Principles of construction on permafrost bases and foundations

For construction outside the permafrost regions it is usually believed that all the load from a structure is transmitted through the base of the foundation to bearing ground while ground in contact with the sides of a foundation only occasionally carries a vertical load (as with piles and deep foundations). The interaction between foundations and ground is assessed differently in the regions of deep seasonal freezing and of permafrost. The load is transmitted to ground here through all the ground surfaces in contact with the foundation. This is associated with the fact that adfreezing of the foundation surface to the ground occurs with the result that tangential and normal stresses are transmitted from the ground to the foundation and from the foundation to the ground. The value of the transmitted stresses is then limited by the strength of adfreezing.

The direction of the stresses arising in the interaction between a foundation and the ground can change with time and depends upon the layer (seasonally or perennially freezing) in which the foundation is situated. The main types of embedding of foundations are shown in Fig. 18.1. Thus, within the layer of seasonal freezing (or thawing), tangential stresses transmitted from ground to the foundation develop during the part of a year in which the layer freezes and heaves, and are directed upwards . In response to adfreezing with the side of the foundation within the layer of perennially frozen ground, the frozen ground takes some portion of the load from the foundation not only through its base but also throughout the side surface. At the same time the side adfreezing of the lower part of the foundation with the perennially frozen ground layers increases foundation resistance to heaving developing in the course of freezing of a seasonally thawed layer. The foundation can be stable or can move (heave or settle) as a result of the combined effect of normal and tangential forces as well as heaving forces (and sometimes of frictional forces). Nonuniform movement of foundations

Frozen Footings

Fig. 18.1. Diagram of foundations (a, b - in the permafrost; c, d - in closed taliks; e,f- in seasonally thawing and seasonally freezing ground, respectively):

1 - 5 - ground (1 - in the frozen state throughout the period of construction

2 - artificially frozen before construction or during the period of operation,

3 - artificially thawed before construction or during the period of operation,

4 - subjected to seasonal thawing and freezing, 5 - in thawed state throughout the period of operation of the structure); 6 - the upper surface of the permafrost (a) and the base of the seasonally frozen ground (b).

Fig. 18.1. Diagram of foundations (a, b - in the permafrost; c, d - in closed taliks; e,f- in seasonally thawing and seasonally freezing ground, respectively):

1 - 5 - ground (1 - in the frozen state throughout the period of construction

2 - artificially frozen before construction or during the period of operation,

3 - artificially thawed before construction or during the period of operation,

4 - subjected to seasonal thawing and freezing, 5 - in thawed state throughout the period of operation of the structure); 6 - the upper surface of the permafrost (a) and the base of the seasonally frozen ground (b).

upward or downward is the main reason for deformation of buildings and constructions. Embedding of foundations below the base of the seasonally freezing or seasonally thawing layer can result in the effect of annual heaving processes and ground settlement being exerted only on some part of the side surface. Foundations embedded entirely in the layer of seasonal freezing-thawing experience heaving and settlement not only through the side surface but also through the base of the foundation.

Measures for ensuring the preservation and durability of structures and required operational qualities must be considered in design and construction on permafrost. This is achieved by selecting the optimal design features of the structure, the type of foundations, methods for enhancing the structural properties of bearing ground and by regulating the heat interaction between structures and bases. It is conventional to call a set of these measures or some part of them the method for ensuring the engineered structure's stability. By convention all these methods are unified into two large groups termed the principles of use of frozen ground as a foundation, according to Building Norms and Regulations 77-18-76.

Principle I: perennially frozen ground is used in foundations in the frozen state, being preserved during the period of construction and throughout the given period of operation of the structure. This includes the following methods for ensuring the structures stability: (a) by keeping the ground base frozen; (b) by limiting the thawing of the frozen ground base; (c) by preliminary freezing of the ground base; (d) by freezing the ground base in the course of construction and operation.

