1 Introduction

Functioning of railway tracks in a harsh environment leads to early deformations of the railroad subgrades due to the variation of yearly average temperatures and the violation of the ecological balance. More than 5,300 kilometers of railroad tracks in Russia go across permafrost territories. Cryogenic processes lead to enhanced deformability of railroad subgrades and substantially hamper their functioning, with the most frequently occurring deformation—e.g., at the Baikal-Amur Railroad (BAR)—being the settling of embankments through thawing (Ashpiz, 2012).

Factors causing the subgrade deformation and specific features of the process were discussed in detail in sources (Ashpiz, 2012; Dydyshko, 2012; Isakov, 2012; Belenkov and Zhdanova, 2013; Isakov and Lavrova; 2015, NIOSP them. NM Gersevanov, 2012a, 2012b, 2013). Among those factors, the main ones are the variation of the temperature regime of the supporting subsoils and the lowering of the upper permafrost boundary. Upon thawing, subsoils in railroad structures represented by clay and large-scale cryogenic soils hosting ice inclusions undergo settling. Subsequently, there proceed lengthy geological processes, during which clay soils come into a fluid state (Dydyshko, 2012). Filtration of groundwaters, whose rate enhances upon thawing, leads to further degradation of soil properties (Ashpiz, 2012).

Advanced developments now allow taking into account specific features of railroad building on permafrost soils. Reliability of such railroad structures directly depends on the human factor and on the capital investments made.

Rebuilding of existing structures is also of interest here. Over Russia's territory, there are railroad infrastructure objects whose subsoils, represented by permafrost soils, keep undergoing deformation over a period longer than one hundred years. That is why the matter of prognostication of deformation processes and the development of reinforcement methods presents a vital problem. Also, it should be noted here that the accomplishment of engineering-geological surveys during inspection of functioning railroad tracks is extremely hampered and has many specific features, in comparison with the surveys for newly built objects. Accordingly, the matter of prognostication of deformations here also presents a difficult problem, to be treated individually for each particular case.

Difficulties in engineering-geological survey include special requirements to be fulfilled in treating permafrost soils. Those requirements are formulated in special specification documents (PNIIS at Gosstroy Rossii, 1998; PNIIS at Gosstroy Rossii, 2000; NIOSP them. NM Gersevanov, 2012b; National Association of Surveyors, 2013). Also, for this study, the survey is to be conducted on the roadbed of a railway in operation.

2 Geological features

Presently, prognosis concerning the evolution of supporting subsoils in railway-track subgrades formed by permafrost soils is based on the determination of the settling class/degree of the soils in terms of the main physical properties. Because, in most cases, the actual values of strength and deformation characteristics of thawing subgrade soils are ignored in making predictions, the predicted supporting strength and settling present only approximate the actual characteristics. In view of such a prognosis, the implementation of measures to be taken prove to be insufficient. Performing a geological survey with complete evaluation of mechanical characteristics of thawed soils in accordance with regulations substantially raises the cost of the survey (PNIIS at Gosstroy Rossii, 1998; PNIIS at Gosstroy Rossii, 2000; NIOSP them. NM Gersevanov, 2012b; National Association of Surveyors, 2013), prolongs its duration, and narrows the range of potential executors. For raising the economic efficiency and quality of a geological survey of a soil subgrade functioning in cold regions, refinement of the survey algorithm on the basis of systematization of features and requirements to the survey is necessary. Those features include, first of all, the subsoil temperature under the railroad structure (Kropachev, 2012); the occurrence, within the compressible subgrade depth, of ice veins and interlayers, underground waters; clay soils' growing weak upon thawing and, contrariwise, rocky soils'; and, also, surface turf (bog) deposits (Figure 1). Using those criteria, one can identify potentially problem-causing railroad sections and systematize requirements for the inspection of those sections.

The temperature of subgrade soils of a railroad structure is one of the main criterions affecting the requirements for an engineering-geological survey. From the subgrade soil temperature, one can judge not only the presence/absence of frozen soils but also the types of the soils. Over of the past three years, the specialists at the Scientific-Engineering Laboratory "Geology, subgrades, foundations and soil roadbed" of the Siberian Transport University have performed engineering-geological surveys of the subgrades of 42 bridges and approach fills to those bridges on the Far Eastern (FERW) and Trans-Baikal (TBRW) Railways. The distribution map of permafrost soils in the territory of the Russian Federation is shown in Figure 2, together with indicated inspection objects. The objects with frozen soils encountered in railroad subgrades are shown as red circles, and the objects with subgrade soils in the melted state, as green circles.

