1 Introduction

This article reveals the developmental features of gravitational and cryogenic processes in tectonically active zones, connected with natural and anthropogenic influences on the environment within the Chulmanskoye Plateau, South Yakutia, Russia. Analysis of field and laboratory research on the proposed topic, started in 2015, highlights the problems facing rapid infrastructural development within the South Yakutia region. The design and building of large manufacture facilities, gas and oil pipelines, federal railways and highways face severe geological, tectonic, geocryological and hydrogeological conditions such as landslide formation, icing and stone run formation, and cryogenic disintegration of rocks (Parise and Jibson, 2000; Geertsema et al., 2009 ; Blais-Stevens et al., 2010 ; Grib et al., 2010 ; Zheng et al., 2012 ; Gagarin et al., 2016 ). Modern climate change as well as neotectonic earth's crust movement, caused by seismic events, which are sometimes of high intensity, contribute greatly to the development of these processes (Delemen' and Konstantinova, 2009). Thereby, evaluation of change in permafrost and hydrogeological conditions of the South Yakutia region is extremely important. Thus, it is necessary to create a complex monitoring network for study of cryolithozone evolution and exogenous processes in connection with infrastructural development.

The authors of this article provide results of their observations on cryogenic and gravitational processes along linear infrastructure: oil pipeline, federal railway and highway within South Yakutia region.

The study sites are located in Neryungrinsky District of Sakha Republic (Yakutia), Russia. The nearest large settlement is Chulman, which is 50 km to the south of the highway Lena.

Cryogenic processes in the territory of South Yakutia are widespread for this reason, this is a subject for future study. In this article we focus mainly on gravitational processes in particular landslides. Landslide processes within the Chulmanskoye Plateau of South Yakutia are rare due to predominant development of hard rock and low thickness of loose deposits (Ospennikov et al., 1980 ). According to the work of Zerkal and Fomenko (2016), landslides in hard rock should be taken as a separate class of landslide processes. Due to the influence of tectonic forces, certain sites develop slope cracks, the development of which can lead to future landslides (Grib et al., 2010 ; Laperdin et al., 2011 ; Gagarin et al., 2016 ).

To understand the genesis and mechanism of the processes described in this article, it is necessary to provide a description of geomorphological, geological, tectonic, permafrost and hydrogeological conditions of the South Yakutia territory.

The Geomorphological study site is located within the central part of the Chulmanskoye Plateau (Figure 1), within the boundaries of a depression of the same name (Southern Yakutia, 1975). Modern tectonic and denudation processes determine relief, among which a range of transverse uplifts and depressions can be distinguished. Thus, a highway segment under study is located in the zone of development of Ungra-Duraiskaya depression (Fotiyev, 1965), elongated approximately in meridional direction. Watersheds have heights of 780~900 m and represent flat and flat-convex surfaces with widths of approximately 1.0~1.5 km. Ledges of higher parts of the plateau, delineating depressions, ledges and steps in the depressions, partly interbedding, and likely modern are of block origin (South Yakulia, 1975). The valleys have incision depths of 150~200 to 300 m and slopes with steep structural ledges. Genetically relief of the territory under study can be divided into structural-denudational, denudational and alluvial (South Yakutia, 1975). Structural-denudational surfaces represent dissected horizontal and sub-horizontal bedding of Jurassic rocks. These surfaces are characterized by sub-horizontal and gently sloping area of mountains and hills with absolute altitudes of 800~900 m and relative elevations of 50~300 m. Denudational relief represents denudational-erosional slopes and ledges of river valleys with inclinations of 25°~30° (South Yakulia, 1975). Alluvial relief appears in a form of alluvial floodplain and fluvial terraces.

The described area is characterized by maximal development of cryogenic processes and related superimposed relief forms, resulting from climate conditions of the territory and widespread development of permafrost.

Within the Duray and Chulmakan river basins, side river erosion develops and forms rudimentary erosive hollows. These are concave and elongated relief forms, originating on the steep erosive slopes predominantly due to meltwaters. Block streams appear during the process of frost-shattered crack formation and slow slippage down the slopes. Block streams are composed of block material and located on the slopes of watersheds. Icing formation is widespread. The relief formed by the icing process is reduced to the development of icing glades, composed of detritus material. Typically woody vegetation is absent and suppressed grass covers icing valley.

The rocks, building up the Chulmanskoye Plateau in the region under study, are represented by gently-sloping alternating formations of sandstone, siltstone and mudstone with coal stratums of Yukhtinskaya and Duraiskaya formations of the Lower and Middle Jurassic, respectively (South Yakulia, 1975).

