Keywords

1 Introduction

Loess is a peculiar quaternary eolic deposit, mainly composed by silt and characterized by specific structure (macroporosity—up to 42–55% with predominance of open large pores) and properties (subsidence). It is widespread on a large territory of the Earth. The important question is to study the regularities of loess formation, properties and variability. The thickness of loess strata can vary from several meters to more than a 100 m. The development of landslides in loesses, which have significant variability of strength characteristics under watering, high sensitivity to dynamic impacts, often causes huge damage, and in some cases is catastrophic, especially in case of earthquakes (Zerkal 1994; Voznesensky and Zerkal 1997; Kalinin et al. 1998). Such peculiarities can cause landslides in regions were loess is widely spread.

The occurrence of landslides in the regions of loess spreading is characterized by certain peculiarities. These peculiarities are caused by the presence of some specific properties of loess:

  • ability of loess in dry condition (at natural humidity less than 10%) to form vertical slopes;

  • significant reduction of loess strength when moistened due to rapid destruction of structural connections formed by carbonates and water-soluble salts;

  • predisposition of loess to subsidence—compaction under moistening under the action of both its own weight and external loads;

  • ability of loess rocks to instantaneous loss of strength under dynamic impacts;

  • cyclic structure of loess strata, which is an interlacing of loess and paleo-soil horizons.

    The paleo-soils present in the loess strata are more clayey and form relative waterfalls, contributing to uneven watering of massifs. As a consequence, the deformations of loess slopes are easily recognized in many parts of the earth and associated landslides are characterized by certain type of peculiarities, later on described in detail.

2 Loess and Its Prevalence

Loess is one of the most widespread type of continental Quaternary sediments. Loess on the Earth globe cover an area of more than 13 million km2 (Trofimov et al. 2001, Ding, Editor 1 et al. (eds), Progress in Landslide Research and Technology, Volume 3 Issue 1, 2023, Book Series of the International Consortium on Landslides. DOI Ding et al. 2019). They cover more than 3% of the Earth’s land area and are found on all continents except Antarctica. The loesses are most widely distributed in Europe and Asia.

In Europe, loess is not evenly widespread. Western and Central Europe are characterized by their “island” distribution (Trofimov et al. 2001; Haase et al. 2007; Jipa 2014). Scattered massifs of loess form a sublatitudinal belt up to 300 km wide, stretching from the north of France (Norman Upland) to Belgium and the Netherlands. To the south, loesses are widespread in the Danube River valley, in the middle and lower part of which they cover significant areas in Hungary, Bulgaria and Romania (Jipa 2014). In the southeastern part of the East European Plain, loess with a thickness of up to 30 m, sometimes even more, become widespread. They are widely common on the territory of Moldova, in the central and southern parts of Ukraine in the middle and lower parts of the valleys of large rivers (Dniester, Southern Bug, Dnieper), and also occupy watershed areas. Landslides in loess in Ukraine account for up to 37% of the total number of landslides in the region.

Within Russia, loess occupies more than 10% of the territory (Trofimov et al. 2001; Zerkal and Ershova 2014; Konishchev 2015; Galay et al. 2017). The main regions of their distribution are (Fig. 1):

  • southern regions of European Russia—the coast of the Azov Sea, the valleys of the Don and Kuban rivers, their tributaries and adjacent watershed areas, the lower part of the Volga River valley, and the foothills of the Caucasus, where loess occupies up to 60% of the territory and its thickness reaches 50–60 m, more rarely up to 100 m;

  • the southern part of Western Siberia—the Priob plateau adjacent to the Ob River valley in its upper reaches, the foothills of the Salair and the Altai-Sayan mountain country;

  • southern regions of Eastern Siberia—areas west of Lake Baikal, in the upper part of the Angara River valley.

In these regions, the largest landslides reach significant volumes. On the right bank of the Kuban River valley near the villages of Grigoropolisskaya and Temizhbekskaya in April 1987, landslide displacements involved a bank section up to 2.5 km long, up to 200 m wide and 55–60 m thick. The total volume of the landslide amounted to 30 million m3 (Zaporodzenko and Derbinjan 1989).

