Abstract: The drop of water table induced by excess drainage after tunnel excavation in waterrich area would cause damage to normal growth of vegetation and ecological balance. As a result, the methods to determine the tunnel drainage volume based on ecological groundwater table are proposed. By taking a highway tunnel under construction in Fuzhou for example, a calculating method for the limit drainage volume of tunnel is proposed based on hydrogeology and the concept of ecological equilibrium embedment depth of groundwater. First, the groundwater seepage model of tunnel is established based on areawell method in groundwater dynamics, which reveals the relationship between the water inflow volume and groundwater table drawdown. Second, according to the proposed groundwater seepage model, the tunnel drainage time that meets the requirements of the ecological equilibrium embedment depth of groundwater is obtained on the premise of the survival of vegetation, and then the influence scope of the drainage funnel is derived. Third, the rainfall supplement volume W within the influence range is obtained by using the method of rainfall infiltration coefficient, and then the W is compared to the total groundwater discharge volume Qt. Finally, through adjusting unit groundwater drainage q to make W equal to Qt, q is the maximum allowable groundwater discharge in the tunnel site. Some conclusions are drawn as follows: (1) When the ecological equilibrium embedment depth of groundwater is constant, with the increase of q, the influence range of the tunnel decreases, W also decreases, and Qt increases gradually. Therefore, there is a unit drainage that makes the total groundwater discharge equal to rainfall supplement. (2) The limit tunnel drainage obtained without considering the time effect of water stress is more conservative than that obtained with water stress. The q of the tunnel under construction is 0.4 m3/(m·d).

Key words: tunnel limit drainage volumn, ecological groundwater table, water stress, groundwater dynamics, areawell method