Analysis of the operation of the railway track in conditions of low-draining mares and high-temperature permafrost (on the example of the area of the Bureysky reservoir bypass).

introduction

             Surveys, design and operation of the railway trackbed on hazy weak-draining areas in the areas of permafrost distribution are accompanied by the emergence of complex tasks to ensure the stability and reliability and uninterrupted operation of lines within them. It is difficult and quite expensive to build a reliable roadbed on permafrost soils at the base. This can be achieved either by preliminary thawing and compaction of the foundation soils to a sufficient depth, or by building an earthen bed on an artificial foundation. Currently, it is not uncommon for standard design decisions to be made for the construction of the roadbed in these areas, which, against the expected effect of preserving permafrost rocks, lead to their degradation, an increase in the capacity of the seasonal thaw layer, the development of thermokarst phenomena, stagnation of water near the roadbed, the formation of reverse slopes in the channels of drainage ditches, etc.The problem of stability of transport structures in complex engineering-geological and geocryological conditions of territories is extremely relevant. It is necessary to name the names of V. I. Vernadsky, V. A. Obrucheva, M. I. Sumgin, P. I. Melnikov, V. P. Melnikov and other scientists who dealt with these issues. These issues are analyzed in the works of foreign experts and at international conferences ^{[1,2,3,4,5,6]}.The analysis of the conditions, causes and patterns of development of exogenous geological processes in the research area with areas of high-temperature permafrost, where adverse processes and phenomena develop, is published in the works of E.A. Kozlovsky ^[16], G.P. Minailov ^[19], V.G. Kondratiev ^[17], S.M. Zhdanova ^[11], Gorshkov N.I.. ^[9] , P.I. Dadyshko ^[10], etc.

PROBLEM STATEMENT

The authors analyze the results of the design and construction of new and reconstruction of operated railways, the construction of second tracks in the areas of permafrost and low-water marys on the basis of scientific consideration of geomorphological, geological, hydrogeological and geocryological conditions of the areas of construction and operation of railways on the basis of existing regulatory documents, generalization of decisions of design organizations and their own research experience. Mari in the areas of permafrost distribution are weak-drained blocked areas with sparse vegetation, mainly with larch and the presence of groundwater of the verkhovodka type. Small slopes of the marei surface significantly complicate the surface runoff. Precipitation in this case is absorbed by a layer of peat, the humidity of which can reach 1000%, increasing in some cases up to 3000% ^[16].Accumulating in this way in the peat layer, atmospheric water filters at a low speed to the places of unloading – talvegs. Moreover, the open water surface on the marys, as a rule, is absent or is located in a thin layer between the hummocks and in a layer of moss-peat vegetation. In summer, when moisture evaporates from peat (transpiration), heat is removed, which contributes to the preservation of permafrost soils at a shallow depth from the surface, usually 0.8 – 1.2 m ^[12].The appearance of an open water space leads to its (space) heating by the sun and heat accumulation and, as a result, to thawing and an increase in the depth of the roof of permafrost soils and further subsidence of the roadbed ^[8]. Often, open water space and stagnation of water is formed in longitudinal drainage ditches arranged by railways or highways, when small values of longitudinal slopes and in places of anti-slopes. In this case, drainage ditches become "catchment ditches" and accumulate water from vast areas of adjacent mares at the sole and at the base of the roadbed ^[20].All this taken together leads to significant watering and thawing of permafrost soils of the base, subsidence and deformation of the roadbed.

THE PURPOSE OF THE RESEARCH

   At the present time, plans for the development of the railway landfill in the Far Eastern region provide for a significant increase in the volume of cargo transported to the non–freezing seaports - Sovetskaya Gavan and Vanino, which means an increase in loads on the bases. The main transport corridor for this is the Baikal-Amur Mainline and the rockad lines to it. These are areas of wide distribution of weak-draining mares on high-temperature frozen soils. The purpose of the work is to identify the conditions and causes of adverse processes and phenomena occurring during the operation of the lines. They cause great problems to ensure the safe and reliable operation of all structures, because in these complex engineering-geological and geocryological conditions, unfavorable geocryological processes and phenomena described in the article are manifested. The authors set themselves the tasks of critical analysis of the structures used, the reasons for their not always successful operation, and also proposed some design recommendations that in their opinion, they will make the operation of railway track devices more reliable and safe, and will reduce operating costs.

