Active Method Accommodate Changes Associated with Permafrost Thawing Under Structure

At first glance, this method looks attractive in cases of degrading permafrost, but in fact it has very few successful applications. Permafrost thawing is accompanied by thaw settlement of soil and foundations if frozen soil is thaw-unstable. Thaw susceptibility of soil is determined by thaw strain — the ratio of thaw settlement to thickness of the soil layer prior to thawing. It is important to define the borderline value of thaw strain below which soil can be considered as thaw-stable. One of the old Russian Building Codes (SN 91-60 1963) defined this value as 0.03 if thaw settlement was evaluated for a load of 100 kPa. Soil with thaw strain greater than 0.03 and smaller than 0.1 is considered thaw-unstable, and soil with thaw strain greater than 0.1 as highly thaw-unstable. According to Velly et al. (1977), even soils with thaw strain equal to 0.02 require special attention. Thaw strain equal to 0.02 and less is typical of gravelly and sandy soils with dry densities greater than 1,900 kg m-3 and water content less than 12%. To be thaw-stable, clayey soils should be well-consolidated, should not have visible ice, and should have dry densities more than 1,800 kg m-3 and water content not exceeding the plastic limit of soil. Most permafrost soils are highly thaw-unstable and have thaw strain exceeding 0.1.

Thaw settlement beneath a building and differential thaw settlement should be less than the tolerable limits for such a building. Most buildings can hardly tolerate thaw settlement greater than 10 cm, and even structurally enhanced buildings can not tolerate thaw settlement greater than 30 cm. This means, for example, that for soils with thaw strain equal to 0.1, thaw depth beneath foundations can not be greater than 3 m. It is costly and difficult to design buildings which can tolerate thaw settlement, and there are numerous examples of unsuccessful applications of this method. High thaw-susceptibility of most permafrost soils, and low tolerance of buildings to settlement, limit application of the method to especially favorable conditions.

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