Evaluation of temporal changes in fracture transmissivity in an excavation damaged zone after backfilling a gallery excavated in mudstone

Aoyagi, Kazuhei; Ishii, Eiichi

Environmental Earth Sciences, 83(3), p.98_1 - 98_15, 2024/02

https://doi.org/10.1007/s12665-023-11416-x

The long-term geological disposal of high-level radioactive waste relies on predictions of future changes in a disposal facility's hydro-mechanical characteristics to assess potential leakage through fractures in the excavation damaged zone (EDZ) after backfilling the facility. This study evaluated the transmissivity of EDZ fractures using in situ hydraulic tests around the area of a full-scale, experimental, engineered barrier system in the Horonobe Underground Research Laboratory, Hokkaido, Japan. After their installation, the buffer blocks swelled, altering the stresses within the EDZ fractures. The effects of these changing stresses on the fractures' transmissivity were assessed over a period of 4 years. The transmissivity continuously decreased in this period to about 41% of its value measured prior to the swelling. Using the Barton-Bandis normal-stress-dependent fracture-closure model, the decrease in transmissivity is quantitatively attributed to closure of the EDZ fractures, which was caused by the swelling pressure increasing up to 0.88 MPa. Evidence of fracture closure came from seismic tomography surveying, which revealed a slight increase in seismic velocity in the study area with increasing swelling pressure. The results show that EDZ fractures were closed by swelling of the full-scale buffer material. They also demonstrate the applicability of the Barton-Bandis model to preliminary estimation of the long-term transmissivity of EDZ fractures in facilities for the geological disposal of radioactive waste.

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

https://doi.org/10.1007/s10040-023-02628-3

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.

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

https://doi.org/10.1016/j.ijrmms.2022.105229

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.

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

https://doi.org/10.1016/j.gete.2022.100311

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.

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

https://doi.org/10.1007/s10040-022-02466-9

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 (D and 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.

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

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.

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.

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.

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.