THE EFFECT OF BOTTOM BOUNDARY RIGIDITY OF SAND CUSHION ON ROCK FALL IMPACT FORCE USING DEM

Cushioning materials, such as sand cushion and granular mats placed on rock sheds, can effectively disperse and reduce rock fall energy before rocks collide with protection works. Although the construction costs for sedimentary layers are low, these layers are attracting attention as construction devices that can substantially improve the margin of safety ratio. To elucidate the relationship between the rigidity of the sand cushion bottom boundary and the impact force generated by the collision of a rock on sand cushion, this study examines the stress propagation behavior in the sand cushion with different bottom boundary conditions by 2D-DEM simulation. The results are summarized as follows: (1) According to the obtained experimental result with different rigidity of bottom boundary of sand cushion, the maximum impact force is smaller in the low rigidity condition than in the high rigidity condition, and the duration of the impact force is longer in the low rigidity condition than in the other condition. If the rock shed is simplified modeled by elastic beam in DEM analysis, the obtained experimental result can be qualitatively reproduced by using DEM. (2) The stress waves generated by the collision of a rock on sand cushion propagate forward the bottom of sand cushion, and the stress waves are reflected on the bottom boundary. After the reflect stress waves pass through a part of sand cushion, the high rigidity condition and the low rigidity condition value, which are the mean principle stress waveform generated in the part of sand cushion, become separated from each other. (3) The mean principle stress of reflected stress waves are smaller in the low rigidity condition than in the high rigidity condition. Because in the case of low rigidity condition, the lower part of the sand cushion become low-density due to the loading point displacement is large.