Analysis and design of an innovative solution for tunnels using elastic-plastic stress fields

For the conception of cut-and-cover tunnel, two solutions are usually considered to define the cross-section : an arch/vault or a frame. The vault enables an optimal static behavior in case of a large soil cover and symmetric loads. However, this solution faces some constructive issues. The construction process of a frame is simpler and more cost efficient, but its static behavior turns out to be less suitable for large soil covers. A research performed at EPFL allowed defining an innovative solution for the transverse section. This solution is a mix of a frame and a vault. It combines the static and constructive advantages of both solutions. This shape was chosen for several projects that are currently under way in Switzerland. The performance of this transverse section depends on the strength and ductility of its nodes. This paper presents an overview of the first results following both theoretical and experimental investigations that are currently under way about the behavior of nodes subjected to various load configurations. The theoretical part is used for the conceptual design and the optimization of bars details aiming at satisfying both ductility and strength requirements. The specimens were modeled and analyzed with the elastic-plastic stress fields method. This method enables to predict the failure modes and strengths of the specimens. The experimental part consists of a series of tests on four full scale specimens. These tests pointed out that the loading type and the bar disposition significantly influence the nodes areas. 1.