# Stability Analysis of the Longitudinal Slope Linear Shield Tunnel Excavation Face
## 1 Introduction
The results indicate that interlayer interactions in multi-layered foundations influence the extent of the disturbed zone, particularly at soft-hard interfaces, where the disturbance mode becomes more complex, leading to localized shear failure. [PERSON] et al. [14] conducted centrifuge model tests to investigate tunnel face stability in dry sand under different overburden-to-diameter ratios. Using DIC-based measurement, the failure mechanism was characterized by a double slip surface and chimney-shaped collapse. [PERSON] and [PERSON] [15] conducted transparent soil model tests to investigate tunnel face stability in saturated sand, and the observed failure developed as a chimney-shaped zone extending from the tunnel face to the surface under shallow cover conditions. [PERSON] et al. [16] conducted small-scale physical model tests to study the stability of tunnel faces in dry sand. Their results showed that dense sand exhibited abrupt, localized settlements after a delay, while loose sand experienced gradual settlement from the beginning of face retraction, highlighting the influence of soil density on collapse mechanisms. [PERSON] [17] conducted a series of small-scale physical model tests to investigate the face stability of shallow tunnels in dry sand. The study revealed that the initial sand density significantly influences failure mechanisms, while support pressures showed limited sensitivity to overburden depth. [PERSON] et al. [18] employed an EPB shield model to simulate the shield tunneling process and ground collapse. Their findings revealed that low-moisture-content sandy soil is prone to stepwise collapse under shield-induced disturbances. Transparent soil materials, due to their similar physical and mechanical properties to natural sand and clay [15], provide an ideal experimental approach for visualizing internal soil deformation in model tests. By integrating Digital Image Correlation (DIC) and Particle Image Velocimetry (PIV), transparent soil enables high-resolution tracking and analysis of soil displacement and strain evolution [19; 20; 21; 22]. [PERSON] et al. [20] conducted a systematic study on the stability of the excavation face in shield tunneling within clay and composite clay-gravel strata using a three-dimensional transparent soil model test. By employing Computed Tomography (CT) scanning technology and a three-dimensional reconstruction program, they accurately captured the three-dimensional soil deformation in the excavation face instability region. [PERSON] et al. [21] conducted instability model tests of shield tunnel excavation faces using transparent clay, combined with Particle Image Velocimetry (PIV) techniques. Their analysis revealed that the soil slippage in front of the excavation face exhibits a 'basin-shaped' failure pattern, which differs from the traditional wedge or prism-shaped failure modes described in conventional models. The current research on tunnel excavation face stability, both in China and internationally, has primarily focused on the instability mechanisms of excavation faces in horizontal tunnels. However, studies specifically addressing shield tunnel excavation faces under longitudinal slope conditions remain relatively limited. [PERSON] et al. [23] investigated the pitch angle of shield tunneling in steep slope construction, analyzing the interaction forces between the shield machine and the surrounding soil. They proposed a three-dimensional limit equilibrium analysis method specifically for longitudinal slopes. [PERSON] et al. [24], through a combination of experimental studies and theoretical analysis, concluded that as the longitudinal slope angle increases, the risk of soil instability ahead of the excavation face rises, accompanied by changes in the failure mode. [PERSON] et al. [25] focused on the impact of groundwater seepage on excavation face stability. Their analysis revealed that a high groundwater level reduces excavation face stability, and seepage effects may exacerbate soil instability.