Effects of particle size and cushioning thickness on the performance of rock-filled gabions used in protection against boulder impact

A gabion is one of the most commonly used cushioning layers to shield protection structures against boulders entrained in debris flow. Despite the prevalence of gabions, their cushioning performance is highly variable because of the wide range of rock sizes and cushioning thicknesses that are recommended in the literature. Correspondingly, the dynamic response of gabion cushioning layers varies dramatically. In this study, large-scale pendulum impact tests were used to calibrate a discrete element model. Subsequently, a parametric study was carried out to discern the effects of particle size and cushioning thickness on the impact load and transmitted load exerted by a boulder. Results reveal that as the particle size in the cushioning layer decreases, the force chains collapse more easily, and the expansion angle of strain energy increases. To optimize the performance of a gabion cushioning layer, practitioners should reduce the size of the particles to a normalized particle radius of about 0.1. A normalized particle radius less than 0.2 ensures that the expansion angle of strain energy is large enough — greater than 45° in this study — so as to enable load spreading across the barrier. To eliminate the effects of energy reflecting off the barrier and directed back to the point of impact, which augments the impact load, the cushioning layer thickness should be greater than three times the radius of the boulder.