Keywords

1 Introduction

At present, landslide typology is based on analysis of the mechanism of slope deformation and consideration landslide-forming material types. The main types of landslide movement are considered as fall, topple, slide, spread and flow (Varnes 1978; Zolotarev and Janič 1980; WP/WLI 1993b; Cruden and Varnes 1996; Hungr et al. 2014). Of course, there are many intermediate, “complex” landslide types in nature, combining different displacement mechanism. Such “polyphase landslides” have been called “complex” (where at least two types of movement are involved) or “composite” (when two different types of movement occur in different areas of the displaced mass) (Varnes 1978; Cruden and Varnes 1996; WP/WLI 1993a). In the Varnes (1978) landslide classification system, rock, debris, and soil are distinguished among landslide-forming material types. In specialized classifications, one can additionally distinguish landslides in permafrost (or cryogenic landslides) (McRoberts and Morgenstern 1974; Zerkal and Strom 2021), slides in periglacial clays (Hutchinson 1988), and slides in sensitive clays (or quick clays) (Mitchell and Markell 1977; Locat et al. 2011). Hungr et al. (2014) suggested using geotechnical material typing (rock, clay, mud, non-plastic (or very low plasticity) sorted soil, debris, peat and ice) when updating the Varnes classification system (Hungr et al. 2014). Recent studies of landslide activity in the plains and lowland areas of European Russia, recognize a specific type of mass wasting and slope deformation formed through suffosion. This paper outlines our current research into formation of suffosion landslides.

2 Suffosion

The widespread occurrence of suffosion throughout the territory of the East European Plain was first noticed by A. P. Pavlov, who introduced the term “suffosion” (Pavlov 1898), and is now recognized as the process of sediment washout—the mechanical removal of small particles from saturated earth material—under the influence of filtering groundwater flow. In contrast to karst, in which dissolution rock material takes place, the main process in suffosion is mechanical removal of poorly consolidated bedrock and unconsolidated surface deposits. In natural conditions, mixed (chemical–mechanical) removal of material from rock massifs is, of course, not uncommon. For example, when dissolution of cement in rocks occurs, followed by the mechanical removal of small particles from a deposit. Another difference between suffosion and karst is the speed of the process. While the rate of formation of karst cavities can be very slow (especially in carbonates due to their low solubility), the rate of formation of suffosion cavities is primarily controlled by the intensity of the filtering groundwater flow speed and head gradient.

The main conditions influencing suffosion activity are: (1) the presence of unstable earth materials susceptible to suffosion; and (2) the possibility of intensive groundwater flow, especially with large groundwater level fluctuations. The free movement of the smallest particles in soil pores, under the influence of the filtered groundwater flow, determines the suffosion stability of the slope. Suffosion failure begins when the velocity and head gradient of groundwater flow reaches critical values. At which time, fine particles smaller than the diameter of the pore channels can be transported and carried away by groundwater. The removal of large quantities of fine particles from the soil is accompanied by a gradual increase in the diameter of the pore channels.

Suffosion failure can result in an increase in soil porosity and permeability that can extend to a considerable depth and cause a reduction in slope strength. Under conditions of favorable transport (removal) of fracture products by the filtration flow beyond the zone of suffosion development, underground cavities in massifs, depressions, sinkholes and closed depressions develop on the surface, while niches and grottos form on the slopes.

Massifs, composed of fine-grained soils with mixed-grained sands or with internal structures characterized by interstratification of sand, silt and clay are most susceptible to suffosion. High suffosion activity is typical for slopes of riverbanks, lakeshores and coastlines as well as around water reservoirs. Such slopes are located in the zones affected by surface water level changes and accompanied by sharp changes—increasing to critical values—in groundwater level gradients.

3 Suffosion Landslides. General Concepts

3.1 Formation Mechanism and Delineation Signs of Suffosion Landslides

“Suffosion landslides” or “landslides of suffosion genesis” are triggered by a reduction of soil strength characteristics at the base of a slope due to suffosion decompaction of the soil, or by destruction of suffosion cavity roofs and flanks of suffosion niches on a slope, followed by landsliding of material.

