Parametric and numerical modeling tools to forecast hydrogeological impacts of a tunnel

The project of interest involving a hydroelectrical diversion tunnel through crystalline rock massif in the Alps required detailed hydrogeological study to forecast magnitude water inflows within and possible effects on groundwater flow. exhibits length 9.5 km is located right side Toce River Crevoladossola (Verbania Province, Piedmont region, northern Italy). Under geological framework Alps, Lower Penninic Nappes footwall Simplon Normal Fault, succession mostly represented by Antigorio gneiss (meta-granites) Baceno metasediments (metacarbonates). Due presence important mineralized springs for commercial mineral purposes, abovementioned focused both quantity quality aspects via rainfall data analysis, monitoring major spring flow rates, hydraulic heads pumping rates existing wells/boreholes, hydrochemical isotopic analysis boreholes tests (Lefranc Lugeon). resulting conceptual model indicated dominant low-permeability (aquitard) behavior gneissic masses, except under conditions intense fracturing due tectonization, aquifer metasedimentary rocks, particularly when interested dissolution. Groundwater systems are mainly controlled gravity. near were characterized high mineralization ratios, indicating long paths. Based all collected analyzed, two parametric methods applied: 1) Dematteis method, slightly adapted case available data, which allows assessment potential impacts (codified as drawdown hazard index; DHI); 2) Cesano only tunnel, thereby discriminating between minor inflows. Contemporarily, was implemented with equivalent porous medium (EPM) approach MODFLOW-2000. This calibrated steady-state against (groundwater levels inside wells/piezometers elevation rate springs). method demonstrated be more reliable suitable site than method. validated considering this considered intrinsic parameters masses notably morphological geomorphological factors considered. relatively overestimated inflows, variations topography overburden above tunnel. Sensitivity revealed low sensitivity these parameter values, designation (RQD) employed represent degree. numerical ante-operam conditions, evaluated influence uncertainties conductivity (K) values different units. head distribution after excavation forecasted three scenarios, namely, draining loss source, sealed along its sectors, 3 K reduction. Tunnel impermeabilization very effective, thus lowering drainage impact springs. quantitatively defined at surface terms decrease. combined obtain final risk likely because assigned chosen conservative way, simulations also (the transient state setting supposedly lasts 1–3 years). Monitoring during boring could facilitate feedback process.