Soil nailing at the tunnel face in difficult conditions: A case study

In presence of difficult conditions in tunnelling, an adequate stabilization of the excavation boundary and face is required to ensure a safe progress of the construction. The stabilization of the tunnel face can be improved by fibreglass soil nailing, offering properties optimal to the purpose and versatility in use. This paper reports a tunnelling experience where difficult conditions, induced by poor mechanical properties of the rock mass and relevant pore water pressures, were faced adopting innovative soil nailing techniques. Some details about the site geological conditions allow to recognize the causes of the difficult conditions which drove towards specific choices about the features of soil nails, acting on both aspects of mechanical reinforcement and drainage. The soil nail mechanical contribution to the tunnel face stabilization is taken into account at the design stage by an equivalent increase of effective cohesion in the improved ground mass. The tunnel face stability is then assessed by analytical approach. The performance and the specific advantages of the innovative technique are discussed also on the basis of results from on site testing.