Multiple barriers are usually installed to effectively mitigate large-volume debris flows. Existing multiple-barrier design recommendations ignore the effects of solid-fluid interaction and may cause uncertainties in estimating the flow impact force on subsequent barriers. In this study, a fully coupled, two-layer, two-phase material point method (MPM) with an incompressible fluid phase is implemented and validated using the experimental results of dry sand, water, and sand-water mixture flows impacting on rigid barriers. Numerical parametric study is carried out using the validated model to investigate the effects of Froude number (Fr) and barrier spacing on second barrier impact force. Debris flow volume of 500 m3 is modelled with the Fr ranging from 2 to 6, which are relevant to gentle and steep terrains in Hong Kong. Simulation results show the importance of changes in fluidisation ratio, which is the ratio of basal pore fluid pressure and total stress. The debris flow after impacting on the first barrier, overflows, and lands on the slope between the two barriers. The fluidisation ratio of debris flow after landing increases by up to 30%, leading to an increase of impact velocity at the second barrier by up to 80%. Consequently, the impact force of debris flows on second barrier is underestimated by up to 50% compared with existing design guidelines. This implies neglecting fluidisation ratio may lead to non-conservative design of multiple barriers.
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