Passive Method Maintain Frozen State of Soil

This method is the main one used in the permafrost regions, but it was not fully appreciated or widely used until the 1950s after a long period of unsuccessful attempts to accommodate changes associated with permafrost thawing under structures. Numerous buildings on permafrost experienced substantial deformations because of thawing of permafrost and thaw subsidence of foundation bases. This has happened throughout the entire Russian permafrost region when methods based on accommodation of changes related to thawing permafrost were mainly applied. Even with the relatively low ice content of the silty clays in Vorkuta, many buildings have been destroyed (Zhukov 1958). Engineering means used for preservation of permafrost under buildings greatly reduced the percentage of deformed buildings.

This method is the only one which can protect structures from excessive deformations associated with thawing of ice-rich fine soils. Foundations built according to this method bear heavy load, have minimal settlement, and can be easily protected from frost heave.

The method is generally recommended for areas with a permafrost temperature of -3°C and below. "As a rough guide, the situation should be critically evaluated when the mean ground temperature is warmer than about -3°C, if the ground is to be maintained in a frozen condition following construction" (Johnston 1981, p 251). The first Russian building code for survey, design, and construction of railroads and their infrastructure in the permafrost regions recommended the active method as technically sound and economical in areas with warm permafrost (temperature above -3°C). Permafrost in the town of Skovorodino was the first example of a place recommended for application of this method. Bykov and Kapterev (1940) showed that such an approach was erroneous.

Successful applications of the passive method in areas with warm permafrost in Russia, such as Chita, Vorkuta, Igarka, and Skovorodino, and many others, showed that the passive method can be used in regions with warm permafrost.

There are several engineering means for maintaining frozen soil beneath buildings, and ventilated air space (crawl space) beneath elevated buildings is the most widely used. In Alaska and Canada this space is usually completely open, in Russia it is ventilated through relatively small openings (vents) in a foundation wall or a wall beam. The total area of openings is evaluated by using the so-called modulus of ventilation (MV), which is the ratio of the total area of openings to the footprint of a building. For buildings with the open crawl space, the MV is equal to the height of a crawl space multiplied by its perimeter. The Russian building code and some other sources provide methods evaluating MV. Saltykov (1959) presented a table which can be used for preliminary evaluation of the MV (Table 17.1). Similar MV values are recommended by the Handbook on Construction on Permafrost (Velly et al. 1977) and by Tsytovich (1975).

Design of ventilated crawl space in Russia has traditionally aimed at two goals. The first is to keep the soil beneath the buildings in the frozen state, and the second is to provide a comfortable temperature at the floor above the ventilated crawl space with minimal thermal insulation to reduce its cost. The MV approach reflects both these goals.

Table 17.1 Recommended modulus of ventilation (based on Saltykov 1959)

Thermal resistance of structure above crawl space (m2 h°C kkai-1)

Indoor air temperature

Modulus of ventilation for permafrost zones

Northern

Central

Southern

Table 17.1 Recommended modulus of ventilation (based on Saltykov 1959)

Thermal resistance of structure above crawl space (m2 h°C kkai-1)

Indoor air temperature

Modulus of ventilation for permafrost zones

Northern

Central

Southern

1

15

0.0025-0.005

0.005-0.02

0.02-0.03

30

0.0075-0.015

0.015-0.05

0.05-0.08

2

15

0.0015-0.003

0.003-0.01

0.01-0.015

30

0.0035-0.007

0.007-0.02

0.02-0.03

3

15

0.0008-0.002

0.002-0.006

0.006-0.009

30

0.002-0.0035

0.0035-0.01

0.01-0.015

In Norilsk (northern permafrost zone), ventilation of the crawl space is designed with an MV ranging from 0.00225 to 0.004 (Shamshura 1959; Maksimov et al. 1978). For example, for a building 50 m by 20 m in Norilsk, if thermal resistance over the crawl space is equal to 3 m2 h°C kkai-1 and air temperature in rooms on the first floor is equal to 15°C, the total area of opening for ventilation of the crawl space can be between 0.8 m2 and 2 m2 (for an open crawl space with a height of 1 m, the area open for ventilation is equal to 140 m2). Velly et al. (1977) presented an example of the evaluation of the total area of vents in the wall beam of the crawl space for a building 60 m long and 20 m wide at Dikson, a seaport in the Russian Arctic. It is expected that, during the lifetime of the building, soil temperature will increase from -6 to -3.6°C and the total area of vents should be equal to 0.66 m2 with MV = 0.00055 (Velly et al. 1977). The ventilated area of the open crawl space of 1 m height would be equal to 160 m2, or 240 times greater. Mean annual soil temperature under the open crawl space would decrease to about -10°C.

Thermal resistance of insulation above ventilated crawl spaces in Russia is 3-5 times smaller than required in Alaska. As a result, mean air temperature in the crawl space is intentionally kept warmer than it could be in an open crawl space, and resources in chilling permafrost remain unused when MV depends on thermal resistance of the floor above the crawl space. The example for the building in Dikson (see above) shows that the opportunity to keep the permafrost at a lower temperature was greatly reduced in an attempt to satisfy both conditions. Such an approach in reaching two competing goals has been implemented in Russian building codes for permafrost regions. This approach is at least questionable and some Russian arctic engineers do not support it. According to Dokuchaev (1963, p 121): "Preference should be given to an open crawl space (especially in regions where mean annual permafrost temperature is above -3°C) because open crawl space guarantees low permafrost temperatures. Money saved on wall beams around the crawl space could be spent on increased thermal insulation." This advice of one of the best Russian permafrost engineers has not been followed. For example in Chita, where mean annual permafrost temperature is about -0.3°C to -0.5°C, MV is equal to 0.015-0.03 for a building with continuous foundations, which is 10-15 times less that could be provided by the open crawl space.

