ABSTRACT
This study proposes a method for evaluating the influence of tunnel excavation on the surrounding groundwater and countermeasures against its adverse effects at Kanegasaka Tunnel. The mountainous part of Kanegasaka Tunnel passes through the Tanba Group, which is composed mainly of Late Triassic shale, sandstone and chert, and the portal part passes through alluvial gravel layers that hold a considerable amount of groundwater. Since portal section is covered by a thin overburden layer and there are many water sources such as rivers and wells in the vicinity, it was thought likely that tunnel excavation would decrease river flows and change groundwater level. It was important, therefore, to predict groundwater inflow into the tunnel. In the present study, this challenge was addressed by using three-dimensional seepage analysis. Application of 3-D analysis enabled simulation of planned seepage control measures and proved helpful in evaluating seepage control works. This paper describes the procedure of evaluation using 3-D analysis, discusses the approach’s superiority over other methods, and reports some of the analytical results.
Efficiency and safety of water control works used to be among the principal groundwater-related considerations in tunnel excavation. In recent years, however, there has been a growing need to address groundwater problems with a view to minimizing environmental impact. Although there are cases where conventional two-dimensional and quasi-three-dimensional analyses give satisfactory results in predicting the influence of tunnel excavation on the surrounding groundwater, their applicability to tunnel construction is limited because of the three-dimensionality of topographic and geologic distribution and the complexity of tunnel structure. Furthermore, there are many cases where evaluation of environmental impacts requires a rigorous 3-D groundwater flow analysis over a large area. In the Kanegasaka Tunnel project reported in this paper, 3-D seepage analysis was performed by the finite element method (FEM) to predict groundwater behavior accurately, and the results obtained were used to explain to the local residents the groundwater conditions induced by tunnel excavation and the effectiveness of different types of control works by which to reduce the lowering of the groundwater level. The talk with residents was successful to show the predictions based on 3-D simulation results and finally a suitable construction and counter-measure method was decided. Accountability of the project was accepted.
