Study On Stress Mechanism Of Arched Protection Shed Of Bridge Under Rock-fall Impaction

The railway building in mountain areas often crosses over the val ey,surrounding by mountains and rivers.The tunnel entrance is usual y located in the "V" type of val ey,the slopes of val ey is cliff and the rock-fall disasters occur frequently,so tunnel entrances have become the most hazardous place.The arch open tunnel,as an effective protective engineering,has the advantages of good anti-impact performance,reasonable load,lower geological disturbance impact and environmental protection.Currently,the arch open tunnel becomes mainly protection measurement for railway building in rock-fall hazards regions.Based on the arch open tunnel structure of passenger transport line,the mechanical responses characteristics and mechanism of arch open tunnel structure under rock-fall impaction is researched by literature study,theoretical analysis and numerical simulation.The main research contents and results are as follows:(1)The mechanism of arch structure and flat top shed structure under rock-fall impaction is analyzed by dynamic finite element method.The results show that the weight and height of rock-fall are the important factors influencing the rock-fall impact force,rock-fall impact load and impact depth,but it is not the critical factor that affects the influence scope of Rock-fall impacts.Moreover,we found that there is a good correlation between rock-fall impact force and rock-fall impact load,a suitable mathematical expression can be built to reflect their relationship.Then,on the basis of the numerical simulation results,the relation between rockfall impact force and rock-fall impact load is established.(2)The mechanical responses characteristics of arch open tunnel structure under rockfall impaction is analyzed using numerical simulation method.We find out that the most dangerous position under rock-fall impaction is the vault and pile foundation,and makes clear the change law of stress,strain,displacement,rock-fall impact force and rock-fall impact load with time.On the base of the above,we further analyzed the influences of impact direction on arch open tunnel structure,the results show that the mechanical responses characteristics along vertical direction is larger than other directions,So we consider that the vertical impact should be as the unfavorable condition to calculate bearing capacity of the structure in the design of the arch open tunnel structure.(3)The influence of different backfill modes on the mechanical performance of arch open tunnel structure is analyzed using numerical simulation method.The results show that buffer layer is able to absorb and disperse the energy of rock-fall impaction,and then improve the stress states of the structure.And the reasonable backfill modes was obtained: The more hard backfill material which being fil ed between arch ring and wing wal,can improve the stress states of the superstructure;The more soft backfill material which being fil ed on top of vault,it is more beneficial to structure performance.