Cushion mechanisms of coarse granular and foam materials subjected to successive boulder impacts

Abstract

Debris flows entrain and transport large boulders that are capable of destroying structures along their flow path. Clusters of boulders generally accumulate at the front of a torrent via particle-size segregation. To shield structures against impacts by boulders directly and to extend their working life, cushion layers are commonly installed in front of barriers. Cushions, such as gabions, are often installed to shield debris-resisting barriers from boulder impact. One of the critical considerations in the design of a cushion layer is its ability to attenuate successive impacts from boulders commonly found at the front of geophysical flows. However, most relevant works only focus on single impact and the performance of different cushion materials subjected to successive loading is still not well understood. The objectives of this research are to understand the cushion mechanisms of different cushion materials including gabions, cellular glass and EVA subjected to successive boulder impacts. Field tests and numerical modelling were carried out in this study. A new large-scale pendulum facility was established to induce an impact energy of up to 70 kJ on an instrumented rigid barrier shielded by 1-m thick cushion layer. The mechanical responses of gabion, cellular glass and EVA under successive impacts were investigated. Numerical modelling using DEM was used to back-analyse the field test results and to investigate the effects of particle size and thickness of the gabion cushion layer on its performance. Test results show that the recommended load-reduction factor (K_c) used in practice to account for the over-conservative of the Hertz equation with the adoption of cushion materials can be reduced by a factor of two. Cell wall crushing and cell wall buckling exhibited by cellular glass and EVA can attenuate up to 25% and 50% for the maximum boulder impact force and transmitted load, respectively, compared to gabion under 70 kJ of impact energy for a single impact. Based on the test results, EVA provides the best cushion performances. Numerical simulations by the DEM method reveal that force chains collapse more easily and the load diffusion angle increases as the normalized particle radius (𝑅_p/𝑅_b) decreases. To optimise the performance on transmitted loads on the rigid barrier, practitioners should reduce the size of the particles used in gabions. Normalised particle radius (𝑅_p/𝑅_b) of less than 0.20 ensures load diffusion angles that are large enough, (> 45° in this study), to spread loading across the barrier. To eliminate the effect of energy reflecting off the barrier and back to the point of impact, which augments the boulder impact force, the optimum cushion thickness should be three times larger compared to the boulder radius.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology