Analytical and experimental evaluation of the use of fibers as partial reinforcement in shotcrete for tunnels in Chile

This work evaluates experimentally the use of steel or polypropylene fibers in shotcrete as a partial replacement for the traditional reinforcement of electrowelded mesh used at the tunnel support for the subway in Santiago (Chile). For the experimental part, 8 tests were performed on section-scaled (1:2) slender (half-span to depth ratio a/d ∼ 5) specimens under transversal (flexure and shear) and axial loads (0.02f′cAg and 0.07f′cAg) and with different reinforcements layout; and another 4 tests of similar specimens, but with low half-span to depth ratio (a/d ∼ 1.5). The section includes welded mesh (one face), a central reticulated frame and a plane shotcrete with a welded mesh (opposite face) or reinforced shotcrete (steel or polypropylene). The results showed a very important contribution of the reticulated frame in flexion (main internal reinforcement) and a modest contribution of the welded mesh and fibers. A two-dimensional finite element modeling of the tunnel-ground system using OpenSees is also performed using calibrated models for the tunnel section based on the experimental part. The soil is modeled with 9-node quadrilateral elements, the lining is modeled with beam-column elements with fiber sections, and the interaction between the lining and the soil is modeled by the Winkler approach without tension in the direction normal to the contact surface and with perfect adherence in the tangential direction. The model is subjected to the static loads from the excavation, modeled considering the constructive sequence of the tunnel using the α method, and then to a seismic analysis by means of the shear wave method (distortion). The results show that the safety factors implicit in the traditional design are high, implying that the structure remains elastic.