Study On The Microscopic Mechanism And Permeability Model Of Granular Soils Considering The Particle Size Distribution

Permeability is the basic physical property of granular media,which directly affects the mechanical behavior of soil and other media under the action of pore fluid seepage,and it may even result in internal erosion of the soil,deformation,instability,and destruction of geotechnical facilities.The permeability of granular media is also an important basis for studying energy and environmental issues such as oil and gas exploitation and migration of groundwater and soil pollutants.The permeability of granular media is characterized by the permeability coefficient,which can be obtained through experimental testing.In addition,the theoretical models of the permeability coefficient of granular media are mostly fitted based on experimental results,have many empirical parameters,and ignore the important characteristic of granular media-gradation.Therefore,it is of great scientific significance and engineering value for the research of soil mechanics,geotechnical engineering and related fields to deeply reveal the influence mechanism of gradation factors on the permeability coefficient of granular media and propose a theoretical model of permeability coefficient considering particle size distribution.In this study,the granular media were tested and analyzed by computed tomography（CT）scanning and three-dimensional reconstruction technology,and the pore structure characteristics under different gradation,particle shape and porosity were studied by using the maximum sphere algorithm.Through the design of granular media groups with different gradations,the constant water head test of granular media was carried out,and the influence of gradation on the permeability coefficient of granular media was analyzed from a macro perspective.Based on CT and model reconstruction technology,computational fluid dynamics（CFD）numerical simulation of seepage in pore-scale granular media was carried out,and the influence of gradation,particle shape and porosity on pore-scale flow field was analyzed.Finally,a pore size distribution model considering particle gradation was established by combining particle combination and fractal theory,and a theoretical model of permeability coefficient of granular media considering gradation was derived by combining the pipe flow model,which was verified by numerical simulation and experiment.The main conclusions include:（1）The pore structure of granular media is affected by gradation,particle shape,and porosity,and the pore size of granular media has a bi-fractal distribution characteristic.The characteristic distribution of pore structure of narrow-graded irregular particles is similar to that of spherical particles,and the peak content of pore throat size of wide-graded spherical particles is about 7.5%different from that of irregular particles.The irregularity of the particles will amplify the effect of gradation on the pore structure,and the narrower the gradation,the less obvious the effect.For spherical granular media,the change of porosity mainly causes the change of the peak content of pore distribution curve and the content of large pores;for the irregular sandy soil granular media,the change of porosity will cause the overall change of pore size.（2）Numerical simulation of seepage in pore-scale granular media shows that pore seepage is similar to pipeline flow characteristics,and seepage velocity field and pressure field are affected by gradation characteristics,particle shape,and porosity.The flow field of irregular particles is more inhomogeneous,the maximum cross-sectional velocity variation can reach 26%,while the maximum of spherical particles is only 10%,and the dominant path is more obvious.When the porosity of the granular medium is the same,the distribution of the narrow gradation flow field is more uniform,and the content of fine particles in the gradation has a great influence on the velocity distribution.The more fine particles there are,the lower the seepage velocity is.（3）A pore size distribution prediction model and a theoretical model of permeability coefficient that can directly use the particle gradation curve as input parameters are proposed,which can fully consider the influence of grading on the permeability coefficient.For spherical granular media,the mean absolute percentage error of the prediction results of the proposed model’s permeability coefficient is only18%,and in the comparison of 73 sets of permeability coefficient data for sandy soil media,more than 84%of the predicted values can meet the standard allowable error requirements of geotechnical tests.Compared with the existing permeability coefficient models,the model proposed in this study can predict the permeability coefficient of granular media with gradation characteristics,and has better applicability and calculation accuracy.（4）Particle grading has a significant impact on the permeability coefficient of the medium,and it cannot be measured by a single characteristic particle size(such as d₁₀,d₅₀,etc.),but is related to the combination of particles.When the porosity and characteristic particle size d₅₀ are equal,the permeability coefficient can increase by80%as the particle size distribution narrows and the gradation range decreases.While for granular media with equal characteristic particle size d₁₀ and porosity,the permeability coefficient can even increase up to 3 times with a wider particle size distribution range.When the granular media d₅₀ is the same,the gradation is narrower and the fine particle content is less,so the pore distribution of the medium is more uniform and the tortuosity is lower;while the d₁₀ is the same,the narrowing of the gradation will reduce the particle penetration in the medium,resulting in tortuosity decrease,but the increase of fine particle content will significantly reduce the media permeability.