Piezometric damming effect and mitigation measures related to the new underground High Speed railway station of Florence (Arno river alluvial aquifer): modelling scenarios and challenges

The new High Speed railway station of the city of Florence will be located below ground level inside a huge excavation, down to 25 m b.g.s, of 450 m length and 50 m width; the pit, already built, is surrounded by concrete cutoff walls determining a piezometric damming effect against the main aquifer of the Arno river, formed by alluvial gravels and sands (K range values between 1E-5 and 1E-3 m/s) and extended between 2 and 12 m b.g.s. Local groundwater flow direction is almost perpendicular to the longer side of the excavation. As permanent mitigation measures of the piezometric mounding (temporarily attenuated by pumping and recharging wells on both sides of the excavation) the design considers a series of drain pipes drilled horizontally on up and down gradient sides of the pit and connected by blind wall tubes. A numerical finite differences code (Modflow) was implemented in order to verify the damming effect and to properly design the drain-based mitigation measures (frequency and length of the drains). One of the major challenges of the approach was related to how simulate the drains and how integrate in the same model the drain system (connection between a porous medium and an empty pipe through the filter) and the blind wall tubes system connecting the upgradient and downgradient sides of the excavation. Drain system is not flowing in open air conditions but in fully saturated conditions in relation to the transient head distribution inside the area of influence of the aquifer. The implementation of the model and the calibration phase was accomplished by the results of a not-in-scale physical model, rebuilt in the lab, useful to calibrate the coupling between the porous medium and the drainage system. In order to guarantee an acceptable head differential between both sides of the excavation, the simulation indicates that the system should permit a discharge flow rate that is consistent with the pumping rate of the wells temporarily installed to prevent the damming effect, so confirming, indirectly, the physical consistency of the modeling approach.