Displacement and stress pattern-based thickness recommendation of seismic buffers for depth ranges of a circular rock tunnel

The extent and forms of seismic damage vary with the tunnel depth range, according to which seismic measures should be designed. A seismic buffer of expanded polystyrene geofoam that is widely applied in retaining walls may be wrapped around the tunnel lining to improve the seismic performance. In this study, seismic buffer thicknesses are recommended based on pairs of displacement patterns (DPs) and stress patterns (SPs) in the linings. They were determined using three-dimensional displacement direction cosines and a stress-space-transformed Mohr–Coulomb criterion. Using raw data from numerical models, four displacement forms (DFs) related to practical seismic damage in four lining domains are presented, with a group of DFs denoting a DP and a group of failure statuses denoting an SP. The results demonstrate that without a buffer, at most depths, the four domains are in a composite DF denoting a perilous DP I and are in failure SP I. As the buffer thickens, the linings are gradually mitigated to lower-risk DP and SP II as well as DP and SP III, depending on whether the buffer thickness is appropriate for a certain tunnel depth. Based on the DP and SP pairs, appropriate buffer thicknesses are recommended for each tunnel depth range.