Keywords

1 Introduction

Landslide phenomena are one of the most widespread and dangerous for humans, biota and technosphere of natural and natural-man-made exogenous processes.

In the Moscow region, block landslides are widely developed, the displacement of which is associated with the Upper Jurassic clays. They are, as a rule, large in area and with a great depth of capture, which leads to the possibility of serious damage from their development and greatly complicates stabilizing measures. Landslides are characterized by very long (tens and hundreds of years) periods of slow (1–30 cm/year) deformations, alternating with short periods of their activation with displacement values from several to tens of meters [1,2,3,4,5,6].

In Moscow, many areas of the development of these landslides are also important recreation areas of the metropolis (the landslide areas “Vorobyovy Gory”, “Kolomenskoye”, “Fili-Kuntsevo” are specially protected natural areas), and the activation of landslides sharply worsens the recreational qualities of landscapes. In addition, in some areas, these landslide processes can lead to the destruction of large sewers and storage areas for highly toxic waste, which will lead to catastrophic environmental consequences.

The unevenness of the relief of the rocky base, apparently, can affect the development of landslide processes, changing the distribution of stresses in the bottom part of dispersed soils and, in some cases, controlling the maximum possible depth and configuration of the displacement zone in the overlying strata. Partially conformal bedding of individual layers at the bottom of the dispersed strata, including the weakest ones, can, with appropriate inclinations and orientation, also contribute to landslide formation or control the direction of secondary displacements of already slidden bodies.

The landslide area “Vorobyovy Gory” is active, large (its length along the Moskva River reaches 3 km, the width along the dip of the slope, including the underwater part, is up to 450 m, the height from the edge to the river's water level is up to 70 m, to the displacement zone – 60–100 m), consists of several landslide systems. The central part of Vorobyovy Gory is a unique landslide object, where, for almost a century, about 150 wells were drilled in a relatively small area, which opened the very bottom of the Jurassic strata, and several hundred smaller wells. Of these, more than 13 wells are equipped for directional survey. More than 60 ground benchmarks have been installed with an observation period from 10–12 to 40 years and more. Thus, this territory is a natural testing ground for studying and decoding the history of development, mechanism, dynamics and kinematics of large block landslides in dispersed soils.

The patterns identified in this area can then be extended to other areas of the development of large frontal block landslides.

2 Buried Paleotopography of the Area

In the opinion of almost all researchers, the main displacements of the Moscow landslides occurred in the deposits of the predominantly Oxfordian stage of the Upper Jurassic, in some cases - in the overlying Upper Jurassic and Middle Jurassic deposits of the Callovian stage. At the Vorobyovy Gory area, within these strata, the minimum values of the angles of internal friction and specific adhesion when tested by the single-plane cut method in the “die-on-die” option are φ = 4°, C = 16 kPa and are recorded in the soils of the lower part of the Oxford Stage and the transition to the Callovian stage (Moscow region, Ratkovskaya and Podosinkovskaya formations) [7]. Earlier, in the course of work carried out by the Institute of Geoecology named after E.M. Sergeev of the RAS by order of the State Budgetary Institution “Mosecomonitoring”, the lowest strength parameters were also noted for the soils of the Ratkovskaya formation - parameters of resistance to cut “die-on-die” φ = 4°, C = 11 kPa (according to the report of the State Budgetary Institution “Mosecomonitoring” for the general customer JSC “Geocenter-Moscow”, prepared in 2008 “Conducting local monitoring of geoecological processes in the Vorobyovy Gory area”).

To test the hypotheses set forth in the introduction of this paper, schematic maps of the top of the Callovian stage of the middle Jurassic and Carboniferous systems were compiled (Figs. 1 and 2). It makes no sense to analyze the position of the top of Oxfordian strata and deposits lying even higher on a landslide slope in this way, because it is already controlled by the displacements of the landslide blocks.

