Experimentation on Stress Analysis of Arch-Shaped Open Tunnel Structure under Rockfall Impaction

In order to study the stress characteristics of the arch-shaped open tunnel structure under rockfall impaction, the magnitude and distribution characteristics of crosswise strain, axial force and moment in different parts of the impaction cross-section of the structure were analysed by a 1∶30 reduced scale model test, with changes of the rockfall mass and the backfill thickness. The trend of maximum internal force peak with backfill thickness was also analysed. Finally, the stress pattern of arched open tunnel structure covered with backfill under rockfall impaction was summarized, in terms of the deformation, moment and axial force of the structure. The results indicate that the mechanical response of the vault is the most significant, followed by the spandrel, inverted arch and the hance. The stress pattern of the structure for the vault is axial tension, and for the spandrel and the lower part is axial compression.This is different from the load-structure mode of the static analysis lining-structure of the tunnel or the open tunnel. Hence safety check and computation of the vault structure cannot be carried out according to the eccentric compression component of plain concrete in the tunnel design code, and other solutions should be devised. The increase of backfill thickness is beneficial to the structure of vault, spandrel and inverted arch, but is not necessarily good for hance, and it needs to be analysed by combining the size of rockfall with the form of side wall and backfill way in the design.