Investigation of the cause of shield-driven tunnel instability in soil with a soft upper layer and hard lower layer.

• We determine tunnel stability in soil with a soft upper layer and hard lower layer. • The effects of multiple soil layers and different soft soil parameters on an underground tunnel in China are studied. • We present the upper-bound finite element method with a mesh adaptive strategy. • Two main reasons for tunnel instability and large surface settlement are presented. • Two construction proposals are presented to ensure normal working conditions and safety of engineering. This paper presents a limit analysis of a tunnel in non-homogeneous soil using the upper-bound limit analysis in combination with finite element technology and a mesh adaptive strategy by investigating the critical loads and failure mechanisms of a tunnel cross-section and tunnel face. A case study of an underground tunnel in China is conducted, with a focus on the effects of multiple soil layers and different soft soil parameters. This work demonstrates that the variation in the soil layering plays an important role in the initiation of tunnel collapse, possibly shifting the failure region and even changing the failure mechanism. When the soil changes from a strongly weathered conglomerate to a soft soil, not increasing the low tunnel pressure quickly enough and using the wrong excavation mode are the two main reasons for tunnel instability and large surface settlement. [ABSTRACT FROM AUTHOR]