Redefining metro tunnel linings: The first full‐scale application of a hybrid FRC–GFRP solution in Rome Metro Line C

Over the past two decades, fiber-reinforced concrete (FRC) has been increasingly adopted in mechanized tunnel construction due to its proven advantages. In urban areas, sometimes FRC alone may not meet performance requirements. A hybrid solution combining glass fiber-reinforced polymer bars with FRC has been proposed to enhance tunnel lining performance and durability. This approach was implemented for the first time in the T3-section of Rome Metro Line C. Full-scale experimental tests, including TBM thrust loading and three-point bending, were conducted to assess segment behavior during TBM excavation and under ultimate flexural demand. Results were compared with both pure FRC and conventional reinforced segments; the latter adopted in the old part of the same Line C. A design analytical study is developed according to the current design Codes to obtain bending moment-axial load interaction diagrams at the transitional phases and final stages. Furthermore, axial load-shear strength interaction diagrams are developed to account for the shear effects on the flexural capacity reduction when the FRC is considered. Both experimental and analytical results confirm the effectiveness of this solution, highlighting its potential as a model for future projects. Finally, the sustainability and economic aspects of the proposed solution are considered, compared to the traditional one.