Analysis of the results of studies of the thermal regime of natural and man-made kurums of the cryolithozone

Galkin Aleksandr Fyodorovich ORCID: 0000-0002-5924-876X Doctor of Technical Science Chief Researcher; P.I. Melnikov Permafrost Institute SB RAS 677010, Russia, Yakutsk, Permafrost str., 36, IMZ SB RAS. Cryolithozone Geothermy Laboratory afgalkin@yandex.ru Other publications by this author     Zhirkov Aleksandr Fedotovich ORCID: 0000-0001-6721-5338 PhD in Technical Science Head of the Laboratory; Cryolithozone Geothermy Laboratory; P.I. Melnikov Permafrost Institute SB RAS 677010, Russia, Republic of the RS(I), Yakutsk, Permafrost str., 36 zhirkov_af@mail.ru Pankov Vladimir Yur'evich PhD in Geology and Mineralogy Associate Professor, Department of Construction of Roads and Airfields, North-Eastern Federal University 677027, Russia, respublika Sakha(Yakutiya), g. Yakutsk, ul. Belinskogo, 58 pankov1956@gmail.ru Other publications by this author

Plotnikov Nikolay Afanasievich ORCID: 0000-0001-6013-931X Postgraduate student; P.I.Melnikov Institute of Permafrost Science SB RAS 677010, Russia, Republic of Sakha(Yakutia), Yakutsk, Permafrost str., 36 plotnikov-nikolay96@mail.ru Other publications by this author

Introduction. According to the modern terminology of artificial intelligence (AI), the neural network provides the following consolidated explanation of the term "kurums" (translated from the Turkic language, "kurum" is a stone). "Kurums are one of the varieties of placers of coarse—grained material in the form of stone cloaks and streams on the slopes of mountains. They are common in mountainous areas of permafrost development and deep seasonal freezing. Kurums are very unstable formations, with man—made impacts, as a rule, they accelerate movement." At the same time, AI asks not to confuse the terms "kurums" and "glaciers", which are also one of the varieties of placers of coarse-grained material. "Stone glaciers are large landforms representing clusters of ice—cemented coarse-grained material of various genesis. The speed of movement of most stone glaciers, as a rule, ranges from tenths to the first meters per year."

Thus, the main difference is that kurums are placers of coarse—grained material, and stone glaciers are accumulations of ice—cemented material. Accordingly, the thermophysical processes occurring in glaciers and kurums will differ when interacting with the atmosphere. Many works of Russian scientists are devoted to the study of kurums of natural and man-made origin. Recently (2021), a monograph by the famous geocryologist and glaciologist, Professor V. R. Aleseev ^[1] was published, which summarizes the results of research by domestic and foreign scientists. As noted in the annotation, "the book is devoted to the stone placers-ruins, known as "kurums". On permafrost, kurums form peculiar seas, streams, rivers, the area of which is measured in total in tens and hundreds of square kilometers. These are cloak-like accumulations of rock fragments that slowly move down the slopes, mixing and sorting under the influence of cryogenic processes. In spring, thawed snow waters "fall through" into the cavities of loosely folded rocks and freeze in the form of so-called char ice. The char ice feeds numerous underground streams that process sharp-angled stones, turning them into boulders and pebbles. Like ball bearings, coarse-grained material can "slide down" on them, turning into crushing landslides, avalanches and mudslides." The scientist also believes that kurums are not only of academic interest, but are also an underestimated source of building materials ^[2]. In the works of A. I. Tyurin and co-authors ^[3,4,5,6], the issues of genetic classification of kurums, the peculiarities of their structure in different latitudes are considered, and for the first time an engineering geocryological assessment of kurums is made, which confirm these conclusions. The results of D. O. Sergeev's research with co-authors ^[7] allow us to judge how the temperature regime of permafrost strata and seasonally thawed layer is formed, including in the areas of distribution of natural kurums, in the mountains of Northern Transbaikalia. It is interesting to note that the conclusions and terminology of AI given at the beginning of the article are largely due to the fundamental works of A.P. Gorbunov and E.V. Seversky with co-authors devoted to the study of the mountainous regions of Kazakhstan and Central Asia ^[8,9,10,11]. The results of studies of the cryogenic structure and temperature regime of large-scale deposits in the Northern Tien Shan are presented in ^[12,13]. Cryohypergenesis of large-block and rocky rocks of the cryolithozone (on the example of the Trans-Baikal region), was studied in the works of Professor D. M. Shesternev ^[14,15,16]. In the works of Professor V. A. Stetyukha, the results of studies on the stability of thawing slopes of man-made mountain ranges, which are a variant of man-made kurums formed as a result of overburden of rocks during the development of mineral deposits by an open method, are presented ^[17,18,19]. In the works of E. V. Seversky, a qualitative and quantitative assessment of the degree of danger of the main geocryological processes and phenomena in the mountainous regions of Kazakhstan is given. Moreover, in the areas of permafrost and seasonally frozen rocks, along with solifluction, thermokarst, frost heaving and cracking, ice and glacial mudflows, these include stone glaciers and kurums ^[20,21].

