Analysis and prediction on the long-term deformation of the tunnel foundation in diatomite stratum based on creep effect

Despite the unique properties of diatomite, relatively few studies have focused on its geotechnical engineering characteristics, especially those investigating the engineering performance of tunnel foundation pile treatment technologies that account for the creep behavior of diatomite. This study is grounded on the Feifengshan Tunnel Project, a key component of the Hangzhou-Shaoxing-Taizhou High-Speed Railway. To address the geotechnical challenges of this project, the creep deformation behavior of diatomite under different saturation levels was investigated by combining laboratory creep tests and numerical simulations. Subsequently, a comparative analysis was conducted on the long-term deformation of tunnel foundations (LTDTF) in diatomite strata, with consideration of different reinforcement schemes. The findings reveal that under the same creep stress levels, the creep deformation of diatomite first decreases and then increases as saturation rises. Creep parameters obtained through the inversion of field monitoring and laboratory test data are more consistent with practical engineering conditions. The miniature steel pipe pile (MSPP) reinforcement technique improves the load-bearing capacity of tunnel foundations in diatomite strata. For tunnel foundations treated with MSPP, the LTDTF decreases by 70.6% in the 100th year after the completion of the secondary support construction. Furthermore, increasing the length (0-8 m) or diameter (0-150 mm) of MSPP can effectively control the magnitude and range of surrounding rock deformation at three key locations: the tunnel foundation (Point A), the horizontal observation line (Line B-C), and the vertical observation line (Line A-D). The findings provide crucial insights and a practical methodology for predicting and controlling long-term deformation in tunnel projects in diatomite strata.