1 Introduction

Any large-scale program aimed at growth of traffic on the Russian Far East northern railway lines requires development of railway infrastructure, including not only reconstruction of existing lines but also construction of new ones (Piotrovich and Zhdanova, 2015).

The Russian Federation government has adopted "The Strategy of the Russian Federation Railway Transport Development Until 2030." The strategy forecasts an increase in traffic on the Baikal-Amur Mainline (BAM) due to the growth of industrial production, development of transport nodes adjacent to the Pacific coast, development of a number of mineral deposits, and completion of the railway line to Yakutsk. On October 2, 1995, the Joint-Stock Company Zheleznye Dorogi Yakutii (Yakutia Railways) was established for the management and completion of the Amur-Yakutsk Mainline (AYaM).

The construction of the 800-km-long mainline in Yakutia began in 1985. The starting point of AYaM is the railway station Berkakit on a side branch of BAM, the Tynda-Berkakit line. On August 28, 2004, the Aldan-Tommot line was officially opened. The start-up complex (pre-commission facilities) of the Tommot-Yakutsk (Nizhniy Bestyakh) 439-km line were put into temporary operation on August 30, 2014; it should be finished in 2017 (Start-up complex of Tommot-Yakutsk (Nizhniy Bestyakh) is to be finished in 2017, 2015).

Difficult construction conditions in the area, especially being permafrost-bounded, have caused a lot of issues in construction and operation of railways and highways. These conditions mostly affect the railway subgrade, which is exposed to deformations of a different nature that those experienced at the surface.

The main causes of phenomena that occur in the soils during freezing and thawing at the different stages of operation under the impact of various external factors were identified. These causes, along with data describing geotechnical system "roadbed-base" deformations in the conditions of BAM and AYaM, are described in this paper. The anthropogenic impact of a subgrade on the permafrost icy grounds of a base, along with drainage flaws and the vibration stress load from the trains, had raised the ground temperature and caused thawing of soils, which in turn leads to subsidence and imbalance of a subgrade. For the last 15 years, the railway subgrade was affected by various types of cryogenic deformations: hazardous aufeis processes, heaving and subsidence deformations, ice mounds (hydrolaccolyths), aufeis piping of weak base grounds, etc..

Some of the authors' developments for subgrade stabilization in the foregoing conditions are given as examples. The main benefit and difference of these developments from the works of other researchers are in the detailed investigation and consideration of the regional environment. Original engineering projects of subgrade reinforcement for the sections of the Berkakit-Tommot Railway line most affected by the deformations are introduced. Counter-deformation measures (preventing and management of consequences) protecting the subgrade from dangerous subsidence deformations (sinkholes and craters) are observed. A set of measures for eliminating the deformations (separation of the rock mass in the near-bottom zone of a subgrade) caused by watering of a slope; and a cut-base is described.

2 Conditions of a construction area

The Amur-Yakutsk Mainline (AYaM) is a railway line in Yakutia (in the East of Russia) and the largest railway construction project of the last two decades in Russia. The mainline goes in the submeridional direction across Southern and Central Yakutia, through the areas with a high diversity of climate conditions. It crosses plains, plateaus, and aggradational environments of the Aldan Anteclise (large geological element of the Siberian Platform). It goes through various terrains with height differences of more than 1,000 meters. The area belongs to the basins of the Aldan and the Chul'man rivers.

The new mainline became the first railway connecting Central Yakutia and the middle course of the Lena River with the state railway network. In the future, it may become part of a new line to Magadan and further to the Bering Strait.

Climate of the area can be characterized as extremely continental. There are subzero annual average temperatures throughout the area, depending greatly on the altitude (from −6.3 °C down to −10.2 °C).

The average annual precipitation is 380~689 mm, and about 80% of the precipitation falls in the summer months.

The snow cover remains from mid-October until the end of April-mid-May. Its maximum height is 50~60 cm in the open, up to 60~75 cm in the woods, and up to 1.5~2.0 m in the lower terrain.

The average depth of the seasonal soil freezing is 4.0 m on the watershed areas and 1.5 m in the valleys of rivers and streams. The soil temperature at a depth of 0.8 m ranges from −0.4 °C to −9.5 °C during winter and from +2.1 °C to +11.8 °C in the summer.

Insular permafrost is a common phenomenon for the area.

