| Debris flow is an extremely dangerous natural disaster,which routinely destroys property and claims human lives.To intercept this hazardous phenomenon,structural countermeasures such as rigid barriers are commonly installed in mountainous regions.Due to the complexity of interaction between debris flow and a rigid barrier,there is still a lack of understanding of its mechanism.In previous studies,debris flow is usually simplified as a particle flow or an equivalent fluid to simulate the interaction with a rigid barrier,without considering the interaction between solid and fluid and the associated energy transmission and dissipation.This may have led to a non-conservative interpretation of the impact and mobility of the debris hitting the barrier.This thesis aims to understand the influence of solid-fluid interaction on two phase debris flows impacting a rigid barrier,and the underlying micro-mechanical mechanisms,via a coupled CFD-DEM numerical modelling and small-scale flume model testing.Based on the physical and numerical investigations,the following conclusions may be drawn:(1)A coupled CFD-DEM method is used to study the effect of solid-fluid interaction on impact by varying the solid fraction from 0 to 0.5,which covers the range from water flow to debris flow.The mesoscopic insight from CFD-DEM modelling reveals that the particle-fluid interaction plays two roles in increasing the kinematic energy of particles:(a)imposing driving force to the particles;(b)reducing inter-particle contact forces and therefore energy dissipation from shearing among grains by applying buoyancy to the particles.Consequently,the run-up height and impact pressure of the water-particle mixture flows at the barrier are higher than those of dry granular flows with the same Froude number.While impacting the rigid barrier,most of the energy of water-particle mixtures(exhibiting a run-up mechanism)is dissipated as the fluid phase segregates from the mixture and rolls-back towards subsequent flow.This differs from the conventional impact mechanism observed for dry granular flows,where the energy is mainly dissipated via shearing between layers of sand piling up behind a barrier.(2)A series of small flume model tests were performed to investigate the effect of particle size distribution on the mobility and impact mechanism of dry granular flows and water-particle mixture flows.The presence or absence of the fluid phase has led to distinct mechanisms for the mobility of debris flows with the same Froude number.For the dry granular flow,the coarse and fine particles are segregated,with the former mainly concentrated in the front flow.After the debris impact,most of the coarse particles deposit on the surface of the debris resting on the rigid barrier.On the other hand,the presence of fluid in the water-particle mixture flow results in a more evenly distributed coarse particles among fine particles during both sliding and deposition process of the debris flow.It is also revealed that in both cases,the increase of fine grain contents would have led to more particle contact,higher energy dissipation during the sliding,and consequently smaller impact load hitting the barrier.(3)Small flume model tests were also performed to investigate the effect of a novel rigid barrier(i.e.,slit dam)on retaining both water-particle mixture and dry granular flows.Special attention was paid to the pile up height and the relationship between the number and size of slit-dam and trapping efficiency.It is found that the slit dam with a given opening size is less effective in retaining water-particle mixture flows,than dry granular flows with the same Froude number.This is because the presence of water has reduced the friction between particles(mainly due to the buoyance),and the ability to form soil arching between the opening,resulting a reduced grain-trapping efficiency.The tests also reveal that with a given total opening size,the slit dam with two smaller openings could be more effective in grain-trapping,than a dam with one big opening. |
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