 RESEARCH METHODOLOGY

The main methods of field work are reconnaissance studies and full–scale field inspections of sites, decryption of high-resolution satellite images posted on Google Earth services, morphometric description of sites of dangerous geocryological processes, detection of water occurrences, GPR studies, thermal engineering calculations. A significant number of stock and literary sources on the operation of lines within the eastern section of the BAMA permafrost station of JSC Russian Railways, institutes of Dalzheldorproject, Dalgiprotrans, Far Eastern State University of Railways, archives of JSC Russian Railways, materials of engineering and geological drilling have been analyzed. A detailed analysis of the geomorphological, engineering-geological, hydrogeological and geocryological conditions of the area was carried out.

RESEARCH RESULTS

The studied section of the line, called "mesopotamia", crosses the valley of the Adnikan River and its tributaries, the valley of the Southern Elga River, the watershed between the valleys of the Adnikan and Dublikan rivers, the valley of the Dublikan River, the valley of the Soloni River and its tributaries (Fig. 1).Characteristic features of the "mesopotamia" and other parts of the region are the widespread low– and high-temperature permafrost soils of the merging type, significant swampiness of the territory, continuous distribution of weak clay soils in the upper part of the geological section. So if, on average, frozen soils account for 60-65% of the area on the territory of the Verkhnebureinskaya Depression, then along the bypass route taliki are found only at crossings over rivers and large streams, as well as on well-drained slopes ^[13].The temperature regime of frozen soils is influenced by geomorphological conditions, microrelief, exposure and steepness of slopes, the presence and type of vegetation, the groundwater level. The areas of the high floodplain of the Bureya River and its tributaries are the most cooled within the depression – the temperature of rocks at a depth of 10 meters here is about minus 3.0 ° C (low-temperature permafrost), with a thickness of the permafrost layer up to 75 meters. On the first above-floodplain terrace, the thickness of frozen rocks is reduced to 50-55 meters, their temperature rises to minus 2.5 ° C and above (low-temperature permafrost). On the second above-floodplain terrace, where drainage conditions are better, the thickness of frozen rocks is reduced to 40 m, and the temperature does not exceed minus 2.0 ° C.Low permafrost temperatures are caused not only by the excessive severity of the climate (the average annual long-term air temperatures in Urgal are minus 3.7 ° C), but are mainly due to the presence of specific ground covers (peat, moss), their high moisture content and relatively small thickness of snow cover. The engineering and geological conditions of the "mesopotamia" are characterized by a wide spread of thawed and frozen peat of the IV category of subsidence with a capacity of up to 2 meters, including a layer of thawed peat with a capacity of up to 1.2 meters, sandy loam with gravel and pebbles of the frozen III category of subsidence or frozen loam III and, only in some places, IV category of subsidence.

1 - section of the Lime – Novy Urgal line;  2 - rivers of the district; 3 – villages and roads. 4 – crossing of a local watershed with small height differences in a straight line to ensure the required longitudinal slope of drainage ditches and the exclusion of three curves

Fig. 1. The area of the "interfluve" of the Adnikan, South Elga, Dublikani and Soloni rivers within the Upper Mesian Depression is the section of the Bureinsky bypass.

The valleys of the Adnikan, South Elga, Dublikan and Soloni rivers are composed of frozen pebble soils (III), underlain by siltstones (II) or sandstones (II) of the subsidence category indicated in parentheses. The watersheds between the valleys of the listed rivers are composed of frozen sandy loams of the III category of subsidence or loam III and, only in some places, IV category of subsidence with a capacity of up to 2.5-4.0 meters, sometimes increasing to 6 or more meters.  The hydrogeological conditions of the "mesopotamia" are characterized by the presence of permafrost groundwater lying at the surface and up to a depth of 1 meter ^[19].One of the factors inevitable in the technogenic development of the territory is a change in these conditions (elimination and replacement of natural ground covers, drainage of the area), as well as the construction of heat-absorbing structures, leads to an increase in the capacity of the seasonal thawing layer-freezing, temperature increase and deep thawing of permafrost soils of the base, which is the main factor in the manifestation of a large number and significant in size deformations of the railway track ^[16].Fig. 2 shows the results of forecast thermal engineering calculations performed at the time of maximum soil thawing (starting from 2012) – October 2017, 2022 and 2032 of the design year, respectively, for 5, 10 and 20 years of operation of the constructed embankment.

