Mechanical Responses of Single-Pressure Arch-Shaped Open Tunnel Structure under Rockfall Impaction

To learn the stress state and provide a design basis for open tunnel structures, the mechanical response of a single-pressure arch-shaped open tunnel structure on a double-line passenger dedicated line is analyzed using dynamic finite element method. Firstly, regarding the vertical drop impaction as the basic condition, interactions between rockfall and backfill soil on the tunnel top, as well as the motion trajectory of rock-fall, are analyzed to study responses of the tunnel structure in terms of stress, strain, strain rate, displacement, velocity and acceleration. Secondly, according to the law of energy conversion and transmission among rockfall, concrete backfill materials and tunnel structure, the mechanisms of backfill soils acting as buffering cushions and concrete-filled materials as structure protection to rockfall impaction are explained through stress responses of the side wall and ground basement, and influences of the structural form of the side wall on the mechanical performance of the ground basement and the inverted arch of open tunnel are also discussed. Finally, the dynamic responses of the tunnel structure under 45 oblique rockfall impaction are analyzed and compared with those in the basic condition. The results show that the designed open tunnel can bear 700 kJ vertical impaction energy of rockfall while substructures such as apex of arch, spandrel, inner side of the arch wall and corner of the ear wall are relatively weak parts in bearing the stress; thus the way of backfill and the structural form of the side wall in the open tunnel need to be further optimized. Strain rates of the open tunnel structure caused by rockfall impaction are found limited in the range of 10-3~10-2 s-1, which is the same as that caused by earthquake. Under the condition of this research, the effect on the open tunnel structure from oblique rockfall impaction is generally less than that from vertical drop impaction.