Soil deformations due to frost action
Under frost action, free stagnant water freezes at 0°C and in doing so expands by 9% of its volume. However, water in narrow capillary tubes does not freeze at 0°C, but at lower temperatures, ranging from -1°C to -78°C. This latter freezing temperature was established by laboratory tests in America. The narrower the capillary tube, the lower the freezing temperature.
Soil contains pores interconnected in all directions, forming capillary tubes. In sand, the pores are large and form wide capillary tubes in which water freezes at 0°C or at a slightly lower temperature. The water freezes uniformly, forming ice crystals in the pores, whose volume increase of 9% does not cause any significant soil deformation. In soil with a fine-grained composition, the pores are smaller and form narrow capillary tubes in which water freezes at temperatures lower than 0°C. However, in the surface layer the soil is always looser, with larger pores, in which water freezes at 0°C and forms ice crystals. These crystals attract chilled water from the narrow pores with great force, the so-called crystallization force. When water thus attracted comes into contact with the ice crystals and freezes itself, it increases the volume of the ice crystals, which become ice lenses, whose thickness may be about 0,1 to 10 mm. The formed ice lenses cause significant soil deformation during frost action, which under particularly unfavorable conditions may reach 30 and 50 cm.
The occurrence and magnitude of soil deformation due to frost action depend mainly on the following 3 factors:
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Groundwater level
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Duration of frost.
Ad 1) With respect to granulometric composition, the greatest frost action occurs in fine-grained soil, whose pores are sufficiently small to form narrow capillary tubes, while at the same time it has sufficiently high permeability, which enables rapid water circulation during frost action. Such soil is the most sensitive to frost, both in terms of the speed of occurrence and the magnitude of deformations. These are silty soils, which consist predominantly of grains of size 0,02-0,002 mm. Grains larger than 0,02 mm have a low capillary rise height, while grains smaller than 0,002 mm have slow capillary rise, so in both cases only a smaller amount of water can reach the frost-action zone during the frost period. If clay soil contains more than 25% particles smaller than 0,002 mm, the soil permeability is very low and water circulation in the pores is so slow that the water freezes in the pores before ice lenses can be attracted.
Ad 2) In addition to the condition regarding the granulometric composition of the soil, another condition must be met for frost heave deformation of the soil to occur. This is that water must be able to supply the frost-affected zone. If this condition were not met, i.e. if the soil capillaries were not in contact with water that would continuously flow through them during frost action, no frost deformation would occur, regardless of the soil’s granulometric composition.
The higher the groundwater level, the greater the possibility of soil deformation due to frost action. If the groundwater level is at a greater depth than the height of capillary rise of water in the given soil, frost-related deformations are negligible.
Ad 3) The longer the period of uninterrupted frost lasts, the greater the frost-related deformations, due to the formation of larger and more numerous lenses. Strong and short-lived frost causes smaller deformations, because in that case the water in the pores freezes suddenly and capillary water flow is interrupted. On the other hand, moderate and long-lasting frost causes greater deformations due to the unhindered formation of ice lenses.
Decramping effect
During thawing, the ice lenses in the upper part of the surface soil melt first, while immediately below them a frozen, and therefore impermeable, layer remains. Since the melted ice lenses release a large amount of water that cannot percolate through the frozen lower layer, this increased amount of water in the surface layer oversaturates the soil and turns it into a liquid mass with no strength, which is instead laterally squeezed out under pressure. Soil deformations caused by thawing can be considerable and in some cases dangerous. These deformations are common in cuts on railway lines and roads, where in spring partial sliding of slopes occurs due to soil oversaturation with water from melted ice lenses, as well as on roads whose pavements deform under traffic on soil that has lost bearing capacity due to water saturation.
Criterion for assessing the effect of frost on soil
By studying the effect of frost on various types of soil, it was concluded that, with regard to frost behavior, two types of soil must be distinguished: safe and frost-prone.
