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Journal Articles

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.

Journal Articles

${it In situ}$ stress measurement in the Mizunami Underground Research Laboratory, Japan

Sato, Toshinori; Tanno, Takeo; Hikima, Ryoichi*; Sanada, Hiroyuki; Kato, Harumi*

Proceedings of 6th International Symposium on In-situ Rock Stress (RS 2013) (CD-ROM), p.354 - 359, 2013/08

Japan Atomic Energy Agency is operating underground research laboratory projects in order to establish a firm scientific basis for safe geological disposal of HLW. One of these is the Mizunami Underground Research Laboratory (MIU) project focused on crystalline rock. Rock mechanical studies, including ${it in situ}$ stress measurements, are being conducted as part of the projects. This paper describes the current status of in situ stress studies conducted in the MIU. Applicability of high-compliance system for hydraulic fracturing was evaluated and the maximum stress values were revised. Core disking was observed with groundwater flow in galleries at the depth of GL.-200m, during the Compact Conical-ended Borehole Overcoring (CCBO) test was adopted. To simultaneously estimate regional stress fields and the elastic modulus of rock mass from in situ stress measurements, back analysis methods based on three-dimensional finite element analysis and the boundary element method were developed.

Journal Articles

Rock spalling and countermeasures in shaft sinking at the Horonobe Underground Research Laboratory

Tsusaka, Kimikazu; Inagaki, Daisuke; Nago, Makito*; Aoki, Tomoyuki*; Shigehiro, Michiko*

Proceedings of 6th International Symposium on In-situ Rock Stress (RS 2013) (CD-ROM), p.339 - 346, 2013/08

The Horonobe Underground Research Laboratory is planned to consist of the Ventilation Shaft (4.5 m in diameter), the East and West Access Shafts (6.5 m in diameter). The host rock of the URL site comprises Neogene sedimentary rocks. The unconfined compressive strength of the rocks is less than 20 MPa on average. Anisotropic stress distribution around the URL is also confirmed. Because several highly permeable fractures (hydraulic transmissivity: approximately 10$$^{-5}$$m$$^{2}$$/s) with the size greater than the shaft diameter develop under the condition of around 2 in competence factor (i.e., the ratio of the unconfined compressive strength of rock to the initial stress) below a depth of 250 m, shaft sinking is a challenging issue from the viewpoint of tunnel engineering in the Horonobe URL Project. In this paper, the construction of the Ventilation Shaft below a depth of 250 m at the Horonobe URL is reported. During shaft sinking, fracture mapping of the shaft wall was performed. The geometry of the shaft wall was also measured using a three-dimensional laser scanner in order to investigate the shape and volume of rock spalling in the shaft wall resulting from the excavation work. Rock spalling was predominantly observed on the south and north wall rock corresponding to the direction of the minimum horizontal initial stress. A large amount of rock spalling also developed along a large-scale fault. With respect to the lining span and the layout of rockbolts, several support patterns were designed and installed as the countermeasures to prevent the development of excessive rock spalling. A flowchart for selecting the optimum support design was then developed.

Journal Articles

Inference of in-situ stress by Blasthole Damage Method (BDM) at Mizunami URL and its comparison with other direct and indirect methods

Aydan, $"O$.*; Sato, Toshinori; Hikima, Ryoichi*; Tanno, Takeo

Proceedings of 6th International Symposium on In-situ Rock Stress (RS 2013) (CD-ROM), p.360 - 369, 2013/08

There are various indirect stress inference techniques utilizing borehole breakouts, fault striations and earthquake focal mechanism solutions. Recently, a new stress inference technique utilizing damage zone around blastholes has been proposed and applied to several sites in Japan and Turkey. This method has been utilized in Mizunami Underground Research Laboratory (MIU) used to infer the stress state from the damage around blasted holes. The inferences are compared with in-situ stress measurements and also inferences from other indirect stress inference techniques such as fault striation, earthquake focal mechanism solutions. In this article the studies undertaken so far have been presented and the possibility of the BHD method as an effective indirect tool for actual engineering applications has been discussed when extensive network of underground openings have to be excavated.

Oral presentation

The Distinct element analysis for the de-stress drilling in the near field of the HLW repository

Shimizu, Hiroyuki*; Koyama, Tomofumi*; Chijimatsu, Masakazu*; Nakama, Shigeo; Fujita, Tomoo

no journal, , 

In the DECOVALEX-2011, an international cooperative research project, the coupled Thermal-Mechanical process in the $"A$sp$"o$ pillar stability experiment (APSE) was simulated. In this paper, a Distinct Element Method (DEM) model focusing on the effect of pre-existing cracks that present in the field scales rock mass has been developed. This model is applied to APSE. From this study the stress release of the pillar was successfully simulated by DEM, and the fracture generation and propagation during the destressing agreed qualitatively well with the observation at the site. The mechanical properties of a DEM model were successfully changed by introducing the pre-existing cracks even if the same particle arrangement and input parameters were used. This result is similar to the size/scale effect of actual rock mass. However, it was also found that the microcracks widely distributed and a large fracture did not formed in the rock model containing a lot of pre-existing cracks.

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