Numerical modeling of yielding linings in squeezing conditions

The growing demand for rapid interconnections within our society has resulted in the development of deep tunnels. This situation presents significant challenges in their design and requires the advancement of cutting-edge technology to accomplish these goals. Currently, in Italy alone, the three major tunnel construction projects (Terzo Valico dei Giovi, Turin-Lyon European Tunnel, and Brennero-Base Tunnel) exceed costs of 15 billion €. When deep tunnels are excavated in weak soil, squeezing conditions can arise, necessitating a design based on the yielding principle. This involves incorporating elasto-plastic elements into the initial tunnel lining, enabling deformations that reduce ground pressure while maintaining the support’s bearing capacity. This innovative approach is highly reliant on the particular technology used, prompting an extensive investigation of a newly developed technology, the high deformability steel element (hiDSte). If the lining is engineered according to the yielding principle, accurately assessing the load acting on it using simplified analytical methods such as the Convergence Confinement method becomes infeasible. Consequently, this study develops 2D numerical analyses modeling a vertical cross-section of a traditional circular tunnel, specifically aiming to examine the ground-lining interaction of a yielding lining. These analyses have become feasible due to prior research on the hiDSte element, which laid the foundation for a simplified equivalent modeling of the element at the tunnel-scale level. Particular focus will be directed towards the influence of the key parameters that define this interaction: the ground-lining interface, the elasto plastic element’s yielding compressive strength, and the anisotropy of in-situ stress. Furthermore, To create an intuitive and manageable framework for comprehending and evaluating the interaction between ground and elasto-plastic preliminary lining, analytical tools have been devised. These tools provide a clear explanation of the mechanics involved in the interaction problem and furnish initial parameter values for numerical analysis. The objective of the thesis is to fill the existing knowledge gap in the current state of art related to the interaction between the ground and elasto-plastic lining within the cross-section of vertical tunnels, by examining: (1) the relative displacements at the ground-lining interface and (2) the anisotropic stress condition of the ground. It provides a lucid explanation of the physical phenomena involved. Ultimately, based on the findings of this study, the thesis offers significant insights for both academics and practitioners into understanding and designing tunnels subjected to squeezing conditions.