As such, suffosion landslides are displacements of saturated soil masses caused by suffosion and subsequent collapse of suffosion cavity sides. Such landslides develop regressively through successive cycles of collapse in suffosion cavities or niches due to loss of sand, sandy loam or clay loam through groundwater flow in the aquifer. Collapsed, saturated and unstructured masses move in the direction of the slope in the form of a flow (Fig. 1). This process often masks the primary genesis of the slope deformation, making it difficult to identify suffosion landslides.

Fig. 1
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A general scheme for the formation of a suffosion landslide. 1. Loam and sandy loam. 2. Saturated sands. 3. Clays. 4. Pre-slope relief. 5. Suffosion landslide. 6. A fan of fine material removal. 7. Landslide crack

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Suffosion landslides do not have a clearly defined sliding surface. Soil displacement occurs over a layer of heavily watered (i.e. saturated) sand. The landslide masses move impulsively by leaps, sometimes at a very high velocity, depending on the scale and speed of development of mechanical suffosion of sand particles at the base of the slope. Sub-vertical displacement often prevails over horizontal movement at the crown and head of suffosion landslides when underlying water-saturated sandy sediments have a significant thickness.

Several features indicate the leading role of suffosion in landslide formation. At the sites where suffosion landslides spread, removal fans composed of fine-grained sediment are often observed prograding at the base of the slope and over adjacent territory, whose contours often exceed the landslide boundaries. Another genetic characteristic of suffosion landslides is a large number of scarps of different sizes across the landslide body. These scarps form as a result of irregular settling of soil blocks into the roof of underlying suffosion cavities, and along the sides of suffosion depressions. Suffosion landslides are often elongated, horseshoe-shaped, or ∞-shaped with a constriction in the central part formed in the area of a breach of saturated soils from an underground suffosion cavity. The direction of the long axis of the landslide basin inherits the direction of the underground suffosion channel formed in the slope massif.

Hutchinson (1981) described suffosion-induced landslides across England from Cromer to Overstrand (Norfolk), in the Christchurch Bay coastal cliff (Hampshire), and in Newhaven (East Sussex). In these landslides, suffosion in aquifer sand interlayers enclosed in beds of clay, silty clay and silt caused overlying sand layers to collapse. Collapsed soils were crushed, then clay and sand were mixed with flowing groundwater to form mudslides. Hutchinson (1981) did not present results of in-situ measurements of the rate of development of such landslides, but noted that in Newhaven, according to a comparison of the topographic maps of 1898 and 1926, the rate of coastal retreat was 0.2 m per year. This rate was assumed to be consistent with the rate at which the sand bed collapsed under the action of suffosion.

3.2 Suffosion Landslides in Landslide Classification Systems

Suffosion landslides are specific in the nature of their development, and characterized by their displacement patterns. Therefore, in many landslide classification systems, where “suffosion landslides” are recognized as a separate type of displacement, they are classified as a group/class of “complex” or “composite” landslides.

Pavlov (1903) was the first to describe suffosion landslides as a separate type of slope displacement observed in the Volga River valley. Rodionov (1937) presented a scheme for the classification of landslide phenomena on the Black Sea coast of the Caucasus and distinguished sixteen landslide types united in three groups according to the structure of landslide-forming earth material. He included suffosion landslides in third group (special cases of displacement), distinguishing as a separate type of displacement the slope deformations caused by suffosion-induced removal of soil particles. Among the main causes of suffosion landslides, Rodionov (1937) indicated high content of dust particles and significant groundwater gradients. Later Rodionov (1939) proposed a division of suffosion landslides into three sub-types (Table 1). Klevtsov (1964), who studied landslides in the foothill areas of the Caucasus, proposed different sub-types of suffosion landslides formed where fine-grained “dusty sands” lay at the base of loess strata (Table 2).

Table 1 Sub-types of suffosion landslides after Rodionov (1939)
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Table 2 Sub-types of suffosion landslides after Klevtsov (1964)
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Hutchinson (1988) identified landslides caused by suffosion material removal at the base of a slope as “slides caused by seepage erosion” within complex slope movements, where as Tikhvinsky (1988) distinguished two sub-types caused by the removal of suffosion material at the base of a slope (Table 3). Most recently, Khomenko (2011) proposed three types of displacements to distinguish among suffosion landslides caused by suffosion removal of material (Table 4).

