Structural Behavior of Connections between Tunnel Roof and Diaphragm Wall: FE-analysis of the connection in Korsvägen, part of the West Link Project in Gothenburg

Abstract

Due to densification of cities and strict demands regarding material use, permanent Diaphragm walls (D-walls) have become a more common option to use as support ing structures of underground tunnels. D-walls require less construction space than more traditional foundation methods and is currently being used in the West Link Project in Gothenburg. The tunnel is only connected to the D-walls through the tunnel roof and thus, the connections become a critical part of the structure. The aim of this project was to investigate a certain connection used in the West Link Project, the load effects on the connection and its behavior regarding cracks. To study the behavior of the connection, three types of analyses were carried out: two global analyses (one elastoplastic and one elastic) and one local non-linear anal ysis. An idealized case of the structure in the sub-project Korsvägen in the West Link Project was chosen as a case study. First, a finite element model using plane strain-elements of both the surrounding soil and the structure was analyzed. This model was then compared to a beam model where the effect of the soil was modeled as applied loads. Section forces and moments from the beam model was inserted into a detailed local model, of the critical connection. The local model was analyzed to study the cracks and inelastic behavior of the concrete. The results showed that the cracking in the connection was extensive, and a large crack developed in the upper corner between the D-wall and the tunnel roof. It was concluded that: • A tunnel located at a large depth, built with a top-down method results in a critical situation with respect to the connection. The situation in Korsvägen is thus more critical than the other sub-projects in the West Link project. • The earth pressure developed in the plane strain model, including the non linear behavior of the soil, showed less mobilization of passive earth pressure than the analyses using a simplified beam model. One reason may be because the soil-structure interaction was included in the plane strain model. • The assumption of elastic structure, and not including the soil-structure inter action in the simplified beam-model analyses result in too high mobilization of passive earth pressure, and thus the crack widths in the connection were overestimated. • The FE-analyses indicated that the design of the connection at Korsvägen is not sufficient regarding allowable crack widths. Additional measures such as a haunch, additional struts or lighter backfill, may be used to reduce the crack widths.