Effects of groundwater seepage in steady-state conditions in deep tunnel shafts

摘要

The aim of this paper is to study the effects of groundwater seepage in steady-state conditions in the excavation of a 12m diameter and 90m deep tunnel shaft. Several of the soil strata located in the excavation area of the tunnel shaft are constituted by medium to high hydraulic conductivity materials, which generate important flow velocities and flow rates. Additionally, the presence of lower hydraulic conductivity strata interbedded between more permeable layers suggests the occurrence of significant hydraulic gradients, and therefore assessment of the uplift pressure in certain stages of the tunnel shaft excavation becomes necessary. A methodology to carry out this type of analyses by two-dimensional numerical modeling based on the finite element method is exposed. The results presented refer to the hydraulic head field, velocity vectors, among others, with emphasis on exit hydraulic gradients and flow rate at different stages of the tunnel shaft excavation. In particular, graphs related to the exit hydraulic gradient, exit seepage velocity, and flow rate as a function of the excavation depth are provided. Finally, conclusions of the analyses performed and recommendations in order to control the groundwater seepage and to mitigate uplift problems into the tunnel shaft are provided.