Prediction of ground ahead of tunnel boring machine (TBM) using electrical resistivity

Abstract

Various tunnels and underground structures have been constructed due to the influence of the gradual urbanization. The tunneling with explosives in the urban area is avoided due to the blasting vibration and noise that can occur the civil complaint. The mechanical excavation machines whose diameter is less than 5 m has been usually employed instead of the blasting method. The site investigations such as coring and surface geophysical survey are conducted to predict the ground condition along the tunnel before construction. However, the site investigation cannot be performed in intended position owing to the deeper depth of tunnel and the pavement of the surface and they induce the inaccurate prediction of the ground condition. A sudden appearance of the anomaly which is not detected during the site investigation can occur the loss of the budget and time. Therefore, the prediction ahead of the tunnel should be conducted during the tunnel excavation. Among various tunnel ahead prediction methods, the electromagnetic method is most suitable. Previous researches with electromagnetic method still have problems such as no consideration of the electrode shape and size during the virgin ground resistivity measurement and conductive materials near the TBM. In addition, the prediction range is certainly reduced by the small diameter of the tunnel. In this dissertation, the theoretical electrical resistance and the geometric factor were derived with consideration of the shape (spherical, disc, conical and cylindrical) and size of the electrodes and they were verified with the experimental tests. The theoretical solutions for layered media were derived and verified with the experimental and numerical method. Arrangements of the electrodes were suggested to prevent the reduction of the prediction range due to the small diameter of the tunnel. Theoretical equations were derived for the arrangements with considering the change of the layers. Finally, the theoretical equations were applied on three field tests and the numerical simulations were conducted in ideal situation. Back analysis was performed with the derived equations from this dissertation.