Microscopic Structure Analysis Of Gap-grading Non-cohesive Soil Under Stress

Internal erosion was the main causation of dam failure.Historical data shows that 46% of the embankment dam failures around the world were caused by internal erosion(ICOLD,2013).Internal instability is a form of internal erosion occurred in broadly or gap-graded cohesionless soils.It refers to a phenomenon in which fine particles migrate through the interstices of coarse particles due to seepage.A well-known qualitative Venn diagram showing the framework of three adverse conditions,i.e.material susceptibility,critical hydraulic load,and critical stress condition,on internal instability.Existing studies have focused primarily on the influence of material properties and hydraulic conditions,while the studies about the influence of stress conditions are scarcely.The existing studies about the influence of stress condition were carried out mainly based on one or a few soils.The combined effect of particle grading and stress conditions and its mechanism are still unclear.In view of the above research background,the numerical simulation with discrete element method was conducted to investigate the combined influence of material and stress state on the soil fabric of binary granular mixtures based on experimental results of Chang & Zhang(2013).It is helpful to understand the mechanism of influence of complex stress state on the internal erosion.The main conclusions are drawn as follows:(1)Based on the research of Chang & Zhang(2013)from gap-grading test,in order to enhance the calculation speed,the lower limit of the coarse particle material is the coarse particle size and the upper limit of the fine particle material is fine-grained fine particles.10%,20%,25%,30%,35%,40%,45%,and 50% binary three-dimensional cubic particle models were produced,and then we studied various models in isobaric,biased,and intermediate Porosity,coordination number,normalized force probability distribution,stress sharing coefficient,contact types and other fabric and mechanical structural parameters under different stress states.(2)When the fine particle content is less than 25%,the influence of the confining pressure level on the coordination number,strong contact coordination number,and stress sharing coefficient is very small,and the variation between the peak value of the normalized force probability density function and the confining pressure level is complicated.When the fine particle content is more than 25%,the larger the confining pressure level,the larger the coordination number,the strong contact coordination number,the stress sharing coefficient,and the higher the peak value of the normalized force probability density function(PDF).This is consistent with the experimental results of Duberg and Nystr?m(1986),Sheng et al(2004),Frenning(2010),Joanna(2014)and Kawano et al(2018).(3)When the fine particle content is less than 25% or more than 45%,the influence of the deviatoric stress on the fabric of the mixture is very small,and the peak of the normalized force probability density function is irregularly changed by the deviatoric stress.When the content of fine particles is between 25% and 45%,the increase of the deviator stress causes the coordination number to decrease,the strong contact coordination number increases first and then decreases,the stress sharing coefficient increases,and the peak value of PDF increases.(4)When the fine particle content is less than 25% or more than 45%,the influence of the intermediate principal stress on the fabric is very small,and the peak of the normalized force probability density function is not changed regularly.When the fine particle content is between 25% and 45%,the increase of the intermediate principal stress causes the coordination number,the strong contact coordination number,the stress sharing coefficient,and the peak value of PDF to increase.