Assessment of the barrier effect caused by underground constructions on porous aquifers with low hydraulic gradient: A case study of the metro construction in Barcelona, Spain

• Impervious tunnels may modify the natural groundwater behavior.
• Impact caused by tunnels may not be perceptible if the groundwater gradient is small.
• The reduction in aquifer connectivity caused by a tunnel can be assessed by pumping tests.
• Pumping tests must be performed before and after the construction of the tunnel.

Construction of tunnels can impact aquifers because of the changes produced in the natural groundwater behavior. The drain effect, which is one of the most important impacts, can be eliminated using a tunnel-boring machine (TBM) to drill a tunnel with an impervious lining. However, the use of impermeable linings results in aquifer obstruction, giving rise to the barrier effect, which may cause an increase and decrease of the hydraulic head upgradient and downgradient of the tunnel, respectively. This modification of the hydraulic head, which can be predicted analytically and is proportional to the natural hydraulic gradient of the aquifer perpendicular to the tunnel (iN) (before it is constructed), is negligible for aquifers with values of iN that are very small or null (approximately 0). In these cases, the analytical solutions are not useful to estimate the real impact because the head distribution is not largely affected.This study proposes a methodology to evaluate the hydrogeological impact produced by the construction of underground impervious structures in aquifers, which have a small or null iN. The method, which is based on the analysis of the groundwater response to pumping tests performed before and after construction, was tested in a stratified porous aquifer and was used along with numerical modeling to assess the barrier effect in an experimental site (Sant Cosme, El Prat de Llobregat, Barcelona). The impact on the head distribution was negligible. However, the reduction of the connectivity was considerable. Pumping tests can determine the changes in aquifer connectivity caused by the construction of an underground impervious structure. The behavior of the groundwater during the post-tunneling pumping changes with regard to the pre-tunneling tests. A delay in the response to the pumping and a decrease of the drawdown are observed in the piezometers located on the opposite side of the tunnel where the well is placed, whereas an increase of drawdown occurs in the piezometers situated on the same side of the well. The procedure explained in this paper reveals a useful tool for determining the impact caused by underground impermeable constructions in aquifers, where iN is small or even 0.