Principle IP. the perennially frozen ground is used in the thawed state (being allowed to thaw in the course of operation of the structure or with thawing to the design depth before construction). This includes the following methods for ensuring structural stability: (a) by adapting the above-foundation structure to permit differential settlement of the foundation with allowable thawing of the permafrost in the course of operation (a constructive method); (b) by prethawing of the permafrost; (c) by stabilizing the initial position of the upper surface of the permafrost.

Selection of the principle of construction on permafrost and of the method for its control is based, on the one hand, on comprehensive study of the geocryological conditions of the site taking into account possible change in the course of construction and operation of the structures and, on the other hand, on complete consideration of the features of the construction (sizes of foundations, structural materials being used and the service life of the structures), and of the mode of operation (with heat release or without heat release, using a wet or dry process, etc.). Data on the availability and transportation conditions of structural materials, on power supplies for construction and on the seasonal dependence of the work are important. Let us consider the methods for realization of Principle I of construction in the permafrost zone.

A method for ensuring structural stability by keeping the ground frozen or by cooling it

This method began to be used widely once N.A. Tsytovich had developed a procedure of thermal-physical and strength calculations for perennially frozen ground in foundations in 1928. The method is based on complete removal of the heat released by a building or a structure, i.e. on conservation of the existing temperature regime of perennially frozen ground, or its cooling to a lower than natural temperature. Ventilated cellars under floors, which had received wide recognition in practice due to the ease of construction and the reliability in operation, are the most widespread constructions for maintaining heat removal from the structure. From 1928 on, a great number of engineering structures in which the foundation was kept frozen were constructed in the North and the Russian North-East (cities of Vorkuta, Noril'sk, Yakutsk, Magadan, etc.). Half a century of experience of continuous operation has justified that this method of construction is thriving. Later on, it has been used widely by foreign specialists in the course of construction in Alaska and in the north of Canada. Basements of one of several types (Fig. 18.2a,b,c) are usually used in this case. These are: 1) open, i.e. a space open from all sides under a building

Fig. 18.2. Structures of buildings according to the first principle of using the ground as the base: a-c - cellars (a - ventilated open, b - ventilated enclosure with some air holes, c - with cold ground floor), d-f- building constructions (d - on fill with cooling pipes in the base, e - with limited thawing of the permafrost,/- with preliminary ground freezing); 1-3 - boundaries of the permafrost (1 - under natural conditions, 2 - under the conditions developing in the course of operation of the structure, 3 - after preliminary freezing).

Fig. 18.2. Structures of buildings according to the first principle of using the ground as the base: a-c - cellars (a - ventilated open, b - ventilated enclosure with some air holes, c - with cold ground floor), d-f- building constructions (d - on fill with cooling pipes in the base, e - with limited thawing of the permafrost,/- with preliminary ground freezing); 1-3 - boundaries of the permafrost (1 - under natural conditions, 2 - under the conditions developing in the course of operation of the structure, 3 - after preliminary freezing).

or under a structure ventilated with air throughout the year (both in winter and in summer); 2) closed, with air holes, by way of cellar space enclosed in a socle and by making ventilation openings in it such that the air required for keeping the base frozen or for its cooling circulates; 3) with a cold socle without air holes being constructed - as a rule, in regions with strong winds where the holes would be plugged with snow in winter.

Special heat engineering calculations based on the assumption of complete heat removal from structures, are carried out to determine the height of ventilated and non-ventilated cellars. The procedure of such calculations put forward by N.A. Tsytovich was developed by N.I. Saltykov, G.V. Porkhayev and others. Experience suggests that the height of cellars can vary from 0.5 to 1.8-2.0 m depending on the severity of the geocryological conditions and size of structures. In the North airtight multi-layered floors with heat insulation are used to reduce heat losses through the floor above the cellar.

If natural ventilation turns out to be inadequate according to the results of the heat engineering calculations, forced ventilation or artificial cooling of the ground is initiated. One such structural variant is shown in Fig. 18.2d.

