Analysis of the behavior of the two-component grout around a tunnel segmental lining on the basis of experimental results and analytical approaches

The mechanized excavation of tunnels generally requires the injection of a grout material between the segmental lining and the ground. This material has an important role for the support system and governs the segmental lining loading, also because of the significant evolution of its mechanical characteristics over time. In this work, an accurate experimentation on the two-component material was conducted. Normally, hand mixing is performed in laboratory to test the mechanical performance of the two-component mix-design. In this paper an innovative instrumentation able to mix appropriately the components was used in order to obtain samples truly representative of the real site conditions. Laboratory compression tests performed on these samples allowed to accurately quantify the evolution of the strength and stiffness of the filling material during the setting time. In order to properly take into account the evolution over the time of the mechanical behaviour of the filling material and of the support system more generally, a new procedure was developed and illustrated in the work. The analysis procedure is based on the convergence-confinement method and is able to describe in some detail the reaction line of the support system during its loading. Due to the presence of the filling material, which improves its mechanical characteristics over time, the reaction line assumes a curved shape. The intersection with the tunnel convergence-confinement curve allows to determine the radial loads acting on the support system and to design the support system to ensure the stability of the tunnel. The innovative procedure was then applied to a real case of a tunnel excavated in Northern Italy, confirming the validity of the main project data adopted for segmental lining and filling material. A sensitivity analysis on the same case allowed to detect how some aspects of the support system loading have an influence, not only on the final radial load acting on the segmental lining, but also on the final stress state induced inside the filling material. © 2021 Elsevier Ltd