Geotechnical problems of construction on permafrost in Mongolia

Transcription

1 Sciences in Cold and Arid Regions 2013, 5(5): DOI: /SP.J Geotechnical problems of construction on permafrost in Mongolia D. Dashjamts *, Z. Binderya, J. Altantsetseg Mongolian University of Science and Technology, Baga toiruu 34, Sukhbaatar District, Ulaanbaatar, Mongolia *Correspondence to: Prof. D. Dashjamts, Mongolian University of Science and Technology, Baga toiruu 34, Sukhbaatar District, Ulaanbaatar, Mongolia. Received: May 27, 2013 Accepted: July 26, 2013 ABSTRACT Permafrost is found on 63% of the territory of Mongolia. This paper provides evidence that the main influences on permafrost formation are meso and micro factors of climate and geographical location. Regional classifications of permafrost areas in order to select the optimal principle of foundation design on permafrost are suggested based on many years experience and lessons learned from past construction works in Mongolian geotechnical and climatic conditions. Finally, optimal alternatives for designing foundations for construction on permafrost are presented based on the specific characteristics of permafrost in the classification areas and certain building dimensions. Keywords: permafrost; ventilation hollow; foundation; frozen soils 1 Climatic and geophysical conditions of permafrost in Mongolia Mongolia is a land-locked country in central Asia, located between Russia and China. It has an area of 1,565,000 km 2 and a population of 2.7 million (Figure 1). The formation of its permafrost deposits is connected with the mutual relationship of climatic factors of the warm and cold seasons of the year, particularly certain aspects of the heat exchange between continuous frozen and thawed soil layers. Permafrost formation is influenced by many factors such as seasonal changes, composition of soil deposits, water cycles on the ground surface, air temperature, precipitation, elevation above sea level, and surface relief. Figure 1 Map of Mongolia

2 668 The climatic impact can be classified into macro, meso, and micro factors. Macro factors include geographical position, general heat flux from solar radiation, and the positions of seas and oceans that influence the continental climate of a specific area. Meso factors include the basic relief of the area and elevation above sea level. Micro factors include comparatively low plateaus, surface relief, vegetation layer, mineral composition of soil and its moisture, and human activities such as manufacturing and farming. Depending on the climate, all these factors influence the process of permafrost formation. Due to the influence of climatic macro factors during the winter season in Mongolia, soil becomes frozen throughout the country. Then in the summer season the upper soil layer thaws. Therefore, it can be concluded that the permafrost distribution across Mongolian territory is influenced by the meso and micro factors of climate (Gravis, 1973). Soil becomes permanently frozen when the average annual temperature of the ground is below 0 C and little solar heat penetrates into the ground. The following are the main factors that influence the formation and development of permafrost in Mongolia: ¾ Climatic and geophysical conditions such as ground surface and air temperature, heat exchange between ground and air, solar radiation balance, and seasonal changes in climate. ¾ Exchange of moisture between soil deposits and atmosphere, and underground water locations, quantity, and dynamic changes. ¾ Geologic tectonic changes, formation, and composition of hard rock layers and friable deposits. ¾ Geographical location, mountain formations, surface relief, absolute and comparative altitudes, and conditions of depressions and elevations. 2 Aspects of climate and surface heat balance Mongolia has an extreme continental climate; its average annual temperature fluctuates between 7 C and +7 C. The average air temperature is a mean of measuring heat accumulation (Figure 2). The annual sum of temperature is estimated as 3,700 C in the northwestern part, 2,000 to 3,000 C in the central part within the high mountainous areas, and 1,500 to 2,000 C in the southern part of the country. This indicates that the permafrost in Mongolia is formed in much higher temperatures than in Siberia, Russia (Lonjid, 1969). The main factors influencing the ground heat balance include surface coverage (snow, vegetation, and water coverage), relief (surface altitude, slope