Effects of seismic buffer thickness on a circular rock tunnel considering seismic damage form and failure state

Conventional seismic designs are currently inadequate to withstand earthquakes in mountain tunnels, which have historically suffered devastating seismic damage. Seismic buffers made of expanded polystyrene geofoam, which are widely used in aboveground structures, have potential applications in tunnels. However, it is not known what the optimal thickness should be, and the seismic effects on such buffers and their compatibility with tunnel construction require investigation. In this study, the effects of seismic buffer thickness on the tunnel lining are investigated. A novel direction-based displacement approach associated with practical seismic damage forms was devised, and the Mohr–Coulomb criterion was integrated into a meridian space to understand the failure of the lining. The states and forms of lining displacement and stress were probed, and the results were validated through the seismic deformation method, shaking table tests, and on-site investigations. The results indicate that unsafe multiple displacement forms of the lining can be mitigated to a relatively uniform vertical shearing form with a seismic buffer no more than 20 cm thick; moreover, lining failure can be prevented, without changing lining tensile and compression forms. However, adverse effects occur with buffers thicker than 20 cm, leading to the resumption of the multiple lining displacement forms and failure. Buffer thicknesses of 10–20 cm should be considered in future seismic designs of mountain tunnels, combined with a trade-off among the seismic effects, manufacturing, and installation of buffers under specific construction conditions.