Structural Reliability Design of Non-Backfilling Arch Open Tunnel Structure under Rock-fall Impaction

A structural reliability design method for a non-backfilling arch open tunnel structure under rockfall impaction was investigated to improve the reliability of structural design. Firstly, in view of the characteristics of the failure modes of a non-backfilling arch open tunnel under rockfall impaction, a four-fixed square reinforced concrete slab was simplified to a rigid perfectly-plastic slab, and the assumed failure of the structure's ultimate load or structural resistance was calculated as a statics problem using plastic limit theory. Secondly, numerical modelling was performed using a particle flow code method, and rockfall impaction load was expressed as a power function of the gravitational potential energy of rockfall through the regression analysis of modelling results. Thirdly, the limit state equation was established by combining structural resistance and the rockfall impaction load expression. Then, the structural reliability index was obtained using a MATLAB software programming method, and structure reliability design and optimization were performed by comparing the calculated structural reliability index with the target reliability index. Finally, using the established method, the structural reliability indexes of the double-line arch open tunnel of a passenger transport line were analysed. The structural reliability index of the original structure design is up to 4.2 for a height of 5-15 m and a rockfall weight of 1-2 kN. Moreover, the structural reliability index is up to 5.4 if the concrete grade is adjusted from the original C35 to C40 and the reinforcing steel bar grade is changed from HRB335 to HRB500, for a height of 5-20 m and a rockfall weight of 1-2 kN.