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International Society for Soil Mechanics and Geotechnical Engineering INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. 51 SUB-SECTIONVId S PECI AL PBORI . EMfi I K FOUNDATION BHGIBEgEIHQ STABILITY OF FOOTDATIOSB nw PEBUAHENTLY FBOZEB GROOHD haery CARLSQK Corps of Engineers, St. Paul District Sn MMAHT The stability of foundations on permanently frozen ground depends on the soil characteristics, the quantity of ice in the 6oil, a nd the depth of thawing below the foundation. Heaving and settling common to fine - grained or frost action soils in temperate zones are also common to similar soils in arctic and subarctic re­ gions which are subject to seasonal freezing and th awing. Tests of insulating materials and deep sand- gravel fills indicated that they did not appreciably reduce heat transfer into the ground under structures. The use of piling and an air space under heated structures appears at this time to be the most effective method of in­ suring the stability of such structures constructed on permanently frozen ground which loses bearing capacity upon thawing. During the recent war years, numerous frozen, even though the composition, texture, structural failures of runways, roads, and and moisture content are the same. The pres­ buildings occurred at military installations ence of underlying frozen ground affects the in Alaska and northern Canada. Generally, the physical properties of the unfrozen ground failures were caused by the melting of perma­ above. Ice fills some or all of the voids be­ frost (permanently frozen ground) due to the tween the soil particles in frozen ground and removal of the natural ground insulation in­ acts as a cement. The strength of frozen cident to construction operations and, in the ground therefore approaches that of ice. The case of buildings, the introduction of arti­ compressive strength of frozen ground in­ ficial heat to the ground. An investigation creases as the temperature is reduced and it is now being made by the Corps of Engineers, also varies with the amount of voids filled St. Paul District, for the purpose of develop­ with ice, in general, increasing until the ing methods for the design and construction voids are filled. of airfields in arctic and subarctic regions. In permafrost regions the relative in­ Permafrost is found at depths of one to tensity of heaving of ground during freezing many feet below the surface in Alaska, Canada, is determined by the texture of the soil,the Siberia and other parts of the world where thickness of the soil layer above permafrost, the mean annual temperature is below 0° C. the availability of a ground water supply,and Seasonal freezing may penetrate to permafrost the duration and intensity of cold weather. in many locations, thus providing a continu­ Essentially, the causes and effects of freez­ ously frozen layer from the ground surface to ing and thawing are the same as in temperate the bottom of permafrost during the winter zones for similar conditions. Where the layer season. Permafrost exists as a continuous lay­ of soil above the permafrost is relatively er or as discontinuous layers. thin, as in northern Alaska, there is gener­ The natural temperature of the ground ally no appreciable heaving when the ground follows a cyclic variation with the seasons. freezes during the winter. Farther south The amplitude of these cyclic variations is where the soil layer is thicker and more greatest at the ground surface and decreases ground water is held above the permafrost, to zero at some depth below the surface.There more heaving can be expected where other con­ is an increasing time lag with depth for sur­ ditions are similar. Generally, the effect of face temperatures to penetrate into the ground freezing and thawing is more severe in arctic under natural conditions. When the natural in­ and subarctic regions than in temperate reg­ sulating cover is removed and an artificial ions because of the greater intensity and cover of relatively high thermal conductivity duration of cold weather. such as a road or runway surface is placed on When frozen fine- grained soil thaws, it the ground, the cyclic temperature variations generally becomes very plastic and the excess are generally increased. Where a heated struc­ water flows out from under any building or ture is placed on the ground, seasonal temper­ pavement load which has been placed upon it ature variations may be largely eliminated resulting in settlement and damage to the due to the nearly uniform heat transfer into structure. The magnitude of the ensuing set­ the ground during all seasons of the year.The tlement depends on the soil characteristics, general effect of the replacement of the na­ the quantity of ice in the soil, and the depth tural ground cover with an artificial cover of thaw. The rate of settlement depends large­ is to cause an increase in the amount of heat ly on the rate of thawing as well as on the transfer into the ground a consequent in­ type of construction and dimensions and orien­ crease in the normal ground temperature. Thus, tation of the structure. There is a definite where permafrost existed prior to construct­ tendency for structures to settle most on the ion, the effect of the construction is to south side because the ground on this side cause a lowering of the permafrost surface. receives the normal amount of solar radiation Where the permafrost is of a silty character plus the reflection from the south wall of and melting occurs below the foundation level the structure. Ground on the shaded side of a of a structure, the stability of the structure structure receives less solar heat and, as a is endangered. result, the depth of thaw as well as the re­ Physical properties of frozen ground de­ sultant settlement is less thaA on the south pend on its composition, texture, ice content, side which is directly exposed to the sun. ice distribution, and temperature. Thawed Sliding or flowing of surface ground ma­ ground has somewhat different physical char­ terials is common in the permafrost region acteristics than ground that has not been during the summer when it is thawed and in a 52 Construction of foundation supported by steel piles in permafrost. Note ground ice in back­ ground . 4 April 1946. Earth crack In roadway shoulder at Northway Airfield, Alaska. 4 April 1946.Differential settlement of garage building at Northway Airfield, Alska. permafrost, temperature of the ground, and the type of foundation construction to be em­ Silt soil subgrade after stripping of vegetation ployed. rihere the soil is largely silt or very and topsoil. Note water from thawing of frozen fine sand, elimination of the frozen condition ground, will convert the ground into a plastic and un­ stable mass upon which construction operations plastic condition. Such actions occur on hill­ are extremely difficult. sides There there is a concentration of ground Methods used in arctic and subarctic moisture at shallow depth immediately above regions for construction of foundations on the permafrost or seasonal frost level. Con­ ground which does not heave are not appreci­ sideration should be given to the possibility ably different from those which are used in of sliding or flowing of ground, especially temperate climates. In planning a foundation in connection with the location of roads. that will rest on or in permafrost, the bear­ Removal of the natural vegetative cover, ing capacity of the underlying ground should such as trees, grass, and peat, through the be determined both when frozen and when thaw­ action of fire and man, results in greater ed. Unless positive measures are taken to pre­ heat transfer into the ground, melting of the serve the permafrost, foundations must be de­ ground ice, and settlement or caving of the signed for the bearing capacity of the founda­ ground. Such action produces depressions of tion material in a thawed condition, where it various sizes and shapes varying in width is desired to preserve the permafrost under a from a few inches to several miles and up to building, it is necessary to provide an air many feet in depth. space between the floor of the building and In building structures in permafrost the ground and to control the circulation of areas, the permafrost may be retained or eli­ air in this space. Vent shields are sometimes minated depending on the character of the constructed around the base of a building be­ soil, ground water conditions, thickness of tween the ground and the floor. The vents are 53 Settlement of floor in motor pool building at Horthway Airfield, Alaska. Floor orinally flush with top of pipe coupling. Condition on 11 April 194- 7. grained fill material in the base courses. This raised type of construction allows the Settlement of floor in motor pool building at wind to remove the snow and provides a well Northway Airfield, Alaska. Floor originally drained roadway and base. In many cases, it flush with top of pipe coupling. Condition an is more economical to place fill material 4 April 1946. over the natural ground than to strip the ve­ opened in winter to permit cold air to circu­ getation or remove soil which is unstable late above the ground surface, thus permit­ when thawed. However, the tests reported on ting deep freezing. They are closed in summer herein indicate the inadequacy of insulating to retain the cold in the ground and to keep qualities of such fills which are composed of the permafrost from thawing.
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