Loesses and loess-like loams: formation, structure and interesting facts

2024 Author: Henry Conors | [email protected]. Last modified: 2024-02-12 02:41

On the outskirts of the deserts and steppes adjacent to them, on the mountain slopes, a special type of clay deposits is formed. They are called loess and loess-like loams. It is a low-cohesive, easily rubbed non-layered rock. Loesses are usually fawn-yellow, fawn or light yellow in color. Loess-like loams - a rock in which there is no property characteristic of loess. It has high porosity and calcium carbonate content.

Loess-like loam: characteristics

In some properties and granulometric composition, the rock approaches mantle loams. As a rule, loess does not contain sand particles larger than 0.25 mm. However, this rock contains a large amount of coarse dust fraction (0.05-0.01 mm). Its content usually reaches 60-70%.

The rock is characterized by weak layering, microaggregation, high water permeability. Loesses are carbonate rocks. In dry areas, they may be saline and contain gypsum particles.

Thancaused by the subsidence of loess-like loams?

The rock is characterized by high macroporosity. In loess-like loams, there are relatively large, vertical tubules (pores) left by dead roots and stems of plants. Their size is much larger than the size of the inclusions that make up the rock. The tubules are impregnated with lime, due to which they acquire a certain strength. That is why, when blurring, vertical walls are formed. When soaked, the rock gives a large drawdown due to the presence of tubules, gypsum, carbonates, easily soluble s alts and colloids in the helium state. This leads to large deformations of engineering structures.

Origin of the breed

Currently, there is no consensus on the causes of the formation of loess-like loams. Among all existing hypotheses, one can single out eolian and water-glacial. The first was proposed by Academician Obruchev. His hypothesis was supplemented by Mirchinok, Arkhangelsky and other scientists. According to the eolian hypothesis, loess-like loams were formed as a result of the combined activity of vegetation, rain and wind.

The glacial-water theory connects the origin of the rock with silt deposited from glacial waters that spread over the entire surface south of the glacial melting line. This hypothesis is supported by such scientists as Dokuchaev, Glinka and others.

Relief features

In outcrops, loess-like loams form cliffs. In areas of loess deposits, as a rule, deep ravines appear. They are fastexpand to the sides and in depth due to the erosion of the walls by groundwater.

Integumentary loess-like loams are widespread in Western Siberia, on the territory of Uzbekistan, Kazakhstan and China.

The thickness of the soil fluctuates over 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 loams can reach 100 and even exceed this value.

The designation of loess-like loams is given in the Interstate standard GOST 21.302-96.

Use in road construction

Loess-like loams are considered unsuitable soil for road infrastructure. In the dry season they are heavily dusted. Due to insufficient connectivity of inclusions, soil abrasion occurs, as a result of which a layer of dust up to several tens of centimeters appears on the roads. This period is called "dry thaw". When moisture gets in, the soil quickly soaks, taking on a fluid state. At the same time, load resistance drops significantly.

Before laying the roadbed on loess-like loam, special measures must be taken to prevent slope erosion.

Breed differentiation

Loess-like loams are more coarse-grained and low in carbonate. Carbonate loams are found everywhere on poorly drained flat surfaces with a slight development of an erosion network and a small incision of river valleys.

SpatialThe differentiation of loess-like carbonate loams indicates the time 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 less drained the area, the higher the carbonate horizon in the soil profile.

Sporadic distribution of loess-like carbonate loams in the strata of carbonate-free rocks indicates the secondary nature of carbonization of the cover loamy massif in arid conditions. The presence of massifs consisting of carbonate loams indicates the incompleteness of the geomorphological cycle.

Mineralogical composition

It is similar for all loess-like loams and the European and Asian parts. The rocks contain 50–70% quartz, 5–10% carbonate minerals, and 10–20% potassium-sodium feldspars.

In the loess, an insignificant amount of minerals containing iron is found. Their concentration does not exceed 2–4.5%. Carbonate inclusions are found mainly in the silt fraction. They are represented by films and accumulations in cracks and pores in the form of impregnation.

Gypsum and silica are deposited together with carbonate inclusions. Accordingly, clay minerals, quartz, mica, feldspars, as well as dolomite and calcite are found in the mineralogical composition, the content of which is greater in the Central Asian loess. In addition, easily soluble s alts and heavy metals (in small quantities) may be present in the composition.

Grain size distribution

In the rocks there is a small content of large fractions. On average, sandy inclusions account for 4.4% in loess, 11% in loess-like loams. The silt content ranges from 5–35%. At the same time, its level rises as humidity increases and loess moves away from the sources of its formation.

On the territory of the Russian Plain, loess acquires a more clayey structure from north to south. A distinctive feature of the rocks is a large amount of coarse dust. Its level reaches 28-55%.

P. S

Loesses are distinguished by their low cation exchange capacity. The composition of exchange cations contains calcium and magnesium in a ratio of 3:1, as well as sodium and potassium. Loesses are characterized by an alkaline reaction of the environment.

The rock has a number of properties useful for soil formation. The process, in particular, is facilitated by physical (high moisture capacity, porosity, water permeability), physicochemical and mechanical properties. In addition, they are rich in nutrients. Chernozem, gray forest, chestnut and other highly fertile soils are formed on loess-like carbonate loams and loesses.

High carbonate contributes to the formation of humate-calcium humus. It also ensures its static nature and accumulation under vegetation. Loesses give the soil useful properties: increase carbonate content, microaggregation and porosity.