size, and direction), and swamp lands and their hydrogeological conditions, composition, and soil moisture. Figure 2 Average annual air temperatures in Mongolia In Mongolia, discontinuous permafrost is formed at 2 C, in contrast to western Siberia where it is formed at 3 C. This difference can be explained by the extreme continental climate of Mongolia. In winter about 75% days are clear and comparatively thin snow coverage. While this thin snow coverage is insufficient to maintain the warmth of the soil, it blocks absorption of solar energy into the ground by reflecting solar radiation. Under the influence of strong air flows, this reflected radiation is absorbed in the turbulent heat exchange of the atmospheric layer. As a result, permafrost in Mongolia has existed for a comparatively long period of time. Mongolia has little snow coverage compared with Siberia, Canada, and the Scandinavian countries. In the winter and spring seasons, the relative air humidity declines as much as 30% 40% and the air becomes dry. Soil moisture, even when in the form of snow and ice, evaporates to a much greater extent. Consequently, this dryness reduces the quantity of heat energy from the sun that is accumulated in the ground. Furthermore, due to climatic conditions, the seasonal freezing of the ground is intensified at the end of winter and at beginning of spring. Results of the measurement of frozen soils in Ulaanbaatar (the capital city of Mongolia) indicated that the thickness of frozen soil increased by an additional cm.

3 669 3 Distribution, classification and assessment of temperature regime of permafrost in Mongolia Mongolia is located in the southern part of the world s permafrost deposit. Permafrost deposits formed for centuries in Mongolia equal the size of 15% of the country s territory. Permafrost is distributed on 63% of Mongolian territory. Permafrost in Mongolia has been formed as a result of glaciation and syngenetic and epigenetic permafrost rocks formed since the Pleistocene Age (Lonjid, 1969). Permafrost is found in Mongol-Altai, Gobi-Altai, Khangai, Khovsgol, and the Khentii Mountains, and most of it is discontinuous. Lonjid and Tumurbaatar (1977) classified frozen soil into five basic groups in Mongolia: (1) continuous permafrost; (2) discontinuous permafrost; (3) scattered permafrost; (4) sporadic permafrost, and (5) seasonally frozen soil (Figure 3 and Table 1). Figure 3 Distribution of permafrost in the territory of Mongolia Table 1 Classifications of permafrost in Mongolia No. Type Thickness (m) Temperature at 10 m deep ( С) Temperature at 12 m deep ( С) Average Max Average Max Average Max 1 Continuous permafrost to to Discontinuous permafrost to to Scattered permafrost to to Sporadic permafrost to to Seasonally frozen soil to to Note: The average temperatures are given for two depth intervals because they include different average temperatures for different areas. The average annual temperature of permafrost continuously formed in Mongolia fluctuates between 4 C and 0 C (Table 1). For instance, in the Khentii mountainous region the permafrost temperature ranges from 1 C to 2 C and reaches 4 C at altitudes over 2,000 m. In the valleys and depressions between short mountains, such as the Nalaikh depression located 45 km from the capital city, the permafrost temperature ranges between 0 C and 1 C. In the Khangai mountainous region with altitudes above 3,000 m, the average annual temperature range is 2 C to 4 C. In the forest areas facing north and northeast, the temperature is between 1 C and 2 C; in the areas facing south and southeast, it is 0 C. In the Mongol-Altai region, the average annual temperature of frozen soil at altitudes of 2,400 3,000 m is between 2 C and 4 C and drops below 4 C in the areas above 3,000 m. In the areas with altitudes below 2,400 2,500 m, it ranges from 1 C to 2 C, and in the inclined areas toward the south, it fluctuates around 0 C. In the Gobi-Altai mountainous region, the permafrost temperature at altitudes of 2,700 3,000 m is between 1 C and 2 C, and drops up to 4 C in more elevated areas. In the Khovsgol region, the temperature is between 2 C and 4 C at altitudes above 1,800 2,000 m, and goes below 4 C at altitudes of 2,400 2,600 m. In the Darkhad depression, permafrost temperature ranges from

