The tunnel face stability in composite strata, commonly referred to as the soft upper and hard lower condition, is a critical challenge in tunnel construction. The soft–hard ratio (SA) strongly influences the limit support pressure as well as the failure mechanism experienced by a tunnel face. This study focused on the Xingye Tunnel project in the Xiangzhou District of Zhuhai City. By conducting numerical simulations, the impact of different SAs on the limit support pressure was investigated. Furthermore, a limit equilibrium model was established on the basis of the analysis of the results of numerical simulation. The findings were then compared and analyzed alongside those of relevant theoretical models. In the event of tunnel face instability of composite strata, the deformation tends to be concentrated mainly in the soft soil layer, with less noticeable deformation in the hard rock layer. The investigation of different SAs revealed a linear decrease in the limit support pressure ratio of the tunnel face in composite strata as SA decreases. The self-stability of the tunnel face was observed when SA ≤ 0.125. Moreover, the limit support pressure ratio predicted by the truncated log-spiral model (TLSM) exhibited a higher degree of agreement with the results of numerical simulation than those of other relevant models. The superiority of TLSM was mainly demonstrated in the range of SA = 0.25 to 1.0.

In coastal cities, some paralleled twin tunnels must be constructed in soft soil layers, and stratum reinforcement is necessary to enhance the stability of the tunnel. However, the influence of construction of the following tunnel on the preceding tunnel in reinforced strata is not clear. This study focuses on four kinds of reinforcement strategies: ring reinforcement of the right tunnel (RR), ring reinforcement of the left tunnel (LR), ring reinforcement of twin tunnels (TR), and cement wall reinforcement between left and right tunnels (CR). The interaction mechanism of the twin tunnels during construction, in terms of surface settlement, tunnel deformation, convergence deformation of the tunnel lining, and internal force of the tunnel lining, is revealed by a series of three-dimensional numerical analyses. It was found that the LR offers an appropriate strategy in terms of both engineering safety and cost savings, and it can significantly reduce the disturbance of the following tunnel excavation on the preceding tunnel.