An experimental study on the use of polypropylene fibers in precast segments for hydraulic and metro tunnel lining

There is a growing interest in the scientific community on the structural applicability of polypropylene fibers (in combination or not with conventional reinforcement) as spread reinforcement of precast tunnel segmental linings. Polypropylene fibers could be used in precast tunnel segments as shear reinforcement and for withstanding splitting and spalling tensile stresses which occur under and between TBM rams, respectively. In addition, fibers could also simplify and boost the tunnel element production process. In this context, the present study investigates the possibility of using polypropylene macro fiber reinforcement in precast tunnel segments for both hydraulic and metro tunnel linings. Four full-scale segments (two counter-key segments of a hydraulic tunnel and two trapezoidal shaped segments of a metro tunnel) were experimentally evaluated under point load tests. The latter simulate the TBM actions on segments during the excavation process (TBM thrust jack phase), which is generally the most critical temporary loading condition for segments. Two reinforcement solutions were studied: a typical conventional reinforcement (reference samples) and a combination of polypropylene fibers and conventional reinforcement (hybrid solution). Polypropylene fibers were observed to be very effective in combination with the conventional reinforcement, underlining the fibers ability in controlling both splitting and spalling phenomena under TBM thrust jacks.