Abstract: Natural soil layers often exhibit overconsolidation due to their deposition history, which significantly affects soil mechanical properties. However, traditional analytical methods for determining critical tunnel face pressure are ineffective in considering the overconsolidation effect. This study introduces a nonlinear Hvorslev surface as the strength criterion for overconsolidated soil. The equivalent Mohr-Coulomb strength parameters are derived using the tangent technique and then incorporated into the modified three-dimensional collapse analysis. A new model is established to predict the critical face pressure of tunnel faces in clay layers with varying overconsolidation ratios (OCR). The model’s validity is confirmed by comparing it with the existing model in its simplified form. The critical tunnel face pressure (σ_c ) in overconsolidated soil is influenced by the overconsolidation ratio (OCR), tunnel diameter (D), the ratio of the swelling line slope to the compression line slope (κ*/λ* ), pore water pressure coefficient (r_u), soil lateral pressure coefficient (K₀), tunnel depth-to-diameter ratio (C/D), and the stress ratio at critical state (M). The findings show that with increasing OCR, the collapse zone at the tunnel face shrinks, leading to a decrease in the critical tunnel face pressure (σ_c). When OCR is constant, σ_c  positively correlates with D, κ*/λ*, and r_u, while negatively correlating with K₀, C/D, and M. The impact of κ*/λ*  on σ_c  is significant at high OCR values, and C/D and K₀ have a high sensitivity at low OCR values. Therefore, to enhance the design of tunnel face pressure in overconsolidated soil, engineers should consider factors like stress history, OCR, tunnel dimensions, and depth.

Key words: tunnel face, critical face pressure, nonlinear Hvorslev surface, overconsolidation ratio, limit analysis