The excavation of a tunnel is thought to induce hydrogeological risk due to regional groundwater supply quali-quantitative pauperization and to unexpected water inflow into the tunnel itself. Predictability of such effects is related to different parameters whose careful modeling is required. This study deals with the influence of the permeability tensor, characterizing the excavated rock, on the tunnel water inflow, the groundwater drawdown and the radius of influence amplitude. The phenomenon has been examined by the development of a finite element model (using software MIDAS/GTS), considering a simplified geometrical structure in order to compare the obtained results to those derived from an analytic method (Goodman’s formula). Subsequently, the model has been implemented to a real case concerning the works of the Brenner Base Tunnel, for which a comparison was made between simulated and observed results. Simulations using different vertical and horizontal permeability combinations has been done in order to quantify the influence of anisotropy on the examined variables. Our data showed that the horizontal permeability has the major impact on the radius of influence amplitude, and furthermore, when vertical permeability is particularly high, influences the ground-water drawdown. The tunnel inflow resulted affected by both examined parameters, whose individual importance changes in relation to the anisotropy. The analysis showed the possibility that, in strongly anisotropic conditions and low permeability, the use of the Goodman’s formula could lead to an undestimation of the tunnel inflow. Therefore with the calibration of an empiric equation depending on the anisotropy ratio kx/kz it has been possible to reduce Goodman’s overestimation from 16% to 2% and its underestimation from 67% to 15% for tunnel inflows greater than 100 l/s. In conclusion, a careful modeling of the system anisotropy must be considered of relevant importance because a calculation based on the bare average value of permeability of the excavated rock (as Goodman’s formula) could underestimate the water inflow up to 70%.
La realizzazione di una galleria è nota comportare un rischio di tipo idrogeologico legato all’impoverimento quali-quantitativo delle risorse idriche della zona e alle possibili venute d’acqua in galleria. La prevedibilità di questi effetti è legata a numerose variabili di cui è necessaria un’attenta modellazione. Questo studio tratta la sensibilità della portata drenata dalla galleria, dell’abbassamento piezometrico indotto e dell’ampiezza della zona di influenza, al variare delle componenti principali del tensore di permeabilità idraulica caratterizzante l’ammasso roccioso sottoposto ad escavazione. Il problema è stato studiato mediante un modello sviluppato ad elementi finiti(con il software MIDAS/GTS),considerando una geometria semplice in modo da poter confrontare i risultati ottenuti con quelli derivati da un metodo analitico (formula di Goodman). Successivamente è stato applicato ad un caso reale inerente alla costruzione della Galleria di Base del Brennero, per il quale è seguito un confronto tra risultati simulati ed osservati. In base alle valutazioni effettuate si è osservato che la permeabilità in direzione orizzontale risulta maggiormente influente nella determinazione del raggio di influenza e, solo nel caso di permeabilità verticali elevate, dell’abbassamento piezometrico in asse alla galleria. La portata in galleria è risultata sensibile ad entrambi i parametri studiati, la cui specifica influenza si modifica in relazione al variare reciproco. Dalle analisi emerge la possibilità che in condizioni di forte anisotropia e bassi valori di permeabilità, l’utilizzo della formula di Goodman induca una sottostima del valore di portata drenata. Pertanto, è stata calibrata una relazione empirica, funzione del rapporto di anisotropia kx/kz, che ha permesso di ridurre dal 16% al 2% la sovrastima di Goodman e dal 67% al 15% la sua sottostima per valori di portata drenata superiori a 100 l/s. Si può pertanto concludere che ai fini progettuali emerge l’importanza di un’accurata modellazione dell’anisotropia del sistema poiché considerare un valore rappresentativo di permeabilità media dell’ammasso roccioso può indurre anche sottostime di portata intorno al 70%.
Studio idrogeologico dei processi di drenaggio indotti da gallerie in ammassi rocciosi anisotropi
MAZZA, ISABELLA
2012/2013
Abstract
The excavation of a tunnel is thought to induce hydrogeological risk due to regional groundwater supply quali-quantitative pauperization and to unexpected water inflow into the tunnel itself. Predictability of such effects is related to different parameters whose careful modeling is required. This study deals with the influence of the permeability tensor, characterizing the excavated rock, on the tunnel water inflow, the groundwater drawdown and the radius of influence amplitude. The phenomenon has been examined by the development of a finite element model (using software MIDAS/GTS), considering a simplified geometrical structure in order to compare the obtained results to those derived from an analytic method (Goodman’s formula). Subsequently, the model has been implemented to a real case concerning the works of the Brenner Base Tunnel, for which a comparison was made between simulated and observed results. Simulations using different vertical and horizontal permeability combinations has been done in order to quantify the influence of anisotropy on the examined variables. Our data showed that the horizontal permeability has the major impact on the radius of influence amplitude, and furthermore, when vertical permeability is particularly high, influences the ground-water drawdown. The tunnel inflow resulted affected by both examined parameters, whose individual importance changes in relation to the anisotropy. The analysis showed the possibility that, in strongly anisotropic conditions and low permeability, the use of the Goodman’s formula could lead to an undestimation of the tunnel inflow. Therefore with the calibration of an empiric equation depending on the anisotropy ratio kx/kz it has been possible to reduce Goodman’s overestimation from 16% to 2% and its underestimation from 67% to 15% for tunnel inflows greater than 100 l/s. In conclusion, a careful modeling of the system anisotropy must be considered of relevant importance because a calculation based on the bare average value of permeability of the excavated rock (as Goodman’s formula) could underestimate the water inflow up to 70%.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/88134