When compiling the maps shown in Figs. 1 and 2, 131 deep wells were used, which uncovered deposits of the Callovian stage, 127 of them also penetrated the Carboniferous deposits. Both maps show a significant indented surface of deposits in valley-like depressions oriented generally from north to south.

Fig. 1.
figure 1

Map of the top of Callovian deposits of the Middle Jurassic system. Legend: 1 – contours of the topography of the Callovian deposits and its absolute elevation; 2 – wells with a roof mark, m; 3 – boundaries of the road network; 4 – the edge of the Moskva River.

Full size image
Fig. 2.
figure 2

Map of the top of the Carboniferous system deposits. Legend: 1 – contours of the relief of the top of Carboniferous deposits and its absolute elevation; 2 – wells with a top mark, m; 3 – wells in which pebbles and gravel above the limestone top are noted; 4 – boundaries of the road network; 5 – the edge of the Moskva River.

Full size image

Attention is drawn to a certain closeness of the topography of the Callovian deposits of the Middle Jurassic and the top of the Carboniferous deposits. The reasons for such a correspondence can be both neotectonic processes and synformal formation of the Callovian stratum, covering the Carboniferous deposits like a cape. It is also clearly seen that there are no deep paleo-incisions in the riverine territory of not only pre-Jurassic, but also pre-Quaternary and pre-Holocene ages. In general, the territory of the Vorobyovy Gory area is confined to a very gentle slope (0.5–1.2°) of the pre-Jurassic paleovalley, the thalweg of which is located much further south (Figs. 3 and 4). This slope is complicated by local forms of neotectonic and erosional genesis and, possibly, even shallower in terms of steep depressions of karst origin. Thus, in the area of Metromost near the embankment, sharp local depressions in the top of limestones and, in some cases, deposits of the Callovian stage were recorded at three sites (see Figs. 1 and 2).

Fig. 3.
figure 3

Geomorphological scheme of the Luzhnetskaya bend area and the position of the section 1-1 [8] with changes.

Full size image
Fig. 4.
figure 4

Schematic geological section along line 1-1 [8] with changes.

Full size image

The angles of inclination of the surface of Callovian deposits, associated with the local relief, apparently do not exceed several degrees with a prevailing steepness of 1–3°, but it should be borne in mind that such angles are not much less than the angles of internal friction in the weakest horizons of the Jurassic clays, as indicated above. This leads to a very large influence of these slopes on the calculated stability factors and, accordingly, the possibility of landslide displacements.

3 Displacements of Benchmarks in a Landslide Area

Further, the vectors of displacements of ground benchmarks were calculated for the central and eastern sections of Vorobyovy Gory (Figs. 5 and 6) according to the data of regime observations carried out by State Environmental Budgetary Institution “Mosecomonitoring” and State Unitary Enterprise “Mosgorgeotrest” since 2008. With a very large difference in the vectors of displacement of the benchmarks, two main features draw attention to themselves: firstly, the vectors are grouped into explicit systems in magnitude and direction, and secondly, these directions are very different from traditional representations. In each landslide system, the direction of movement of benchmarks is traced to certain conditional focal points, and these directions often do not coincide with the direction of the relief dip.

Fig. 5.
figure 5

Vectors of planned displacements of soil deformation benchmarks in the central part of the “Vorobyovy Gory” area. Legend: 1 – soil deformation benchmarks and their numbers; 2, 3 – vectors of total displacement (2 – for 2008–2017, 3 – for 2008–2019); 4 – border of the zone of abnormal displacements of benchmarks; 5 – boundaries of the road network.

Full size image
Fig. 6.
figure 6

Location of ground benchmarks and vectors of their planned displacements for 2008---2016 in the eastern part of the landslide area “Vorobyovy Gory”. Legend: 1 – ground benchmarks of SEBI “Mosecominitoring”, 2 – ground benchmarks of SUE “Mosgorgeotrest” [8, 9].