Table 3 Sub-types of suffosion landslides after Hutchinson (1988) and Tikhvinsky (1988)
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Table 4 Sub-types of suffosion landslides after Khomenko (2011)
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4 Suffosion Landslides in the European Part of Russia

Much of the European part of Russia is occupied by the East European Plain, framed to the south by the Caucasus Mountains and to the west by the Ural Mountains. The main areas of landslide activity in the Eastern European Plain are confined to the valleys of large rivers and their tributaries (Volga, Oka, Don, etc.) (Zerkal and Strom 2017). Suffosion landslides are most widespread in the middle and lower parts of the right (west, high) side of the Volga River valley. This is due to the peculiarities of the hydrogeological properties and structure of Quaternary deposits in the area, which are characterized by alternating horizons of unsaturated and saturated sands, loams and clays in the section (Fig. 2).

Fig. 2
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The European part of Russia—the research territory of suffosion landslides

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In the area of Quaternary glaciations, the section is formed by alternating glacial loams and fluvioglacial sands of different glaciation stages. Outside the area of glaciations, the structure of the section includes subaerial deposits (loess, elastic silt), alternating alluvial (of different-age terraces) and marine (different stages of transgression) sands, loams and clays.

4.1 Suffosion Landslides in the Volga River Valley

The first detailed descriptions of landslides caused by “friable sediments from springs flowing out of the mountain” date back to 1724, when the buildings of the Uspensky Monastery in Simbirsk (now Ulyanovsk) were deformed as a result of landslide displacement.

Active study of landslides on the slopes of the right (west, high) side in the middle and lower parts of the Volga River valley started at the beginning of the 20th Century and was connected with construction of the railway network in this region (Martin 1911; Vasilevsky 1929). One of the largest landslides occurred on July 12, 1941 on the bank of the Volga in the southern part of Stalingrad (now Volgograd). The landslide formed on a slope composed of Upper Pleistocene Khvalyn clays (about 20 m thick), underlain by Middle Quaternary Khazar sands. The landslide covered the entire thickness of the Khvalyn clays up to their base. The landslide, which lasted about 40 min, formed a depression 10–12 m deep, 250 m along the axis, and up to 275 m along the front. The basin exited to the Volga River through a relatively narrow neck 110–140 m wide. In the part of the landslide that advanced into the Volga River and in the half of the landslide basin adjacent to the bank, the displaced soil was a non-structural saturated mass (Cheprasov 1972; Tikhvinsky 1988). The upper part of the basin was filled with displaced blocks of Khvalyn clay. Researchers noted a decrease in the average thickness of 0.6 m in the Khazar sands beneath the landslide body, which is about one third less than the thickness of the Khazar sands outside the landslide. This reduced thickness can be seen as a consequence of their removal by suffosion during the preparation and development of the displacement.

In addition to the lower part of the Volga valley near Volgograd, suffosion landslides are widespread in the middle part of the Volga valley between Ulyanovsk (formerly Simbirsk) and Saratov (Cheprasov 1972; Rogozin and Dunaeva 1962; Rogozin and Kiseleva 1965; Tikhvinsky 1988) and in the piedmont areas of the Caucasus (Klevtsov 1964).

4.2 Suffosion Landslides in Moscow Region

A peculiarity of the geological structure of the Moscow region is the wide distribution of erosion-cut Quaternary fluvioglacial and alluvial-fluvioglacial silty sands interbedded with glacial clay loams (with a total thickness up to 35–40 m), overlying fine-grained silty sands with interlayers of loams and clays of Lower Cretaceous age (with a total thickness up to 50–55 m). As a rule, these strata contain groundwater horizons, including pressurized (artesian) groundwater. These hydrogeological condition are favorable for the development of suffosion and the formation of suffosion landslides (Fig. 3).

Fig. 3
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The suffosion niches at the “Karamyshevo” landslide site, left bank of the Moscow River valley, Moscow. A—the landslide deformations; B—the suffosion niche; C—the suffosion fan

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The “Vorobyovy Gory” landslide site, located on the right bank of the Moscow River valley, is one of the areas where landslide displacements are associated with active suffosion. In this section of the Moscow River valley, the right bank is up to 70 m high (Barykina et al. 2021). It is composed of Quaternary fluvioglacial sands interbedded with glacial clay loams that are underlain by Cretaceous sands with interlayers of loams and clay. Jurassic clay beds occur only at the base of the slope. In the middle and lower parts of the slope, numerous springs are observed as a consequence of groundwater discharge.

