Characteristics of Permafrost and Foundation Design in Mongolia

CHARACTERISTICS OF PERMAFROST AND FOUNDATION DESIGN IN MONGOLIA

Namdag Choibalsan

Mongolian Association of Civil Engineers, Ulaanbaatar, Mongolia

Abstract

Mongolian permafrost varies between Ð0.3¡C and Ð4.0¡C, and underlies substantial areas of the country. About 80 settlements are located in permafrost regions. This paper describes Mongolian permafrost and expe- riences in the design and construction of buildings on perennially frozen ground in Mongolia.

Physical setting influenced by aspect, altitude and soil composition. Russian scientists N.M. Marinov and V.N. Popov (1963) Mongolia is located in the central Asia mountainous and Mongolian scientist N. Lonjid (1969) studied zone, far from the sea, and has a continental dry cli- Mongolian permafrost. Based on their results and those mate. Forest-steppe, steppe and desert (Mongolian of other scientists, maps of Mongolian permafrost at a Gobi) zones are present, with mountainous zones domi- scale of 1:1,500,000 (1987) were produced. Permafrost is nant. Seventy-five percent of the total territory of divided into four altitudinal zones: (1) mainly conti- Mongolia is between 560-1000 m a.s.l. and 25% is more nuous (over 80% of the area is permafrost); (2) discon- than 2000 m a.s.l. The annual average temperature is tinuous distribution (50-80% of the area is permafrost); generally below 0¡C (-1.0 to -8.7¡C) and the cold winter (3) scattered ÒislandÓ distribution (5-50% of the area is season lasts almost 5 months. The amplitude of the air permafrost); (4) ÒislandÓ distribution (less than 5% of temperature is high (+80¡C), precipitation is rare and the area is permafrost). The thickness of permafrost in winter snowcover thin (average 8 cm). As Mongolia is the four subzones varies between 5 and 200 m. located at 42¡-52¡N, the intensity of solar radiation is high. The following regional factors play an important role in determining the extent of the area with ground tem- Winters are mostly sunny and clear, the season being peratures defining the presence of permafrost. These stable and cold. Spring is relatively windy and cold, are slope, location, composition and physical properties and the summer is short. During summer, wind is rare of rocks, and vegetation characteristics. These combine and sometimes it rains for a long time. Autumn is short, to produce permafrost with temperatures down to clear skies are common, with fewer rainy days. Ð4¼C, but in some places, such as DarkhadÕs Hollow, even lower temperatures are observed. We have stu- Permafrost conditions died the permafrost temperature regime in a number of cities using a specifically equipped case study station Frozen ground occupies nearly 63% of the total terri- for 5-7 years. Observations in foundation soils below 14 tory of Mongolia and more than 15% of it is permafrost buildings in the above mentioned 4 subzones, allowed distributed in an ÒislandÓ form. Permafrost occurs heat exchange processes to be examined (Table 1). mainly in the mountainous areas. The temperature of Mongolian permafrost varies between -0.3¡C and -4.0¡C Mongolian permafrost is generally characterized by (Table 1). The depth of seasonal freezing and thawing low to moderate ice contents. High-ice content per- ranges from 1.0 to 5.0 m. Freezing and thawing >2.5 m mafrost appears only in river valleys and near springs. is directly related to coarse clastic soils which cover Frost heave, ground rupture and icing formation are about 70% of the territory of Mongolia. The characteris- very common. The most widespread frost heaving is tics of climate, as they pertain to foundation construc- observed in coarse-grained soils with sand, clay and silt tion, are presented in Figure 1. contents, especially near streams. Heaving can reach 16 cm. The depth and annual average temperature of permafrost and the active layer thickness is strongly Based on the results of our studies, the characteristics of Mongolian permafrost are as follows:

Namdag Choibalsan 157 Figure 1. Map of climatic zones for foundation construction. Zones labeled I to IV are characterized as follows:

(1) The annual average temperature of the permafrost (7) The permafrost table varies between 2.0 and 4.8 m is near zero (0¡C to -4¡C). in depth.

(2) Mongolian permafrost occurs in mountainous (8) In almost all permafrost construction, supra- regions of seismic activity, where earthquake forces permafrost water is apparent. exceed 7 on the Richter scale. (9) The thermal conductivity of frozen and thawing (3) Permafrost predominates in coarse, clastic ground. ground is relatively high (frozen, greater than 1.45 W/m¡C; thawed, greater than 1.35 W/m¡C). (4) Eighty percent of the towns and villages are located in permafrost areas formed in coarse clastic (10) Mongolian permafrost, if it remains frozen, has materials. a high bearing capacity and is suitable for other eco- nomic purposes, for instance, ice storage facilities. (5) In general, ice contents in permafrost are not high. The highest ice contents are observed at the bottom of Construction on permafrost the active layer or at the top of permafrost. The most obvious ice feature in Mongolia is the icing. About 80 towns and villages are located in permafrost regions. Construction on permafrost is carried out (6) Depths of seasonal freezing and thawing under mostly in continuous, discontinuous and scattered natural conditions vary between 2 and 5 m, while ÒislandÓ subzones. In areas where permafrost is distri- depths of seasonal thaw range from 2.5-6 m. buted in single ÒislandsÓ form, foundations are built

158 The 7th International Permafrost Conference Table 1. Typical characteristics of permafrost soils in Mongolia

upon thawed ground. The author, under supervision of maintain frozen ground. In addition, construction is Professor L.N. Khrustalev, Moscow University, deve- undertaken in winter to avoid disturbance of the terrain loped a thermal algorithm for the interaction between and thereby preserve the bearing strength of per- permafrost and construction. Consequently, the author mafrost. More than about 140 structures on permafrost and his collaborators have composed a regional per- have been completed since 1979 (Figure 3). mafrost map at a scale of 1:1,500,000 for planning construction activity. Acknowledgments

Construction on permafrost is complicated. Because of The assistance of Dr. H.M. French and Dr. A. the permafrost, the population centres of many Lewkowicz, University of Ottawa, in reworking the provinces have been relocated. The author carried out a original manuscript to make it suitable for the study during 1983-1990 and established that, because of Permafrost Conference is gratefully acknowledged. Ms. damage to about 250 structures and buildings cons- Shawne Clarke, University of Ottawa, kindly re-typed tructed since 1960 in 42 towns and villages, the the text. Mongolian government suffered damage of about 10 million USD (Figure 2).

The reason for this damage was the change of permafrost temperatures beneath the construction. Realizing that the formation of the permafrost temperature regime is quite different under natural conditions compared to under a building`s foundation, a new approach for the calculation of the permafrost temperature regime beneath foundations for Mongolian conditions has been developed. The main technique used is to elevate buildings in order to allow air circulation to

Namdag Choibalsan 159 a)

Figure 2. Examples of buildings built on permafrost in Mongolia that have Figure 3. Examples of buildings built on permafrost in Mongolia that have suffered structural damage due to lack of consideration of geocryological con- remained structurally sound due to appropriate design and construction ditions. techniques.

160 The 7th International Permafrost Conference