Fig. 1
A map of northern Eurasia marks the distribution of loess in a single shade, along with the locations of the Black Sea, Caspian Sea, Laptev Sea, and Okhotsk Sea. The highlighted cities include Kyiv, Beijing, Moscow, Minsk, Astana, Ulaanbaatar, Baku, and Vladivostok.

Loess occurrence in Northern Eurasia

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In Central Asia, loess is widespread in Uzbekistan, Kyrgyzstan, and Tajikistan. In the Chirchik River valley in the Chatkal-Kuramin region of Uzbekistan, landslides in loess occupy 42% of the territory (Niyazov 1974).

One of the largest landslides in loess in Uzbekistan is the landslide “Sarybulak”, formed on 27.11.1976 in the side of the same name gully (Niyazov 2015). The formation of the landslide occurred in 30 m thick loess underlain by Jurassic clays, on which a groundwater horizon was formed as a result of prolonged precipitation. The displacement of the landslide occurred in several phases. In the initial phase of slope deformations, apparently as a result of loess continuity disruption by subsidence phenomena, there was formation of a loamy avalanche with a volume of 800 thousand m3, moving towards the Sarybulak gully channel with a velocity of more than 8 m/sec. The resulting air shock wave knocked down a shepherd on the opposite side of the shed. The second stage of deformations was a landslide-flow of strongly moistened loess (with humidity up to 30–32%) with the volume of more than two million m3, displaced at a speed of 6–15 m/h during 4 days. Further, the development of landslide deformations continued for several years. In 1980, the total volume of the Sarybulak landslide reached 8.5 million m3 (Niyazov 2015).

In Tajikistan, landslides in loess rocks, which is covering 16% of the territory and count for 27–28% of all landslides, and even up to 84–85% taking into account shallow landslides.

The highest intensity of landslide development in loess is characterized by medium-low-lying areas of arid regions of Tajikistan. In some areas (the valley of the Kharangon River, the left bank of the middle course of the Obi-Mazar River) landslides in loess occupy 35% of the total area (Skvaletsky 1988).

The largest landslides in loess in Tajikistan are the sesmogenic phenomena formed during Gissar earthquake 23.01.1989 in the northern part of Urtaboz plateau. Landslides were formed in the thickness of 73–90 m in the loess strata. The main factor of landslide formation was liquefaction of water-saturated loess under seismic impact (Zerkal 1994, 1996; Voznesensky and Zerkal 1997). The total cumulative volume of loess involved in displacements during the Gissar earthquake reached 65 million m3. The largest of the seismogenic landslides is the Okuli landslide, 3.8 km long. Its velocity of flow was up to 3–3.1 m/sec (Zerkal 1994, 1996). The most catastrophic was the Sharora landslide with a total volume of up to six million m3 (Zerkal 1994). This landslide partially destroyed the village of Sharora, located at the foot of the Urtaboz Plateau, killing more than 270 people.

In China, loess covers 6.6% of the land area, most of which forms the Loess Plateau with an area of about 430,000 km2 (Bian et al. 2022; Zhuang et al. 2016). The thickness of loess strata ranges from a few meters to 300 m.

In North America, loess is distributed within the Great Plains in the Ohio, Missouri, and Mississippi River basins. In South America, loess covers vast areas in Argentina and in some parts of Uruguay.

In Africa and Australia, there are no large massifs of loess, being deposited locally. The loess is widespread in New Zealand, mainly in the South Island, where it covers 10% (Carey et al. 2017).

Loesses, widely developed in areas of intensive economic development, occur in a variety of conditions, composing surfaces of different morphology—from plains to slopes of considerable steepness. Specific properties of loesses associated with their genesis and subsequent diagenetic transformations cause the development of a whole complex of geological processes (subsidence, suffosion, pseudokarst, etc.), among which landslides are one of the most dangerous.

3 Specific Features of Landslide Occurrence in Loess Rocks

In general, among the slope deformations involving loess formation, several groups can be distinguished (in relation to their structure (by (Zerkal and Ershova 2014):

  • Type I - landslides, whose deformation zone is located in the loess strata;

  • Type II - landslides whose deformation zone is confined to the contact of loess and underlying rocks;

  • Type III - landslides where the loess plays a passive role in displacement, i.e. it is involved in displacements without deforming (e.g. in the upper part of the landslide block).

