| Loess is a Quaternary wind-formed sediment formed under arid and semi-arid climatic conditions,with temporal and spatial variations in particle morphology,pore structure and material composition.The special hydrological properties of loess,such as water sensitivity,collapsibility and disintegration,have led to the frequent occurrence of geological hazards such as collapse,landslide and debris flow in the loess distribution area,and the widespread distribution of engineering geological problems such as ground settlement,ground cracks,soil erosion and dam foundation instability,which have caused serious hazards to industrial and agricultural construction and people’s lives.The microstructure of loess determines the hydrogeological and engineering geological properties of loess.Modern photoelectric observation instruments and image information processing technology can establish a microstructure map of loess,and the research results can provide a basic scientific basis for revealing the influence of microstructure on the hydrological and engineering properties of loess.While some important results have been obtained by domestic scholars on the microstructure of the Malan loess,relatively little research has been carried out on the microstructure of the Lishi loess and the Wucheng loess,which are buried at a greater depth.In this paper,the Quaternary loess from the Liujiapo profile in Lantian County,Xi’an City,is used as the object of study,and microstructural observations such as scanning electron microscopy（SEM）,X-ray diffractometer（XRD）and energy spectrometer（EDS）are applied to explore the establishment of a microstructural map of the loess since about 900 thousand years,and to study the evolution of the temporal characteristics of the loess microstructure.The main research results are as follows:（1）The microstructure of the 27 layers of soil in the Liujiapo profile was observed at the micron scale,and the particles,pores and cementation forms were described and classified to establish a qualitative description map of the soil microstructure in the Liujiapo profile.Along the profile,from top to bottom,the microstructure transitions from granular particles,bracket pores and microagglomerates forms,to aggregated particles,mosaic pores and semi-glomerates forms,and then to clotted-cemented particles,cemented pores,and cemented forms.In the upper part of the profile,loess structure is loose while the paleosol is relatively dense,with obvious differences;in the middle part of the profile,the differences between the particles,pores and cementation forms of the loess and paleosol gradually decrease;in the lower part of the profile,the particles,pores and cementation forms of the loess and paleosol are basically the same,both showing a dense honeycomb structure.The changes in microstructure well reflect the weathering process of soil formation in the Liujiapo profile.（2）The nine analytical parameters of particle size percentage,particle size frequency,particle gradation,mean particle size,median particle size,sorting coefficient,skewness,kurtosis and fractal dimension were combined to establish a quantitative analysis profile of the particles in the Liujiapo profile.The highest content of powder grains between 5 and50μm in the profile is followed by clay grains≤5μm,and the lowest content of sand grains>50μm,and the variation curve of powder content along the profile can well reflect the local paleoclimatic characteristics.The loess has a relatively good particle gradation,and the mean grain size,median grain size and sorting coefficient are generally larger than those of the paleosol,which have a higher fractal dimension.The particle size frequency curve,except for the chalk layer L’9,show the form of three peaks,and the bias of the frequency curve of all layers are positively or very positively skewed,reflecting the high content of clay particles in the Liujiapo profile.（3）Based on Matlab software and IPP software,the SEM images were processed and parameters extracted to calculate five analytical indexes:pore size,pore area rate,pore shape,pore fractal dimension and pore probability entropy to establish a quantitative analysis map of pores in the Liujiapo profile.Along the profile from top to bottom,the number of microscopic pores<2μm in diameter and small pores between 2-8μm in diameter tended to increase,while the number of medium pores between 8-32μm in diameter and large pores>32μm in diameter tended to decrease.The maximum pore size and pore area ratio of both loess and paleosol gradually decrease as the layers move downwards.Round pores dominate the pores in all layers,and most of the microscopic pores are rounded,while small pores are predominantly irregular and medium pores are predominantly elongated.The pore fractal dimension of the loess and palaeosoil ranges between 1.148 and 1.377,with little difference among the layers.The probability entropy of the loess layers averages 0.69,while that of the palaeosoil layers averages 0.65.The palaeosoil shows a higher order than the loess.The variation of microscopic pore space along the profile reflects to a certain extent the variation of macroscopic properties such as wetting and permeability in the Liujiapo profile.（4）Based on XRD and EDS techniques,a mineral composition profile was established for the Liujiapo profile.The mineral composition mainly consists of seven minerals:quartz,sodium feldspar,potassium feldspar,illite,chlorite,hematite and calcite.The content of clay minerals such as illite tends to increase from the top downwards,while the content of clastic minerals such as quartz and sodium feldspar tends to decrease.The presence of quartz,sodium feldspar and potassium feldspar in the form of granular grains,which play the role of skeletal support;illite and chlorite mainly adhere to the surface of the grains and the contact zone of the grains,which play the role of cementation;calcite is relatively small,but uniformly distributed under the microscopic view;and hematite is often attached to the pore wall. |
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