## 5 Conclusions  
(1) By focusing on the stability of the tunnel face in composite formation shield tunnels beneath high-speed railway bases, a comprehensive test plan was devised. This plan takes into account the impacts of high-speed railway vibration loads and shield thrust on the stability of the tunnel soil, face, formation stress, deformation, and settlement. It is capable of intuitively replicating the formation-deformation characteristics influenced by shield and high-speed rail factors. The test plan presented herein enables research testing of the stability of tunnel faces in composite formation shield tunnels under high-speed railway base sections. The plan is proven to be feasible, and the test results are highly reliable. (2) The ultimate support force of the tunnel face, as determined through indoor scaled testing, is 387.8 kPa. Meanwhile, the FLAC\({}^{\text{3D}}\) numerical simulation method yields an ultimate support force of 412 kPa. This value is strikingly close to the limit support force obtained via the indoor scaled test, with a difference of 6.2%, indicating that the indoor scaled test model constructed in this study effectively predicts and evaluates the tunnel face failure characteristics and limit support force. (3) In laboratory-based similar model tests, different initial conditions may affect the test results. If the strength of the model material does not match that of the prototype, the stress distribution and failure mode may be distorted. The loading rate directly affects the stress-strain response and time-dependent behavior of the material. Fast loading may cause the material to behave in a brittle manner, ignoring time-dependent deformation (e.g., creep, consolidation). Slow loading allows time-dependent deformation to develop closer to actual conditions. The boundary condition is the key to simulating the actual condition in the model test, and its change will significantly affect stress distribution and deformation behavior. Rigid boundaries can lead to stress concentration, affecting stress distribution and failure modes. A flexible boundary can better simulate the actual working conditions, but it is difficult to design and realize. (4) The variations in tunnel face load during shield excavation can be categorized into three distinct phases: the deceleration and growth stage, the stable growth stage, and the slight acceleration stage. Tunnel face load and displacement can be further divided into the first and second critical loads and displacements. The tunnel face predominantly exhibits a horn-like failure feature. However, the damage range of the tunnel face under train load conditions is significantly more extensive than in the absence of train loads. In the initial phase of shield propulsion, the damage range of the tunnel face is primarily characterized by an increase in the width of the horizontal damage area. As the process progresses, both the width and height of the damaged area expand. (5) Laboratory-based similar model tests serve as a dependable methodology for studying the influence of overlying high-speed rail dynamic loads on the stability of shield tunnel faces during excavation. Through this experimental approach, a more precise comprehension of the actual response characteristics of the strata during shield tunneling can be achieved, with particular emphasis on the deformation and potential instability of the tunnel face under the influence of dynamic load. This research can provide a reliable basis for determining safe shield thrust in the design and construction of similar projects and offer important references for adjusting the design of tunneling parameters during the shield tunneling process. **Author Contributions:** Conceptualization, Y.P. and H.L. ; methodology, Y.P. ; software, Y.P. and G.M. ; validation, Y.P., H.L. and P.C. ; formal analysis, Y.P. ; investigation, Y.P. ; resources, P.C. ; data curation, H.L. ; writing--original draft preparation, Y.P. and H.L. ; writing--review and editing, Y.P. and H.L. ; visualization, Y.P. ; supervision, H.L.