Structural behavior of precast tunnel segments with macro-synthetic fibers during TBM operations: a numerical study

The use of fiber reinforced concrete in tunnel linings, with or without conventional rebars, has increased in the two last decades, especially in segmental linings. In the meanwhile, in the scientific community there was a growing interest on macro-synthetic fibers for use in underground structures. Within this framework, the present study investigates the possibility of using macro-synthetic fiber reinforcement in precast tunnel segments by means of a numerical study. Firstly, an experimental program based on three point bending tests was carried out on polypropylene fiber reinforced concretes (PFRCs) characterized by different fiber contents in order to assess their post-cracking residual strength. Secondly, the corresponding stress vs. crack opening laws, representative of the PFRCs investigated, were calculated through inverse analysis procedure. Then, a segment of a typical tunnel lining having small diameter was adopted as reference to optimize the reinforcement solution (macro-synthetic fibers and conventional rebars, i.e. hybrid solution) and to study its structural behavior by numerical analyses. Particular attention was devoted to the Tunnel Boring Machine (TBM) thrust jack phase, in which the TBM moves forward by pushing the thrust jacks on the bearing pads of the latest assembled ring, introducing high-concentrated forces in the lining.