In this paper some failure phenomena observed in the crossing of brittle rock formations at the Lötschberg base tunnel are presented. A brittle failure criteria, recently developed by Canadian researchers, is applied through finite element calculations together with a semi-empirical failure criterion, to explain these behaviours. Good agreement is found between predicted and observed failures at the tunnel walls. This study also shows some of the limitations of this approach.

Dans cet article sont presentes certains phenomènes de rupture observes durant la traversee de zones de roches fragiles lors de l'excavation du tunnel du Lötschberg. Un critère de rupture developpe recemment par la recherche canadienne est utilise à travers des calculs par elements finis. En parallèle, les resultats sont compares avec ceux d'un critère semi-empirique. Au niveau des parements, l'ampleur des ruptures observees est assez proche des resultats de calcul. Cette etude a egalement permis de mettre en avant certaines limites de l'approche choisie.

In diesem Artikel werden verschiedene Brucharten dargestellt, die bei der Durchfahrung von sprödem Gestein im Rahmen des Ausbaus des Lötschberg-Basistunnels beobachtet wurden. Zur Erklarung des vorliegenden Verhaltens wurden die erst kuerzlich in Kanada entwickelten Theorien zur Beschreibung von Sprödbruechen herangezogen und bei der Berechnung mittels der Finite- Elemente-Methode und eines semi-empirischen Kriteriums angewendet. Die so vorhergesagten Versagenszonen an den Tunnelwanden stimmen mit den Beobachtungen gut ueberein. Die Studie hat auch dazu beigetragen, die bei der Anwendung des Lösungsansatzes angetroffenen Schwierigkeiten aufzuzeigen.

Introduction

The Lötschberg Base Tunnel is undoubtedly one of the largest projects of the end of the 20th century for European civil engineers and researchers. It has been under construction since July 1999 in the Swiss Alps. The great diversity of the rock-stratigraphic formations, the various stress conditions encountered - from low to very high coverage - and the various construction techniques used make it a perfect case for back-analyses and research in rock mechanics.

In this context, the team of the Rock Mechanics Laboratory (LMR), EPFL, in cooperation with MIRARCO (Canada), BLS AlpTransit AG and IGWS, have carried out several studies to analyse and explain the diverse behaviours observed in the tunnel. This paper is centred on the brittle failure phenomena that occurred during the excavation of the Steg lateral adit in 2002, mostly in massive granite. It yields the first results of a series of back-analyses based on the recent theories of brittle failure from the AECL Underground Research Laboratory, Canada. The advantages and some limits of these new methods are highlighted through the comparison between theoretical results and field data.

The Lötschberg base tunnel project
General overview

The Lötschberg base tunnel is located in Switzerland and links Frutigen, in the Lanker valley, to Raron, in the Rhône Valley. Combined with the existing Simplon tunnel it will constitute the first transalpine rail axis with end-to-end enlarged tunnel cross-sections. It can be expected that this new line will play a predominant role in the global European transportation structure as it will provide a direct link between Germany and Italy through Switzerland.

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