Fracture characterization and rock mass behavior induced by blasting and mechanical excavation of shafts in Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Tokiwa, Tetsuya*; Sato, Toshinori; Hayano, Akira

Proceedings of 2019 Rock Dynamics Summit in Okinawa (USB Flash Drive), p.682 - 687, 2019/05

In high-level radioactive disposal projects, it is important to investigate the extent of the excavation damaged zone (EDZ) for safety assessment because EDZ can provide a migration pathway for radionuclides from the facility. To investigate the quantitative differences between EDZs formed because of blasting and mechanical excavation, we studied the characteristics of fractures induced by excavation based on fracture mapping performed during shaft sinking (V- and E-Shafts). As a result, it was found that blasting excavation can lead to the formation of a large number of newly created fractures (EDZ fractures) compared with mechanical excavation. In addition, the seismic velocity (P-wave velocity) measured during blasting excavation (E-Shaft) was lower than that measured during mechanical excavation (V-Shaft). Furthermore, we found that the support pattern that reinforces forward rocks to be appropriate for limiting damage to the shaft wall.

A Study of efficient excavation limiting the extent of an excavation damaged zone in Horonobe URL

Aoyagi, Kazuhei; Tokiwa, Tetsuya*; Fujita, Tomoo

Rock Mechanics and Rock Engineering; From the Past to the Future (EUROCK 2016), p.1023 - 1028, 2016/07

Investigations for av change of an excavation damaged zone with time at the 250 m gallery in the Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Tsusaka, Kimikazu; Kubota, Kenji*; Tokiwa, Tetsuya; Kondo, Keiji; Inagaki, Daisuke

Doboku Gakkai Rombunshu, C (Chiken Kogaku) (Internet), 70(4), p.412 - 423, 2014/12

https://doi.org/10.2208/jscejge.70.412

The authors have been conducting seismic and resistivity tomography surveys in a gallery of the Horonobe Underground Research Laboratory in order to investigate an extent of an excavation damaged zone (EDZ) along time. The objective of this paper is to discuss an influence of fracture distribution and water saturation of a rock mass on variations in seismic velocity and the value of apparent resistivity in an EDZ. Based on the result of seismic tomography survey, the extent of a layer which has low seismic velocity was about 1.0 m from the gallery wall after excavation of the tomography area. From the results of resistivity tomography survey, the value of apparent resistivity has not changed remarkably along time. To investigate a relationship between variations in seismic velocity and density of fracture in the survey area, the authors built a three dimensional fracture model around the tomography area. From the comparison of seismic velocity with density of fracture, seismic velocity decreased almost linearly as the density of fracture increased. Also, it was found that density of fracture in the layer of low seismic velocity could be estimated using a simple numeric model. From this result, seismic tomography survey and investigation of density of fracture are suitable method for evaluation of an EDZ.

Seven-year history of vertical hydraulic diffusivity related to excavation around an underground facility

Miyakawa, Kazuya; Nohara, Tsuyoshi; Tokiwa, Tetsuya; Yamazaki, Masanori*

International Journal of Rock Mechanics and Mining Sciences, 70, p.332 - 342, 2014/09

https://doi.org/10.1016/j.ijrmms.2014.02.023

A large amount of groundwater drain induced by an excavation of an underground facility will influence parameters of site formations such as permeability. In this study, a spectral analysis was applied to long-term records of atmospheric response of pore pressure in order to obtain time history of rock permeability change at the Horonobe Underground Research Laboratory (URL) in Japan. This study revealed that the excavation of the URL has influenced aquifers around the URL, and has reduced the vertical hydraulic diffusivity in the aquifers. At the point of ca. 130 m distant away from the URL, the hydraulic diffusivity has decreased in approximately 70% after 5 years from start of excavation of the URL. At the point of ca. 860 m distant away from the URL, the vertical hydraulic diffusivity has decreased in approximately 26% even though pore pressure has not changed remarkably. These results indicate that groundwater consequently becomes difficult to flow, and then safe-side estimation with respect to geological disposal system was obtained. This study confirmed that long-term history of permeability change can be monitored by using long-period records of pore pressure and atmospheric pressure. During the development of a geological disposal system, which takes several tens of years, the vertical permeability can be monitored continuously from the start of the excavation through the post-closure of the repository.

The Origin of muddy sand sediments associated with mud volcanism in the Horonobe area of northern Hokkaido, Japan

Miyakawa, Kazuya; Tokiwa, Tetsuya; Murakami, Hiroaki

Geochemistry, Geophysics, Geosystems (Internet), 14(12), p.4980 - 4988, 2013/12

https://doi.org/10.1002/2013GC004924

The origin of mud from the mud-volcano-like sediments in the Horonobe area of northern Hokkaido, Japan was characterized by analyzing the mineral and chemical compositions of the mud. Mineral and chemical compositions of the mud are similar to those of the Hakobuchi Formation located at approximately 1500-2500 m depth. The results suggest that the mud originates from the Hakobuchi Formation, and also that the mud ascended from a depth of 1500-2500 m to ground surface. This result could be an exemplification of the occurrence of mud volcanism in oil and gas field and tectonic compressional region. This is useful information that should be taken into consideration in the planning and construction of underground facilities for geological disposal of HLW.