4 670 1 C to 2 C, but in the regions with sporadic permafrost, the average annual temperature of the soil is 0 1 C. The map of Mongolia s geocryological conditions (Figure 3) illustrates permafrost distribution of Mongolia and can serve as a basis for general planning of regional development, construction of industrial and civil facilities, and in the selection of road locations in infrastructure development. In the future, it will be necessary to conduct detailed studies on permafrost in connection with sustainable economic and social development of the region through a long-term program. A study on the temperature cycle of permafrost in Mongolia has been conducted for 35 years on the territory of 16 districts in the mountainous region of Altai and Khangai by the State Central Institute for Construction. Data from that study show that the temperature of the permafrost fluctuates in a wide range between 0 C and 3.7 C. The lowest temperature was recorded in Gurvanbulag of Bayankhongor (t 0 = 3.7 C) and Arbulag of Khovsgol (t 0 = 3.7 C). The lower temperature range in Gurvanbulag is connected with the lower average annual air temperature (t m = 7.8 C) and large differences between the sum of summer temperature (Σt 3 =85.1 C) and the sum of winter temperature (Σt a = C). In contrast, the climate condition in the Arbulag district is mild, where Σt m = 4.1 C, Σt 3 =46.3 C, and Σt a = 99.3 C. The air temperature has such lower values because of comparatively little snow coverage, t w = 3.0 C. Therefore, it is necessary to prioritize consideration of the geocryological study findings in selecting the principles for construction on permafrost. The key points of the above-mentioned permafrost study are: (1) Formation of permafrost in Mongolia is a result of world glaciation, but today there are almost no macro factors influencing it. The permafrost condition in Mongolia is comparatively stable because of meso and micro factor influences on it. On a macro scale there is a relationship between the average annual temperature and the ground temperature throughout the entire territory of Mongolia, but no functional relationship was observed in the local areas (Table 1). This is connected with evidence that the influence of meso and macro factors differs from area to area. (2) The temperature of the permafrost is comparatively low due to the dry climate and thin snow coverage. In such conditions, turbulent warm exchange between the ground surface and atmosphere intensifies and results in low temperatures of the permafrost. (3) In areas with an average annual temperature below 2 C, it is possible to keep the permafrost in a frozen condition (Figure 2). This fact can influence the design of structural foundations in those areas. These conclusions were based on five key factors: average air temperature, temperature stability of the foundation soil of permafrost, snow coverage thickness that influences the quantity of solar radiation accumulated in the soil, wind speed, and the relative insignificance of anthropogenic effects on the ground surface due to the remoteness of towns and settled areas (Table 2). No. Place Table 2 Factors influencing the condition of frozen soil in some settled areas where permafrost is distributed Average annual air temp. ( C) Permafrost thickness (m) Permafrost temp. ( C) Ground surface temp. ( C) Average annual wind speed (m/s) Snow coverage (cm) 1 Khatgal of Khovsgol Tosontsengel of Zavkhan Bayantes of Zavkhan Tariat of Arkhangai Gurvanbulag of Bayankhongor Ulaanbaatar area Mongonmorit of Tov Nalaikh District of Ulaanbaatar Bayan of Tov Study on stability of construction on permafrost in Mongolia As previously mentioned, permafrost is distributed over 63% of Mongolia s territory, including 244 of the total 324 districts in the country. The settlement centers of many districts are located on permafrost deposits. Of these 324 districts, 140 are located on continuous, discontinuous, or scattered permafrost (Table 3). At present, construction works experience difficulties in 28 districts. Deformation of buildings in these areas has occurred due to construction done without previous research on the permafrost in these areas, resulting in substantial economic loss to reconstruct them. Therefore, it is necessary to conduct geologic, hydrogeologic, and geocryologic engineering studies in the region prior to constructing buildings on permafrost there. It is important to study the temperature regime of the frozen soil; its physicomechanical and thermophysical characteristics must be analyzed and, based on the findings, the optimal construction methods for foundation design should be selected.