According to the classification given in ^[22], all rocky block massifs (kurums) are proposed to be divided into three categories according to genetic characteristics: natural (in engineering geology, called kurums), man-made (rock dumps, road embankments, poured onto rocky and dispersed massifs, stone sketches used in construction) and natural and man-made (blocky geotechnical structures on blocky natural foundations). According to their location relative to the earth's surface, blocky massifs can be divided into surface and underground (formed in the cavities of natural (caves) and man-made mine workings as a result of roof collapses, falls from walls or rock breaking by explosion). The natural type of blocky rock massifs is represented by clusters on the slopes and at the foot of the mountains of blocky and large fragments of rocky rocks formed as a result of various geotectonic and geomorphological processes (stone glaciers, kurums, kurumo-glaciers, cones of avalanche and mudflow, etc.). They occur in the form of areal or local elongated deposits, the thickness of which ranges from several tens of centimeters to several tens of meters ^[23]. The basis of such massifs are solid rock formations. In practical terms, the problem of using blocky rock massifs as the bases of geotechnical structures, primarily upland overburden dumps in open-pit mineral deposits, as well as the roadbed of transport communications in high-altitude areas, in particular Northern Transbaikalia, is of particular interest.

The purpose of the work is to analyze the main literary sources, in which the results of research on the formation of the thermal regime of rock dumps (kurums) are published.

Results and discussion. When analyzing the thermal balance of the surfaces of blocky rock massifs of man-made and natural formations, it was found ^[22,24], "that the highest values of heat transfer are characteristic during the entire thawing period for open areas of the slope of the southern exposure. This, in turn, sufficiently explains the maximum power of the STS 2.3–2.5 m on such terrain elements. With a change in exposure and the nature of the surface of the blocky massif, the values of the thermal balance also change – they decrease significantly, which leads to a reduction of the STS to 2.2 m in the open area of the relief of the northern exposure and 0.9–1.6 m in areas covered with vegetation." The peculiarities of heat transfer in a blocky rock mass are associated with changes in the amount of total solar radiation, which, decreasing in autumn, leads to a decrease in heat flow, as well as changes in the structure of the thermal balance: heat costs for evaporation compared to the summer cycle, they increase following an increase in humidity, which further reduces the magnitude of the heat flow. A reduction in the values of the heat flux cannot cause an increase in the depth of thawing^[24]

Experimental and theoretical studies of the thermal regime of natural kurums are devoted to the work of the staff of the Institute of Permafrost named after P.I.Melnikov SB RAS ^{[25,26,27,28]} Experiments were carried out on a site typical of the char belt of the Kolyma Highlands, which is composed of steep blocky-gravelly scree, almost completely devoid of vegetation. The average power of the STS is 1.2 m, the diameter of the debris in it varies mainly from 4 to 20 cm with an average value of 10 cm and a soil porosity of 0.35. At depths approximately coinciding with the sole of the STS, the composition of sediments changes dramatically due to filling the pores of the coarse-grained skeleton with sandy-sandy loam material. As a result of field studies, an important result of applied importance has been established. In particular, it is noted that when calculating the thermal regime of kurums, it is advisable to use the concept of an effective coefficient of thermal conductivity. Using data from field observations, it was found that the effective thermal conductivity of soils at the experimental site does not exceed 1.0 W/(m⋅K).