2.1 Geological structure

Geological conditions along the line are extremely diverse. The area under study contains deluvial-solifluction deposits, represented by boulder material with an admixture of gravel, sandy loam, and loam. The presence of the latter at the base of deluvial deposits contributes to the development of solifluction phenomena. The average capacity of the deposits is about 3 m. Deluvial-solifluction deposits are seasonally frozen and commonly overlap bedrock permafrost presented by small-and medium-granular Jurassic sandstones, and siltstones.

2.2 The hydrogeological conditions

The hydrogeological conditions are largely determined by permafrost conditions. There are several types of waters in relation to the permafrost depths: suprapermafrost, interpermafrost, subpermafrost; and waters of the through taliks.

Suprapermafrost waters are spread almost over the entire area in the seasonally thawing layer of quaternary deposits. The source of feeding is atmosphere.

Waters of blind and through under-lake and under-river taliks have limited distribution. Their sources of feeding are surface-water courses, and waters of the seasonally thawed layer, artesian interpermafrost, and subpermafrost waters.

The essential structural feature of the upper layers of the permafrost is the presence of suprapermafrost (blind) and interpermafrost taliks.

2.3 Geocriological conditions

Geocriological conditions of the mainline are characterized by considerable complexity and heterogeneity. The mildest permafrost conditions—mainly with insular permafrost of 50~100 m, with an average annual temperature of 0 °C to −1 °C are typical for the Priyanginskye and Prialdanskoye plateaus and are confined to the watershed areas.

The maximum depth of seasonal thawing varies widely, from 0.4~0.6 m for heavily iced soils to as much as 3~5 m in the watersheds; the respective level of freezing ranges from 1.5 to 5 m or more.

3 Subgrade construction and operation issues

Adverse exogenous-geological and cryogenic processes and phenomena are widely developed on the mainline—mostly water, gravity, and cryogenic in nature.

Swamps and wetlands are spread throughout the area, especially on the Aldan Plateau, with both plain and watered areas.

Karst processes are caused by the activities of both surface water and groundwater and are confined to the Cambrian carbonate rocks in the northern part of the Chulmanskoye Plateau. The most active karsts have developed within the river valleys, with the lesser ones on the slopes and watersheds.

Gravity processes are represented by the stone runs that develop on surfaces with a gradient of 5° to 30°. Stone runs are stone stripes and flows 10~20 m wide, rarely to 50~60 m, and up to several hundred meters long. Stone runs change the nature of heat and mass exchange in the rocks: ground temperature drops and highly iced soils form in the body of a stone run.

Cryogenic processes include frost heave, thermokarsts, polygonal-veined ice formations, and aufeis.

Soil heaving is quite common in the area. It causes frost mounds, medallion spots, stone polygons, etc..

Seasonal frost mounds appear at the sites of groundwater discharge. They are up to 1.5 m in height, with a diameter of 20~30 m. Perennial frost mounds are rare. Their height is 8~10 m; diameter is 20~30 m.

The process of buckling of large inclusions and cryogenic sorting of soils is manifested in the form of stone piles, placers of stone material at the foot of the slopes, watersheds, etc..

Polygonal-veined ice is common for the Chulman River terraces, where ice covers have developed. Vein size varies from 1 to 1.5 m in sands, 2.5 to 3 m in loams. The depth of the ice veins depends on the depth of seasonal thawing.

Thermokarst processes and phenomena are spread over almost all the terrain where heavily iced rocks or underground ice have developed.

Aufeis formations are widespread in the areas of the Chul'manskoye and Aldanskoye plateaus. Aufeises are formed by surface, suprapermafrost, and subpermafrost waters. Proper decisions at the early design stages allowed detouring around aufeis areas with minimal construction expense.

Geocryological and hydrogeological conditions of the area are mainly determined by the performance indicators of subgrade condition at the site. In the areas of permafrost and deep seasonal freezing suprapermafrost, (ground) waters have a great impact on the nature of permafrost soil processes and soil deformation, including that of the subgrade and its base (Belenkov and Zhdanova, 2011, 2013). Therefore, subgrade reinforcement should be supported with a very careful approach to the return of the principle of restoring the old boundaries of permafrost.

