery were analyzed, with no pyrite dissolution or precipitation of calcium sulfates being found. It is concluded that oxidizing conditions do not exist in the excavation-damaged zones, which is attributed to the suppression of air intrusion by the release of CH$$_{4}$$ and CO$$_{2}$$ from groundwater as pressures decreased and their accumulation in fractures. The modeling of oxygen diffusion into host rock further indicates that a reducing environment is maintained around the URL drifts.

175 (Records 1-20 displayed on this page)