As can be seen from an analysis of the map, the primary route of the Trans-Siberian Railway is located in the zone of rare, sporadic, and mass permafrost soils; that is why objects in this railroad are colored red or green.

Obviously, requirements for engineering-geological surveys of frozen and thawed subsoils differ fundamentally from each other, including the aspect of regulatory documents. That is why preliminary information on the temperature of supporting soils under given conditions provides a basis for systematization of the soils, and it will allow taking into account local features of objects to be inspected. For instance, such features include the water-and-thermal regime of the structure-base system. Local variations of the water-and-thermal regime due to the building and functioning of the railway roadbed cannot be taken into account uniformly through compiling a general distribution map of permafrost soils because of a dynamic variation pattern of soil temperature.

Another important feature of permafrost subsoils along the Far Eastern and Trans-Baikal Railways is the relatively high temperature of those subsoils (not lower than −3 °С) at a considerable depth of plastic frozen soils. Under such conditions, the lack of preliminary information on subsoil temperature will not allow the assignment of optimal methods and procedure for boring operations with minimum subsoil thawing, with this circumstance causing distorted exploration data.

A not less important criterion affecting the quality of engineering-geological surveys in the distribution regions of permafrost soils is the occurrence of superpermafrost subsoil waters capable of thawing the frozen soils at boring operations.

The occurrence of ice veins and interlayers within the compressible supporting stratum of a railroad structure leaves almost no choice in assignment of the operation principle of subgrades whose thawing will lead to excessive deformation and complete collapse of the railway structure. In the latter case, the main design solutions are to be aimed at preservation of the subgrade in a frozen state. Possessing necessary information prior to performing the engineering-geological surveys, one can simplify the investigation of the mechanical properties of thawing soils and improve the quality of determining the physical, thermal, and engineering properties of frozen soils.

An opposite picture will be observed at the occurrence, within the compressible subgrade stratum of a functioning railroad structure, of low-compressibility rocky soils and clay soils' growing weak upon thawing. Apparently, while designing a railroad, one must analyze, as a potential alternative, complete thawing of subgrade soils with accurate prognosis of their supporting strength and total settling. To this end, particular attention during engineering-geological surveys needs to be paid to the mechanical properties of thawing soils.

In the presence of surface turf deposits (bog) lying over permafrost soils, the extent of the survey should include not only identification of the type of the deposit in term of the content of organic substances but also identification of a full set of physical properties for performing more accurate thermotechnical calculations, as turfs and peaty soils present natural heat insulators.

That is why the main point in developing a geological survey program is the preliminary revealing of the state of the embankment subgrade soils in terms of the above criterions/specific features.

In most cases, geophysical methods fail to yield definite data on the temperature and type of frozen subgrade soils because of specific aspects of interpretation of diagnostics data. The most reliable diagnostic method for permafrost degradation is the monitoring of temperature in temperature wells (NIOSP them. NM Gersevanov, 2012b). Preliminary boring of a well at a railroad object yields accurate data on the subgrade soil temperature and complete data on the above criterions for development of a survey program.

3 Specific features of a geological survey

Taking as an example railroad objects located along the northern route of the Far Eastern Railroad (BAR), let us discuss specific features of geological survey of the soil subgrades of functioning railroads whose bed is formed by permafrost soils. Those soils, for instance, include the soil subgrade at the approaches to the bridges located at the 2,178^th kilometer, at the 2,242^nd kilometer, and at the 2,254^th kilometer of the Lopcha-Khorogochi Railroad section. Engineering surveys of those railroad sections were performed from August to September 2015. Those surveys included a standard scope of works, such as engineering-geodesic, engineering-geophysical, and engineering-geological examinations.

Under permafrost conditions—along with the standard tasks for thawed soils to be performed according to regulatory documents—such as the hot-die tests of frozen soils aimed at determination of their compressibility parameters at thawing and the arranging of temperature wells for measuring the subgrade temperature were carried out. In addition, soils in a frozen state were transported to a laboratory, where physico-mechanical characteristics of the soils on thawing were evaluated. At the analysis stage, the depth of the annually thawed layer was also calculated; and the PU-9 sheet data (database for taking into account railway deformations by operating organizations) were analyzed.

