Fracture Characterization and Rock Mass Damage Induced by Different Excavation Methods in the Horonobe URL of Japan
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We conducted detailed fracture mapping of the soft sedimentary rocks (uniaxial compressive strength of 10–20 MPa) in shaft walls at the Horonobe Underground Research Laboratory to characterize fractures and to understand the influence of different excavation methods on rock mass damage. The mapping indicates that the fractures are numerous and can be divided into shear fractures and extension fractures. On the basis of orientation and frequency, the shear fractures are inferred to be pre-existing fractures, and the extension fractures are considered to be newly formed fractures (EDZ fractures) induced by the shaft excavation. The frequencies of pre-existing and newly formed fractures have a negative correlation, and we infer that stress relief leads to the formation of excavation damaged zone by the generation of the newly formed fractures in the parts of shaft that have intact rock, and by the reactivation of pre-existing fractures where such fractures are numerous. Although more newly formed fractures are formed by blasting excavation than by mechanical excavation, there is little difference in the comparative excavation rates. These results indicate that rock mass damage is caused by the mode of excavation rather than excavation rate. Therefore, the mechanical excavation is preferred to blasting excavation from the viewpoint of minimizing rock mass damage.
KeywordsFracture Excavation EDZ Soft rock Shaft
This work was funded by a collaborative research project between Shinshu University and Japan Atomic Energy Agency under project No. 27k026. The authors would like to thank Mr. Hagihara of Taisei-Obayashi-Mitsuisumitomo joint venture group, Mr. Matsubara of Geoscience Research Laboratory, Co., Ltd. and Mr. Ishikawa of Mitsubishi Materials Techno Corporation for their technical contribution to the acquisition of the data. We would also like to extend our gratitude to Glen McCrank for editing the English and providing helpful suggestions. This paper benefited from critical and helpful reviews by two anonymous reviewers and Dr. Mohammad Hassan Baziar (Editor of International Journal of Civil Engineering).
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