Landslide at “Vorobyovy Gory”site is a complex, multi-stage landslide. The volume of soil involved in the landslide deformation is estimated at 2 million m3 (Barykina et al. 2019). The study of landslide structure showed that the upper tier of landslide blocks is composed of Quaternary sands and loams, as well as Cretaceous sands. Often the landslide is marked by the absence of some horizons of Cretaceous sands, which may be due to their destruction by suffosion.

The Cretaceous-age sands that make up the slope on the “Vorobyovy Gory” site were investigated to determine suffosion stability. According to stratigraphic data, sand strata susceptible to suffosion include: (1) the Volgushinskaya formation (K1vlg), Vorokhobinskaya formation (K1vr), and Ikshinskaya formation (K1ik) (Aptian Stage, 113–121 Ma). (2) The Butovskaya formation (K1bt) (Barremian Stage, 121–131 Ma). (3) The Kotelnikovo formation (K1kt), Gremyachevskaya formation (K1gr), Savelievskaya formation (K1sv), and Dyakovskaya formation (K1dk) (Hauterivian Stage, 131–134 Ma). (4) The Kuntsev formation (K1kn) and the upper part of the Lopatin formation (K1lp2) (the Berriasian Stage, 139–145 Ma).

Quartz with varying degrees of ferruginization dominated the mineral composition of all studied sands. Sands were diverse and dissimilar in their grain-size composition. The results of the grain-size analysis (Fig. 4) show that sands of the Volgushinskaya, Savelievskaya, and Dyakovskaya formations are unimodal, while those of the Ikshinskaya and Gremyachevskaya formations are bimodal, which is typical of sands of marine genesis. The dominant particle dimension in almost all the formations is 0.10–0.25 mm, except for the Volgushinskaya formation (0.25–0.50 mm).

Fig. 4
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Integral graphs of the granulometric composition of the studied soils

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Suffosion-stability of Cretaceous sands was evaluated in two ways: (1) by calculation; and (2) by using physical simulation. To determine the suffosion properties of sands, the computational method used the coefficient of heterogeneity of the grain size distribution of soils, calculated as the ratio of fractions containing 60% and 10% (Kn = d60/d10) (cf. Istomina, 1957). Suffosion-resistant soils include sands with Kn < 10. Suffosion-unstable are sands with Kn > 20. Soils in the transition region can be both suffosion-stable and suffosion-unstable.

The next step was to estimate the size of particles that can move through soil pore spaces under the influence of filtering groundwater flow. For the studied sands, values obtained for the maximum particle sizes that can be moved by the filtration flow as a result of suffosion—ranged from 0.019 mm for the Volgushinskaya formation, 0.131 mm for the Vorokhobinskaya formation, to 0.075 mm for the Ikshinskaya formation, and for the Gremyachevskaya formation, 0.057 mm. The homogeneous sands of the Butovskaya formation proved to be suffosion-resistant (Kn > 10.8).

The determination of suffosion stability of the sands in the physical model confirmed the results of the simulation. The least homogeneous Vorokhobinskii sand was the least resistant to suffosion: the intensity of sand particle removal was 0.000104 g/cm3*s. The same values for suffosion particle removal were obtained for sands of the Ikshinskaya formation (0.000104 g/cm3*s). A slightly lower intensity of particle removal was characteristic of the Volgushinskaya formation sands (0.000094 g/cm3*s) and sands of the Gremyachevskaya formation (0.000083 g/cm3*s). The sizes of grains removed by the filtration flow in the modeling of suffosion mostly coincided with the sizes obtained from calculations.

These results confirm the susceptibility of Cretaceous sands in the “Vorobyovy Gory” area to suffosion processes, which in turn, influences the development of landslide processes. Sands of the Butovskaya formation in the upper tier landslide blocks have a thickness close to that of undisturbed bedding. Sands of other formations in the blocks of the upper landslide tier are characterized by a decrease in thickness, completely disappearing in places.