 1 - man-made bulk soils of the roadbed, 2 - moss-growing peat layer, 3 - thawed loam (frozen), 4 - pebble soils of the foundation of the roadbed frozen, 5 – the position of the VGVM of the 5th billing year, 6 - the position of the VGVM of the 10th billing year, 7 - the position of the VGVM of the 20th billing year, 8 – isotherms October of the 20th billing year

Fig. 2. The results of the forecast thermal engineering calculation of the embankment of the Dublikan-Novy Urgal DVost.zhd

         The calculation results show that the depth of thawing for the 10th billing year in the area of sq. 1/12 (north side) will be 6.1 m, in the area of sq. 2/11 (south side) - 11.3 m, under the axis of the path – 11.8 m (Fig. 2). The absolute value of thawing of permafrost soils at the base of the embankment under the axis of the path for 10 years will be 1.0 m. The average thawing rate will be 10 cm per year, the subsidence of thawed soils at the base of the embankment, at the same time, will be 30-40 mm / year. Thus, the share of thermal deformations in the total annual subsidence (100-120 mm/year) of the roadbed is 25-35%.The rest of the deformations (70-80 mm /year) are realized due to plastic deformations of the spreading of the embankment, the extraction of weak base soils and the introduction of loam of the base into the large-block soils of the body of the embankment. Another factor complicating construction and operation is the widespread occurrence of low– and high-temperature permafrost soils at the base of the roadbed, subsidence during thawing ^[9].  Soils with a relative subsidence from 0.2 to 0.5 are widely found (III and IV categories of subsidence, "weak" and "subsident" according to VSN 61-89). Moreover, this group is represented not only by peat and clay soils, but also by sands of various sizes and even pebble soils (subsidence up to 0.2–0.3). With a significant thickness of these soils at the base and deep thawing, the sediments of the roadbed can reach values from tens of centimeters to 1.5-2 meters or more. The depth of seasonal soil freezing in such areas varies widely depending on the type of underlying soil and the type of ground cover – from 0.6 to 3.5 meters. The greatest depth of freezing – 3.0-3.5 meters – was noted in large-block soils. Thermokarst, technogenic ice and seasonal frost heaving, waterlogging are manifested in the region.

In these conditions, the following design solutions and structures were used in the design and construction:

– embankments up to 3 meters high from draining soils.

– recesses up to 12 m deep in clay soils and in rock softened, developed for an embankment (protective layer), the thickness of the protective layer is established from the condition of limiting the deformations of the path under the influence of frost heaving.

The minimum height of embankments is adopted taking into account snow–bearing capacity and taking into account the permissible uniform heaving ("on wet and wet grounds"), taking into account flooding, and in swamps - taking into account the permissible elastic deformations of the roadbed.

The roadbed at high temperature (– 2 ^oC and above) permafrost soils of the foundations are designed according to the principle of using soils in a thawing thawed state, taking into account changes in the original surface of permafrost soils (thawing and precipitation) during construction and operation, i.e. according to the second principle.

The removal of surface water from the roadbed is carried out: at recesses – by ditches, ditches - by trenches and upland ditches, at embankments – by drainage ditches.

For embankments with a height of less than 1.5 m with a transverse slope of less than 0.02, as well as on marys, drainage ditches are designed on both sides of the embankment. The technological highway is designed on the upper side of the railway, the upland and drainage ditches are designed behind the highway, in the sinus between the highway and the railway.  The estimated costs are assumed with a probability of exceeding 1% for ditches, upland ditches and spillways and 3.3% for longitudinal and transverse ditches.

The slopes of embankments in the areas of flooding by flood waters of the rivers of the district are reinforced with lumpy soil – a "rock outline". The bottom and slopes of cuvettes, upland and drainage ditches in clay soils are reinforced with a 30 cm layer of rock soil.

Currently, after 14 years after construction, a large number of deformable places of the roadbed are listed on the site, which is explained by the following factors.

