Influencing Mechanisms Of Grain Size Distribution On The Mobility Of Granular Flows In A Small Flume

Rock avalanches is characterized with the large volume,the extreme high speed and the unexpected long runout distance,which often occurred in high mountain areas and caused devastating disasters.With the expending of human activities,the threat of rock avalanches to human life and property security was growing,so the moving mechanismof rock avalanche has increasingly attract more attentions.The factors of particle size distribution such as particle size,content and structure,mineral composition,underlying surface and topography,influenced the mobility of rock avalanches together.However,the understanding of influence of particle size distribution is not yet clear enough.The two-section flume test was carried out to investigate the effect of particle size and content on the runout distance,velocity of rock avalanches.Based on the video image analysis,the dynamic reading method of the runout distance is proposed in view of the dispersion and unidentified boundary of the frontmaterial in the complex gradation,and then the variation of the runout distance in different cased and the corresponding relation with the accumulation state and the speed-time history are analyzed.Finally,the front effect and size effect of granular materials in complex gradation during the movement are put forward.Based on the theory of granular materials such as friction theory,sorting mechanismand force chain theory,the mechanism of acceleration,deceleration and accumulation of granular materials is preliminarilydiscussed.The main results are shown as follows:-A set of 10 conditions is designed to investigate the effect of particle size distribution on the mobility of rock avalanches,which could simulate the content variations of different particles from 0%,33.3%,66.7% to 100% and conduct comparative analysis of monodisperse,bidisperse and polydisperse materials.-In addition to the condition of 100% fine particle,the front materials in other conditions has the characteristics of discreteness and unidentified boundary,which can be divided into three distinct areas: continuous accumulation area,discontinuous accumulation area and discrete accumulation area.The acceleration change point in the accumulation process is located in the intermittent accumulation area,which has the uniqueness of the limit and can be used as the identification point of the runout distance.-The runout distance of the condition with 66.7% coarse particles,16.65% middle particles and 16.65% fine particles is the farthest,which shows that the less middle and fine particles is beneficial to maintainthe motion inertia of coarse particles and reduce the friction energy consumption.-In the acceleration phase,the mixed materials of 3 granular materials display the whole movement after the particles sorting;in the deceleration phase,the acceleration begin to appear differentiationand the front materials separate from the whole mass.When the content of middle particles is relatively small,the force chain becomes weaker,which causes the small dynamic friction and farther runout distance,on the other hand,the force chain becomes stronger,the runout distance is shorter.