The groundwater rise in the urban area of Milan and its interactions with underground structures and infrastructures

This paper discusses the effectiveness of different solutions for managing the rise of the groundwater level in the urban area of Milan (Italy). The study area is interested by a regional rising trend of the water table, which can bring about a hydrogeological hazard for the existing underground structures (metro tunnels, stations, deep foundations, etc.). The study was carried out by means of the following steps: (1) monitoring data analysis for reconstructing the regional trend of the water table; (2) calibration of a 3D numerical model of the groundwater flow; (3) scenarios simulation of the aquifer system evolution and evaluation of its effects on underground infrastructures; (4) evaluation of the effectiveness of different solutions for hazard mitigation. In the present day scenario, modelling results pointed out a local increasing of the groundwater levels over wide areas located nearby the tunnels intersections, where the highest increase in water table due to the regional trend is observed (about 10 m in the last 20 years). Results obtained for the future scenarios pointed out a significant increase in water level (from 1 m in short term scenarios to 10 m in the most pessimistic long term scenario), as well as an increase in water thrusts acting on the structures and flow velocity below foundations, bringing about important issues related to their static stability in the long term. Finally, the numerical results for different mitigation systems showed that in the long term the problem could be solved only through a superimposition of the effects of several solutions.