The field hot-die tests of soils on railroad objects were carried out using a facility whose schematic is shown in Figure 3.

Tests were conducted according to the procedure described in the State Standard (NIOSP them. NM Gersevanov, 2013). The main components of the experimental facility were the anchor beams, the stop bar, the hydraulic advancing cylinder equipped with a manometer, the support, the die, and the reference system with flexometers. An advantageous feature of the facility was the anchor stop system used, which made it possible to perform rapid tests in regions with poor transportation accessibility without transporting heavyweight masses. For mounting the die, there is no need for a trawl and a crane; it suffices to have a boring apparatus on off-road chassis. The anchor beams, two or four in number depending on the required load, are screwed into the ground by the boring apparatus. The heat carrier was gas-heated water.

In performing hot-die tests, dependences of die settling versus load and die settling versus the time of stabilization at the load stage were plotted. The tests were performed for two loading/unloading cycles implemented for determining the deformation and elasticity moduli of thawed soils. The thawing factor was evaluated under natural pressure at soil thawing underneath the die, occurring to a depth of half the die diameter.

From the graphs of relative die settling versus load, the values of the thawing factor A_th, and the compressibility coefficient of the frozen soil under study m, necessary for accurate prognostication of subgrade soil settling through thawing, were determined.

The physico-mechanical characteristics of permafrost soils on thawing under laboratory conditions were determined using the procedure of the State Standard (NIOSP them. NM Gersevanov, 2012a). From a plot of relative settling value versus load, the thawing factor A_th and the compressibility factor m of the frozen soil under study were evaluated.

In that study, some difference between the values determined and the hot-die data were revealed. In all tests, a decrease of the deformation modulus was observed, this finding being due to the difference between the stress-strain state of the material underneath the die and under compression (Razuvaev, 2013). Here, the mean values of the thawing coefficient remained almost unchanged, whereas a much wider spread of the values of this coefficient was observed due to the specific influence of ice lenses. For instance, for the supporting subsoil of the bridge at the 2,242^nd kilometer, the data obtained were the following:

(1) Thawing characteristics of soils obtained in hot-die tests: A_th = 0.0078, E = 22.7 MPa (silty clay with sand and granitic subsoil); A_th = 0.0036, E = 34.0 MPa (silty fine gravel (coarse sand) with sand).

(2) Thawing characteristics of soils obtained in laboratory tests (under compression): A_th = 0.0091, E = 13.8 MPa (silty clay with sand and granitic subsoil); A_th = 0.0032, E = 22.0 MPa (silty fine gravel (coarse sand) with sand).

For measuring temperatures, temperature wells made in accordance with the State Standard (NIOSP them. NM Gersevanov, 2012b) (Figure 4) were used.

The battery of temperature wells was provided in the transverse direction, with installation of reference wells in the bog.

For measuring the temperatures, an MTsDT 0922 sensor chain was used. The sensor-chain controller polled the sensors at an interval of 5 to 60 s and transferred the measured data to a display. The spacing between the neighboring sensors was 0.5 m within the first 5 m and, beyond that, 1.0 m to a depth of 10 m.

The soil temperature was measured accurately to 0.1 deg. According to measured data, temperature graphs were applied onto engineering-geological sheets to indicate the permafrost-soil surface in the soil profiles.

On the whole, all the mining operations to be performed under severe engineering-geological conditions on a functioning railroad were specific ones; and they called for application of individual approaches. It was required to determine the properties of subgrade soils not only outside the subgrade base (for which operation presents no difficulty for any boring apparatus) but also the same properties under the sloping parts, the primary site, and even (in some cases) under bridge-abutment bases.

The studies performed have allowed the workers to develop a geological survey algorithm, illustrated in Figure 5.

In general, a geological survey is to be carried out according to a work program specifying the amount of work to be performed, the survey procedure to be applied, and the infrastructure and features pertinent to the railroad object under inspection. The program is to be developed on the basis of systematized criterions pertinent to the railroad object. Those criterions can be revealed while performing an adequate reconnaissance survey and an analysis of the result of arranging of the temperature well. Arranging of at least one temperature well is a necessary condition for gaining data on the lithological profile, on the temperature of layers, and on underground waters. A reconnaissance survey and examination of archival materials yield information on interrelated natural processes affecting, in one way or another, the actual state of permafrost rocks. An exemplary chain of interrelated processes is shown in Figure 5. The precipitation and air temperature are regional climatic features that define the snow cover and the seasonal soil thawing. The seasonal thawing is also affected by the presence of moss (acting as a heat-insulating layer) and by the relief. As a final result, all these factors have an impact on the surface waters, which in turn exert an additional heating action on the soils.

