Due to long and linear properties of tunnel, pre-geological investigation of it is sometimes insufficient and contractors sometimes have to change the plan depending on the geological condition they face during excavation. Especially, as to groundwater, misjudgment of its condition ahead of tunnel face might cause tremendous disaster. However, in the case of large-scale tunnel, it is quite difficult to set monitoring wells on the surface due to its huge overburden. Here, in this study, several types of innovated groundwater surveying and monitoring technique from tunnel inside are developed. At first, the horizontal pilot boring in the tunnel was categorized into three types as boring distance. Then, the special devices to measure ground water pressure for each boring type are respectively developed. Therefore, devices for each boring type have different concept and structure depending on the purpose and timing of each horizontal boring. There three types of devices were respectively tested in tunnel site and were verified their good performance. In the future, the system that combines these three different devices will help site staffs to estimate the ground water condition ahead and judge the appropriate counterplan, and that must leads to safety tunnel excavation.
Since mountain tunnels are long and linear structures, it is often difficult to accurately predict geological, groundwater, and other subterranean conditions through pregeological investigations alone. This means construction plans are often modified in response to unexpected geological conditions encountered during excavation. Of various possible factors, groundwater can cause sudden and high-pressure water inflow during the construction of a large-scale mountain tunnel, resulting in major construction delays or, in worst-case scenarios, disasters. Since it is not always possible to drill monitoring wells from the ground surface due to large overburden, it is critical to obtain reliable groundwater information from inside the tunnel via horizontal boring during the construction phase. In recent years, horizontal bores of various lengths have been drilled during tunnel construction for various purposes, including extra-long horizontal bores.1)-3) We have developed a system of water inflow research monitoring (SWIReMo) capable of acquiring accurate water inflow information; this system comprehensively evaluates conditions ahead of the tunnel face and provides water inflow information to help ensure safe excavation. Various measurement technologies and horizontal bores of different lengths were required to enable SWIReMo to obtain the information needed based on the construction period and purpose. We tested individual techniques and systems in field tests.
