On the outskirts of deserts and steppes adjacent to them, on the mountain slopes, a special type of clay deposits is formed. They are called loesses and loesslike loams. This is a slightly connected, easily rubbed non-layered rock. Loesses are usually fawn yellow, fawn or light yellow. Loesslike loam is a breed in which there is no characteristic characteristic of loesses. It is characterized by high porosity and calcium carbonate content.
Loesslike loam: characteristics
According to some properties and particle size distribution, the rock approaches the cover loam. As a rule, loesses do not contain sand particles larger than 0.25 mm. However, in this rock there is a large amount of coarse dust fraction (0.05-0.01 mm). Its content usually reaches 60-70%.
The breed is characterized by weakly stratified, microaggregated, high water permeability. Loesses are carbonate rocks. In arid areas, they can be salted and contain gypsum particles.
What causes the subsidence of loesslike loams?
The breed is characterized by high macroporosity. Loesslike loams contain relatively large, vertical tubules (pores) left by dead roots and plant stems. Their size is much larger than the size of inclusions that make up the rock. The tubules are impregnated with lime, due to which they acquire a certain strength. That is why vertical walls are formed during erosion. When soaking, the rock gives a large drawdown due to the presence of tubules, gypsum, carbonates, readily soluble salts and colloids in the helium state. This leads to large deformations of engineering structures.
Origin of breed
At present, there is no consensus on the causes of loesslike loam formation. Among all the existing hypotheses, aeolian and water-glacial can be distinguished. The first was proposed by academician Obruchev. His hypothesis was supplemented by Mirchink, Arkhangelsk and other scholars. According to the aeolian hypothesis, loesslike loams were formed due to the combined activity of vegetation, rain and wind.
The glacier-water theory connects the origin of the rock with silt deposited from glacial waters, spreading over the entire surface south of the boundary of the melting glacier. This hypothesis is supported by such scientists as Dokuchaev, Glinka, etc.
Relief features
In outcrops loesslike loams form cliffs. In areas of loess deposits, as a rule, deep ravines arise. They quickly expand to the sides and in depth due to the erosion of the walls by groundwater.
Cover loess-like loams are widespread in Western Siberia, in Uzbekistan, Kazakhstan and China.
The power of the soil varies in a fairly wide range. So, for example, in Western Siberia it is within 5, 090 m, in Central Asia up to 50 m or more. In China, the thickness of loess loam can reach 100 and even exceed this value.
The designation of loesslike loams is given in the Interstate standard GOST 21.302-96.
Use in road construction
Loesslike loams are considered unsuitable soil for the arrangement of road infrastructure. In the dry season, they are very dusty. Due to the lack of connectivity of inclusions, soil abrasion occurs, as a result of which a dust layer of up to several tens of centimeters appears on the roads. This period is called "dry moss." When moisture enters, the soil quickly softens, taking a fluid state. In this case, the resistance to loads drops significantly.
Before laying the roadbed on loesslike loams, special measures must be taken to prevent erosion of the slopes.
Rock differentiation
Loesslike loams are more coarse-grained and low carbonate. Carbonate loams are found everywhere on poorly drained flat surfaces with a slight development of the erosion network and a small incision of river valleys.
The spatial differentiation of loesslike carbonate loams indicates the temporal dependence of soil leaching on the degree of their involvement in the process of geomorphological development due to the natural drainage of the site. The smaller the area is drained, the higher is the carbonate horizon in the soil profile.
The sporadic distribution of loesslike carbonate loams in the thickness of carbon-free rocks indicates the secondary nature of the upsetting of the loamy-loamy massif under arid conditions. The presence of massifs consisting of carbonate loams indicates the incompleteness of the geomorphological cycle.
Mineralogical composition
All loesslike loams and the European and Asian parts are similar. The rocks contain 50–70% of quartz, 5–10% of carbonate minerals, and 10–20% of potassium-sodium feldspars.
A small amount of minerals containing iron is found in loesses. Their concentration does not exceed 2–4.5%. Carbonate inclusions are found mainly in the dust fraction. They are represented by films and clusters in cracks and pores in the form of impregnation.
Along with carbonate inclusions, gypsum and silica are precipitated. Accordingly, clay minerals, quartz, mica, feldspars, as well as dolomite and calcite, the contents of which are greater in Central Asian loesses, are found in the mineralogical composition. In addition, soluble salts and heavy metals (in small quantities) may be present in the composition.
Grading
In the rocks, a small content of large fractions is noted. Sand inclusions account for an average of 4.4% in loesses, 11% in loesslike loams. The sludge content ranges from 5–35%. At the same time, its level increases with increasing humidity and the removal of loess from the sources of its formation.
On the territory of the Russian Plain, loess acquires a more clay structure from north to south. A distinctive feature of the rocks is a large amount of coarse dust. Its level reaches 28–55%.
