Visualization investigation on stability of shield tunnel face with transparent soil, considering different longitudinal inclination angles

•A new 1-g experimental equipment with transparent soil in evaluating tunnel face stability was designed.•A new 3D-PIV algorithm was proposed.•Deformation and failure of the shield tunnel is a three-dimensional problem.•Tunnel face failure mode induced by longitudinal inclination angles and buried depths were investigated. Due to design optimization and geological conditions, shield tunnels often work with different longitudinal inclination angles. In this case, it is very important to understand and master the failure mode of the tunnel face. In this study, an adaptive image update method is introduced to optimize the threshold update method in PIV processing and improve the continuity and smoothness of post-processing images. The failure mechanism of tunnel face in clay stratum under different longitudinal inclination angles δ and the ratio of buried depth to diameter C/D is studied. The results show that the failure shape of the shield tunnel face shows an ‘ellipsoid’ mode under different longitudinal inclination angles δ and C/D conditions. The deformation process of cohesive soil has different development stages, and the failure modes in different areas vary. The deformation zone is mainly divided into subsidence areas and uplift areas. With the increase of the longitudinal inclination angles, from δ = −20°, 0° to 20°, the instability area, the maximum displacement, and the vertical pressure all changes, and the 3D reconstruction or numerical simulation also obtains the same failure mode. Compared with other cases, the tunnel has a higher collapse risk with shallow buried and upward inclination angles. Long-term upward inclined excavation in clay stratum should be avoided in engineering practices.