Ultimate Support Force of Excavation Face in Curved Shield Tunnels in Composite Strata

For curved shield tunnels in composite strata, the excavation face may undergo sliding failure due to the interface effect of soil layers, the over-excavation of shield cutterhead, and the asymmetrical thrust of jacks. Inspired by the silo theory, this paper establishes the calculation model and formula for the ultimate support pressure in the excavation face of irregular curved trapezoidal prism in composite strata, and analyses the variation law of the excavation face at different soil layer thickness ratios and curvature radii through the case study. The results show that the horizontal displacement of the excavation face in curved shield tunnel in composite strata changes suddenly at the interface between hard and soft soil layers; under the eccentric support force, the failure of the excavation face is asymmetrically distributed, and the ultimate support pressure increases first and then decreases with the decline of the curvature radius; the support pressure is biased towards the outside of the curve and greater than that of straight shield tunnel in single stratum; the ultimate support pressure increased with the soil cohesion and internal friction angle of the excavation face, and increased first and then declined with the growth of the rupture angle difference between the left and right sides of the soil mass before the excavation face; when the rupture angle difference exceeds 1.28, the ultimate support pressure gradually stabilizes