Eurocode 7 and design of deep excavations and tunnels using finite element method

Abstract

Advanced numerical methods, such as the finite element method (FEM), are widely used in geotechnical engineering practice to predict displacements induced by deep excavations and tunnel construction. While the finite element method is primarily used to obtain displacements and hence to verify serviceability limit states (SLS), it is also increasingly used for geotechnical ultimate limit state (ULS) design. The introduction of the Eurocode 7 and the associated concept of partial safety factors has replaced the global safety factor concept which had previously been used in geotechnical design practice. Eurocode 7 introduces partial safety factors on actions (loads), materials and resistances. Three design approaches (DA1, DA2 and DA3) that differ in the combination of partial factors are available in the current version of Eurocode 7, however, the guidance as to how the partial factors should be applied in numerical analysis is not provided. The second generation of Eurocode 7 is currently being prepared, and the proposed draft of Eurocode 7 Part 1 (EN 1997-1:202x) contains a new set of rules covering geotechnical design and the verification of limit states using advanced numerical methods. In order to ensure sufficient reliability in relation to the ultimate limit states occurring in the ground and structural elements, two combinations of partial safety factors have to be applied, i.e. ULS verification with numerical models has to be performed by reducing the ground strength parameters and by increasing the structural forces in accordance with applied design approach. Although Eurocode 7 has no specific parts devoted to the design of tunnels, it is increasingly used in practice, at least for shallow tunnels in soil, since there are no European standards for tunnel design.This paper gives an overview of the procedures for applying finite element method and design approaches defined in Eurocode 7 to design of deep excavations and tunnels.