Mechanical Response and Stability Assessment of Tunnels in Fair Rock: A 3D Finite Element and Field Study

Abstract

Tunnel excavation in fair rock masses presents significant challenges due to complex stress redistribution and deformation mechanisms. This study develops and validates a full-scale three-dimensional finite element (3D FEM) model of a road tunnel excavated in fair blocky limestone in eastern Egypt using ABAQUS/CAE 2019. Rock mass parameters were derived using Geological Strength Index (GSI)-based correlations and implemented within a Mohr–Coulomb constitutive framework, while the shotcrete lining was simulated using the Concrete Damaged Plasticity model. Full face excavation in steps following the New Austrian Tunneling Method (NATM) was modeled and validated against 75 days of field monitoring data. The numerical model accurately predicted maximum crown settlement (3 mm) and reasonably estimated surrounding rock pressure (400 kPa simulated versus 348 kPa measured), confirming the reliability of the adopted calibration strategy. A systematic parametric study was subsequently conducted to evaluate the combined influence of tunnel span and overburden depth on deformation behavior and stress redistribution. Results demonstrate the tunnel span exerts a stronger influence on displacement magnitude, while increasing overburden enhances stress arching effects. The validated modeling framework provides a robust basis for stability assessment and support optimization in fair rock tunnels.