Frost-safe soils are those in which ice lenses do not form, no matter how long the frost period lasts. In this type of soil, only the freezing of water in its pores and the formation of ice crystals occur, with minor deformations under the action of frost. This group includes gravel, sand and similar soils.
Frost-susceptible soils are those in which ice lenses form, as a result of which greater deformations may occur. Water accumulated in the surface layer in the form of ice lenses during frost also produces a thawing effect. This group includes certain silty-clayey soils and, in particular, silty soils.
There are several criteria for assessing frost resistance of soil, among which the best known are the criteria of Arthur Casagrande and Rukli, which we will describe here.
Casagrande’s criterion A
This criterion is based on the grain-size composition of the soil and reads:
a) For soil of uniform composition, whose coefficient of non-uniformity U < 5, the soil is frost-susceptible if it contains more than 10% grains smaller than 0,02 mm.
b) In soil of non-uniform composition, whose degree of non-uniformity U > 15, the soil is frost-susceptible if it contains more than 3% of particles smaller than 0,02 mm.
Although this criterion has been applied for a long time and has always given reliable results, its shortcoming is that it is based only on the granulometry of the soil and does not take into account other factors influencing the action of frost in the soil. For this reason, it is now considered that this criterion is too strict and uneconomical.
Rukliev criterion
According to this method, the soil is divided into two fractions based on the coarseness of the solid particles, namely a coarse-grained fraction, grains with diameter d > 2 mm, and a fine-grained fraction, grains with diameter d < 2 mm. To assess the frost resistance of the soil, the dependence of the amount of fine-grained fraction on the amount of coarse-grained fraction is established, and this dependence is given by the lower and upper limits (fig. 1).
The lower limit represents the maximum permissible percentage of solid particles with diameter d < 0,02 mm of the dry weight of the fill, i.e. the fraction 0-2 mm, at which practically no frost damage occurs under unfavorable conditions such as a high groundwater level, unfavorable climatic conditions (prolonged frosts), and high natural soil moisture.
The upper limit represents the maximum permissible percentage of particles d < 0,02 mm of the dry weight of the fine-grained fraction at which practically no frost damage occurs under favorable conditions, such as a low groundwater level, favorable climatic conditions (short-term frosts), and low natural soil moisture.
Fig. 1 Lower and upper limits for assessing the danger of soil frost according to the Rukljev criterion
Protection against the action of frost in the soil
The depth of frost penetration in the soil depends on the climatic conditions of the area. In our country it is generally 0,8 to 1,0 m below ground surface. Accordingly, structures founded at a depth equal to or greater than 1,0 m will not be affected by frost.
Shallow foundations such as road pavements are exposed to frost action, which can cause greater deformations and damage to the pavement both during freezing and during thawing. Therefore, when constructing road pavements and airport runways, the soil must be examined with regard to frost action, and if the subsoil is frost-susceptible, protective measures must be taken. For road and airport pavements, the most effective measure is the construction of a subbase layer, whose thickness is determined so that the total thickness of the pavement and the subbase layer is 0,80 m for heavy traffic and up to 0,60 m for light traffic. In most cases, a subbase layer thickness of 0,30 m beneath the pavement provides sufficient protection against frost action.
The material for making the cushion layer is usually sand or sandy gravel, which should meet the following conditions:
Plasticity index IP < 5
Fraction content d < 0,02 mm less than 3%
Degree of non-uniformity U > 7.
The upper part of the bedding layer, with a thickness of 20 cm, must not be uniform fine-grained sand because of insufficient compaction and limited load-bearing capacity. In this part, the bedding layer should be sandy gravel with 30-70% grain size 2-30 mm. Grains larger than 30 mm are not recommended.
Shallowly founded boundary walls are also affected by frost in the ground, causing them to tilt, which over time, with repeated frost action, leads to overturning and collapse. Therefore, these walls should be founded to a depth of 1,0 m, if the soil is frost-prone.