Type I landslides are formed in areas where loess rocks have significant thickness. Landslides of this type are most widely represented in the Pre-Caucasus. Type II landslides develop in areas where erosion uncovers loess underlying rocks, usually of clay composition. The thickness of loess rocks composing the slopes in this type of displacements can vary widely—from the first meters to 20–25 m and more. The development of landslides of this type in Russia is observed in the valleys of the Don, Kuban and Ob rivers, as well as in the southern regions of Siberia (Pribaikalia). Type III landslides are formed in areas with low thickness loess cover.

Almost all known types of landslides (by displacement mechanism) occur in loess (Varnes 1978; Cruden and Varnes 1996; Hungr et al. 2014):

  1. 1.

    Rotational slide:

    • Rotational loess slide, typical for type I and type III of the selected slope deformations.

    • Translational slide, typical for type II of the selected slope deformations.

    • Loess fall (local block collapses), which are characteristic of type I slope deformations in the areas of vertical scarps within the loess and are, as a rule, local in nature.

  2. 2.

    Flow slide:

    • Loess flow slide, typical of both type I and type II slope deformations.

    • Dry loam avalanche, arising (in the form of deformations, as a rule, of type II) at strong dynamic impacts on the strata of loess in the dry state during earthquakes.

    • Sudden liquefaction loess slide, formed by the effect of liquefaction of moistened loesses during earthquakes (in the form of type I or II deformations) or anthropogenic dynamic impact. Subsequently, the mass movement occurs as earth spreading (sudden spreading failures, by (Hutchinson 1988)).

  3. 3.

    Complex slide and transitional types of slope movements:

    • Rotational loess slide и loess flow slide, transforming into mudflow when reaching river valleys with subsequent turning and movement along river channels, accompanied by water saturation of shifting soil masses.

    • Rotational loess slide, transformed into liquefaction slide under dynamic impact at the moment of impact on the opposite slope of the valley.

    • Сomplex slope movements with cambering and bulging (by (Hutchinson 1988)), which are formed either in the conditions of uplift of the edge part of the landslide block or in the presence of a deeply buried horizon of strongly moistened loess in the massif (Fig. 2).

A special type of combined slope deformations occurring in loess should include suffosion slides (Fig. 3). The formation of suffosion loess slides (by (Zerkal and Barykina 2023) (in the form of type I or II deformations) is associated with suffosion soil removal by groundwater flow. Also, a special type of combined slope deformations of loess are landslides, the initial phase of displacements of which is caused by subsidence phenomena.

Fig. 2
A photograph of a valley landscape features loess deposits with a convex shape, outward bulging, and less vegetation. A car is parked at a distance.

Loess slide with cambering and bulging (Tadjikistan)

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Fig. 3
A photograph of a muddy landscape with loess deposits and few trees. The landslide crack and 2 suffusion sinkholes are indicated by 3 arrows. The mountains are in the background.

Site of suffosion landslide formation in loess (Tajikistan)

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3.1 Rotational Loess Slides

Rotational loess slides (Fig. 4) in Russia are widely developed in the Pre-Caucasus, in the south of Western and Eastern Siberia, where their thickness can reach 40–100 m (Zerkal and Ershova 2014). The formation of sliding landslides occurs in loess rock massifs with low humidity (We < Wp). Landslide deformations occur in the form of displacement of one or several large blocks forming a typical terrace-like surface in the relief. The displacements occur along a small (3–5 cm, rarely more) zone, a sliding surface with a shape close to circular-cylindrical and uniform for each individual block.

Fig. 4
A photograph of a valley landscape features loess deposits on a sloped, curved surface with vegetation on and along the river banks. The water in the river has ripples on its surface.

Rotational loess slide (valley Kuban river, Russia)

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Progressive motion of detached blocks down the slope is accompanied by rotational motion, which leads to overturning of blocks. Vertical amplitudes of one-time motions may be insignificant and therefore the probability of repeated deformations is high.