In tectonic terms, the research site is situated in the northern part of the Chulmanskaya depression, structurally of the Predstanovoy (South Yakutia) deflection, which refers to the Mesozoic substage of the platform mantle of the Aldano-Stanovoy shield.

Formation of the Chulmanskaya depression began in the Mesozoic Era as a result of thrust fault of Archaean rocks, referred to as the Stanovoy block of the Aldano-Stanovoy shield (South Yakulia, 1975). Thus, formation of foreland basin was accompanied by accumulation of heavy carboniferous strata. Small epeirogenic movements of the crust took place in the southern part (Muostahskaya zone) of this structure, where Mesozoic deposits are more crumpled into folds and cut through by numerous faults in northwest and latitudinal directions. The northern part of the Chulmanskaya depression (Chulmanskaya zone) is a zone of flat dislocations. The key sites of our research are located within the last zone.

Modern tectonic movements of the crust are evidenced by steep faults of lengthwise river profile, characterizing the differential block movements (Laperdin et al., 2011 ). Convincing proof of modern tectonics is the abundance of mineral and hot springs near the faults, such as sulphurous springs in the lower and middle reaches of the Duray River.

Hydrogeological conditions of the territory have difficult composition, on the formation of which tectonic, exogenous processes and also dynamics of cryolithozone have significant influence.

According to the map of permafrost and hydrogeological zoning of Eastern Siberia, the study site is located within the Chulmansky artesian basin (Ivanova et al., 1984 ). It is difficult to classify aquiferous by the basis age complexes due to high rock fracturing, and absence of sustained regional aquiclude which divides formations of different ages (Fotiyev, 1965). The most convenient classification characteristic of groundwater is water exchange intensity. In this way, within the Chulmansky artesian basin the zones of intensive and increasingly hampered water exchange can be delineated.

The lower boundary of the zone of intensive water exchange is set conventionally on the level of absolute altitudes of regional drainage, e.g., Aldan, Ungra and Timpton rivers at 270~600 m. The factor complicating water exchange is permafrost. This influence is reflected in the conditions of supply, regime and discharge of groundwater. Interaction of permafrost with groundwater mostly defines the temperature of the second, which is 6~8 °С in the Chulmanskaya depression (South Yakutia, 1975). The processes on the key sites, which are described in this work, are connected with development of the zone of free water exchange.

In the limits of the Chulmanskoye Plateau permafrost has insular distribution on the watersheds, discontinuous and continuous in the valleys of rivers and brooks (South Yakutia, 1975). Geocryological conditions of the given territory are defined by conditions such as terrain roughness, slope exposition, fracturing of rocks, and presence of groundwater springs.

In the Duray River Valley, permafrost has continuous distribution associated with significant erosion incision (up to 300 m). Permafrost thickness can reach 100~150 m and temperatures can reach to −2 °С. According to the literature, within the watersheds of Duray, Nizhnyaya Talumas and other rivers, permafrost can have discontinuous distribution (South Yakutia, 1975). Taliks occupy 30% of the territory and predominantly are of snow-radiation type.

Within the framework of our research, two key sites were chosen. The first site is located in the upper reach of the Duray River, where the river crosses the highway Lena. The second site is confined to the lower reach of the Chulmakan River and is located 400 meters to the east of the ESPO highway (Figure 1).

2 Materials and methods

In the framework of this research, complex studies were conducted, which included analysis of geological, geocryological, hydrogeological information and application of GIS technologies (Harris et al., 2001 ). Onsite field work included research on quantitative assessment of landslide movement, identification of heterogeneity in the geological environment by geophysical methods, sampling of groundwater springs, and aerial survey of landscapes.

Within the polygons, where landslide processes were studied (Duray and Chulmakan river valleys), a network of reference benchmarks were organized. Along the perimeter of the landslides, five fixed observation points were established by means of a laser rangefinder. For the Duray River site, during the summer of 2016, 3 iterations of measurements were carried out, and 1 iteration for the Chulmakan River site. Furthermore, we also conducted chemical sampling of groundwaters which are drained by joint fissures for each site. For the Duray River site water sampling was conducted in July of 2015, and March, July and September of 2016. For the Chulmakan River site, water sampling was carried out only once, in July. For estimation of geometric parameters of landslide bodies, aerial surveys were performed using a quadcopter, and digital imagery processing. Chemical analyses were carried out in the Laboratory of Permafrost Groundwater and Geochemistry of the Melnikov Permafrost Institute (Siberian Branch, Russian Academy of Sciences).

