Analytical solution for the limiting drainage of a mountain tunnel based on area-well theory

Abstract

Water seepage from a tunnel may induce the drawdown of the groundwater table and destroy vegetation and the ecological environment in the tunnel area. One of the primary objectives in tunnel engineering is to develop effective methods to estimate the amount of the limiting drainage of a mountain tunnel. Based on the area-well theory, a groundwater seepage model of a tunnel is established. Due to the model and groundwater dynamics, the equation describing the relationship between the amount of water inflow and drawdown of the groundwater table for a tunnel is derived under unsteady flow conditions. After the ecological groundwater table for vegetation is introduced, a method of calculating the amount of the limiting drainage for a mountain tunnel is proposed to prevent the drawdown of groundwater table from exceeding the ecological groundwater table required to support vegetation. According to the proposed method, the three-dimensional form of the groundwater drawdown funnels and the influence scope of tunnel water inflow are determined. The rainfall recharge amount Wt within the influence scope is obtained based on the rainfall infiltration coefficient method, and the value is compared to the total groundwater discharge Q. These values Wt and Q are then equated through adjusting the unit groundwater inflow q0. The values less than or equal to q0 can be determined as the limiting drainage value of groundwater for the tunnel, and the groundwater table will not drop and balances the groundwater and the vegetation ecological environment. The results and case study show that the proposed method exhibits good agreement with actual conditions and is suitable for use in tunnel construction.