According to data from subgrade monitoring of the AYaM line Berkakit-Tommot implemented by the authors in 2015, the main types of deformations are spread along the line by the following way:

Section KM 4~KM 14 (7 sites total): sedimentary and subsidence processes prevail; between KM 11 and KM 12, there are sinkholes in the subgrade due to melting of the underground ice veins. The main cause is the filtration of groundwater and surface water from the slopes of cuttings and through the embankment due to the lack of a drainage systems.

Section KM 18~KM 28 (10 sites total): along with water filtration through a subgrade (25 KM), there is soil heaving, especially on the zero level and in the cuttings (KM 28, Chul'man station).

On the sections KM 30~KM 56 (13 sites total), KM 65~KM 72 (3 sites total), KM 87~KM 142 (18 sites total), KM 121~KM 142 (7 sites total), KM 157~KM 270 (39 sites total), and KM 292~KM 357 (14 sites total), there is subgrade subsidence.

The main cause of that subsidence is mostly a violation of temperature and humidity conditions of frozen soils in the bases of embankments. As a result of their thawing, depression landforms occur. In turn, these cause the pressure filtration of groundwater through the subgrade, due to uneven freezing of the near-bottom zones. Decrease of the effective cross section of the groundwater flow causes the water under pressure either to pour near the construction or to form ice mounds in the subgrade grounds. Water piping occasionally flushes out small soil particles from a weak base of a subgrade. Depending on the lithological and geological structure, as well as climatic, geo-engineering, hydrogeological, and permafrost conditions, suprapermafrost water under pressure may contribute to the development of aufeis processes, heaving and subsidence deformations, ice mounds, aufeis piping, and injected ice.

On the section KM 58~KM 59, KM 165~KM 166, besides the foregoing phenomena, there are also sinkholes in the form of holes and craters up to 1.2 m deep under the subgrade base (Figure 1).

On the sections KM 60~KM 61 and KM 64~KM 65, there are slope slides.

On the sections KM 114~KM 116, there are fractures in the rock mass, which partially consist of dolomites. When dolomites become wet, they lose durability; and karsts are forming.

On this section, sinkholes in the base of the subgrade caused subsidence.

On the section KM 176~KM 178, there are washouts on the cutting slopes.

On the section KM 291~KM 292, a separation of the rock mass can be seen in the form of fractures in the near-bottom zone of the subgrade (Figure 2).

4 Causes of dangerous deformations

To develop design solutions for existing railway subgrade reconstruction and reinforcement in the permafrost zone, and for the new lines in the northern regions, it is essential to have a clear knowledge about subgrade-base geotechnical system behavior.

Far Eastern State Transport University (FESTU, Khabarovsk, Russia) is located close to the areas of BAM and the Amur-Yakutsk Mainline (AYaM). FESTU researchers have been engaged in the development of infrastructure stability issues of BAM since the 1960s and of AYaM since the 1980s. The results of their research work were summarized and published (Pereselenkov et al., 1982 ; Piotrovich et al., 1997 ; Piotrovich, 2003; Zhdanova et al., 2003 ; Zhdanova and Dydyshko, 2005). These two railways are the test areas for research and engineering application of permafrost construction techniques.

In previous publications, the authors described the common flaws that occurred during construction and operation of BAM. The most notable are the following (Zhdanova and Piotrovich, 2001; Zhdanova and Dydyshko, 2005; Piotrovich and Zhdanova, 2015).

First, drainage-system design flaws. Replacement of the culverts with substitute measures led to immediate deformations soon after the construction was finished. These substitute measures included connection of neighboring water pools, especially in swampy areas, with water diversion to nearby watercourses and filling them with draining ground; and using water-channeling bunds instead of ditches or gutters and berms made of drainage grounds.

Second, building the highways close to the railway subgrade caused water accumulation in the spaces between the railway and highway, leading to uneven permafrost thawing and other deformations in these locations.

Third, subgrade operating conditions do not always contribute to its stability. The main flaws here are repair of railway track without subgrade reinforcement, lack of a proper drainage system, and not enough culverts in the areas where groundwater may cause hazardous permafrost processes, causing subgrade deformations.

5 Search for solutions and examples of implementation

The issue of stabilizing the natural and constructed systems of the railway infrastructure during construction and long-term operation under permafrost conditions is very urgent, especially in the context of changing external factors (seasonal processes of freezing and thawing; increase of traffic load; etc.).