Firstly, during the design, decisions were made that did not correspond to the specifics of the natural conditions of the area, the use of unreliable and erroneous data of engineering-geological and hydrogeological surveys. These are, first of all, design solutions (mainly drainage systems) and the use of soils unsuitable for filling the roadbed.

An example is a recess with a depth of more than 20 meters, projected in weak soils, represented by a sandy loam of a fluid consistency, which has almost silted up the ditch (Fig. 3). On the same slope of the recess, a bulge of heaving at a height of 5-7 m is clearly visible (Fig. 4). With additional watering by atmospheric precipitation, the slope of the recess can be washed away.

In addition, a concentrated groundwater outlet was found in the lower part of the slope of the excavation, the remnants of ice formed due to the discharge of groundwater are visible (Fig. 3). Nevertheless, during engineering and geological surveys, the presence of groundwater was not detected.

Secondly, the cause of deformations is also a large amount of excavation work on the construction of elements of the roadbed, mainly drainage ditches designed and built both from the upper and lower sides of the roadbed.

The design of the bypass route, mainly of the plan, was carried out without taking into account the requirements for the provided drainage of surface water and sufficient design slope of the longitudinal profile of the ditches.

In the first years after construction, this led to excessive watering, flooding, and in some areas – to filtration of water in the soils of the foundation of the roadbed (Fig. 5), to accumulation and stagnation of water in the ditches in front of the roadbed, arranged, including from the bottom side (Fig. 6).

Three years after the construction, the drainage ditches opened from both the upper and lower sides of the roadbed are almost completely silted up and overgrown with hummocks, grass and shrubs, swamped. This made it impossible to divert surface water and led to the formation of numerous areas of stagnant water.

The accumulated heat in places of stagnation of water led, in turn, to the thawing of permafrost soils under the channel of the ditches with the seizure of the base of the roadbed and, as a consequence, subsidence of embankments and berms with the separation of slopes (Fig. 7, 8).

         It should be emphasized that when designing a railway track plan in such conditions, it is most effective to trace the line through local elevated terrain in order to ensure the necessary slope of the design profile of drainage ditches of at least 4 ppm to ensure water flow. Figure 1 shows a section of such straightening, the intersection of a local watershed with small height differences in a straight line to ensure the required longitudinal slope of drainage ditches and the exclusion of three curves

Progressive degradation of permafrost soils of the base leads to subsidence of berm and the formation of new places of stagnation of water, which again contributes to the degradation of permafrost "in breadth" and "in depth" (Fig. 9).  

In addition, an increase in the depth of the sole of the seasonally frozen layer under the channel of the ditches contributes to the formation of man-made ice (Fig. 10) ^[18].

 In adverse weather conditions, the volume of ice can be significantly large and threaten the safety and continuity of train traffic.

It should be noted that within the Mari, crossed by the bypass route, there are 17 rivers and streams that divert all atmospheric water from the Mari and without the formation of an open water surface. Nevertheless, about 30 small bridges and pipes have been designed and built on the bypass section.

During the construction of bridges, excessive deepening of the watercourse bed to depths of 3.5 meters and accumulation of large volumes of water is sometimes allowed (Fig. 11, 12).

 In such areas, during operation, there is a landslide of the slopes of the ditches and, subsequently, a violation of the longitudinal profile of the ditch and its silting. This worsens the drainage conditions, which contributes to the growth of uneven sediments of the roadbed.

conclusions

           For the studied area, the characteristic features are the significant swampiness of the territory and the wide distribution of low– and high-temperature permafrost soils of the merging type. This structure is typical for many sections of the region's lines. In these difficult conditions, there are cases of irrational and inefficient design, which leads to flooding of the foundation of the roadbed, degradation of the upper boundary of frozen soils, and deformations of the foundations of the roadbed and artificial structures.  It is not uncommon to use unsuitable soils for the filling of the roadbed and its elements and to exceed the volume of excavation work during construction. Under these conditions, it is recommended that when designing a railway track plan, trace the line through local elevated terrain in order to ensure the necessary slope of the design profile of drainage ditches of at least 4 ppm. Drainage ditches along the path should be located at a distance that excludes the inflow of water into the base of the roadbed. Do not allow deviations from design decisions and violations of work production technology.