Inituation,Development Mechanism Of Seepage Erosion And Its Consequences On Subgrade Fillings With Different Gradations

In recent years,due to the rampaging of the El Nino phenomenon,frequent rainstorms occur and the river water levels rise,leading to seepage erosion,resulting in major disasters in the infrastructure of coastal cities,posing a great threat to the personal and property safety of the Chinese people and affecting the harmonious development of the economy and society.The phenomenon of erosion usually occurs in water conservancy buildings,and it often happens in subgrade engineering under heavy rain as well.Due to the effect of seepage,the fine particles of soil move in the pores of the coarse particles or even wash away.In the long term,the fine particles in the roadbed filler are gradually hollowed out,and the physical and mechanical properties of the soil change,which in turn affects the soil stress in the subgrade and causes local settlement of the subgrade or collapse of the road surface.Therefore,it is necessary to carry out related research on the seepage erosion in the roadbed.This paper mainly discusses the induced factors,development laws and influences of seepage erosion in the subgrade.The following are the work and achieved results carried out in this article:(1)Using PFC software combined with the Darcy flow calculation program in porous media to simulate the soil erosion process of different particle gradations,verifying the suffusion criterion proposed by Kezdi.The changes of the mesoscopic parameters of the internal stable soil and internal unstable soil during the erosion process are analyzed.The influence of the initial flow velocity on the development of seepage erosion is discussed.The research shows that the erosion's initial stage of the internal unstable soil is short,the erosion rate is high in the accelerated development stage,and the erosion rate still has a larger increase trend in the continuation stage.The initial stage and the development stage of internal stable soil erosion is not affected by the fine particle content,but in the continuation stage,the larger the fine particle content,the greater the erosion rate.When the initial flow velocity is larger,the initial stage of erosion curve is shorter and the erosion rate in development stage is greater.For internal unstable soil,the erosion is continuous when the water flow velocity exceeds the critical one,but for internal stable soil,the flow velocity does not affect that the erosion rate of the continuation stage tends to be stable.(2)Using the CFDEM fluid-solid coupling calculation program,the classical Trapdoor test is simulated,and the formation process of soil arches and the final shape of soil arches with different particle gradations are analyzed.Based on the soil arching effect,the influences of different fluid pressures and particle gradation on soil arching effect are studied.The research shows that when the gap-ratio of soil particles is larger,the arch profile is more obvious.The larger the fine content of the soil,the higher the height of the arch.The soil erosion is greatly affected by the stress of particles,due to the particles below the arch are looser than other areas,the contact force between the particles is small and the average stress level is low,the erosion rate is large.The average coordination number of fine particle increases rapidly when the erosion begins,and that will be gradually stable until it is close to four.The change of soil arching effect is caused by both the drag force and the loss of fine particles when erosion begins.The greater the erosion rate,the more significant the soil arching effect is weakened.(3)Designing and building an erosion test platform based on pile-supported subgrade.Through the physical model tests,the loss process of fine grains in the gap-graded roadbed filler under the effect of seepage erosion is explored,and the variation of pile-soil stress ratio during the erosion process is analyzed.The distribution of fines content in the roadbed model after erosion is obtained.The experimental result shows that a large quantity of fine particles of internal stable soil loss in the early stage of erosion,and the internal unstable soil has less fine particles lost.For the possible-eroded soil,partial fine particles loss in the initial stage,and erosion occurs again in the middle stage,the fine particles at the downstream are intermittently lost to form a seepage channel.The pile-soil stress ratio gradually increases during the development of erosion.When the direction of seepage is perpendicular to the soil arch plane,the soil stress at the top of the piles increases,but the soil stress between the piles decreases when the direction of seepage is parallel to the soil arch plane.The change in spatial distribution of fine fractions in the internal unstable soil induced by drag force and gravity is obvious,and the loss of fine particles gathers at the inlet and outlet of water stream.The fine contents of possible-eroded soil in each area have minor changes under the seepage,but there are more regions where fine particles are eroded.