Data close to this value are also given in ^[29] (1.0-1.2 W/m3), which summarizes the results of more than forty years of research on technogenic kurums conducted by JSC TSNIIS^[29-33] in order to study their effectiveness in the construction of the Baikal-Amur mainline. At the same time, results were obtained that have both scientific and practical significance. In particular, the essence of the work of the stone outline, characterized by summer and winter cooling effects on the soils of the base, is revealed and the main characteristics of the thermal processes occurring in different periods of the year are determined. The main patterns of the influence of the properties of the outline on the temperature regime of the underlying soils are revealed: the geometric dimensions of the stone filling; orientation in space (the magnitude of the slope, the nature of the location in relation to the engineering structure, the degree of embedding into the main body of the earth, etc.); daily, monthly, seasonal fluctuations in outdoor temperature; solar radiation, precipitation, wind; thickness snow cover; granulometric composition of the outline (the size of the stone, the presence of atypical inclusions, etc.). These research results are perhaps the most complete and comprehensive of the available data in the literature. G.P. Minailov, who summarized these results ^[29], notes that more than 250 facilities have been built and successfully operated for a long time (15-20 years) using rock sketches used as the main measure for the preservation of permafrost at the base of the roadbed and bridge cones on the BAM and AYAM highways. The materials of long-term comprehensive field and theoretical studies were used in the preparation of 15 regulatory and advisory documents ^[30,31,32]. For the purposes of this work, the following results obtained by JSC TSNIIS are primarily relevant. "It has been established that the cooling effect of the stone outline on the underlying soils of the foundations consists of two components: cooling influences in summer and in winter. In the summer, the outline is a thermal insulation layer that prevents heat from reaching the surface of the structure, and due to convection with daily fluctuations in outdoor temperature, it negates the warming effect of solar radiation. In winter, the outline reduces the warming effect of the snow cover due to its partial or complete "rupture"; due to air convection (due to the temperature difference between the outside air and the underlying soil), it completely eliminates the warming effect of the outline material. In this regard, in winter, the sketch cannot be considered separately without correlation with the snow cover."

