Comparison of displacement field predicted by 2D and 3D finite element modelling of shallow NATM tunnels in clays

The 2D load-reduction method for simulating NATM tunnels using plane strain finite elements is evaluated in the paper by comparison with fully 3D simulations. Three real shallow tunnels in urban environment in different stiff clays were simulated. The soil behaviour was described by an advanced non-linear soil constitutive model based on the hypoplasticity theory. Time-dependent behaviour of shotcrete lining was considered in 3D simulations, whereas constant final stiffness was used in the plane strain analyses. The 2D analyses were thus controlled by a single parameter that accounts for 3D effects. It is shown that for an optimum value of this parameter, the displacement field predicted by the 2D method agrees well with the 3D simulations. In some cases only, a discrepancy was observed in the close vicinity of the tunnel. The controlling parameter was, however, found to be dependent on the problem simulated (for the same material) and also on the material properties (for the same tunnelling problem). Considering the material properties, the shear modulus at very small strain was found to be more influential than the shear modulus at large strain. The initial K0 stress state did not influence the controlling parameter substantially.