4 Example of the geological examination algorithm implemented

An example of the survey algorithm implemented is the work on geological inspection of the approach fill to the bridge at the 2,242^nd kilometer.

According to the PU-9 sheet data, the railroad bed was built in 1977. In 1996, this railroad section was registered as a 253-meter-long sore spot (monitoring over this spot was initiated). The main deformations revealed were the railroad settling through the thawing of icy subgrade soils (settling value during the observation period in excess of 150 mm) and, as a consequence, the track-raising-induced narrowing of the track-bed bench (narrowing by about 50 cm).

Arranging a temperature well and performing a reconnaissance survey allowed the engineers to develop an efficient geological survey program. From the data gained, detailed geological profiles of the approach fills were constructed (Figure 6) and standard and calculated properties of thawed and frozen soils were evaluated.

From the results obtained, it was found that the railroad bed, formed by coarse gravel, was lying on a frozen-sand-loam subgrade with cobble and sandy gravel, the granitic bedrock being located at a depth down to 12 m from the ground surface.

In spite of the initial adoption of principle 1 (NIOSP them. NM Gersevanov, 2012c) of using the subgrade (keeping of subgrade soils in a frozen state during the operation period of the structure), during the whole period of use of the subgrade, a thawing bed had formed in which the lower mark of permafrost soil had descended to 3 m. The subgrade sandy loam had passed over into a plastic state, and the subgrade had settled from the time of building by 30 cm. The calculation of the depth of the annually thawed layer (whose boundary is indicated in Figure 6 with the red dashed line) showed that the permafrost outside the fill was of the closing type, and the thawing bed underneath the subgrade never became completely frozen. One of the causes of the indicated deformations is an inadequate pre-building engineering-geological survey of the object in which the main geological features were not taken into account in full measure (Figure 1). This approach had led to an inadequate thermotechnical prognosis and subgrade soil thawing.

To predict the settling value through ever-frozen-soil degradation at the subgrade of the approach fill to the bridge, MidasGTS calculations were performed (Figure 7). In those calculations, the frozen-soil degradation was assumed to reach bedrock soils.

Calculations of the railway embankment were carried out using the standard components of the MIDAS software. First, the MIDAS graphic editor was used to develop a geometric model of the railway embankment. Then, the geometric model developed of the planar carcass was integrated in structural units, and a finite-element grid was generated. An operational continuity control of generated elements was performed.

The soil medium was constructed based on the perfectly plastic Coulomb-Mohr model in which the following plasticity characteristics were used: specific cohesion C, angle of internal friction φ, deformation modulus E, and Poisson's ratio ν.

The calculations were performed for two sets of thawing-soil characteristics obtained under field and laboratory conditions.

To this end, a calculation scheme was developed and, then, worsening of soil conditions through permafrost degradation was modeled.

From the calculated data, the following conclusions were drawn:

(1) The subgrade settling as estimated using data gained in full-scale tests amounts to 29.1 mm.

(2) The subgrade settling as estimated using laboratory compressibility data amounts to 47.2 mm.

As evaluated for all inspected objects, depending on the physical characteristics of the frozen soil such as the density, the indicator of fluidity, the plasticity number, the total humidity, the ice content, and the cryogenic texture, the predicted subgrade settling through permafrost degradation as evaluated from the full-scale tests of the soils proved to be 20%~40% smaller than the data obtained in calculations based on laboratory data only.

5 Conclusions

On the whole, the following conclusions can be drawn from the results obtained:

(1) The quality of a geological survey may have a substantial influence on the choice of further design decisions. The effectiveness of a geological survey and its cost both directly depend on the quality of the program developed. This quality can be improved via implementation of the survey algorithm proposed in the present publication.

(2) In prognostication of the behavior of thawing soil subgrade in the distribution regions of permafrost soils, one must use physico-mechanical characteristics obtained through establishing a comparative correlation of laboratory and field evaluations.

(3) A comparison of observation data over temperature wells and boring data permits an exact revealing of distribution boundaries of frozen soils, plastic frozen soils, and thawed soils.

(4) In the cases in which the field hot-die test turn out to be impossible for reasons beyond the researchers' control, correction factors to laboratory data to be used in calculations must be found.