Much attention was paid to the study of the processes of thermal regime formation in natural and man-made block massifs at the Chita Institute of Natural Resources ^{[22,24,34,35,36]}. Technogenic blocky rock massifs in the natural and climatic conditions of the mountainous regions of the Subarctic in the territory of Northern Transbaikalia were studied. Both field studies were carried out on the technogenic dumps of the Udokan deposit, and theoretical studies based on mathematical modeling of heat and mass transfer processes in blocky massifs of various nature. In a way, an article by the famous geocryologist I.I. Zheleznyak ^[36] summarizes the results of many years of research. The author examines in detail the mechanism of heat transfer in a blocky massif and compares theoretical results with the results of practical field studies, noting their qualitative and quantitative features. In particular, it is emphasized (important for our case), and previously noted in the writings of G.P. Minailov ^[29,33,34], the seasonality of the heat transfer level in a low-power stone outline (0.3-0.7 m), which turned out to be characteristic of large-block massifs up to 20 meters high. The bottom line is as follows. When the temperature in the upper part of the blocky massif (in winter) is lower than at some depth from the surface, the convective mechanism of energy transfer from the lower layers of the massif to the upper ones works. The rest of the time, when the temperature at the surface is higher than at depth (summer) The convective heat transfer mechanism does not work. The heating of fragments (pieces of rock) is carried out only due to the conductive component of heat exchange (thermal conductivity of rocks). The rate of energy transfer in this type of heat transfer is several times lower. Therefore, the heating of the massif is carried out less intensively than cooling at the same temperature gradient between the rock layers. The author notes that "during long-term cycles, additional cooling of the underlying surfaces and the accumulation of cold under the block deposits of natural and man-made nature occur, which ensures the formation of a frozen rock stratum under them." This fundamental scientific position is fully confirmed by numerous comprehensive studies of the effectiveness of various types of stone sketches (man-made kurums) conducted by Chinese scientists in order to substantiate new ways and means to improve the safety of railway operation in the permafrost zone ^[37-46]. Their conclusions completely coincide with the results of research by Russian permafrost scientists. In particular, in the works ^{[37,38,39,40]}, a long-term cooling effect of an embankment of large crushed stone (up to 0.2 m) is noted to preserve the frozen state of the road base. The authors, based on experimental and laboratory studies, claim that "a ventilated crushed stone embankment can effectively reduce the temperature of the permafrost under it, increasing the thickness of the permafrost under the embankment, ensuring the long-term stability of the permafrost under the embankment." In addition, theoretical calculations show that "given the significant possible increase in air temperature, it is recommended to use such an embankment for roads in permafrost areas to ensure the stability of permafrost and the normal functioning of transport infrastructure." That is, the global warming expected in the scientific world will not affect the stability of the foundations covered with a stone ventilated outline. This conclusion is quite suitable for the foundations of dumps. The positive experience gained in China, based on protection in the cryolithozone, proves the effectiveness of using man-made curums in the construction industry. All researchers note, for example, that "existing field tests and practical engineering applications show that a rubble embankment can provide the best cooling effect of the base."

Analyzing the research results published in our country and abroad, one generalizing conclusion can be drawn. There is a strong opinion in the scientific community of geocryologists and practical engineers (based on the results of many years of comprehensive: theoretical, experimental and field studies and practical experience) that any types of natural and man-made kurums (including rock dumps) lead to progressive cooling of the base on which they are located. Regardless of the type of origin and geometric dimensions of the kurums. This authoritative opinion is the basis for further specific forecast calculations on the formation of the thermal regime of rock dumps. In particular, there is no need to calculate long-term temperature cycles during the formation of the thermal regime of the blade bases. It is enough to calculate the first cycle of "freezing-thawing". Subsequently, as follows from experience (if there is no melting of the array in the first cycle), only additional cooling of the array will occur. Although the cooling rate will be lower with each cycle due to the additional increase in the height of the blade, but this does not matter in principle. It is important that the melting of the rock base of the dump will not occur. And the presence of a barrier whole made of frozen rocks for a long period is not in doubt.

Conclusion. An analytical review of literary sources containing theoretical and practical results of research on the formation of the thermal regime of natural and man-made kurums is carried out. The results of the analysis allow us to draw the following main conclusions. 1. There is a strong opinion in the scientific community of geocryologists (based on the results of many years of complex: theoretical, experimental and field studies) that any types of natural and man-made kurums (including rock dumps) lead to progressive cooling of the base on which they are located. Regardless of the type of origin and geometric dimensions of the kurums and the rock formations composing them (pieces, blocks). 2. When interacting with the rock base, the technogenic kurum works as an active thermal protection, changing its thermal resistance during the annual cycle. This makes it possible to successfully use man-made kurums to preserve the frozen state of engineering structures for various purposes in the cryolithozone, without resorting to active methods of cooling rocks. 3. When modeling the process of thermal interaction of technogenic kurum, it is quite acceptable, confirmed by the results of long-term experimental studies and field observations and their comparison with theoretical calculations, to consider kurum as a heat-protective layer, the thermal resistance of which depends on the effective coefficient of thermal conductivity, which changes its value when the sign of atmospheric air temperature changes.

The work was carried out according to the state assignment on the topic: "Thermal field and cryogenic strata of the North-East of Russia. Features of formation and dynamics" (No. 122011800062-5).