Research On Three-dimensional Microstructure Evolution And Deformation Mechanism Of Loess During Loading

Loess Plateau in China is the most concentrated and largest loess sedimentary area on the earth.The Silk Road economic belt runs through Loess Plateau,which is the main area of economic construction and social development in the western region of China.However,Loess Plateau is one of the regions with the most serious geological disasters in China due to the fragile geological environment.Frequent geological disasters seriously restrict the economic and social development of Loess Plateau.The special microstructural characteristics of loess are the essential causes of geological disasters.Mastering the microstructural characteristics of loess is the basis of studying its mechanical behavior.However,the study on the control mechanism of loess microstructure to its deformation is still insufficient.This dissertation took Q₃ Malan loess in Dongzhiyuan as a case study.Based on the field investigation,laboratory test,triaxial test,SEM,energy spectrum and micro-CT,firstly,a quantitative model of three-dimensional(3D)microstructure of Malan loess is established,and the 3D quantitative index system is proposed.Secondly,the process of microstructure evolution during loading is observed for the first time,and the 3D microstructure characteristics before and after deformation are studied.Thirdly,the response law of microstructural parameters to macro deformation of loess is studied,and the sensitivity and order of these parameters to deformation are determined.Finally,the relationship between macro deformation and microstructure evolution,defect formation of loess is established,and the micro mechanism of loess deformation and failure is revealed.The main innovative findings are shown as follows.(1)Based on the the 3D microstructure model of Q₃ Malan loess,we have a comprehensive understanding of its 3D microstructure characteristics.The 3D morphology of particles can be divided into three categories:I-sphere or ellipsoid(3.4%),II plate,disk or triangle(77.6%),III column(19.0%).Particles are randomly distributed in the horizontal direction,23%of the particles with angle less than 45°(_?).Three particle units appear in the loess:individual silt or sand,clay-coated silt and aggregate.There are direct contact and indirect contact between particle units,and point contact is widespread.The main pore types are spaced pore,inter-aggregrate pore and intra-aggregrate pore.The loose porosity,weak cementation,spaced arrangement and unstable contact constitute the metastable structure of loess.(2)According to the triaxial shear test results of Q₃ Malan loess,with the increase of water content,the hardening degree of the stress-strain curve decreases,and the shear strength decreases;with the increase of confining pressure,the curve changes from strain softening to hardening,and the shear strength increases.With the increase of water content,cohesion decrease,and the internal friction angle is basically unchanged.When the soil becomes soft plastic and flow plastic,the internal friction angle decreases.(3)Through the observation of microstructure and macro deformation during shearing,it is found that:1)The structure is not uniformly compacted during shearing,the average porosity in the middle part is higher than that of the upper and lower parts,and the standard deviation of local porosity increases gradually.2)The intact Malan loess particles are loose and evenly distributed,the structure is uniformly compacted at the intital stage of shear;at the strength peak,oversized pores(>200?m)collapse,weak cementation chain is broken,scattered particle fill into the pores,and microcracks are formed;after the peak,the pore structure continued to be compacted,the pore and throat size decreased but the number increased,the flatness decreased,weak cementation sites continue to be damaged,and more microcracks frmed;with the increase of strain,the compressed pores will be compressed into many pores,and some microcracks will gradually connected.3)After loess failure,the contact between particles shifts from point to face contact,the arrangement shifts from spaced to inlay arrangement.4)The morphology and size of particles are basically unchanged,the pores,larger than 43?m and 34?m in and out the shear zone respectively,provide main space for loess collapse.(4)According to the results of relative entropy of microstructure indexes:the sensitivity of pore index is higher than that of particle index;the response of pore size volume distribution to deformation is the largest,followed by pore throat size(channel length)and pore number,followed by flatness,direction angle and elongation.(5)Based on the above results,the micro mechanism of loess deformation and failure is mainly considered from material composition and structural characteristics.For the loess studied in this paper,the precondition of deformation and failure is its loose and porous structural characteristics,the failure of weak cemented chain and spaced structure is the direct cause,and the composition is the indirect cause.