Face stability of shallow shield tunnels in inclined sand-clay strata: effects of dip angle and interface elevation

The stability of shield tunnel faces in inclined sand-clay strata remains an underexplored problem in geotechnical engineering. This study investigates failure mechanisms and critical support pressure through centrifuge testing and three-dimensional discrete element simulations. Thirteen centrifuge tests were conducted by systematically varying interface elevation, dip angle, and cover-to-diameter ratio. Support pressure was monitored using a load cell and embedded earth pressure cells, vertical stress distributions ahead of the tunnel face were recorded by miniature pressure sensors, and failure evolution was captured by high-speed photography. DEM simulations employed the rolling resistance linear contact model for sand and the Hertz-Mindlin model with JKR cohesion for clay, both calibrated against triaxial test data. Particle displacement fields, stress concentration ratios, and contact force networks were extracted to characterise instability zone evolution, soil arching development, and stress redistribution ahead of the tunnel face. Results demonstrate that the vertical position of the sand-clay interface governs critical support pressure far more strongly than interface inclination. Raising the interface from the tunnel axis to the crown reduces the normalised limit support pressure by more than 50% across all tested inclination angles. Varying the inclination angle from −60° to +60° at a fixed interface position results in variations not exceeding 20–30%. When the interface is low and steeply negatively inclined, support pressure demand reaches approximately 0.32 times the initial overburden pressure, compared to around 0.20 under horizontal interface conditions. Failure evolves in two stages. Curved shear bands develop first within the clay layer and subsequently act as preferential pathways along which overlying sand is mobilised toward the tunnel face. Positive inclination angles restrict the lateral extent of the instability zone and produce wedge angles steeper than classical predictions, while negative inclination promotes extensive sand mobilisation even when the tunnel face is fully embedded in clay. Earth pressure measurements reveal pronounced horizontal soil arching with arch crowns located approximately 1.5–2.0 tunnel diameters ahead of the face, while vertical arching remains underdeveloped under shallow cover. Contact force analysis shows that the loosened zone is characterised by reduced contact numbers, weak forces, and randomly oriented distributions. Stable regions beyond sustain abundant oblique force chains oriented at 30°–60° that maintain the arching mechanism. These findings indicate that the interface position relative to the tunnel face warrants careful consideration in the design and assessment of face support pressure in sand-clay composite strata, with particular attention to configurations where the interface is low and negatively inclined.