5 671 Table 3 Towns and settled areas located on permafrost in Mongolia No. Type of frozen soil Area ( 10 3 ha) Depth of Number of districts and percent permafrost (m) and percent 1 Seasonally frozen soil (37.0%) (24.7%) 2 Sporadic permafrost (29.4%) (32.1%) 3 Scattered permafrost (22.4%) (33.9%) 4 Continuous and discontinuous permafrost (11.2%) > (9.3%) 5 Total areas with permafrost (63.0%) 244 (75.0%) Total area 1, Some examples of stability studies of construction facilities on permafrost 5.1 Continuous permafrost in Khatgal City Khatgal City is located in the northern part of Mongolia between the Khoridol Saridag Mountains in the upper zone of the Khovsgol Mountains in the Khangai region. It has an extreme continental climate: it is cold in winter and cool in summer, with an average annual temperature 3.8 C. The soil in the local area is predominantly gravels and gravelly-sandy soil. However, in the northern parts of the region, sandy and clayey soils interspersed with gravels are widespread. The thickness of the permafrost is m, in some areas as thick as 100 m, and the temperature of the frozen soil is 0.5 C to 2.0 C. Construction works began in Khatgal in A wool-washing factory, one of the country s first factories, was established there, and during several stone and brick structures were built. New buildings for a school, a hospital, and the wool-washing factory were constructed. In this area, continuous permafrost is contiguous to the upper layer of soil which freezes and thaws seasonally (the active layer), and this caused the school and hospital buildings to break down after one year, and cracks appeared in their walls. During the reconstruction, reinforced concrete was placed outside the stone foundations of the buildings but this had no positive results; settlement of these buildings continued and rendered them unusable. For example, the two-storey factory administration building had brick walls, 8m 48m (Figure 4). The design of the building was developed at the Central Institute of Construction Design in In October, 1976 the building maintenance was started but one month later cracks appeared in the walls. After four months of operation, permafrost melting resulted in significant structural damage of building so the building operation was stopped in February, Upon reconsideration, the first principle for constructing a building on permafrost should have been followed, specifically, installation of ventilation hollows based on thermo-technical estimations and calculation of changes in the permafrost temperature regime. The permafrost temperature around Khatgal City is relatively stable, the ice concentration in the soil is high, and the average annual temperature is above 3.8 C. All of these factors should have been incorporated into the structural design of those buildings. Figure 4 Cracks on a factory administration building constructed on permafrost in Khatgal City 5.2 Discontinuous permafrost in Nalaikh Town Nalaikh Town is located in the central part of the country in a tectonic depression 36 km from Ulaanbaatar, the capital city of Mongolia. The permafrost in this area was assessed by the Russian scientist Tsytovich (1950) as a northern part of the world permafrost plateau and a continuation of the Siberian permafrost deposit. The soil composition consists of gravel, sand, and clay. The first modern study on the physical and mechanical properties of permafrost in Nalaikh Town was conducted by Tsytovich (1972) and later by Lonjid and Tumurbaatar (1977) who investigated its geotechnical characteristics. In the Nalaikh depression the average annual temperature is 3.5 C and discontinuous permafrost exists. The thickness of the permafrost is m and its temperature ranges between 0.7 C and 1.0 C. Construction of buildings for a mining plant (Figure 5), one of the largest industrial facilities in Nalaikh, was accomplished with ventilation hollows by specialists from the Kuzbass Project Institute of the Soviet Union (now Ukraine), based on the principle of keeping the permafrost conditions unchanged. This was an initiative by the Russian professor N. A. Tsytovich. The buildings

