Microstructure Characteristics Of Different Loess Layers In Bailu Plateau

As a special medium,loess is well known for its strong structural,water sensitivity and seismic vulnerability,among which water sensitivity and seismic vulnerability are often derived from its structural characteristics,which are secondary characteristics of structure.Loess has high strength under natural humidity,but its complex physical characteristics make its skeleton structure change and destroy rapidly after immersion,which causes disasters such as loess collapse,loess liquefaction and foundation settlement,and poses a great threat to people's lives and property.These urgent problems need to be solved in-depth study on the special properties of loess.As a kind of granular material,the macroscopical catastrophic mechanics of loess is controlled by its internal structure system,and the core of this system is the special structure composed of pore,granule and skeleton,and the contact system and force chain structure based on it.Therefore,it is of great significance to study the changes of loess microstructures for evaluating the engineering properties of loess.Taking the different loess strata in Bailuyuan area of Shaanxi Province as the research object,the characteristics of Loess grain,pore and micro-structure were systematically studied by means of data collection,field investigation,in-situ test,micro-structure test and image reconstruction,which provided scientific basis for the study of loess catastrophic mechanics theory and loess major engineering construction in this area.The main results are summarized as follows:(1)The three-dimensional micro-structure characteristics of different loess layers in Bailu Plateau were studied,and the quantitative characteristics of Loess Grain Index(particle shape,particle size distribution,particle sphericity distribution,particle orientation),pore index(pore and pore throat),structure index(particle contact mode,skeleton particle arrangement and micro-structure type)were determined.(2)The three-dimensional morphology of particles in loess can be divided into five categories: plate-like particles,strip-like particles,polygonal and angular particles,subspherical particles and prismatic particles.According to the sphericity,the particles can be divided into strip-like and flake-like(0-0.1),substrip-like and subflake-like(0-0.3),polygonal-angular(0.3-0.7),subspherical(0.9)and spherical(0.9-1).The number of angular particles ranged from 71.3% to 94.7%,followed by spherical particles,ranging from 4% to 22.7%.(3)According to the analysis of the directional distribution characteristics of particles,it can be seen that the angle between the long axis and Z axis of most of the particles in the nine strata of S1,L2,S2,L3,L5,L6,S7,L9 and S9 in Bailuyuan area is between 70 degrees and 90 degrees,i.e.they are nearly horizontal distribution,and are more likely to form mosaic structures supported by each other;the angle between the long axis and Z axis of most of the six strata of L1,S3,L4,S5,S10 and L11 is less than 40 degrees,i.e.,they are nearly vertical or inclined.Bu,these strata are more likely to form metastable porous oblique textures.(4)Based on the three-dimensional micro-structure model of pore in different loess strata in Bailuyuan area,the pore volume and pore volume are skewed distribution,and the distribution parameters decrease with the increase of burial depth,which shows that the variation of pore type with burial depth is as follows: pore increases with the increase of burial depth,the number of small pore increases gradually,the degree of pore aggregation decreases,and the distribution density increases.By comparing the total pore throat area with the corresponding formation permeability coefficient,it is found that the pore three-dimensional micro-structure model is more suitable for loess samples with large and medium pore.The contact modes of loess particles in Bailuyuan area include point-point,point-line,surface-surface,point-surface and cementation contact.The skeleton arrangement is dominated by overhead arrangement and mosaic arrangement.Microstructure types in Bailuyuan area are divided into honeycomb structure and flocculation structure according to the connection mode.