Application of Ground Penetration Radar (GPR) research in the study of landslides is described in Kurlenya et al. (2016) . Four profiles were studied at the Duray River site. Certified GPR with central frequency of 300 MHz and antenna unit ABDL Triton (Omel'yanenko, 2012, 2013) with central frequency of 50 mHz (LLC LogiS-Geoteh) were used. GPR data were processed with GeoScan-32 software. For referencing measured data to the real depths of the section, a parameter of permittivity was set to 6, which corresponds to rocks susceptible to fracturing.

Electrical sounding was conducted for one profile, the length of which amounted to 155 m. The profile is situated along the old highway Lena, located 20 m up slope compared to the level of modern highway Lena, and stretched in a southeastern direction. Field tests were carried out by means of BIKS equipment, produced by design engineering bureau of Seysmicheskoye Priborostroyeniye (Modin et al., 2016 ). For spatial referencing, a Garmin 70 GPS receiver was used.

Researchers using the electrical sounding method rely on dipolar capacitive lines (Nakhabtsev et al., 1985 ; Douma et al., 1994 ; Kuras et al., 2006 ; Glazunov and Burlutskiy, 2014). In our study, 10 benchmarks were installed at 5 m intervals along the joint fissure and west boundary of the landslide (Figure 2b). On every benchmark receiving and producing dipoles were installed. Measurements were acquired by dipole and axial installation with different distances between centers of dipoles. Fieldwork parameters are provided in Table 1. After the acquired measurement at the first benchmark, installation was moved 5 m to the next benchmark. The choice of geometry was preceded by experimental work, during which it was set at a maximum possible distance between the centers of dipoles due to evaluation of the signal/noise relationship. Maximum span was chosen when the relationship equaled 3.

Processing of the gathered data was produced by ZondRes2D software (Kaminsky, 2012), which allows inversion of apparent electrical resistance into values of the true resistivity of rocks of which a geoelectrical profile can be obtained.

Aerial images taken by the Phantom 3 Adv. quadcopter were analyzed to characterize the landscape environment. Photogrammetric processing of these images was accomplished by Agisoft Photoscan Professional software. This software uses a set of classical photogrammetric methods (Lobanov, 1984) and computer vision Structure from Motion (SfM) technique (Lucieer et al., 2014 ). Entirely automated technology allows one to obtain orthophoto plans, digital terrain models (DTM) and 3D models of objects from imagery and computed coordinates. The creation of orthophoto plans and DTM is based on a dense cloud point, which is obtained during the first steps of data processing.

3 Discussion and conclusions

Field work at the Duray River site began in 2015 where boundaries of the site are connected with the highway Lena. Back in 2012 road services registered a landslide onto the road along the right slope of the Duray River Valley. This landside was due to seismic events which occurred in 2011 with M_s=6.1~300 km to the south from the site as well as mining projects in association with federal road construction, all of which resulted in a significant increase in the gradient of slope. As a result, the drainage of groundwater occurred in the lower part of the slope along the perimeter of the slumping massive rocks. This landslide is related to the slope of the southwest exposure and is represented by two blocks, allowing groundwater discharge into the base of the lower block along the whole perimeter. Absolute altitudes of the landslide body is 690~750 m. It should be noted that joint fissures of the top block are higher up the slope in a northwestern direction, coming out of the boundaries of the landslide body. This feature indicates the seismic nature of the landslide. Interpretation of aerial and satellite imagery also points to the confinement of the joint fissures to the lineament of the southeast strike (Figures 1, 2).

By the authors in 2015 a geological description of outcrops exposed as a result of road construction was acquired. Within the range of absolute altitudes of 690~750 m, the strata consist of interbedding of light grey sandstone, mudstone, and fractured siltstone with thin lenses of coal of the Lower Jurassic, Yukhtinskaya Formation (Figure 3). Interpretation of satellite imagery and geological map analysis (website www.vsegei.com; scale 1:200,000) allowed us to determine that the boundary of the Lower Jurassic and Durayskaya Formation of the Middle Jurassic passes through the absolute altitude of 850 m, which is 100 m higher than the location of the opening mode crack. It should be noted that slope angle of the strata is northwest, i.e., turned towards the slope. This geological structure is not favorable for the development of landslide processes. Hence, this fact confirms the seismic origin of the development of the opening mode crack.

