Samoilych K. O., Mokritskaya T. P., Posudiievska O. R.
Oles Honchar Dnipropetrovsk National University
ABOUT THE MICROSTRUCTURE OF LOESS SOILS
The continuous accumulation of dust particles during the Quaternary period formed a heavy loess layer on the north continent. Mainly it covers the territory of China, Europe and Central Asia. The history of research of loess soils began as early as the XIX century, however, nowadays many questions remained still unsolved. The texture and structural features of soils are an important issue in the study of loess soils. As Valentina Bykova states, the loess of the central part, where Dnipropetrovsk is situated and the south-west part of Ukraine belong to the zone of water-parting plains and slopes, subsideed under their own weight. Thus the greater part of Ukraine is covered with loesses of water-parting plains and slopes which are characterized by subsidence deformations, mainly at the additional loading.
The main typical texture difference of loesses, is their macroporosity. During the analysis of clay and dust aggregates on the REM – images the scientists stated high aggregation to be the main feature of their structure. The main factor that influences the subsidence of loess sedimentations is their specific structure, namely the size and the form of solid structural elements, special character of structural links (interaction of particles), structure of soil space. Sand and large dust grains that named «globul’s» have a specific structure. In its center there is a core that consists of quartz microassemblies, further there is a holed shell of calcite, that is overlaid by a clay «shirt» (continuous coating of particles of clay minerals), impregnated with the oxides of iron and amorphous silica. It is the specific character of solid structural elements in loesses, that stipulates the formation of the so-called aggregative or grainy microstructures in them. For loesslike soils there were determined four basic types of microstructure: skeletal, aggregated, mixed and matrix.
For a long time an optical method was the basic method of study of microstructure. It allowed to diagnose the minerals of clay faction in petrographic microsections, to determine the degree of orientation of structural elements, as well as the types of contacts. At present time raster electronic microscopy – REM is considered to be the most informative. It helps to define the measure and character of aggregation, the sizes, morphological features and structure of aggregates and grains, as well as the form of micropores, the closeness of packing of elements of microstructure. According to some scientists, a microstructure reflects the influence of various physical and chemical factors on the processes of structure formation. Therefore, a microstructure is considered as a certain «photo» of the conditions which influenced the breed. However, for quality research of loess soils we need the complex study of breed microstructure determination of grain-size distribution and the analysis of REM – images.
On the basis of visual analysis of REM-images of the sample. Dnepr – vPІІ dn (increase of Х200) it was found out that the percentage of grains sized d >50 mkm equals 18 %, and d <50 mkm equels 72 %. The correlation between these dimensions K1 =0.25 indicates that the sample belongs to class II of aggregative granular microstructure. The groups of microstructure are distinguished by the size of active porosity of the breed, obtained during the analysis of REM-images of the sample surface. The pores were marked on the quadratic panel, their percent correlation was calculated. As it was found, active porosity of L equals 22 %, which corresponds to the group A (microstructure with low porosity). The sub-groups of microstructure are distinguished according to the morphometric sign – character of distribution of pores a long the sizes, which represents the character and durability of structural connections. At visual analysis it becomes evident, that breed consists of interaggregate (intergranular) and large internally aggregate pores.
The obtained type of microstructure (II-A-2) is typical for the breeds being subsided at natural loads (namely, for loesses), so Bykova’s hypothesis was proved in our region.