OPTIMIZATION OF SEGMENTAL TUNNEL LININGS: A PARAMETRIC STUDY ON THE EFFECTS OF GSI AND OVERBURDEN DEPTH

Abstract

In mechanized tunneling using Tunnel Boring Machines (TBMs), accommodating severe geostatic loading typically requires increasing the thickness of precast concrete segmental linings, which introduces critical logistical constraints, elevates excavation volumes, and increases overall material consumption. To address these engineering challenges, this thesis presents a comprehensive numerical investigation into an alternative support strategy that couples thin precast concrete segments with internal, high-stiffness structural steel rings installed in an alternating sequence. Three-dimensional finite element simulations were conducted using PLAXIS 3D to capture the multi-axial ground-structure interaction and staged excavation dynamics of mechanized shield tunneling, employing the elastoplastic Hoek-Brown failure criterion for the surrounding rock mass. The system was evaluated in two main directions: (a) variation of the Geological Strength Index (GSI) with a constant depth and (b) variation of the operating overburden depth with uniform ground conditions, using a complete parametric framework. The structural response was systematically analyzed by means of key performance indicators such as total displacements, hoop thrust forces, in-plane shear forces, in-plane bending moments, as well as secondary longitudinal and outof-plane internal forces. The numerical results demonstrate that the alternating i stiffness configuration activates an active load-redistribution mechanism along the tunnel drive; the structural steel rings of high stiffness receive disproportionately higher axial forces and flexural demands, thus shielding the neighboring thin concrete segments from localized stress concentrations and eccentric loading. The longitudinal force profiles show a typical periodic variation with demand maxima at the locations of the steel ring and a significant relief in the zones of the intermediate concrete segment. Internal force magnitude is strongly dependent on rock mass competence and a better ground quality triggered by mobilization of an effective natural ground arch leads to a sharp reduction of structural bending demands. Rather, an increase in overburden depth will raise the cross-sectional thrust and sidewall bending actions. However, the structural utilization levels of all components were observed to remain within the allowable limit state thresholds under the investigated conditions. To summarize, the study demonstrates the potential applicability of internal steel rings to significantly reduce the lining thickness compared to conventional solutions, and provides a structurally stable, logistically feasible and material-efficient alternative for deep mechanized tunneling in challenging geological conditions.