## 1 Introduction  
With the continuous expansion of China's subway project scale, the laying route and surrounding environment of the shield tunnel become more and more harsh and it will inevitably pass through the abrupt geological environment [1] with uneven hardness. Different from the conventional single stratum or stratum with similar mechanical properties, when the shield machine crosses this kind of geological environment, it is affected by factors such as different mechanical properties of strong and weak geology and geological interface effects. It is easy to cause engineering problems [2], such as the instability of the excavation face, sliding failure of the geological interface, and stratum deformation. In the process of shield construction in an abrupt stratum, due to the uneven stress of the cutter head [3], it is easy to shift the actual tunneling direction, which leads to a decrease in tunneling efficiency and affects the smooth progress of the project. [PERSON] [4] thinks that when shield construction crosses abrupt geology, the reason for the drastic deformation of the stratum is that the complexity of the geological body causes the excavation parameters to fluctuate greatly during shield construction and then leads to the collapse of the soil around the excavation face. [PERSON] [5] obtained the law of ground settlement during shield tunneling by studying the composite soil layers with different ratios of soft to hard. When shield tunneling is carried out in the upper soft and lower hard strata, as the proportion of hard soil increases, the smaller the settlement value of the surface becomes. Based on the elastic-plastic theory, [PERSON] [6] studied the mechanical properties and deformation of surrounding rock in the process of shield tunnel crossing abrupt geological interface. [PERSON] [7] systematically analyzed the loosening mechanism of excavated surrounding rock in abrupt strata and studied the influence of slip and joint on the loosening zone. [PERSON] [8] adopted the ideal elastic constitutive model. The influence of the stress coefficient on surrounding rock deformation is studied. The above research results play an important role in guiding the stability of excavation face and stratum deformation in abrupt strata, but the research on interface instability mechanism, interface failure characteristics, and interface parameter sensitivity of shield tunneling through abrupt geology with uneven hardness is relatively scarce. The existing studies on the interface stability of different geological bodies generally focus on the compression test and meso-failure mechanism of coal-rock combination in mining engineering, but there is a big difference between the interface failure mechanism and shield tunneling through abrupt geology. [PERSON] [9] used numerical simulation to explore the influence of support of the excavation face of an abrupt geological body on surface deformation. The compound EPB shield machine independently developed by the [PERSON] [10] team, which has the function of pneumatic obstacle clearing, provides a technical guarantee for efficient support and stratum stability in shield construction. [PERSON] [11] established a three-dimensional finite element model of shield tunnel excavation using MIDAS GTS (2018) software and conducted numerical simulation analysis on surface settlement, soil plastic zone, adjacent building influence, and shield tunnel segment stress caused by shield construction. The results showed that surface settlement caused by shield construction may damage adjacent buildings. [PERSON] [12] and others adopted the axisymmetric half-tunnel model to study the instability process and surface deformation law of a tunnel excavation face under different reinforcement conditions. [PERSON] [13], etc., through field detection and numerical simulation, explored the influence law of the shield method on surface deformation and settlement in transition zones in uneven soft and hard strata.