Experimental Study On Seepage Characteristics Of Levee Clay Layer During Seepage Erosion

Most levees in the middle and lower reaches of the Yangtze River are composed of a binary structure of cohesive soil and non-cohesive soil.With the increasing frequency of extreme weather conditions,the clay layer of the levee foundation is subjected to a large hydraulic gradient due to its weak permeability under the influence of flood.When the hydraulic gradient exceeds the critical hydraulic gradient,the clay layer is at risk of seepage erosion and even seepage failure.In addition,due to the clastic effect of cohesive soil and the wide distribution of particle gradation,this paper conducted seepage erosion tests on wide-gradated clay to explore the influence of soil particle composition,compaction degree,and hydraulic conditions on the seepage erosion of levee clay layers,based on the evolution law of saturation permeability coefficient.A state equation of critical hydraulic gradient in the process of seepage erosion is established to provide a theoretical basis for the safety evaluation of permeation deformation risks of earth-rock levee structures under high water levels.Based on the principle of constant head permeability test in geotechnical experiments,a series of permeation erosion tests from bottom to top were carried out on wide-graded cohesive soils using a self-developed permeation erosion test device.The permeation characteristics were mastered by calculating the fine particle loss rate and the average saturation permeability coefficient of the samples.The influence analysis of factors such as grading parameters,dry density,and fine particle content on the test process and results was also conducted.The main research results and conclusions are as follows:(1)According to the evolution law of the saturated permeability coefficient with the seepage pressure,the seepage erosion process is divided into three stages: the early stage of seepage erosion,the development stage of seepage erosion,and the failure stage of seepage erosion.Based on this,two characteristic hydraulic gradients are defined that are the critical hydraulic gradient of seepage erosion development and the hydraulic gradient of seepage erosion failure.(2)The primary and secondary order of the influences of hydraulic gradient on the critical development and the critical failure seepage of erosion of wide-graded soil are dry density,particle gradation,and fine particle content.Among them,dry density has a significant influence both of them.With the increase of dry density,the critical hydraulic gradient of seepage erosion development presents a nonlinear increasing trend,and the critical hydraulic gradient of seepage erosion failure increases approximately linearly.The gradation parameters have a significant effect,and with the increase of the gradation parameters,the critical hydraulic gradient of seepage erosion development shows a gradually decreasing trend.The fine particle content has no significant effect on both.(3)The dry density and gradation parameters were selected as the main influencing factors of the seepage erosion critical development hydraulic gradient,and dry density was chosen as the main influencing factor of the seepage erosion critical failure hydraulic gradient,the critical hydraulic gradient model was established,and the formulas were in accordance with the power exponential relationship.Through the basic physical property parameter testing of the actual embankment engineering soil,the seepage erosion model experiment was carried out to verify the effectiveness of the formula.Through the conduct of seepage erosion tests with different sample heights,it is found that when the height of the soil sample is less than 10 times the maximum particle size,it is difficult for fine particles to form a self-filtering stable layer and it will continue to be lost until piping.In actual embankment risk situations,sand and gravel layer can be laid,which can not only increase the critical hydraulic gradient for seepage erosion failure,prolong the time for the protected soil layer to erode out of the filter layer,but also can effectively prevent the soil from undergoing large deformations,which is beneficial for the rescue work.