Model test research on the uneven stress and deformation characteristics of high-speed railway tunnel bottom structures

To investigate the stress and deformation characteristics of the tunnel bottom structure (the lower load-bearing system consisting of the invert and invert backfill below the springline) of the SJZ tunnel on the Lanzhou–Xinjiang high-Speed railway under surrounding rock softening, swelling, and train-induced vibration, a 1:25 scaled tunnel model test was designed and conducted. The effects of three working conditions on the stress, deformation, and failure modes of the tunnel bottom structure were analyzed, clarifying the internal mechanisms of tunnel damage. The results show that: (1) Under the softening condition, the maximum soil pressure on the left tunnel line after backfilling was 179.97 kPa, while the minimum at the right arch foot was 20.72 kPa. As softening intensified, the contact pressure at the left and right invert bottoms showed opposite trends. Deformation and vertical displacement of the invert backfill layer increased, with continuous compression at the right spandrel and predominant tensile strain in other areas, ultimately leading to longitudinal, transverse, and oblique cracking. (2) Under the swelling condition, the maximum contact pressure after backfilling occurred at the right arch waist (62.91 kPa), and the minimum at the right invert (6.34 kPa). With increasing swelling force, the contact pressure at the spandrel, arch waist, and arch bottom decreased on the left side and increased on the right. The right backfill layer exhibited enhanced transverse tensile strain and uplift, while the left side underwent initial compression followed by settlement, eventually resulting in longitudinal and circumferential cracks. (3) After backfilling, the maximum pressure at the arch crown was 95.87 kPa. As vibration intensity increased, the central pressure at the invert bottom rose to 128.44 kPa and then sharply decreased by 33.00 kPa. After the first vibration, pressure at the arch waist decreased, and the structure was predominantly subjected to tensile strain. After four vibration cycles, compressive strain at the arch crown and left arch foot increased significantly, leading to the development of horizontal and oblique cracks. The settlement of the invert backfill layer ranged from 1.80 mm to 2.80 mm. (4) Both surrounding rock softening and swelling resulted in uneven tensile cracking of the invert structure, characterized by settlement, concentrated deformation, and structural instability. This study reveals the deformation and failure mechanisms of tunnel bottom structures under complex conditions, providing a theoretical basis for the design and disease prevention of tunnels.