Analysis of face stability for a shield tunnel in the inclined strata with soft upper and hard lower layers

Tunnel excavation often encounters inclined strata with a soft upper layer and a hard lower layer at the excavation face. Currently, limited research has been conducted on the mechanisms underlying the instability of such strata. This paper proposes a new excavation face failure mechanism considering the inclination angle and soil arching effect, and classify the failure models into two types according to the inclination angle. The results of numerical simulations and existing theories are used to validate our model. Through sensitivity analysis of soil strength parameters, the analytical results indicate that as the inclination angle changes from counterclockwise to clockwise, the limit support pressure first decreases, then increases, and then gradually decreases. The internal friction angle of the inclined strata significantly affects the limit support pressure. The influence of cohesion difference on the limit support pressure gradually weakens. Under clockwise inclination angles, the limit support pressure mainly depends on the strength parameters of the upper layer. This paper provides a more accurate theoretical basis for the stability analysis of shield tunnels in inclined strata with soft upper and hard lower layers, which is of great significance for guiding the design and construction of shield tunnels in complex geological conditions. • A new excavation face failure mechanism is proposed to consider the soft upper and hard lower inclined strata. • Failure model has been divided into two types based on different inclination angles. • Presented method agrees well with the numerical analysis and other existing studies. • The effect of soil strength parameters in inclined strata on limit support pressure was investigated analytically.