Study on Mechanical Response of Shed Tunnel with Rockfall Impacting Sand-EPE Composite Cushion

In order to find a more effective energy dissipation and vibration reduction shed tunnel structure, this study uses model tests and LS-DYNA numerical calculation methods to explore the mechanical response mechanism of shed tunnel structure with pure sand cushion and sand EPE composite cushion under the impact of falling ball. Through research, it is concluded that the peak strain at the center of the top plate abdomen under the pure sand cushion is 48.78×10^-6, the peak stress is 1.46 MPa, close to its ultimate tensile strength of 1.78 MPa. After adding 6 cm thick EPE cushion, the peak strain can be reduced by 83.42%, the equivalent stress can be reduced to 0.24 MPa, and the safety factor can reach 7.42. When the thickness of EPE cushion is increased from 0 cm to 4 cm, the strain reduction at Z1 and Z2 characteristic positions can reach 57.14% and 65.22% respectively. When the thickness of EPE cushion is increased from 4 cm to 6 cm, the strain reduction at Z1 and Z2 positions is 19.05% and 18.75% respectively. When the thickness of EPE cushion is set to 4 cm, the vibration reduction effect reaches the inflection point. Build a numerical calculation model with the size of 1:1 with the model test. After adding 2, 4, and 6 cm thick EPE cushion, the central stress of the roof abdomen decreases from 1.16 MPa to 0.699, 0.418, and 0.313 MPa, with a decrease of 39.74%, 63.96%, and 73.02%. It is recommended that the actual protective structure should be equipped with 40~60 cm thick EPE cushion. The calculation model of the actual shed tunnel on site was established. After the 40 cm thick EPE cushion was added, the peak stress of the central unit decreased from 5.71 MPa to 1.72 MPa, with a decrease of 69.88%, and the peak ball fall acceleration decreased from 546 m/s² to 432 m/s², with a decrease of 20.88%. It was verified that the sand EPE composite cushion can play an obvious protective role on the shed tunnel structure.