Full size image

The directions of displacement of benchmarks, in the author's opinion, are explained not only by the inclination of the weakest layers of the section, but also by other factors: the presence of a displacement zone developed by previous movements, incomplete coincidence of the conditional weak horizon with the top of Callovian deposits, differences in hydrogeological conditions and pore pressures, mutual dynamics of displacement of individual blocks, in which the displacement of more active blocks reduces the stresses at the boundaries with neighboring blocks, allowing them to move towards lower stresses, etc.

4 Territory Schematization

On the basis of a set of data, a scheme was drawn up for dividing the entire landslide area “Vorobyovy Gory” into separate landslide systems (Fig. 7). The scheme is based on the analysis of the general structure of landslide systems in different areas, the morphology of landslide forms, the direction and magnitude of modern displacements of ground benchmarks and the topography of the top of rocky soils. The concept of “landslide system” was first used by V.N. Slavyanov in 1951. The features of landslide systems were analyzed in detail by N.F. Petrov. The material components of the landslide system are landslide bodies [10]. In this case, the real elements are blocks, i.e. bodies, all points of which are characterized by a similar type of movement and deformational behavior. Accordingly, in the landslide systems under consideration, the history of development and the nature of modern displacements are different from the adjacent areas. Currently, the most active displacements are developing in landslide systems C and D. It should be noted that the D2 system was most active several decades ago.

Fig. 7.
figure 7

Scheme of dismemberment of the “Vorobyovy Gory” landslide area into landslide systems. Explanation of designations – in the text. A plan of the Moscow city 1937–1949 from the Retromap open electronic resource was used as a topographic base, which is the only one of sufficiently accurate materials that reflects the landslide relief with minimal man-made changes.

Full size image

The noted features of deformations, apparently, must be taken into account in the methods used for assessing stability. Initially, calculations of the stability of landslide and landslide-prone slopes were carried out everywhere in a flat setting with the orientation of the calculated profiles, as a rule, strictly along the relief dip.

Subsequently, the practice of calculating stability began to include three-dimensional calculations, in which initially it was meant mainly to take into account the additional shear resistance on the lateral sides of the landslide body. For frontal landslides, this leads to an increase in the stability coefficient by several percent. Then more and more complex programs of three-dimensional calculations began to be used more actively, and an established to some extent opinion was formed that volumetric calculations show a stability coefficient that is 20–30% higher than when using two-dimensional models. In reality, this is not always the case. The difference in the stability coefficients can be minimal, under certain conditions, perhaps even Кy3d < Кy2d. The mechanism of rock mass destruction can be quite complex, including progressive destruction from a specific focus, “pushing” of the central part by unstable sides (which causes deformations to “focal” points), etc.

However, there is no doubt that at the stages of the main displacement, the blocks move in a direction much closer to the perpendicular to the course of the displacement basis. Thus, before the main displacement, apparently, there are changes in the vectors in magnitude and direction (by sub-perpendicular to the course of the displacement basis). Perhaps this will provide some clue to predicting the main displacements, and will also help to reveal whether the landslide process in general is attenuating or progressing.

5 Conclusion

A significant irregularity of the top of deposits of both the Carboniferous and the Middle Jurassic was revealed in the landslide area “Vorobyovy Gory”, which also determines the inclined occurrence of weak horizons contained in the Upper Jurassic strata. At insignificant angles of internal friction, the slopes of these soil horizons, even in the first degrees, have a significant effect on the stability of the massifs and the direction of movement. Ground benchmarks at the current stage of slow deformations in each landslide cirque are displaced in the direction of certain “focal” points. There are also areas of landslide systems that are displaced “abnormally” parallel to the river, generally confined to the slopes of buried paleo-incisions.

To predict large activations of such landslides, it is likely that not only an increase in the rates of displacement of benchmarks can be used, i.e. scalar values, but also a change in the direction of displacement, i.e. analysis of the restructuring of the entire pattern of displacement vectors.