A method for ensuring structural stability by thawing ofperennially frozen ground

With this method perennially frozen ground is allowed to thaw to a depth determined by heat engineering calculations. Foundations are put deeper into permafrost, below the stationary thaw basin (see Fig. 18.2e). One can gain some reduction in the foundation construction cost by partial or complete abandonment of a cooling system. However some increase of the mean annual temperature of frozen base ground takes place with thaw basin formation resulting in a bearing capacity decrease. In order for the bearing capacity decrease to be compensated the foundations must be put still deeper into the ground resulting in a rise in the cost of these foundations. In addition, the method requires very close control of the position of the upper boundary of the permafrost in the thaw basin.

A method for ensuring the stability of the structure by preliminary freezing of underlying ground

In the regions with permafrost islands it is rather difficult to site a building entirely within a frozen mass. In this case preliminary freezing of an unfrozen ground layer is carried out (see Fig. 18.2f) and then it is kept frozen with the help of a cooling system. The essential drawbacks of this method are its high cost and the extending of the construction time by the period of freezing, covering usually 3-6 months. Therefore this method has not been used widely in practice in civil and industrial engineering. However it is used widely in hydrotechnical construction: during the construction of dams within the permafrost zone for creating the counter-seepage frozen ground core in their body; for driving tunnels in ground with high water pressures and in the course of mine operations in regions of unfrozen ground.

A method for ensuring structural stability using ground freezing in the course of construction and operation

This method is used usually in regions where frozen ground of the unconnected type occurs where the permafrost table is separated from the layer of seasonal freezing. The base of the foundation is laid in the layer of thawed ground underlain by the permafrost. After construction of the foundations cooling systems are put into the permafrost at a depth of 0.5-f.0m, installed adjacent to each part of the foundations and then construction of the above-foundation structure begins. Freezing of the ground around the cooling systems causes an increase in its volume, with great horizontal pressures developing in the unfrozen ground between the cooling systems. Expansive forces fix the foundation preventing it from moving upward under the effect of heaving forces. With the passage of time complete ground freezing with retention of foundation stability occurs.

Types of foundations

In the course of foundation construction according to Principle I, shallow (surface) posts and piles are used (Fig. 18.3). Shallow foundations are used mainly for light-weight, one-storey, most commonly wooden buildings. They are placed in shallow excavations within the layer of seasonal thawing or directly on the ground surface. Operation of the structures with these types of foundations is accompanied by deformations with a pulsating pattern in the course of seasonal freezing and ground heaving as well as in the course of seasonal thawing and settlement. Pressure on a pile in a pile foundation is balanced by counter forces of the frozen ground on the pile face and by forces of the adfreezing of the ground and the pile side surface. If the permafrost strength does not ensure the stability of the structure on piles, post foundations are used, with the shoes being put into the base of a pile (see Fig. 18.3 II).

To fabricate the piles, reinforced concrete, wood or metal is used. Placing of single piles, pairs of piles and pile clusters has to be considered in the design. Piles may be placed by boring, and by lowering into position and by boring and driving (see Fig. 18.3 III). In the first case piles are installed in a preliminarily bored hole of diameter greater than the pile diameter. The space between the pile and the hole wall is filled with mud or mortar which freezes later. The bored-driven piles are driven into holes of diameter less than the pile diameter. This method can be used when the mean annual temperature is near to 0°C. The lowered piles are installed in preheated and prethawed ground.

As noted above, the second Principle of construction within the permafrost zone includes several methods for ensuring structural stability.

A method for ensuring structural stability by adapting the above-ground structures to differential settlements under permissible permafrost thawing in the course of operation This is a constructive method for designing and construction, and is the oldest method in foundation engineering in the North. When construction using this method is carried out a thaw basin is formed below buildings and structures (Fig. 18.4a) as the result of heat release from them and ground settlement occurs. This settlement is differential as a rule because of lithological nonuniformity of the ground, nonuniformity of heat release from the building and of load distribution in it and because thaw occurs

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  • anni
    How can constructions take place in permafrost?
    5 months ago

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