Kotlov (1962) earlier suggested a suffosion genesis for some of the landslide deformations in the “Vorobyovy Gory” area: “Natural suffosion is most widely developed on the right (west) high bank of the Moscow River in places where the groundwater horizon, enclosed in Jurassic, Cretaceous and Quaternary fluvioglacial sands, outcrops at surface. On the “Leninskie Gory” (now the “Vorobyovy Gory”) one can see hundreds of springs, most of which serve as foci of suffosion. Currently, there are several such sites on the slope of the “Vorobyovy Gory” (Fig. 5).

Fig. 5
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Mid-slope suffosion niche at the “Vorobyovy Gory” site. A—the suffosion niche

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Suffosion outflow is promoted by significant hydrodynamic pressure during filtration flow. In the central part of the “Vorobyovy Gory” (Kotlov 1962), the drop in groundwater level across the landslide body is 20.7 m over 330 m, with the average values of i = 0.066. At a number of sites, the groundwater level slope increases up to an i = 0.08–0.1 and more. A localized suffosion fan of finely dispersed earth material is clearly seen in the middle and lower parts of the slope.

Estimates of the amount of material eroded showed that the volume (of Cretaceous sand loss) was from 50 to 100 mg/l (Kotlov 1962). Considering groundwater discharge is confined to Lower Cretaceous and Quaternary fluvioglacial sands at the base of slope with the addition of more outlets (sources), removal of earth material by suffosion significant in scale. Over geological time, this has led to the formation of suffosion cavities and niches on the slope, that further contributed to active landslide deformations.

5 Conclusion

Suffosion is the process of washing out and mechanical removal of small particles from saturated soils under the influence of filtrating groundwater flow. Suffosion particle removal rate is controlled by the intensity of the filtering groundwater flow and head gradient. Destruction of soils by suffosion can result in their de-consolidation at the base of a slope, or in the formation of underground cavities in massifs, niches and grottos, which undoubtedly affect the stability of slopes.

Suffosion is only rarely considered in the analysis of landslide activity. Suffosion landslides are described in regions with alternating of horizons of unsaturated and saturated sands, loams and clays in section.

Displacement of saturated soil masses occurs as a result of sediment removal and the subsequent collapse of suffosion niche sides associated with it. Suffosion landslides develop regressively in the form of successive cycles of cavity or niche collapse. Resulting unstructured masses move (as flows) in the direction of the surface slope. At the landslide head, sub-vertical movements often prevail over horizontal movements when displacement occurs.

Signs of suffosion landslide genesis are: (1) the presence removal fans comprising fine material at the base of slope and extending into adjacent territory the contours of which often exceed the landslide boundaries. (2) The presence of a large number of ledges of different size on the surface of the landslide body, formed by irregular subsidence of soil blocks into the roof of suffosion cavities and on the sides of the suffosion niches. Often, when compared to the undisturbed part of the slope, a significant decrease the thickness of suffosion-unstable sandy soils in the landslide body is recorded.

Suffosion landslides are often elongated, horseshoe-shaped, or ∞-shaped with a narrowing in the central part formed in the area of breakthrough of saturated soils from an underground suffosion cavity. The long axis of the suffosion landslide is inherited from the direction of the underground suffosion channel formed in the slope massif.

Thus, suffosion landslides are specific in their development and are characterized by peculiarities of their displacement. Therefore, in many landslide classification systems, where “suffosion landslides” are recognized as a separate type of displacement, they are referred to a group/class of “complex” or “composite” landslides. Suffosion landslides are divided into several sub-types according to the peculiarities of formation and displacement (Tables 1, 2, 3 and 4).

One of the regions where suffosion landslides are widespread is the East European Plain, which occupies the European part of Russia. The main areas of occurrence of suffosion landslides are confined to the valleys of large rivers and their tributaries (Volga, Oka, Don, etc.), where their volume can be up to several million m3 (e.g., the landslide of July 12, 1941 on the bank of the Volga). Suffosion landslides are also widespread in the Moscow area. The study of the landslide at “Vorobyovy Gory” site (a complex, multi-storeyed landslide) has shown that suffosion plays a significant role in the formation of the upper tier of landslide blocks. Our simulation and physical modeling confirm the susceptibility of sandy earth materials to suffosion processes.

In conclusions, future development of landslide classification systems should pay attention to suffosion landslides as a separate type of displacement in regions underlain by unconsolidated fine-grained clastic bedrock and surface earth materials.