There is no standard geological investigation method to acquire geological information along a tunnel route. In this study we applied the time-domain electromagnetic (TDEM) method and acquired geological information along the routes of 25 tunnels to improve the accuracy of geological and geophysical data used for tunnel design and construction. The TDEM method is used mainly for oil and mineral resource explorations and is reliable at depths of 200 m or more. Tunnel-face observations recorded during tunnel construction as well as data of various geophysical survey techniques were analyzed by comparing the correlation coefficients based on a quantitative theory. Quantitative analysis of the comparisons between the TDEM resistivity results and the tunnel-face observations at the same site showed good correlation. The resistivity obtained by the TDEM method was found to have similar survey precision as the seismic velocity obtained by a seismic refraction survey when the overburden was less than 100 m thick. The accuracy of the TDEM results was very high even for an overburden of 200 m. Moreover, TDEM can investigate the low-velocity speed layer below a high-velocity speed layer, where the seismic refraction method does not perform well. The resistivity had strong correlation with the crack intervals in the rocks, and the seismic velocity had strong correlation with the uniaxial compressive strength. Moreover, the resistivity data obtained by the direct-current electrical method in the same way revealed clear differences in the survey accuracy when the overburden exceeded 100 m. Based on this study, we propose the application of the TDEM method to geological surveys along tunnel routes passing deeper than 200 meters.

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We have developed a new technology for surveying natural ground ahead of a tunnel face. The technique, called “DRi-Scope,” visualizes the state of ground forward of a tunnel face using an industrial endoscope. The natural ground at the point of a bit is observed directly by inserting an industrial endoscope in the water supply conduit of a rod on a hydraulic drill jumbo. Accurate geological information is obtained while minimizing the impact on the tunneling process. This paper provides an outline of the technology and describes its features. It then introduces application examples for visualization of natural ground ahead of tunnel faces.

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This article is the summary of the author’ thesis (Hashimoto, 2017), which is the winning paper of the Japanese Society for Rock Mechanics (JSRM) Doctoral Thesis Award 2017. The Angkor ruins, a World Heritage in Cambodia, includes many masonry structures in danger of collapse due to geotechnical problems, such as uneven settlement of the foundation ground. In order to select suitable restoration methods, stability evaluation considering the mechanical interaction between the masonry building (discontinuum) and the foundation ground (continuum) is required. In this study, to establish the stability evaluation method for the historic masonry structures, a numeridcal method for the composite structure of the soils and the masonry stones is newly proposed based on the coupled Numerical Manifold Method and Discontinuous Deformation Analysis (NMM-DDA), a discontinuum-based numerical method. Using the developed method, the failure modes/mechanisms and the bearing capacity characteraistics of masonry structure foundations were investigated. Finally, incorporating the results by the numerical analyses, a simplified design scheme that can be easily implemented in the actual restoration projects was also proposed.

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Prediction of geological and hydraulic conditions ahead of tunnel face is very important because misjudgment of the ground conditions may lead to a tremendous disaster and a delay in schedule planning. However, due to the long and linear nature of tunnel, pre-geological investigation is sometimes insufficient. Especially, as to the groundwater, installation of observation wells is quite difficult due to huge overburden. In order to solve this problem, the authors have developed a new system of monitoring groundwater information in combination with several types of pilot boring ahead of the tunnel face. The horizontal pilot boring for the tunnel was categorized into three types by drilling length; extra-long boring, moderate-length boring and short boring. Also the authors have developed new devices to measure water inflow information by each three drilling system. These devices have different concept and structure depending on the purpose of measurement. By applying these systems at appropriate construction stages in turn, hydraulic information can be obtained quantitatively. Three types of the device were respectively tested in a tunnel site and were verified their good performance. In the future, the system that combines three different techniques will be applied to the estimation of the groundwater conditions ahead of tunnel face and judgement of the appropriate countermeasures that could lead to safety tunnel excavations.

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