Investigation On The Impact Mechanism Of High-speed Debris Avalanche And The Engineering Design Of Barrier Structure

Under the influence of global warming and frequent extreme rainfall events,as well as the long-term effects of strong earthquakes,debris avalanches create long-lasting risks that should not be underestimated.In China,engineering construction activities are often undertaken in complicated geological,geographic,and geomorphic conditions close to densely populated areas.As the occurrence of granular-flow related disasters becomes more sudden and frequent,there is the possibility of significant risks to human lives and extreme economic losses.Thus,the design of disaster prevention schemes is a difficult and urgent task.Focusing on the rapid nature of debris avalanches within meizoseismal areas,this paper reveals the impact mechanism of high-speed debris avalanches and proposes a dynamic-process-based engineering design for a baffle barrier structure.Based on centrifuge modeling and the discrete element method（DEM）,we model the impact effects of high-speed debris avalanches,identify the impact mechanism of such flows against barrier structures,and establish a physics-based design for a barrier structure that can resist the associated impact.The contributions of this paper are threefold:（i）we establish a novel centrifuge model to investigate the effect of flow speed and volume on the impact behavior of superspeed debris avalanches and assess the Coriolis effect related to centrifuge modeling of superspeed debris avalanches;（ii）we investigate the impact mechanism and impact effect of superspeed debris avalanches on closed-type rigid barriers;and（iii）we propose a dynamic-process-based design strategy for a baffle structure by revealing the debris–baffle interaction mechanism during the deceleration of superspeed debris avalanches.The main results are as follows:（1）Centrifuge modeling of superspeed debris avalanche.A novel centrifuge model allows us to investigate the effect of the flow speed and volume on the impact behavior of superspeed debris avalanches and to assess the Coriolis effect related to the centrifuge modeling of superspeed debris avalanches.Based on centrifuge modeling,the influence of Coriolis acceleration on the debris avalanche and its impact is revealed,and DEM simulations elucidate the mechanism of the Coriolis effect on the debris avalanche and its impact under a quasi-plane-strain condition.Variations in the granular behavior because of the Coriolis effect are assessed,whereupon some practical suggestions for the physical modeling of superspeed debris avalanches in a centrifuge are proposed.（2）Closed-type barrier.Based on the proposed centrifuge model,we first investigate the influence of the granular speed,volume,and particle characteristics on the impact effect against a closed-type rigid barrier.We show that the differences in the impact dynamics between high-and low-speed processes lie in the dynamic properties of the superspeed debris avalanche,which dominate the impact effect,and the reduced importance of the static force caused by the dead zone.The mode of the impact pressure distribution along the barrier height and the failure mode of the dead zone behind the barrier are investigated.The evolution of energy consumption during the debris–barrier interaction is analyzed and the contribution of the dead zone in attenuating the flow impact energy is quantitively assessed.To better understand the components of the impact force exerted by a debris avalanche on a barrier,we directly track the force generated by different parts of the debris avalanche during impact.The different force components and their sensitivity to the flow speed,volume,and particle characteristics are analyzed.And besides,the impact force calculation method and design strategy of engineering structures based the impact mechanism and inhomogeneous material composition of landslides debris avalanches are proposed.（3）Baffle structure.During the debris–baffle interaction,the particles undergo rearrangement,which promotes the development of a unique meso-structure represented by a granular arch across the baffle slits.The breaking and reconstitution of the granular arch results in a significant deceleration of the superspeed debris avalanche.Thus,the destructive power of the superspeed debris avalanche is reduced.We explain the debris–baffle interaction mechanism in terms of both energy consumption and the contact behavior of particles within the granular arches,and also propose that the baffle deceleration potential should be quantitively assessed based on the strength of the granular arches.We systematically investigate the coupled effect of the Froude number and particle characteristics(,/)of the debris avalanche on the deceleration process,run-up process,and impact effect during the debris–baffle interaction,and reveal the dominant mechanisms controlling the different impact processes.We suggest that the first array of the baffle structure should be taken as a reference in baffle design.Following this strategy,granular-jump-based and jet-based impact models that can be used to estimate the peak run-up height and impact force for baffle design are proposed and verified using numerical data.In addition,considering the highly unsteady nature of debris avalanches,we suggest that the global maximum values of the flow depth and velocity should be used for baffle design.When dealing with material inhomogeneity,we suggest that the particle size of the dominant flow content should be used to estimate the baffle deceleration potential.The largest particle size,constituting no less than 10%of the debris flow,can be used to estimate the impact force on the baffle（excluding boulder impacts）.The results presented in this paper are expected to advance the physical modeling of superspeed granular systems in centrifuges and offer some fundamental guidance for disaster prevention design for resisting superspeed debris avalanches.This study has a number of limitations.Based on existing studies,further research into the disaster-causing mechanism of flows within meizoseismal areas should be encouraged.Future studies should consider the extremely large debris volume and complex disaster environment,including the effect of strong ground motion and the frequent recurrence of landslides or debris flows after earthquakes.This would improve our knowledge of geo-disasters and enhance the reliability of protective structures.