6 672 were constructed on reinforced concrete foundations at depth of 4 m. Around the entrance of the mining vertical shaft there are double vertical walls constructed. In this way, the principle of maintaining the permafrost condition was successfully implemented in the southern boundary of the world permafrost plateau. Figure 5 Administrative building (left) and entrance to the vertical shaft of the Nalaikh coal mining plant with ventilation hollows (right) 5.3 Scattered permafrost in Ulaanbaatar Ulaanbaatar City is in the Hentii mountainous zone, which has scattered permafrost. Construction works were conducted in the area between 1930 and Small islands of permafrost which were found under the buildings of the industrial complex, Thermo Power Station No. 1, the Construction Material Producing Complex, and the National University of Mongolia are now nonexistent due to gradual thawing over many years (Figure 6). These buildings were built without ventilation hollows but using the second principle for building on permafrost, which is to thaw and compact the permafrost before construction, or to take into account the thawing process and construct directly on permafrost. Permafrost thawing resulted in foundation settlement in these buildings, but it was comparatively minor due to the gravel contained in the soil. Figure 6 Industrial complex and Thermo Power Station No. 1 in Ulaanbaatar (1934) After a year of operation, cracks appeared in building walls of the National University of Mongolia, the Children s Palace, and some administrative buildings in Ulaanbaatar (all constructed between 1950 and 1960), apparently due to thawing of permafrost under the foundations. However, the settlement eventually stopped due to the comparatively small thickness of the permafrost layer. The buildings were reconstructed in the 1960s and the 1970s, and now maintenance is normal. Review of the construction experiences on permafrost in Khatgal, Nalaikh, Ulaanbaatar, and other districts of the country demonstrate that most buildings have been damaged by permafrost thawing. 6 Principles for optimal foundation design on permafrost in Mongolia As referenced above, there are two principles of foundation design to be followed in constructing buildings on permafrost: Principle I, keeping the soil permanently frozen; or Principle II, thawing and compacting the permafrost before the construction, or taking into account the thawing process and constructing directly on the permafrost. According to Dashjamts (1999), the territory of Mongolia can be divided into two main zones based on consideration of its geocryological and climatic conditions (Figure 7). In the Region I areas, where the climate is colder with an average annual temperature falling below 2 C, soil freezes permanently and continuous, discontinuous, and scattered types of permafrost have been developed. In such conditions, Principle I of foundation design on permafrost (keeping the permafrost in a frozen condition) is preferred. In the Region II areas, with an average annual temperature ranging from 0 C to 2 C, the permafrost is relatively thin and sporadic. In such areas, Principle II of foundation design on permafrost should be followed, which offers solutions to deal with the possibility of permafrost thawing. The Principle II method of construction on permafrost should be used in areas with sporadic permafrost distribution, and it offers two options. The first option takes into account the possibility of thawing during building operation, and the foundation is designed to be able to endure slight settlement (the constructive method). The second option is to thaw and compact the frozen soil

7 prior to constructing buildings. The first option should be used when soil settlement resulting from permafrost melting is expected to be within the permissible limit, whereas the second method can be used when rock and 673 gravel deposits are located under the thin permafrost layer. If the thermal regime of the permafrost is unstable, pre-thawing and compacting of the frozen soil is suggested as a permanent solution (Figure 8). Figure 7 Regional classification of permafrost in Mongolia Figure 8 Scheme for optimal foundation design on permafrost in Mongolia

8 674 By Principle I, there are three basic methods of maintaining permafrost in the frozen condition when constructing buildings: Construction of the building on a soil embankment (Figure 9а); Raising the building above the surface by a shallow pile foundation (Figure 9b); and/or Constructing an air duct foundation (Figure 9c). In areas of Principle I construction of buildings with widths exceeding 12 m, installation of ventilation hollows is preferred (Figure 9b). In summer, larger structures should have mechanical refrigerating devices to cool the pad. For smaller structures with widths less than 12 m, air duct foundations and soil embankments are required. Figure 9 Basic methods of maintaining permafrost in the frozen condition In order to maintain permafrost in the frozen condition, it is necessary to assess the thermal regime of the building base (Figure 10) and take measures such as ventilation hollow or use thermoisolative materials to prevent heat from the building from penetrating into the base. Although various methods of maintaining soil in a frozen state have been used in construction of many buildings in Mongolia, it has been proven from many years experience that the best way to maintain permafrost is by installing ventilation hollows under the building (Figure 11). To successfully do this, accurate assessment