The size and shape selection of a tunnel cross-section is based primarily on spaceproofing considerations. Decisive factors are among others the function of the tunnel, the kinematic envelope with the associated margins including the various installations, surface and underground property restrictions and user comfort and safety criteria. Geotechnical conditions of the surrounding ground and particularly the direction and magnitude of the stress field as well as the available excavation and construction methods and equipment may affect the way that the design is carried out among the industry. The above generic and indicative factors are part of a shape optimisation procedure which entails unavoidable compromises due to inherent counteractions. Nevertheless, the common engineering practice lacks the ability to precisely quantify the impacts of such dictated geometry selections on the robustness of the tunnel lining capacity given the uncertainty and the spatial variability of the geotechnical parameters along the tunnel alignment.

The present paper investigates and quantifies the impact of such predefined geotechnical parameters – known to be associated with the stress distribution of a given shape- on the robustness of the tunnel lining capacity using probabilistic 2D finite element analysis when different shapes of the widespread three arc tunnel cross-section are adopted. The analyses are conducted using the software Phase2 combined with the Rosenblueth point-estimate method. The investigated cross-sections are equivalent in terms of area but with varying dimensions and curvatures. The models simulate a relatively shallow tunnel in a uniform soft soil stratum with the stochastic parameters being the coefficient of lateral earth pressure K0, the relative stiffness between ground and lining affected by the soil’s undrained Young modulus Eu, the undrained shear strength Su and the ground relaxation factor β. The results are assessed in terms of the stresses’ and forces’ distribution around the tunnel cross-section, on the basis of a sensitivity/parametric analysis. In conclusion, the present paper offers a better insight into the design and shape optimisation procedure with potential benefits being increased structural reliability of the structure, as well as cost savings resulting from improved stress flow in the lining and less support requirements due to more informed selections in light of the specific geotechnical circumstances.