Backcalculation of internal forces in the segmental lining of a tunnel: the experience of Line 1 in Naples

The prediction of the internal forces in the segmental lining of a bored tunnel is a rather complex task as they are significantly influenced by even apparently minor details of the installation process. Typically experimental researches on the performance of tunnel linings are based on monitoring the strains in the structural elements which are subsequently converted into forces. This last passage is not at all straightforward and the paper will deal with some of the difficulties encountered in carrying out such step for the data provided by a case history in Napoli. The twin tunnels of Line 1 Underground Extension in Napoli (Italy) were bored by EPB shields and lined with pre-cast reinforced concrete segments. Vibrating wire gauges were embedded in the segments of four lining rings and the strain values were recorded since the segments were concreted in the manufacturer’s factory, during their installation in the tunnel and for several months afterwards. The back-calculated values of hoop forces and bending moments in the instrumented rings of the tunnel lining are reported and discussed in the paper.