A Local-Scale Numerical Model to Quantify Groundwater Infiltrations into Underground Infrastructures: The Case of Milan

Urbanization, worldwide, led to progressive increase in the use of the subsurface. Therefore, interactions between groundwater and underground infrastructures (UIs) (i.e. subway lines, public and private car parks) come to attention. As a results, episodes of infiltrations and corrosion can occur for these subsurface elements. In this sense, numerical models are widely applied to manage these issues. Infiltration episodes were documented over the years also for the city of Milan. In the framework of a further subsurface development to be achieved by 2030, the Plan of Government of the Territory has already defined the construction of new UIs. Hence, proper tools must be adopted to support stakeholders in urban planning, also considering these hydrogeologic aspects. Thus, a steady-state numerical model, calibrated against a groundwater maximum condition, was developed for the western portion of Milan metropolitan city to simulate the UIs (i.e. subway lines and public car parks) and quantify their infiltrations. MODFLOW-USG was used, combining the Wall (HFB) and DRN packages to model the UIs. Model cells range from 100 up to 6.25 meters close to the UIs. A total of 18 layers modelled 100 – 120 meters of depth, with the UIs falling inside the first 8 layers (25 meters of depth). Different scenarios were simulated to cope with the wall conductance uncertainty of non-waterproofed subway lines, testing respectively: intact walls, a progressive saturation over time (i.e. a prolonged interaction with groundwater), and leaky walls. Results pointed out those UIs suffering major groundwater infiltrations that were historically identified as critical in previous studies (i.e. submerged by the water table). Particularly, this occurred for the western branch of subway line M1 (i.e. Bisceglie station), and for the surroundings of Sant’Agostino (subway line M2), showing a maximum infiltration of 5.29 L/d. The use of MODFLOW-USG was pivotal, as the HFB package was used to simulate not only the lateral walls of the UIs, but also their vertical sides (i.e. top and bottom). Hence, UIs were modelled with their real depth and volume, properly simulating both their interactions with the water table and the consequent infiltrations. Model outputs could assist stakeholders to adopt the needed measures to avoid these UIs from being flooded in the future, also considering the already planned subsurface development of the city.