Geological conditions associated with alpine environments in combination with high overburden in many tunnelling projects lead to squeezing behaviour. The main problems associated with such conditions include large deformations, overstressing of supports, and lack of face stability. In the past during Alpine tunnelling the problem of large displacements incompatible with rockbolt - shotcrete support systems have been solved by leaving gaps in the lining to accommodate excessive deformations. In the late nineties, yielding elements have been developed, which are integrated into the lining. The paper deals with the basic concept, the design and the application of that system, which has been successfully used in several Alpine tunnels.
Die geologischen Verhaltnisse in den Alpen fuehren bei großer Überlagerung haufig zu starken Deformationen wahrend des Baues von Tunneln. Die großen Verformungen fuehren in vielen Fallen zu einer Überbeanspruchung des Ausbaues und Problemen mit der Ortsbruststabilitat. Frueher ist man dem Problem der Unvertraglichkeit des Ausbaues mit den hohen Dehnungen durch Freilassen von Deformationschlitzen im Spritzbeton begegnet. Vor etwa 10 Jahren wurden duktile Stahlelemente entwickelt, welche in die Spritzbetonschale integriert werden. Der Beitrag beschaftigt sich mit dem Konzept, der Planung und der Ausfuehrung.
Les conditions geologiques dans les Alpes conduisent frequemment lors d'un grand recouvrement à des deformations fortes pendant la construction des tunnels. Les grandes deformations conduisent dans beaucoup de cas à une surtension du revêtement et à des problèmes avec la stabilite de front de taille. On a fait face plus tôt au problème de l'incompatibilite du revêtement avec les extensions elevees par liberer des rainures dans le beton projete. Il y a environ 10 ans, des elements siderurgiques ductiles qui sont integres dans le beton projete ont ete developpes. La contribution s'occupe de concept, de planification et de mise en oeuvre.
Often due to the geological conditions and high overburden associated with alpine tunnels, large deformations (squeezing conditions) are expected and/or encountered. There are two basic design methodologies for tunnelling through squeezing ground. One is based on a stiff support concept the other on a ductile or yielding support system.
In order to evaluate which support concept is appropriate for a given tunnel project, a systematic procedure has been developed (Schubert et.al 2000, OEGG 2001) that is based on assessing project and rock mass specific key parameters and the associated rock mass behaviour types considering the local influencing factors (stress state, groundwater, etc.) and boundary conditions (worker safety, tunnel deformations, costs). Once the rock mass behaviour is determined, different support methods and intensities are evaluated to determine the potential system behaviours. After suitable support concepts have been identified, cost and time analyses are performed to identify which excavation and support concept is optimal for the project.
There are a limited number of discussions in the literature on which factors or parameters govern the design of a yielding support system. To identify key factors for the design of ductile support systems experience from tunnel projects in which yielding support methods were applied was used.
