Modelling the interference of underground structures with groundwater flow and remedial solutions in Milan

Underground structures and infrastructures within urban areas cause disturbance to the groundwater flow. Groundwater abstraction in Milan decreased of more than 30% since the 1970s generating a groundwater rebound which caused submergence and flooding of multiple underground structures and infrastructures. In order to evaluate the impact of underground structures on the groundwater flow in the Milan city area, a detailed analysis based on a groundwater flow model was carried out. The 3D FEM groundwater flow model was calibrated against high resolution monitoring data and used to evaluate both the disturbance induced by underground structures and their sequence of construction, and to verify the performance of mitigation strategies. Major modifications of groundwater head distribution are observed when structures transversally cut the groundwater flow, resulting in the fragmentation of the groundwater flow pathways and leading to changes in groundwater head from about ±0.05 m, at a regional scale, to about ±0.45 in relative proximity (few hundred metres) of underground structures. The effectiveness of a traditional-grey (i.e. a drainage tunnel) and a blue-green infrastructure (i.e. water extraction wells combined with heat pump systems) mitigation scenarios is discussed. Blue-green infrastructures result the most effective and efficient mitigation solutions with direct impact on sustainability and environmental conditions.