Reserch On The Influence Of Agitation On Excess Pore Water Pressure And Its Behavior Evolution Of Debris Flow

The Nujiang River Basin in Yunnan Province is one of the areas most prone to catastrophic debris flows in China.The frequent occurrence of debris flow hazards has caused extremely serious harm to the lives and property safety,economic development and ecological environment of local residents.The debris flow in the basin usually has the characteristics of strong concealment and sudden outbreak,high altitude in the start-up area,strong handling and carrying,strong scraping ability along the way,long transport and diffusion distance,and large breaking force.These macroscopic characteristics shown in the flow process of debris flow depend largely on the development degree and persistence of internal excess pore water pressure,that is,excess pore water pressure strongly affects the rheology and flow behavior of debris flow.However,the existing understanding of the source and evolution of excess pore water pressure is mostly speculative or hypothetical,and it is more dispersed and not widely agreed.Therefore,more systematic research is needed on the influencing factors of the development and maintenance of excess pore water pressure in debris flow,especially for debris flows in low gradient channels.Based on the typical alpine sandy debris flow event in Dongyuege(DYG)Gully in Nujiang River Basin on August 18,2010,on the basis of identifying the background of the hazard,this paper studies the specific influence of the agitation mechanism on the excess pore water pressure and behavior evolution of debris flow through the combination of field detailed investigation,various types and scales of experiments and theoretical analysis.The main conclusions are as follows:1.The elevation drop and channel fluctuation of DYG gully debris flow basin ar e large,and the terrain energy of the starting area is high.The basin area,surface slope and gully morphological characteristics are conducive to the erosion of loose materials by surface runoff and slope flow,and the longitudinal slope gradient,bending coefficient and topographic advantages of the main gully provide strong kinetic energy for the movement of debris flow.The double factors of more precipitation in the study area than in previous years and the ice and snow melting water near the snow line are superimposed to trigger the debris flow disaster to ensure the water source.Under the strong tectonic movement in the debris flow basin and its vicinity,the cracks and joints in the slope rock mass on both sides of the gully bed are relatively developed,and the degree of fragmentation is high and the stability is not good,which also provides a loose source for the outbreak of debris flow events.2.Debris flow deposits cover a gradual transition from fine debris of 0.04μm to large rock blocks above 50 cm,in which the grain gradation with the maximum grain size of 20 mm is good and continuous on the whole,with the characteristics of poor sorting and wide gradation.The matrix part is mainly sandy～silty,and the 20-0.075mm debris from the macro and micro perspectives is mainly equiaxed.The composition is derived from granite,gneiss,schist and quartz crystal.Quartz(non-clay minerals)and mica atypical clay minerals are the majority in the fine debris below 0.005 mm.The permeability coefficient of the debris flow mixture at 10 h is very small,which belongs to the low permeability medium.This low permeability medium will confine the water firmly in the intergranular pores,and the water holding capacity of the debris is strong,which is beneficial to the development and persistence of excess pore water pressure.At the same time,it also contributes to the liquefaction behavior of debris in the shear-agitation stage of debris flow slurry under non-seismic action,which should be one of the key factors for the long-distance migration and diffusion of DYG debris flow in low gradient channels.3.The transition of P_R curve from static state to dynamic state is obvious,which means that the excess pore water pressure increased by agitation in natur al debris flow has a strong response to the transition of debris flow from stagnation state to flow state,and the monitoring time,accumulation rate and peak value in the dynamic stage are 3.13 times,1.12～8.27 times and 1.14～1.34times of those in the static slurry,respectively,agitation obviously promotes the development and maintenance of excess pore water pressure in debris flow slurry.The degree of slurry agitation can be described by the measured temperature of debris flow slurry.When the degree of slurry agitation gradually increases,the solid particles suspended in water will increase,the grain size will become wider and wider,and the excess pore water pressure will be further improved.4.The key mechanism of the excess pore water pressure in the continuous graded debris flow is that the weight of the solid phase is transferred to the water,and the continuous and rapid turnover will increase the gap between the adjacent solid particles,which will promote the dispersion of the solid in the debris flow slurry to increase the excess pore water pressure,and the relative excess pore water pressure in the dynamic slurry does not increase or decrease suddenly,which again shows that the formation of the excess pore water pressure is mainly based on the transfer of the weight of the solid material to the water.5.Under the same upper limit grain size,the viscosity coefficient and yield stress of the slurry have a synchronous change relationship with the solid volume concentration,while the apparent viscosity of the slurry decreases with the increase of the shear rate gradient,showing the characteristics of shear thinning.Similarly,the net torque time series characterizing the flow resistance of the debris flow slurry also shows a downward trend,that is,the slurry also produces a thinning behavior under the action of shear agitation,which helps the debris flow body to turn into a fast flow state with high energy after the thixotropic start.6.Agitation significantly improves the liquefaction degree of debris in debris flow slurry.The peak value of dynamic liquefaction rate is 1.053～1.159 times higher than that of static liquefaction,which is generally at the level of complete liquefaction.The flow resistance of slurry gradually decreases with the improvement of liquefaction capacity of particles,indicating that the low resistance and high fluidity of debris flow are caused by the excess pore water pressure promoted by agitation.The increase of excess pore water pressure leads to the liquefaction and suspension of solid particles,which weakens the contact effect between particles,reduces the shear strength and viscosity of the slurry,and improves its thinning and flow behavior,which is conducive to its rapid flow in the channel.When the degree of agitation increases,that is,the flow behavior becomes stronger,which in turn feeds back to the slurry and the loose solid material along the way to form a high and lasting excess pore water pressure.This cycle further strengthens the viscosity reduction and drag reduct ion effect of the debris flow until the solid material reaches a complete liquefaction state.Therefore,the development degree and durability of excess pore water pressure have a strong evolution effect on the rheological and flow behavior of debris flow.