A Study of the regional stress and the stress state in the galleries of the Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Tsusaka, Kimikazu; Tokiwa, Tetsuya; Kondo, Keiji; Inagaki, Daisuke; Kato, Harumi*

Proceedings of 6th International Symposium on In-situ Rock Stress (RS 2013) (CD-ROM), p.331 - 338, 2013/08

In a high-level radioactive waste disposal, initial stress state is important for designing support and layout of repository. Based on the background, objectives of this paper is to investigate the state of regional stress in detail and change of initial stress state along construction of the Underground Research Laboratory (URL). Hydraulic fracturing tests and observation of the borehole breakouts through borehole televiewer logging have been conducted in the boreholes around the Horonobe URL and in the galleries of the URL. As a result, the values of the stress measured in the boreholes around the URL increased along depth and the orientations of the maximum horizontal stress were different between a map scale fault. In addition, values of initial stresses measured in the galleries were less than those of boreholes around the URL and orientation of the maximum horizontal stress were different in each depth of the gallery. These results suggest that measurements of the stress in the galleries are important for modification of the layout designed before construction of the URL.

Collection of URL measurement data in 2011 fiscal year at the Horonobe Underground Research Laboratory Project

Inagaki, Daisuke; Tokiwa, Tetsuya; Murakami, Hiroaki

JAEA-Data/Code 2012-029, 132 Pages, 2013/02

JAEA-Data-Code-2012-029.pdf:8.0MB
JAEA-Data-Code-2012-029-appendix(DVD-ROM)-1.zip:5339.82MB
JAEA-Data-Code-2012-029-appendix(DVD-ROM)-2.zip:4415.05MB
JAEA-Data-Code-2012-029-appendix(DVD-ROM)-3.zip:127.72MB

In the Horonobe Underground Research Laboratory (URL) Project, construction of the Ventilation Shaft, the East Shaft and the drifts has been conducted as a phase 2 research. In the research, observation of the lithofacies and fracture, and tests, simple elastic wave exploration, schmidt hammer test, equotip test, point load test are conducted in each face, and measuring instruments such as extensometer, rock-bolt axial force meter, shotcrete stress meter and tunnel lining concrete stress meter are installed in particular face. In addition, for the purpose of the validation of the results of predictive analysis, conducted in phase 1, about the amount of spring water during construction of underground facilities, data on the changes of amount of spring water and water quality is obtained. This report summarizes the measurements data acquired at the Ventilation Shaft, the East shaft and the 250 m Gallery.

Occurrence of high-permeability fracture estimated by grouting in Horonobe URL of Japan

Tokiwa, Tetsuya; Sawada, Sumiyuki; Ochiai, Shoji; Miyakawa, Kazuya

Dai-13-Kai Iwa No Rikigaku Kokunai Shimpojiumu Koen Rombunshu (CD-ROM), p.1021 - 1025, 2013/01

In the Horonobe area of Japan, underground facilities have been excavated in soft sedimentary rocks in order to enhance the reliability of relevant technologies for geological disposal of high-level radioactive waste in Japan. Pre-excavation grouting was carried out at 250 to 380 m depth at the location of the planned Ventilation Shaft. After the grouting, the shaft was excavated from 250 to 350 m depth, and fracture mapping was carried out to understand the characteristics of water conductive fractures by focusing on the occurrence of injected grout. Fractures consist mainly of shear fractures, and extension fractures and faults are observed in the shaft. The faults are associated with fault rocks and high continuity. Grout was mainly observed in fractures located on the branching part from faults rather than the fault itself, and therefore high-permeability fractures are not the fault itself (fault core) but fractures located on the branching part in the damage zone of the fault.

Formation mechanism of extension fractures induced by excavation of a gallery in soft sedimentary rock, Horonobe area, Northern Japan

Tokiwa, Tetsuya; Tsusaka, Kimikazu; Matsubara, Makoto*; Ishikawa, Taiki*; Ogawa, Daisuke*

Geoscience Frontiers, 4(1), p.105 - 111, 2013/01

https://doi.org/10.1016/j.gsf.2012.05.003

This paper focuses on the formation mechanism of fractures induced by excavation of a gallery in soft sedimentary rocks in the Horonobe area of Japan. Detailed fracture mapping of the gallery indicates that the fractures consist of both pre-existing fractures (shear fractures) and EDZ fractures (extension fractures). EDZ fractures correspond to bedding planes or potential joints inferred by paleo-stress field, and the EDZ fractures terminate against pre-existing fractures. Therefore, even for excavations in soft sedimentary rocks, formation of the EDZ fractures are controlled by pre-existing fractures and weakness planes associated with bedding planes and potential joints.

Horonobe Underground Research Laboratory Project; Investigation report for the 2011 fiscal year

Nakayama, Masashi; Amano, Kenji; Tokiwa, Tetsuya; Yamamoto, Yoichi; Oyama, Takuya; Amano, Yuki; Murakami, Hiroaki; Inagaki, Daisuke; Tsusaka, Kimikazu; Kondo, Keiji; et al.

JAEA-Review 2012-035, 63 Pages, 2012/09

JAEA-Review-2012-035.pdf:12.23MB

The Horonobe Underground Research Laboratory Project is planned to extend over a period 20 years. 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). This report summarizes the results of the investigations for the 2011 fiscal year (2011/2012). The investigations, which are composed of "Geoscientific research" and "R&D on geological disposal technology", were carried out according to "Horonobe Underground Research Laboratory Project Investigation Program for the 2011 Fiscal year". The results of these investigations, along with the results which were obtained in other departments of Japan Atomic Energy Agency (JAEA), are properly offered to the implementations and the safety regulations. For the sake of this, JAEA has proceeded with the project in collaboration with experts from domestic and overseas research organisations.