The role of inverse analysis in tunnel design

Summary

In modern design of complex tunnelling projects it is best practice to utilise 3D finite element analysis (3D FEA), its main advantage being the ability to capture the impact of the actual construction processes and the very influential three dimensional effects. The latter may include the effect of tunnels constructed in close proximity, the interaction with existing structures and the non-homogeneity of the ground conditions. On the other hand, 2D finite element analyses (2D FEA) can be employed for the execution of simplified, quick models which, as long as the engineer is fully aware of the assumptions and the limitations involved, can prove to be a very useful means of calibrating, supplementing and in some cases substituting the demanding 3D models.

An important question at the beginning of a design project is therefore which type of analysis should be applied. To provide a case example, Crossrail’s Farringdon underground station was considered and presented in the following sections. The Crossrail project comprises a combination of long TBM drives forming the running tunnels with sprayed concrete lined (SCL) tunnels forming the station layouts. In principle for such a project, 2D FEA can be used for the design of the running tunnels but inside the footprint of the stations, 3D FEA are necessary for the aforementioned reasons.

This paper presents a combination of 3D and 2D FEA that has been used to back analyse the actual performance of SCL tunnels. The findings of this exercise as well as the importance of the inverse analysis in tunnel design are highlighted in the context of practical FE modelling.