Evaluation of Fractures in a Rock as Flow Paths Around Tunnel Using Gruond Penetrating Radar

Abstract

Fractures developing around the tunnel during the excavation result in issues related not only to the mechanical stability of the rock cavern, but also to the groundwater flow paths. Specifically, in terms of the safety assessment of geological disposal of radioactive waste, the groundwater management of rock caverns for oil/LPG storage using hydraulic containment system, or planning of countermeasures for water seepage of tunnels, it is important to have a thorough understanding of the groundwater conditions around the tunnel.

The GPR（Ground Penetrating Radar）method is unique technique to survey the groundwater condition in fractures around the tunnel, indirectly, and with no disturbance of original ground water condition. In order to estimate the possibility of application of GPR to estimate the fractures as flow paths, the authors conducted the GPR survey along the side wall of 500 m access tunnel of Mizunami Underground Research Laboratory of JAEA. The results of the profile measurements indicated that water-conducting fractures were detected as a reflected waves using GPR survey. Furthermore, as the results of fixed-point measurements during the injection of the saline water, it could be indicated to estimate the flow paths of saline water in the fractures, in a non-destructive way, based on spectral analysis in the reflected waveforms of GPR.