The construction of deep tunnels is continuously increasing due to both economic and environmental reasons. High cover and poor geotechnical ground make the construction of tunnels difficult (squeezing conditions). Due to the high loads transmitted by the ground, the preliminary lining can be rarely designed following the so-called resistance principle. The state of the art suggests approaching the lining design by allowing the development of the necessary convergences to adequately reduce the rock load (yielding principle). In this paper, a new steel element (HiDSte), yet to be employed in real tunnel works, is described. The behaviour of the lining made of steel ribs along the HiDSte is illustrated through 2D numerical analyses. The numerical results allowed the evaluation of the peculiar way soil and lining interact and, more in general, the ability of the yielding lining to reduce the in-situ stress level undergone by the lining.
When deep tunnels cross poor ground, squeezing conditions occur; thus leading to huge risks, longer times, and higher costs (Hoek 2011). Regarding traditional tunnelling, there may happen extreme convergences and the excess of the preliminary lining bearing capacity. To prevent these, is essential to understand the soil-structure interaction (SSI) in squeezing conditions and develop technology able to reduce the risks. In extreme conditions the state of the art shows that generally it is not possible to design a tunnel with the "resistance principle", but it is necessary to adopt a preliminary lining able to deform without losing the bearing capacity, following the "yielding principle" (Kovári 1998). A new elasto-plastic element (EPE), known as "HiDSte" (High Deformable Steel Element) has been patented by SolExperts and created by Prof. Kovari with the goal of realizing a yielding element for the steel ribs. The HiDSte can be a valid alternative to the sliding joints in the TH steel ribs. Indeed, it is more suitable for a perfect plastic behaviour, that are more regular and with a clear and reproducible yielding threshold. These elements have never been employed before in a real tunnel. In scientific literature, there are only a few works treating the design of preliminary deformable linings in deep tunnels under squeezing conditions. Radončić et al. (2009) and Moritz (2011) compare the different behaviours of the main yielding elements with the aim of studying the expected convergence related to the distance to the face. In particular, they extend the CCM for the design of yielding linings. Wu et al. (2022), similarly to Radončić and Moritz, did a detailed study about the shotcrete with yielding elements embedded, proposing a new form of the equation for the Support Reaction Curve of that kind of linings. Furthermore, Yang et al. (2022) extensively studied the behavior of shotcrete damage and proposed a numerical model to simulate the yielding linings based on the double-node beam element between two interfaces.
