In conceptual and preliminary design stages of tunnel projects a critical decision to be made concerns the method of

excavation and the lining support to be installed. The selection of the structural dimensions and reinforcement of

the lining depends primarily on factors associated with the desired structural robustness and durability of the lining

as well as the geotechnical, hydrological and environmental regime. It is generally anticipated that the inherent

uncertainties of the geotechnical parameters pose greater influence to the structural safety and robustness of the

lining. The contribution discusses the value of using quantitative risk analysis techniques and probabilistic finite

element analysis to define the optimum characteristics of the lining in terms of structural performance and

durability. A typical case of a shallow urban tunnel excavated in soft ground with a Tunnel Boring Machine (TBM) is

considered and the segmental lining, reinforced either with steel fibres or bar reinforcement, is analysed. The results

are presented in the form of structural interaction diagrams and on the basis of a proposed, geometrically defined,

utilisation factor.