3D Stability Analysis of Soil-Rock Tunnel Faces: Pore Water Pressure and Partial Failure Perspectives

This paper addresses stability challenges at excavation faces in shield tunneling through water-rich soil-rock formations, particularly focusing on partial failure caused by significant strength differences between soil and rock layers. A three-dimensional discrete rotational failure mechanism model is developed under the limit analysis upper-bound theorem, considering the influence of pore water pressure. This model leads to a novel method for calculating ultimate support pressure in complex strata, with its reliability confirmed through comparison with existing solutions. Key findings reveal a roughly linear positive correlation between soil layer proportion, water level, soil saturation weight, and ultimate support pressure. Conversely, cohesion, tunnel depth and friction angle demonstrate an inverse correlation. Notably, the relationship between soil layer proportion and ultimate support pressure exhibits significant nonlinearity. Cohesion and water level exert the most significant effects on ultimate support pressure, while the impact of soil layer proportion is notably complex. Additionally, a normalized design method is established using tunnel diameter and soil saturation weight, supported by design charts for varying normalized cohesion, normalized water level, and friction angles. A detailed example of a classic case is provided to illustrate the use of these design charts, aiding practical engineering applications.