The thickness of rotational loess slides is from 6–7 m to 15–25 m, rarely more. N.N. Nilov (personal communication) believes that the depth of landslides in loess does not exceed the thickness of the subsidence zone. Volumes of sliding landslides in loess can be from the first hundreds and thousands of m3, reaching in some cases 1,5–two million m3 and more. The development of rotational slides is activated by watering of loess massifs in spring and autumn periods, as a consequence of snowmelt and heavy precipitation. Displacement rates of landslides of the considered type in the periods of activation range from mm/day to first m/day, rarely more.

3.2 Translational Loess Slides

Translational loess slides in the territory of Russia are predominantly developed in the northern areas of loess distribution, where their thickness is up to 15–20 m, rarely more. In the southern parts of loess distribution on the territory of Russia translational loess slides are also quite widespread in foothill areas (North Caucasus, foothills of Salair and Altai-Sayan mountain country). The formation of translational loess slides occurs in loess rock massifs at sufficiently high moisture content (Wp < We < WL). Landslides of this type with volumes from the first hundreds and thousands of m3, reaching in some cases several tens and first hundreds of thousands of m3, prevail in the northern areas of loess rock distribution. The periods of development and activation of sliding landslides in loess rocks are close to the periods of development of sliding landslides. Displacement rates of landslides of the considered type in the periods of their activation range from mm/day to the first m/day, rarely more.

3.3 Loess Falls

Loess falls (Fig. 5) are usually local deformations of subvertical scarps formed by dry loess (We < <Wp). The volumes of loess falls are thousands, rarely tens of thousands m3.

Fig. 5
A photograph of a small hill features trees and a house at the top, along with loess deposits and patches of vegetation. The hill has an uneven slope, sinking at one end and bulging outward from the other end.

Loess falls on the Azov Sea coast (Russia)

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3.4 Loess Flow Slides

Loess flow slides (Fig. 6) are quite a common type of slope deformations formed in the areas of loess distribution. Within the composition of flow slides in loess rocks can be distinguished:

  • shallow flow landslides—with thicknesses up to 3 m, more rarely up to 5 m, and volumes reaching thousands (shallow slide) and tens of thousands m3;

  • deep flow landslides with thicknesses of 5–10 m and more, with volumes reaching hundreds of thousands m3, rarely several million m3.

Shallow loess flow slides are formed due to infiltration and accumulation of atmospheric precipitation and are often confined to low slope areas—head parts of ravines, runoff troughs—which promote accumulation and infiltration of surface runoff. Shallow loess flow slides often develop in areas of anthropogenic humidifying of the upper part of loess massifs. Deep loess flow slides are formed in areas where their watering occurs both due to infiltration of atmospheric precipitation and groundwater. The formation of loess flow slides occurs in massifs with sufficiently high humidity (We ≥ WL).

Fig. 6
A photograph of a landscape with mountains in the background, sparse vegetation including leafless trees and a few tall trees, and loess deposits causing slope deformations. The land below the mountains is uneven, with a large pit.

Loess flow slides (Tadjikistan)

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In plan, loess flow slides have a glacial, pear-shaped, or, less often, circular-shaped form. The shape of loess flow slides depends both on the character of watering of the loess massif and on the morphology of the slope. Often loess flow slides due to the low viscosity of shifting masses, meeting obstacles on the way of their movement, flowing around them, sharply change the direction of displacement. Having reached the foot of slopes, loess flow slides can turn around and continue their displacement along the watercourse channel (along-channel displacements). This may result in additional watering of landslide masses and loess flow slides may be transformed into mudflow. The length of loess flow slides depends on the height and morphology of slopes and ranges from hundreds of meters to first kilometers with widths of tens and hundreds of meters, and in some cases up to a kilometer. Characteristic for landslides of this type is a significant (from 2–5 times) excess of the distance of spreading of landslide masses “length of the detachment surface”.

The rate of displacement of landslides ranges from the first m/day-m/min to 4–5 m/s. The displacement velocity of landslides and the distance of their movement depend significantly on the day surface gradients and the viscosity of soils, which is determined by their initial moisture content. At the same time, the maximum flow velocities are characteristic of the central parts of the flows, which move, as a rule, for long distances. Having considerable kinetic energy, the flows, even on an almost horizontal surface (1°-1.5°), can move for considerable distances. The displacement of loess flow slides is rather rapid and, as a rule, single-acting.