of the meso and micro factors of the area and the dimensions of the entire building with its thermo-technical indicators is a prerequisite. The appropriate size of the ventilation hollow and the temperature inside it should be estimated for each case individually. Research results conducted on permafrost in Mongolia, involving the thermal regime of building bases and soil temperature measurements, indicated that the main factor affecting the stability of foundations constructed on permafrost is the climatic conditions of the area. Therefore, estimation of the appropriate temperature in the ventilation hollows is the main issue to be solved, since the basic requirement for maintaining soil in a frozen condition is keeping the ventilation hollows constantly cool. Constant cooling of ventilation hollows stabilizes the temperature of the frozen base of the building. In other words, cooling devices help to reduce the temperature of the active soil layer that absorbs heat from the structure, and keep the temperature of the permafrost deposit under this layer constant. The difference between the temperature of the upper boundary of the permafrost deposit (or the bottom of the active soil layer) and the temperature in the ventilation hollows is related to the difference between the thermal conductivity coefficients of frozen and thawed soil. Figure 10 Heat transfer from building to permafrost Figure 11 Calculation scheme for a foundation ventilation hollow on permafrost The temperature of the upper boundary of the permafrost layer should be below or equal to the temperature inside the ventilation hollow (estimation made for summer season):

9 675 T T c, a 0 ' (1) where T 0 ' is the temperature of the upper boundary of the permafrost layer, and T c,a is the average annual temperature inside the ventilation hollow. The temperature of the upper boundary of the permafrost deposit depends on the temperature in the ventilation hollow. Consequently, if the temperature in the ventilation hollow increases, the permafrost layer starts thawing. The relationship between these two temperatures is (Gravis, 1973): ( 1 ) τ th = T c, a λ th λ f T th ca (2) τ y T 0 ' /, where T th,ca is the average summer temperature in the ventilation hollow; τ y is the duration of the year in days, 365 days; and τ th is the duration of the summer in the area. Mongolian Construction Standards Codes suggest the following equation to estimate the average annual temperature in ventilation hollows based on suggestions of Tsytovich (1972): T c, a k 0 T0 ' = (3) where T 0 ' is the temperature of the upper boundary of the permafrost layer, and k 0 is the coefficient related to the ratio of the thermal conductivity coefficients of the frozen and thawed soil (λ f /λ th ) and the duration of the period of the year with temperatures below 0 C. Coefficient k 0 can be estimated by the following equation: k0 = 1 τ th τ y / πtg( τ y 1 πτ th τ y ) λth 1 λ f (4) According to this methodology, coefficient k 0 is estimated in connection with regional soil characteristics and climatic conditions of Mongolia. For our research, 73 towns and settlements with an average annual temperature below 2 C were selected and classified into four basic regions as Khovsgol and the Mongol-Altai, Khangai, and Khentii Mountains areas (Figure 12). Figure 12 Regional classifications for estimating coefficient k 0 for ventilation hollow design in permafrost in Mongolia Our results indicated that the meaning of the heating regime coefficient k 0 of the ventilation hollow determined by the currently followed Building Code is much higher than the Mongolian climate would suggest. This finding means that the Code is unlikely to determine the ventilation hollow temperature correctly. Determining the coefficient k 0 with respect to actual regional soil characteristics and climatic conditions of Mongolia will improve the probability of ensuring frozen soil stability under buildings. In our research, coefficient k 0 was determined by indications of towns and settled areas where durations of 185 to 220 days with below 0 C temperatures were considered. Herein, the k 0 coefficient is identified by estimations of the number of below 0 C temperature days, for example,190, 195, 205, and 215 days in the Khovsgol region; 200 and 220 days in Mongol-Altai; 185, 190, and 220 days in the Khangai Mountains; and 185, 190, and 210 days in the Khentii Mountains. To compare the estimation results with the Construction Standards Code, Table 4 presents our results with five-day increments during the winter, that is, 180, 185, 190, 195, 200, 205, 210, 215, and 220 days, making it possible to select the optimal k 0 coefficient. Table 5 presents our original estimation results. The evidence documents that it is impossible to use the k 0 coefficient in the normal procedures that are followed currently.