There is one more disadvantage of the ventilated crawl space with small vents. It is not easy to observe and, thus, does not allow for easy inspection. Leaks in water or heating lines, which are usually attached to the ceiling of the crawl space, can remain undetected for a long time and badly damage frozen foundation soils before detection.

According to Shamshura (1959), permafrost temperature under an open crawl space becomes almost equal to mean annual air temperature, and permafrost temperature under a crawl space ventilated through vents is several degrees warmer. Maksimov et al. (1978) also found that mean annual soil surface temperature in an open crawl space is the practically equal to the local mean annual air temperature there. They reported that the winter temperature greatly depends on the type of crawl space. In an open crawl space, it is very close to outside temperature. In a poorly ventilated crawl space, the winter mean air temperature can be more than

Annual mean soil temperature, C Fig. 17.2 Change in soil temperature under shoe factory in Yakutsk (based on Voytkovsky 1968)

12°C higher than the outside air temperature. Summer mean air temperature in the crawl space is about 1-2°C colder than outside air temperature (Maksimov et al. 1978).

Figure 17.2 shows a decrease in permafrost temperature in an effectively ventilated crawl space over a 6-year period in Yakutsk, Russia. The gradient in annual mean soil temperature in 1963 showed that the decrease in soil temperature continued.

An effectively ventilated crawl space reduces permafrost temperature by several degrees. It occurs during the years after construction, and can not be taken into account by design if preliminary cooling of soil prior to construction has not been applied. Design relies on permafrost temperatures during construction. Decrease of permafrost temperature under a crawl space during the service life of a building increases the Factor of Safety for the bearing capacity of foundations.

Cooling of permafrost beneath a crawl space takes years, and consequently bearing capacity of soils and foundations increases with time. The Russian Building Code (SNiP 1991) requires a decrease of soil temperature of plastic frozen soils to about -2 to -3°C. To take the advantage of such cooling into account, soil temperature should be reduced prior to construction or during construction of foundations. The simplest way is to plow snow from a site for several years prior to construction and thermally insulate the soil surface in summer. Soil can be also chilled through pipes used as piles or through holes used for the installation of piles (Maksimov et al. 1978).

A ventilated open crawl space provides a continuous decrease in permafrost temperatures and increases design bearing capacity of permafrost up to two-fold (Table 17.2). As a result, an increase permafrost temperature by several degrees due to climate change or other factors can take place without any impact on the structure

Table 17.2 Increase in bearing capacity of piles during service life (based on Lukin 1966)

Design characteristics

Building 1

Building 2

Building 3

1950

1963

1959

1963

1958

1963

Embedding of piles in permafrost (m)

4

5

4

5

4

5

Average design temperature along a

-2.8

-3.4

-1.2

-2.1

-1.5

-2.2

pile (°C)

Design temperature at the tip of

-3.6

-5

-1.6

-3.1

-2.1

-3.2

pile (°C)

Pile bearing capacity (T)

100

140

60

100

55

106

Increase in bearing capacity during

40

67

integrity, and a potential climate change impact would affect buildings with open crawl spaces much later than buildings with crawl spaces with openings designed according to MV.

Permafrost temperature under outer walls determines the properties of soils used in structural design, and it is a function of permafrost temperatures beneath and outside of the building. This temperature can be decreased by several methods, such as the use of thermal piles, thermal insulation of soil outside a building, snow-plowing around a building, and a combination of these methods. A combination of thermal insulation with thermal piles resulted in greatly reduced permafrost temperature at some sites along the Trans Alaska Pipeline. A combination of thermal piles and open crawl space has been used effectively in Alaska and Russia (Vialov et al. 1993). A combination of open crawl space with heat pipes associated with piles and summer seasonal thermal insulation can keep soil in a frozen state even when the mean annual air temperature is a few degrees above 0°C.

Porkhaev (1959), whose contribution to development of methods for evaluating thermal interaction of buildings with frozen and thawing soil has so far been the most significant, found that permafrost under structures can be protected in practically the entire permafrost area. He also found that the lower permafrost temperature and the greater its thickness, the easier it is to protect permafrost. The air temperature is the defining factor, because permafrost temperature can be reduced by cooling systems and eventually becomes close to mean annual air temperature. "The thermal impact of engineering cooling systems such as ventilated crawl space, ventilated ducts and others is several times greater than the impact of natural factors" (Porkhaev 1959, p 19). Contemporary methods of frozen ground engineering have powerful means to protect the frozen state of permafrost in a wide range of climatic conditions (Khrustalev 2005).

Although general approaches to design for permafrost conditions are identical, their applications are different in Russia and North America (Table 17.3). For comparison, a building with ventilated crawl space and pile foundations is considered. The differences are important when evaluating the potential climate-change impact on permafrost as a foundation for buildings. Comparison shows that a building designed with American standards can withstand greater climatic changes.

Table 17.3 Comparison of North American and Russian approaches to designing foundations with ventilated crawl space

Characteristics

North America

Russia

Safety factor

2.5-3

1.05-1.56 (Khrustalev 2001)

Tip bearing capacity of piles

Usually not taken into

Taken into account

account

Type of air space beneath a

Open

Often closed with openings, whose

building

area is calculated from modulus

of ventilation (MV)

Central heating line in crawl

Usually not installed

Often installed

space

Pile material

Steel

Concrete

Building construction

Light

Heavy

material

0 0

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