Although permafrost has a continuous distribution in the Duray River Valley, there are large patches of pine forest with scattered larch within the south slope, as well as in the watershed. According to permafrost research at South Yakutia (1975) similar landscape conditions feature either deep seasonal thawing of rocks, or talik development, as evidenced by year round groundwater runoff, discharged at the foot of the landslide body. The opposite northern bank of the Duray River Valley experiences extreme cold temperature around −1.1 °C in the deep of zero annual amplitude. Also, there is a large patch of larch which indicates such temperatures. Thus, in borehole #1~#15 (Figure 4), located at the foot of the northern slope, in September of 2015 the depth of seasonal thawing was 1.3 m, and the temperature of permafrost was −1.1 °С.

Sampling of groundwater springs, found at the foot of the landslide body, allowed us to analyze the macro-component composition. The variability of chemical composition of water was noted, which depends on time of the year. During the spring, water is classified to the sulphate-bicarbonate calcium–magnesium class (Figure 5) with mineralization from 55 to 90 mg/L and pH 6.7~7.0. During the summer, there is an apparent dilution of groundwater, by infiltrating precipitation and snowmelt water. Mineralization decreases to 30~32 mg/L, with a bicarbonate calcium composition, and pH 6.6~6.8. Also during the spring, at the time of maximum frost penetration of rocks, the appearance of ground water pressure in the form of ascending springs is noted. According to 2015 data, the total production rate of a spring in the water-critical period amounted to 43 L/s. Nevertheless, the ditch, constructed by road services alongside the highway, fully holds the forming icing, excluding its development within the limits of the roadbed. In February to March of 2017, after site inspection of the highway Lena roadside, large clumps of icing were pulled out by road services from the nearby ditch (Figure 2a). At the time of the inspection, this drainage ditch was nearly filled with icing, which indicates continuous icing formation. The timely excavations by road services have temporarily prevented the spreading of icing onto the roadbed. As a result, the calculation and comparison of icing volume for 2016 and 2017 could not be computed. Nevertheless, the reason for such intensive icing formation can be explained by groundwater regime. According to test results of the chemical composition of groundwater springs at the foot of the landslide body, their relation to the zone of free water exchange was determined. Water supply of this type of aquifer is due primarily to infiltration of precipitation and snowmelt water. Figure 6 represents the graph of precipitation distribution for the period 2014~2016, based on data from the meteorological station in Chulman (website http://rp5.ru, 2017). Most of the aquifers water supply occurs during the summer, and maximum precipitation occurs in the second half of summer and autumn (South Yakutia, 1975). According to data from the meteorological station during the summer of 2015, 234.5 mm of precipitation fell, while in the summer of 2016, 568.9 mm, which is 142% higher. Thus, the process of icing formation in the Duray River Valley site can be dangerous for the stability of the highway Lena. The intensity of these processes is mostly determined by water abundance in aquifers, which in turn depends on the amount of fallen precipitation. An important impact on the stability of the landslide slope is made by cryogenic disintegration of rocks, formed within the zone of groundwater discharge during shoulder season. High fracturing of Yukhtinskaya sandstone, mudstone and siltstone contributes to intensive cryogenic weathering due to splitting influence of discharged groundwater when freezing. This fact contributes to additional loss of rocks stability within the base of the landslide body.

For quantitative estimation of landslide movement, a network of observation benchmarks were installed (Figure 2b). According to measurement results of the control points during July and September of 2016, the movement down slope of the upper block was 2~5 cm, and for the lower block it was up to 50 cm (Table 2).

In order to prove the possible application of geophysical methods for revealing heterogeneity of a geological environment, which also allow specifying mechanisms of landslide formation for a given site, we performed the method of electrical sounding and GPR at our sites.

Electrical sounding was conducted for one profile at the Duray River site (Figure 7). The depth of the studied geoelectrical section was 22 m. It was impossible to reach greater depths due to a weak generator. The section is characterized by the following values of resistivity: minimal values of 12~40 Ω·m were fixed in the points PK-70~PK-120 at depths of 0.5~3.0 m; maximal values of 1,100~1,200 Ω·m were fixed in the points PK-20~PK-25 and PK-75~PK-80 at depths of 10~20 m. In the geoelectrical section three blocks of increased resistivity values can be clearly distinguished (more than 500 Ω·m): PK-15~PK35, PK-60~PK95 and PK-112~PK-132. These blocks are divided by areas of low resistivity (from 200 to 400 Ω·m). In the right part of the section from PK-100 to PK-155 there is an area of low resistivity (less than 200 Ω·m) with a power from 1 to 22 m. It is impossible to provide lithological referencing and condition of the rocks due to the absence of a borehole at the study site. During data interpretation, three areas (Figure 8) can be distinguished in the section: increased resistivity values (more than 400 Ω·m); decreased resistivity values (200~400 Ω·m); low resistivity values (less than 200 Ω·m). The first area is related to sites of poorly compressed moist rocks (areas of high resistances), divided by weakened zones (fractures), probably water-saturated (the second area), which is evidenced by springs, located downslope. In the right part of the section (Figure 8) there is a third area of decompressed rocks (zone of low resistances). The last area is probably the edge of the landslide body.

