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

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

Classificazioni geomeccaniche, modellazioni numeriche e metodologie di scavo per opere sotterranee. L’impianto idroelettrico di Pieve Vergonte (Alpi Occidentali)

Le classificazioni geomeccaniche costituiscono la base per la descrizione degli ammassi rocciosi, per la previsione del comportamento degli scavi e per la progettazione dei sostegni. Nel presente lavoro vengono illustrati i dati relativi alla caratterizzazione geologico-geomeccanica, al monitoraggio estensimetrico ed alla modellazione numerica di un impianto idroelettrico in caverna realizzato nelle Alpi occidentali, in provincia di Verbania. Tutte le opere dell’impianto (sistema di gallerie e caverna) ricadono all’interno di litotipi metamorfici marcatamente scistosi che, nella zona della caverna, descrivono una piega aperta di dimensioni paragonabili allo scavo stesso. Lo studio ha incluso analisi petrografiche e geomeccaniche dei principali litotipi, determinazioni in sito dello stato tensionale, classificazioni dell’ammasso roccioso sia nella zona della caverna che nelle aree adiacenti ad essa. Sulla base dei risultati delle precedenti analisi sono stati realizzati due modelli numerici della caverna, basati rispettivamente su codici agli Elementi Finiti ed agli Elementi Distinti, al fine di prevedere le deformazioni dell’ammasso roccioso a seguito dello scavo. Le deformazioni reali registrate dagli estensimetri sono risultate in buon accordo con quelle predette dai modelli numerici. Infine, e’ stata analizzata l’influenza della metodologia di scavo rispetto alle condizioni di stabilità delle pareti: l’utilizzo di metodi meccanizzati (raise-borer e TBM) ha determinato un minore degrado delle qualità dell’ammasso roccioso registrato da variazioni nei valori delle classificazioni geomeccaniche e ciò ha consentito la posa di sostegni meno pesanti rispetto a scavi vicini, realizzati con metodi tradizionali (perforazione ed esplosivo)

Engineering geological characterization and comparison of predicted and measured performance of a cavern in the Italian Alps

A detailed engineering geological characterization and performance monitoring were carried out at the site of an underground powerhouse cavern in the Italian Alps. In the area of the hydroelectric project, consisting of a 4-m diameter and 10-km-long diversion tunnel and a powerhouse cavern (20 m wide, 39 m long and 30 m high), metamorphic anisotropic rocks, are present. A pervasive foliation, whose trend describes an open fold at the cavern site, characterizes the geological structure. The studies include petrographic analyses and geo-mechanical properties of the rocks, in situ stress measurements and rock-mass classifications for the cavern site as well as the surrounding area. Based on field investigations, two numerical models (FEM and DEM codes) were used to investigate the overall stability of the excavation and to predict the expected deformation caused by each excavation phase. The measurements of actual deformations, by multi-base extensometer data, are reasonably close to those predicted through the numerical approaches

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

The project of a hydro electrical diversion tunnel through a crystalline rock massif in the Alps needed a detailed hydrogeological study in order to forecast the magnitude of water inflows inside the tunnel and the possible effects on groundwater flow. The tunnel has a length of 9.5 km and is located on the right side of Toce River at Crevoladossola (Verbania province, Piedmont region, Northern Italy). In the geological framework of the Alps, the tunnel is located inside the Lower Penninic Nappes, in the footwall of the Simplon Normal Fault; the geological succession is mostly represented by Antigorio gneiss (meta-granites) and Baceno metasediments (metacarbonates). Due to the presence of important mineralized springs used for commercial mineral water, the hydrogeological study focuses both on quantity and quality aspects, by means of: rainfall data analyses, monitoring of major springs flow rates, monitoring of hydraulic heads and pumping rates of existing wells/boreholes, hydrochemical and isotopic analyses on springs and boreholes and hydraulic tests (Lefranc and Lugeon). The resulting conceptual model evidences a dominant low permeability (aquitard behaviour) of gneissic rock masses, except for situations of intense fracturing due to tectonization, and an aquifer behaviour of metasediments, particularly when interested by dissolution. Groundwater flow systems are mainly controlled by gravity. Springs located near Toce river are characterized by higher mineralization and isotopic ratios, indicating long groundwater flow paths. Starting from all the data collected and analyzed, two parametric methods are applied: 1) Dematteis method (Dematteis et al., 2000), slightly adapted to the case study and to the available data, that allows assessing both potential inflows inside the tunnel and potential impact on springs (codified as Drawdown Hazard Index); 2) Cesano method (Cesano et al., 2000) that allows only assessing potential inflows inside the tunnel, discriminating between major and minor inflows. Contemporarily a groundwater flow model is implemented with the EPM (Equivalent Porous Medium) approach, using MODFLOW-2000; it is calibrated in steady state conditions on the available data (groundwater levels inside wells/piezometers, elevation and flow rate of springs). Dematteis method proves to be more reliable and more adequate to the site than Cesano one; it was validated on a tunnel in gneissic rock masses and it takes more into account intrinsic parameters of rock masses than morphological and geomorphological factors. Cesano method relatively overestimates tunnel inflows, taking more into account the variations of topography and overburden above the tunnel. A sensitivity analyses evidences a low sensitivity of parametric methods to parameters values, except for RQD (Rock Quality Designation) used to represent fracturation degree. The numerical model is calibrated in ante-operam conditions and a sensitivity analysis evaluates the influence of uncertainties in hydraulic conductivity (K) values of the different hydrogeological units. Hydraulic head distribution after tunnel excavation is forecasted considering three different scenarios: tunnel only draining; tunnel as a losing source of water; tunnel sealed along its aquifer sectors, using 3 different levels of K reduction. Tunnel impermeabilization results very effective, lowering the drainage rate and the impact on springs. The model defines quantitatively the tunnel inflows and the effects on springs flow at the surface in terms of flow rate decrease. Dematteis method and the numerical model are crossed to obtain a final risk of impact on springs. The study is supposed to overestimate the risk, because all the values assigned to parameters are chosen in a conservative way and numerical simulations at steady state are very conservative too (transient state in such a hydrogeological setting is supposed to last 1-3 years). Monitoring of tunnel and springs during tunnel boring will allow the feedback process