Research On The Fine Particle Migration In Diffuse Failure Landslides Based On Rare Earth Elements Tracer Technology

The "5.12" Wenchuan earthquake in the southwest mountains area left a large number of loose granular deposits.With the rainfall or a gradul rise of the groundwater level,the pore pressures in the slopes surge resulting in a sudden collapse and then the fluidization movement which causes long migration distances at high velocity,eventually forming a gentle deposit.Once it occurs,it will bring huge losses in properties and lives.Therefore,it's important to study the surge of pore pressure before failure to clear the mechanism of the diffuse failure landslides.At present,scholars have found that the pore pressure surge is caused by the progressive collapse of the pore channel caused by the internal erosion in the loose granular deposites by the simultaneous analysis of the microseismic signal,the pore pressure and displacement.However,the fine particle migration is not only the separation of the pores to form an open channel.It is also accompanied by deposition and clogging effects.Therefore,the surge of pore pressure before diffuse failure is not only derived from separation and erosion,but also related to particle deposition and clogging.In this paper,through the early flume tests and physical simulation test based on rare earth elements(REEs)tracer technology,the formation conditions of coarse-grained soil and internal fine particle migration are analyzed in depth,which reveals the relationship between pore pressure surge and fine particle migration before diffuse failure to achieve a new understanding of the initiation mechanism of the diffuse failure landslides.It got the following conclusions mainly:(1)By the flume test used by the coarse-grained soil with the fine particle(<2mm)content of 27.4%,it found that the diffuse failure,which causes obvious settlement,occurrs under a certain relative compactness(D_r)whose range is from0.013 to 0.3.The relative compactness is positively correlated with the collapse at the leading edge,but negatively correlated with coarse granulation at the leading edge.When the relative compactness range above 0.3,the artificial slopes exhibit the failure characteritics which are intermittently regressive rather than diffused.In addition,there is a critical water head at the time of diffuse failure,which increases linearly as D_r increases.(2)The adsorption of fine particles with different size to REEs is different.With the increase of particle size,the adsorption increases first and then decreases,and the maximum adsorption value appears in the range of 0.054~0.075?m.The fine particles of same size have a consistent capacity for adsorbing the light,medium and heavy rare earth oxides.The stepwise dilution method is adopted to configure the mixed soil sample with high concentration of rare earth,and the sample non-uniformity is within5%.(3)Based on the rare earth tracer technology,the fine particle migration is studied by the flume test.The fine particles are fan-shaped divergent migration and non-longitudinal linear migration.Due to the seepage,the fine particle flowing out are most in the leading edge,followed by the middle and trailing edges,and the seepage flow increases with the continuous adding water.When the water head in the trailing edge is half of the critical head,the amount of deposited fine particles is small,and the particles show internal erosion and outflow.However,when the water head in the trailing edge is between half of the critical head and critical head,the fine particles are mainly deposited and increase significantly between 10?m and 300?m.At the same time,the fine particle of 500?m or more are deposited in the leading edge.Affected by the rise of the water level at the trailing edge,the fine particles in the slope form a filter cake effect in the middle front part,which prevent the migration of the fine particles in the middle trailing edge,resulting in plugging pores.(4)The fine particles in the coarse-grained soil are filled in the coarse particles by monomers,aggregates,and adsorbents.Under hydraulic conditions,the force acting on a fine particle are the vertical lift force,the adhesion force,the electric force,the gravitational force and the horizontal drag force.When the torque is in an unbalanced state,the fine particle is separated from the coarse grain surface,which in turn causes subsequent deposition and clogging effect.(5)Synchronous analysis of pore pressure,displacement and microseismic with different compactness confirms that the pore pressure increasing sharply is related to the fine particle migration before the diffuse failure.The pore pressure and displacemet by the correlation analysis shows that there is a large amount of early response in the pore pressure of each part of the slope,and the time is kept within 1.5s.Along the direction of the section where the slope first destructed,the pore pressure in the middle and front shows a significant early response.(6)When 0.013<D_r?0.2,the pore pressure increasing before the diffuse failure is dominated by the particle clogging effect.The internal erosion only opens the channel,and the induced pore collapse scale is small.The generalized model is:internal pore channel opening?pore collapse micro-sedimentation(or significant sedimentation)?filter cake effect?particle blockage causes pore pressure increasing sharply.When 0.2<D_r?0.3,the pore pressure surge is mainly caused by internal erosion,and the pore size is reduced due to the progressive collapse of the rich pore channel.The pore pressure surge increases the generalized model:internal erosion pore channel opening?pore collapse settlement?pore pressure surge.