The contribution discusses the quantitative evaluation of design robustness through the use of a new design Index
for the Capacity Utilization of Linings in Tunnels (CULT-I). This index is geometrically defined and uses a simple
concept to measure the proximity of a design point to a lining capacity envelope, in this case the Bending moment-
Axial force capacity limit curve / interaction diagram. The use of this index can allow for a rationalized optimization
and comparison of different designs, particularly on the basis of probabilistic engineering methods. Typical cases of
shallow urban tunnels excavated in soft ground with different methods are presented, demonstrating the practical
application of this novel evaluation index.