Geological mapping of regional structural lineaments for tunnel construction

Abstract

The Hakkoda tunnel is part of the Tohoku Shinkansen, which is under construction. The tunnel is about 40 m deep in the Yakata area. According to former geological data, the tunnel will pass through a regional structural lineament in the base rocks, which are highly fractured and faulted. There is a small river above the tunnel in the area, which provides drinking water for downstream villagers. Due to the low permeability and the mud rocks, shown at the nearby outcrops because of geopressure during the construction of other sections. This can be a potential danger for the tunnel construction at this particular site since river water can potentially leak into the tunnel through the fractures or faults during the construction. There is also a water dam in the neighborhood. There is a possibility of damaging the dam during the construction of the tunnel in this complicated geological environment. For safety reason, it is important to map the geological and hydrological settings in detail before construction. Considering the accuracy of various geological or geophysical methods and their applicability in the region, we employed four geological survey methods. A comprehensive work plan for the tunnel construction was made based on the results. Four geological prospecting methods were employed sequentially during the survey. They are: (1) Electrical Sounding, which was used to map the geological structure, positions of faults or fracture zone and possible water layers in the region. (2) Vertical drilling, which provided more detailed mapping of the geological settings and physical properties of the rocks. (3) Surface wave exploration and Microtremor measurements. Both methods provide more detailed mapping for the regional structural settings, faults or fractured zones. The methods also have much a better three-dimensional resolution in comparison with the electrical method. (4) Horizontal drilling from the work face to obtain more detailed information on properties of the rocks and fractures or faults ahead of the tunnel under building. Water samples were taken from these horizontal wells to evaluate the possibility of possible water invasion by comparing and analyzing the quality of various waters. Correctly integrating various geophysical and geological data was the key to the success of detailed geological mapping in the area, from which a comprehensive work plan was made. The survey results were also accessed by comparing the geological predictions with what we saw at the work face during the construction. In the complex geological environment like this, it is essential to utilize various geological and geophysical methods in order to correctly predict the geological conditions in advance. An optimized working plan can be made based on the detailed geological mappings. Even for areas or zones with the high possibility of water leakage, a proper supplementary work at small scale will be able to prevent the water leakage without injecting cements into the base rock at a much larger scale. This optimized working plan will potentially reduce the construction cost. In conclusion, considering the construction cost and time, favorable geological and geophysical methods were selected based on known geological information and regional geology in the area. The tunnel construction work plan was optimized by correctly integrating various geological and geophysical results. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.