The work of many leading scientists in the transport-construction field served as the basis of methodology elaboration, adoption, and implementation of stabilization of the subgrade, and bases of artificial structures constructed and operated in the cryolithozone (Tsernant, 1998, 2013; Zhinkin and Grachev, 2001; Ashpiz, 2002; Gritsyk, 2003; Kondratiev, 2013; Zhang et al., 2013 ; Périer et al., 2014 ).

Along with many famous companies, institutes, and organizations, Far Eastern State Transport University is actively involved in patent research and patenting of new design and technological solutions for subgrade stabilization (Russian Federation patents Nos. 2186170, 2192517, 2208091, 2392385, 2422577, 2490395).

Analysis of the known methods and devices for ensuring stability of the subgrade showed that many well-known measures for subgrade stabilization useful for the short local sections are less effective for the long sections (i.e., up to several kilometers).

From the vantage point of our experience, we suggest that issues of subgrade reinforcement can and should be addressed through the harmonization principle, i.e., to contribute to natural processes. Such solutions developed by FESTU researchers have already been tested (Piotrovich et al., 1997 ; Piotrovich and Zhdanova, 2001, 2003, 2006; Zhdanova and Piotrovich, 2001, 2014; Krapivny et al., 2003 ; Piotpovich, 2015). Two typical examples are shown below.

In 2015, under the contract with the Joint Stock Company Zheleznye Dorogi Yakutii (Yakutia Railways) the authors developed engineering projects for subgrade reinforcement on particularly deformable sections of the Berkakit-Tommot Railway line. Here, the new design and technological solutions, examples of which are set out below, were employed.

The section KM 58~KM 59 of AYaM was prone to subsidence deformations. Field study using seismic survey had shown that the deformations were caused by sinkholes in the form of pits and craters 1.2 m below the base of the subgrade (Figure 3). Such deformations can be eliminated, for example, by injections of grout mix; but it is impossible to track every single one of them under a subgrade. Therefore a new, original solution was proposed.

With the anti-deformation complex, a construction we called elastic overpass was applied (Russian Federation patent - FESTU research lab "Bases and Foundations"). The structural diagram is shown in Figure 4.

The elastic overpass is the alternating layers of bulk ground and flexible cloths of synthetic nonwoven material stacked in the longitudinal direction over a length of 25 m and across the width of a subgrade.

The structure is supplied with rods, anchored on the both sides in the hard ground. Synthetic nonwoven cloths are shaped in the closed loop belt and are laid on the base of an embankment, while the loop belt is stretched between the rods. The inside of the closed loop belt is filled with soil (e.g., sand and gravel mix), and the rods are supplied with rollers.

The elastic overpass applies the following principle of operation: when a sinkhole forms under it, the lower cloth of "roller conveyor" sags down, thus stretching the upper cloth. The resulting vertical pressure provides extra support for a subgrade above, so the embankment maintains its condition.

According to calculations (Tukmakova and Zhdanova, 2016), stresses in the cloths (3.0044 kN/m²) do not exceed threshold tension that causes cloth rupture (breaking load for the geosynthetic material used, P=10.5 kN/m²). The tension values obtained for the body of an elastic overpass cloth allow us to conclude that most of the stress from the weight of the embankment and rolling stock is absorbed by the subgrade. The suggested reinforced ground structure ensures the integrity and stability of the road embankment under the conditions described.

On the section KM 291~KM 292 km of AYaM, there was a separation of the rock mass in the form of fractures below the subgrade. Diagonal fractures crossed the base of the subgrade. This disruption was followed by constant deformations, such as soil heaving, starting in November and lasting the whole winter period.

Field surveys and lab studies of the ground samples revealed two layers in the area of survey:

Layer 1: blocky, "made" ground was opened from the surface in both boreholes (thickness 0.8~1.7 m). Made ground is a packed, homogeneous soil consisting of blocky ground and 10% to 15% gray, sandy filler. The detrital material is represented with dark gray, fine-grained, durable dolomite with quartz inclusions and gray silicified semidurable granites. At the time of the survey, the grounds were in a thawing condition, with slight water saturation.

Layer 2: softened, fine-grained, durable, fractured dolomites. Fractures are oriented at an angle of 80°~90° to the core axis—in borehole #2, at a depth of 6.5~7.0 m. There is a shatter zone, with cracks randomly oriented to the core axis. Dolomites are spread across the area. Layer 2 is opened under anthropogenic deposits (depth 0.8~1.7 m, thickness 5.2~8.3 m).