10 676 Table 4 Coefficient k 0 estimated based on regional soil characteristics and climatic conditions of Mongolia λ f /λ th Number of the days in a year with temperatures below 0 C Table 5 Duration of winter and coefficient k 0 No. Region Province, Town Duration of the winter (days) k 0 1 Arkhangai, Tariat Khangai Bayanhongor, Galuut Zavkhan, Bayantes Khovsgol, Khatgal Khovsgol 5 Khovsgol, Arbulag Ulaanbaatar, Nalaih Khentii 7 Tuv Aimag, Zuunmod Mongol-Altai Bayan-Ulgii, Tsagaannuur Conclusions The following conclusions have been drawn based on the above estimations: (1) It is necessary to conduct detailed geocryologic studies in the regions of Mongolia where permafrost is widespread, and to establish the functional relationships between the influencing factors of the geocryologic conditions. (2) It is important to choose the optimal principle (either I or II) for construction work and foundation design according to the physical, mechanical, and thermo-physical characteristics of the permafrost in each region. (3) Detailed study on the factors influencing permafrost thawing that result in settlement, such as the pressure of the foundation on the frozen soil (base), the soil composition, and the thermal regime of thawing, is required for optimal foundation design. (4) We have presented a scheme for optimal foundation design on permafrost based on the geocryological conditions of Mongolia and certain building dimensions (Figure 8). (5) Principle I of construction on permafrost is to maintain the permafrost in a frozen condition using ventilation hollows based on the estimation of meso and micro factors of an area, and the dimensions of an entire building and its thermo-technical indicators. Principle I should be used in regions where the annual average air temperature is below 2 C or in areas with continuous, discontinuous, and scattered permafrost. (6) Principle II of construction on permafrost should be used in areas with sporadic permafrost. Principle II offers two methods of construction work on permafrost: if the permafrost thawing beneath the foundation is slight, the structure could be constructed directly on permafrost; the second method is to thaw and compact the frozen soil before constructing buildings. (7) In the Region I areas of Mongolia where the average annual temperature is below 2 C, for buildings with widths greater than 12 m, mechanical cooling devices in addition to ventilation hollows could be used if required to keep the soil in a frozen condition, especially in summer. For small structures with widths less than 12 m that are to be used only for summer seasons, it is recommended to elevate the foundation by soil damps. (8) In the Region II areas with sporadic, comparatively thin permafrost, thawing and compacting the permafrost layers before constructing the foundation is considered as a permanent solution. It is possible to design a foundation for construction directly on permafrost if settlement due to permafrost melting beneath the building is expected to be slight. REFERENCES Dashjamts D, Theoretical and practical bases for foundation design on permafrost of Mongolia. Science Paper of MUST No. 3/35. Gravis GF, Geocryological Conditions of Mongolia. Proceedings of International Conference on Permafrost,Yakutsk, Russia. Lonjid N, Tumurbaatar D, Permafrost distribution in Mongolia and its influence on construction. Proceedings of 7 th Conference on Structural Engineering. Ulaanbaatar, Mongolia, pp. 7. Lonjid N, Mongolian Permafrost. National Publishing House, Ulaanbaatar, Mongolia, Tsytovich NA, Mechanics of Frozen Soils. Higher Education Press, Moscow, Russia. Tsytovich NA, Permafrost in Ulaanbaatar area of Mongolia. Proceedings of Permafrost Institute. Volume YII. Science Academy of USSR, Moscow.