ABSTRACT:

The construction of deep railway tunnels requires the prediction of natural temperatures at depth. At first, a good knowledge of superficial temperature is needed. Other natural conditions must be established then: geological structure of the rock mass, rock conductivities and regional or local geothermal deep flow. We use then a finite element numerical model based on pure conduction to calculate temperatures at depth. This method allows rock heterogeneity and anisotropy to be taken into account. The model is applied to the Maurienne-Ambin tunnel project, a 55 km long tunnel between St-Jean-de-Maurienne (France) and Susa (Italy), which will be the longest tunnel for the planned TGV (high speed train) Lyon-Torino link.

RÉSUMÉ:

La construction des tunnels profonds necessite une bonne prevision des temperatures dans Ie massif rocheux. Le premier element est la temperature de surface, facile à connaître d'après les donnees meteorologiques. II faut y ajouter d'autres paramètres: la structure geologique du massif, la conductivite thermique des roches (souvent anisotrope) et Ie flux geothermique local ou regional. On peut ensuite modeliser Ie massif en conduction pure et determiner les temperatures en tout point. Le modèle est applique à la determination des temperatures pour Ie tunnel ferroviaire profond Maurienne-Ambin (55 km), projete entre St- Jean-de-Maurienne (France) et Susa (ltalie) sur la ligne à grande vitesse Lyon-Turin.

ZUSAMMENFASSUNG:

Der Bau tiefer Bahntunnel fordert eine Vorhersage der Umgebungs-temperaturen in entsprechenden Tiefen. Hierzu muss zunachst die Oberflachen-temperatur bekannt sein, bevor weitere natuerliche Parameter in die Untersuchung miteingeschlossen werden: die geologische Struktur des Bergmassivs, die Warmeleitfahigkeit des Gesteins, sowie der regionale und lokale geothermische Warmefluss, Mit Hilfe eines numerischen Modells finiter Elemente, basierend auf Warmeleitung ohne Massentransport, kann die Temperatur an jedem beliebigen Punkt in beliebiger Tiefe bestimmt werden. Gesteinsheterogenitaten, sowie Anisotropie werden bei unserer Modellierung mitberuecksichtigt. Praktische Anwendung des Modells ist das Maurienne- Ambin Tunnelprojekt, ein 55 km langes Bahntunnel zwischen St-Jean-de-Maurienne (Frankreich) und Susa (ltalien). Es wird das langste Bahntunnel auf der geplanten Hochgeschwindigkeitsbahntrasse (TGV) zwischen Lyon und Turin sein.

1 INTRODUCTION

The prediction of rock temperatures is one of the geotechnical problems which must be solved before the methods of excavation, ventilation and cooling can be chosen for a deep tunnel in alpine regions. High temperatures have been measured for many alpine tunnels under large overburdens, such as the Simplon, St-Gotthard and Mont-Blanc tunnels (see table 1). The Simplon tunnel has higher than average temperatures, compared to other tunnels. In the Mont-Blanc tunnel, the maximum temperature is relatively low and does not correspond to the maximum cover; water circulation may explain the anomalously low temperatures, even though no detailed hydrogeological explanation has been provided so far. Early on, only empirical methods were used to predict temperature. Accurate in-situ measurements were made during the construction of the first long tunnels through the Alps, such as the St-Gotthard (1872- 1881) and the Simplon (1898–1906) tunnels. Theoritical studies were also published during this period (Koenigs berger and Thoma, 1906) In 1948, Professor C. Andreae (E.T.H. Zurich, Switzerland) developed an analytical method which led to a good approximation of temperatures in rock massifs. This method was applied (Andreae, 1958), for example, to the prediction of temperatures in deep hydraulic tunnels planned by Electricite de France. Additional in-situ data were obtained in the seventies, with the construction of the Arc-Isère hydraulic tunnel and new alpine road tunnels (Frejus, St- Gotthard, …). The currently planned railway tunnels through the Alps now offer a valuable opportunity for the application of powerful numerical methods for predicting temperatures. Our thermal model uses a finite element numerical program based on pure conduction which allows us to take heterogeneity and anisotropy into account. The input parameters are the topographic profile, ground surface temperatures, geological structure, thermal conductivity of the rock and geothermal heat flow.

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