During the 2015 field work, four GPR profiles were set up (Figure 7). As a result of data interpretation from geoelectric profiles, low-frequency areas were identified, indicating zones of increased wetness, possibly related to the filtration zones of groundwater (Figure 9). For detection of the geometry of high moisture zones some GPR sections were plotted taking into account the relief, which in turn was obtained from tachometric survey data. The profile section with zones of increased moisture is related to the lower part of the landslide. The depth of the upper boundary of the watered zone was impossible to define, as velocity characteristics of electromagnetic wave propagation in the rocks had a wide range of values. The obtained data show that the width of watered zones reaches 40 m, the boundary of which, in the projection on the surface, coincides with the boundary of the landslide body. Primary conclusions allow for an assumption that highly fractured soils of high moisture content can become a probable plane for sliding of the landslide body within the study site.

The results obtained by the two aforementioned methods for the Duray River site profile show the presence of water saturated zones on the same places, which indicates the reliability of the information received. Future geophysical research is planned, with the use of special equipment, in order to define the geometry of the landslide body.

In 2009, joint fissure formations were discovered at the intersection of right slope of the Chulmakan River Valley close to ESPO route (Grib et al., 2010 ; Imayev et al., 2010 ). These researchers noted that the main reason of joint fissure development was the execution of preliminary ground treatments on the slope: slope trimming, thawing of massive rock, and blasting operations that allowed for surface water runoff. Such fissures are similar to opening mode cracks in a landslide formation. Nevertheless, no sliding surfaces were detected, and such fractures in the massif refer to the joint fissures. The fissures system is traced in the range of the absolute altitudes of 690~755 m. In addition to new fissures, ancient fissures were found, indicating a seismic origin (Grib et al., 2010 ). Moreover, on the opposite bank of the Chulmakan River, a clearly pronounced large lineament of a northeast strike can be observed (Laperdin et al., 2011 ).

According to our observations, two large joint fissures are evident on absolute heights of 734 and 743 m. Inside these fissures, at a depth of 5 m, ice bodies are filling the free space which indicates the presence of permafrost. For quantitative evaluation of the fissure openings, by means of a network of reference points, initial measurements of control points were conducted by the laser rangefinder. At present, the observations are continuing. At the absolute altitude of 727 m downstream springs were found. Groundwater discharge is related to weathering cracks. Groundwater spring is characterized by bicarbonate composition with equal cations, with mineralization of 77 mg/L and a pH of 6.8 (Figure 11). Chemical composition of groundwater and the presence of permafrost at the foot of an aquifer indicate groundwater filtration within a seasonally thawed layer.

Thus, gravitational processes have both natural and technogenic origins on the Chulmakan and Duray river sites. Undoubtedly, the main reason for the development of these processes are seismotectonic events within the Chulmanskoye Plateau, and the technogenic impact probably predetermined their location. At present, there is landslide body movement and size change of joint fissures on the Duray River Valley site. Within this site there is a danger for the stability of the highway Lena due to intensively developing landslide and icing formation. Gravitational geological phenomena for both sites are in the range of absolute altitudes of 690~750 m, confined to the tectonic fault zones. An important factor is the drainage of groundwater, which is the consequence of the described gravitational processes. Within the Duray River Valley site, the aquifer, drained by joint fissures, has different water abundance from year to year. Chemical composition and icing volume, well correlated with precipitation in previous years, indicating the confinement of groundwater to the zone of free water exchange. Within the Chulmakan River Valley site, groundwater discharge probably has a seasonal character. Thus, the process of icing formation will end during the period of groundwater drawdown of the seasonally thawed layer, or the process may not begin at all if the waters are completely drained in late autumn. This icing is not dangerous for the ESPO oil pipeline because of its significant remote location (about 400 m).

Research on the origin and kinematics of the described cryogenic and gravitational processes is ongoing. Thus, it is necessary to continue systematic observations with applications such as borings and methods of isotope research. This acquired data will allow for in-depth analysis of these processes, and provide forecast evaluation for the development of hazardous conditions in relation to linear construction.

Acknowledgments:

The study was supported by Russian Foundation for Basic Research (RFBR, Nos. 16-35-60027, 16-35-60028).