Influence of the Canopy Filling on Shallow Tunnels

The construction of shallow tunnels using a pre-support system based on pipe canopies may require important decisions during the planning stage. Once the decision of using pipe canopies has been made, the engineer has to choose between steel cylinders or steel tubes for the canopy. If the later ones are chosen, it is necessary to decide whether to keep the soil inside, remove the soil or remove the soil and fill the tube with concrete. In order to clarify the benefits and harms that guide this interactive solution, a study about the influence of the filling material in the bending behavior of the canopy was conducted. The results were obtained for steel behaving as an elastic-perfectly plastic material and concrete being represented by the parabola-rectangle diagram, however the methodology may be applied for different stress-strain curves. A case study, based on the site conditions proposed in Domingues et al. (Society for Mining, Metallurgy and Exploration, The World Tunnel Congress 2016, San Francisco, California, United States, 2016) exemplify the usefulness of the methodology proposed. As a result, it was shown that it may be favorable to keep the soil inside the beam instead of replacing it by concrete or use a massive structural element. In other words, as any other structural system, there is an optimal solution that leads to an economic and safe construction. Hence, this paper provides important tools for decision-making as it expands the possibilities of using the pre-supported pipe canopy technology in tunnel construction.