The survey showed that the deformations were caused by watering of a slope and a sidehill cut. The moisture was provided by groundwater and precipitation. The fractures were caused by over-wetting of the area and weathering of the dolomite rocks. The main reason for over-wetting and weathering is the cracking of the rock mass in the upper area of the breaking load of the slope. Among other reasons: the annually increasing accumulation of water in the newly formed fractures, formation of ice, volume gain in winter, and heaving and breaking of rocks territorially and in depth.

To eliminate dangerous deformations, a special set of techniques was suggested. Besides the water drainage, the elastic overpass was used for base reinforcement, as were fracture injections and building of the retaining wall out of auger-cast grout piles.

Alternate calculations for the retaining-wall structure were carried out. The images below (Figures 5 and 6) were used for verifying calculations on using two pile rows with 2-m distance between rows and 1 m distance between piles in a raw. Piles are joined together with a grillage. The pile diameter is 0.8 m; grillage dimensions are 0.6m×0.6m.

Modeling was done in the MidasGTS software. This software provides a suite of solutions by the numerical finite-element method. The main advantage of numerical methods is that they allow you to get a fast solution for different geometrical parameters, different force fields, and other input parameters, as well as to quickly carry out the analysis of the mechanic system. Source data for the calculations are shown in Table 1.

The durability of the bored-piles sections has been calculated by the first group of limiting states. The maximal bending moment of the bored-piles section, according to MIDAS GTX program suite calculations, is M=920 kN·m. The required percentage of reinforcement for longitudinal rods to manage the bending moment is μ=2.0. The percentage of reinforcement is estimated for a bored pile of the following characteristics: diameter 800 mm, B25 concrete $({R_b} = 14.5M\prod a, {R_{bt}} = 1.05M\prod a), $ A400 reinforcement rods $({R_s} = 355M\prod a)$ . Total: eight reinforcement longitudinal rods with diameter of 40 mm (8Ø40), equally distributed along the perimeter of a section. Area of the reinforcement rods, AS=100.48 cm². The protective layer is 50 mm.

The foregoing techniques were implemented in 2016 at the AYaM sites mentioned.

A series of techniques developed by the authors for subgrade stabilization of the lines that were built up and operated in the cryolithozone were used at the Russian Far East facilities in the past 15 years. Introduction of the authors' innovations revealed their engineering and economic efficiency.

For example, a structure called "counterberm for a variable cross-section" was successfully introduced on the AYaM. The construction of a double-sided counterberm was altered to reinforce it and simultaneously allow water drainage from lower spaces instead of a culvert (Zhdanova and Piotrovich, 2014). A thermokarst settlement of an embankment developing on the BAM for 20 years was fully removed with an anti-deformation complex based on the Setkon technology (Piotrovich et al., 2003 ). We tested the "small bypass" structure at the problematic sections of the AYaM with underground ice veins and karsts. The same structure was applied to the section with bedrock falling (at the 205 km of the Pivan-Sovetskaya Gavan line) (Zhdanova and Dydyshko, 2005). The foregoing and other engineering solutions are patented at the Russian Federation. There are also other patented developments. Though the list of deformable spots is quite long, it will take only a small number of types of anti-deformation structures to handle them all. But it is important to use them with proper calculations and theoretical grounds, and to apply them to the real objects.

6 Conclusion

The issues of construction, operation, and further reinforcement of subgrade under the severe conditions of the northern areas of the Russian Far East have become urgent for the development of transport infrastructure in similar regions.

Therefore, the search for and selection of rational, manufacturable, and highly efficient engineering solutions for the reinforcement of subgrade in an environment of cryogenic deformations is very urgent.

Long-term field surveys revealed a tendency for restoration of temperature and humidity conditions of a railway subgrade base after 15 to 20 years of operation in a permafrost area. Subgrade reinforcement issues can and should be resolved using the principle of harmonization: meaning that the counter-deformation measures (preventing or dealing with the aftermath of dangerous deformations) should favour natural processes. The authors' main innovations are intended to function according to that principal for stabilization of a deforming subgrade.

Scientific and practical data obtained by the authors can be used by designers and construction workers in selecting anti-deformation techniques for railways and highways under similar conditions.

To avoid errors and losses in the future, we should raise the scientific and technological levels of adoption and implementation of design and engineering solutions in the permafrost areas.