3.5 Dry Loam Avalanches and Sudden Liquefaction Loess Slides

Dry loam avalanches and sudden liquefaction loess slides on the territory of Russia are noted in areas of increased seismicity (North Caucasus, Altai-Sayan mountain country). At the same time, in recent decades, no strong earthquakes have occurred in the territory of Russia in regions with extensive loess development.

3.6 Suffosion Loess Slides

Suffosion loess slides are formed in loess rocks as a result of slope stability failure due to suffosion of material in the lower part of the slope by groundwater (often hydrostatic pressure). In this case, the hydraulic gradient reaches a certain critical value (Zerkal and Barykina 2023). Under the influence of filtration forces, material is carried out from the massif and a suffusion niche is formed. Landslide deformation begins with the collapse of the niche vaults, and the collapsed rocks together with the suffusion material form an unstructured landslide mass. Displacement of the landslide generally occurs as a flow landslide. Suffosion loess slides are drop-shaped, and their volumes reach tens of thousands of m3. Landslides of this type are characterized by high—up to 5–7 m/min displacement velocities. The development of suffosion slides in loess rocks occurs in places where there is a temporary “barrier” on the way of water filtering in the massif, which leads to a sharp increase in head and creates conditions for a sharp activation of suffosion processes. Suffosion loess slides have been described on high slopes (20°-25° steepness) in the valleys of the Volga River, the Ob River and in the Northern Caucasus (Zerkal and Barykina 2023).

4 Stability of Slopes Composed of Loess Rocks and Factors Influencing Its Changes

The formation and development of landslides occurs under the influence of a variety of complex of factors. Among these, the stability of loess slopes can be influenced by:

  • structure of loess rock massif;

  • climatic (meteorological) conditions and surface water erosion activity depending on the first;

  • character and degree of watering of loess strata, as well as manifestation of specific properties of loess rocks in conditions of moistening and under dynamic influences;

  • morphology of slopes and their exposure;

  • geomorphologic conditions and paleorelief features;

  • tectonic disturbances;

  • seismicity of the area;

  • anthropogenic impact causing changes in hydrogeological conditions and properties of loess.

    In general, the factors affecting the stability of loess slopes, as in other cases, can be divided into slowly and rapidly changing ones. The first type of factors determining the general predisposition of the territory to the development of landslide processes includes the peculiarities of spatial distribution and structure of loess strata, the nature and type of tectonic disturbances of the area, seismicity, geomorphologic conditions, the history of recent geologic development reflected in the paleorelief features, and regional hydrogeologic conditions. The second type of factors includes watering of loess rocks both due to groundwater and under the influence of meteorological conditions and anthropogenic impact, as well as peculiarities of changes in the properties of loess in different conditions.

5 Conclusion

Loesses are widespread over vast areas in Europe and Asia, as well as in North America, where they form covers with thickness from several meters to the first tens of meters, and in the foothill areas and intermountain hollows of the Caucasus, Central Asia, and China—up to 100 m and more. The peculiarity of loess is, on the one hand, its ability to form high (up to tens of meters) subvertical ledges in dry condition, on the other hand, to be characterized by a significant decrease in strength characteristics when moistened, and in waterlogged condition it has the ability to liquefy. A specific feature of loess is the ability to subsidence

  • self-compaction when moistened under the action of both its own weight and external loads.

    One of the consequences of sharp variability of strength properties of loess is high landslide activity in the regions of their distribution, where landslides can occupy up to 30–40% of the total area. The spread of landslides is also promoted by the unstability of loess to water erosion, which contributes to the rapid erosional dismemberment of the territory with the formation of high slopes.

    Almost all known types of landslides are formed in loess:

    • in dry loess—loess falls, rotational and translational loess slides, and dry loam avalanche can be formed during earthquakes;

    • in humidifying loess—loess flow slide and sudden liquefaction loess slide (sudden spreading failures, by (Hutchinson 1988));

    • at interlacing of relatively dry and moistened loess—complex slope movements with cambering and bulging (by (Hutchinson 1988)).

      A special type of combined slope deformations—suffosion loess slides (by (Zerkal and Barykina 2023)) also occurs in the areas of loess distribution.

      The high landslide activity is typical for the regions of loess widespread, the variety of landslide types formed in loess, require much more detailed study when assessing the landslide hazard in loess.