Combined geophysical survey for characterizing bedrocks of a road tunnel
T. Inazaki, H. Kisanuki, K. Okazaki, S. Yamazaki and T. Kurahashi
Event name: SAGEEP 2017 - 30th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Infrastructure II
Publication date: 19 March 2017
Info: Extended abstract, PDF ( 1.26Mb )
We conducted a combined geophysical survey in an abandoned road tunnel for characterizing conditions of pavements and bedrocks. A 2 km long survey line was set along the center line passing through the 1.8 km long tunnel. Capacitively coupled resistivity measurement using OhmMapper and high-frequency surface wave measurement using Land Streamer were employed for the tunnel survey. Engineering geophysics has played an important role in planning and constructing of mountain tunnels. Surface seismic and resistivity surveys are adopted as the standard methods at the planning stage and HSP has been widely applied for long-spanned tunnels during excavation in Japan. Moreover, recent increase in collapsing incidents of active tunnels has led us to utilize geophysical methods for the safety check of existing tunnel structures instead of visual inspection. However, measurements for concrete pavements and the bedrocks of tunnels have been scarcely conducted due to the difficulty in applying them in active tunnels. It is needful to verify field performance and acquisition parameters controlling data quality before applying them in active tunnels. We then conducted field measurements in an abandoned road tunnel to test acquisition parameters of geophysical methods. As a result, near surface S-wave velocity structures up to 20 m beneath the tunnel floor were clearly imaged. The reconstructed structure was concordant with excavation and maintenance history of the tunnel. In contrast, the capacitively coupled resistivity measurement was very sensitive to the state of concrete pavement slab due to its internal rebar structures. In the repaired zone where rebar was densely arranged and connected each other, measured potentials were very low to reconstruct resistivity structures. Conversely, in the unrepaired zone we could obtain potential signals reflecting bedrock resistivity. In conclusion, combined resistivity and surface wave survey is capable for the characterization of physical properties of floor structures and bedrocks of tunnels.