Impulse load characteristics of bouldery debris flow impact

Boulders entrained in debris flows are the main cause of damage to debris-resisting structures. Poly-dispersity leads to grain-size segregation, which causes boulders to migrate to the free surface and then accumulate at the front of the flow. Despite the importance of grain-size segregation, the current design of debris-resisting structures does not explicitly consider its effects on impact. In this study, two series of centrifuge tests were carried out to investigate the impact behaviour of mono-disperse bouldery flows and bi-disperse flows comprising boulders mixed with fine debris material. The diameter of the boulders was varied to study the effects of boulder size on the dynamic response of an instrumented model rigid barrier. The results reveal that, as the boulder size increases, a transition from progressive loading to predominantly impulse loading is observed. Boulders floating on the fine debris can induce even higher peak loads compared with mono-disperse bouldery flow. A new relationship between an equivalent dynamic pressure coefficient for the hydrodynamic approach and boulder size is established. This new relationship serves as a criterion for distinguishing between the boulders and fine debris in the design of structural countermeasures.