In this study, we introduce the hydro-mechanical behavior observed in a fractured granite medium. Rock deformation due to the changes in the hydraulic pressure during the borehole drilling was observed at the 500m stage in the Mizunami Underground Research Laboratory. The obtained data shows an elastic behavior, although the several fractures are observed in the monitoring borehole. We carried out a numerical simulation to evaluate this behavior by the finite element method. The magnitude of the simulated deformation agrees well with the experimental data. From these results, we conclude that the movement of fracture joints is slight, and the subsurface facility is stable to changes in hydraulic response.

1. Introduction

The Mizunami Underground Research Laboratory (MIU) was constructed by the Japan Atomic Energy Agency (JAEA) for a multi-disciplinary study of the deep geological environment, providing a scientific basis for the research and development of technologies needed for geological disposal in crystalline rock. Large amounts of geoscientific data in the fields of geology, hydrology, geochemistry, rock mechanics, and others were collected from the ground surface to the deep subsurface before the construction of the MIU for this purpose. Among the many branches of geoscience, the studies concerning rock mechanics have been tackled to assess the safety and stability of deep subsurface facilities. Data on the physical properties of rocks, initial stress and rock deformation have been collected so far.

A high groundwater pressure is one of the characteristics of the geological environment in the deep underground facilities. During the construction and operation of an underground facility, the water pressure changes due to the drift excavation, borehole investigation, and drift-closure processes. These changes in groundwater pressure affect the rock's stress distribution, so rock deformation also changes during the construction and operation due to the changes in water pressure in addition to the stress redistribution due to the excavation. Therefore, the understanding of the coupled hydro-mechanical process is also important to asses the safety and stability of subsurface facilities. However, the available in-situ data about such hydro-mechanical behavior is scare. In this study, we introduce the in-situ hydromechanical response observed at the MIU site.

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