The water which migrates upward from the water table to the capillary fringe may freeze if the atmospheric temperature falls to the freezing point so that the ice get formed. This results in an increase in the volume of soil, because when water is converted into ice, there is about 9% increase in its volume. If the porosity of the soil is 45% and the soil is saturated, the expansion of the soil would be (0.09 X 45) = 4.05%. In other words, there would be a heave of about 4 cm in every one meter thickness of the soil deposit. Due to frost heave, the soil at the ground surface is lifted. This may cause the lifting of light structures built on the ground.
The frost heave observed in most of the soil is much more than a heave of about 4 cm per meter. This is due to the fact that when tthe ice lenses are formed in the soil due to freezing of water, the water film from the adjacent soil particles are also removed. This disturbs soil suction equilibrium and more water are drawn up from the water table by capillary action to replenish the water deprived by the ice lenses from the soil particles. This process may cause a frost heave of 20-30% of the soil depth.
The soils which are prone to frost action are mainly silts and fine sands. Thses soils has large cappilary rise due to the relatively fine particles. Moreover, water can easily flow through these soils because of fairly good permeability. In coarse-graned soils and clayey soils, the frost heave is relatively small. In coarse-graned soils, the frost heave is limited to about 4%, as there is very little capillary rise. Clayey soils, on the other hand, have very large capillary rise, but their permeability is very low. The water cannot move easily through these soils and therefore, the frost heave is limited. However, if the clayey deposited have fissures and cracks, water moves easily and a large frost heave may occur such soils. If the temperature persists below the freezing point for a long period, frost penetrates the soil further, and the depth of the affected soil increases. The depth upto which the water may freeze is known as the frost line.
The basic condition for the formation of the frost heave may be summerized as under:
- The temperature in the soil is below freezing point and persists for a long period.
- A reservoir of the ground water is available sufficiently close to the frost line to feed the growing ice lenses by capillary action.
- The soil is saturated at the beginning and during the freezing period.
- The soil has sufficiently high cappilary potential to lift the water above the ground water table.
- The soil has good permeability so that water moves quickly through it. The cracks and fissure also permit rapid movement of water.
- The soil particles of size about 0.02 mm are most prone to frost heave. If a uniform soil contains more than 10% particles of the size 0.02 mm or if a well-graded soil contains more than 3% particles of this size, the soil is prone to frost heave.
The foundations of the structures should be carried below the frost depth to avoid possible frost heave after the completion of the structure. However, highways and runways have limited depth below the ground surface and cannot be constructued below the frost line. In such cases, other measures are taken to reduce frost heave.
After the occurance of frost heave, if the temperature rises, the frozen soil thaws and free water is liberated. Thawing process starts from the upper layer and moves downwards. The liberated water is trapped in the upper layer while the lower layers are still frozen. The strength of the soil in the upper layer is reduced due to its softening caused by an increases in water content. The process of softening of soil due to liberation of water during thawing is known as frost boil.
Frost boil affects the structures resting on the ground surface. The effect is more pronounced on highway pavements. A hole is generally formed in the pavement due to extrusion of soft soil and water under the action of wheel loads. In extreme cases, the pavement breaks under the traffic and there is ejection of subgrade soil in a soft and soapy condition.
Coarse-grained soils are not affected much by frost boil, as the quantity of liberated water is small, and that too is drained away quickly. The soils most prone to the softeningeffect are silty soils. These soils have low plasticity index and become very soft with a small increase in water content. Clayey soils are not affected as much as silty soils since the quantity of liberated water is small and the plasticity index is high.
How to Prevent Frost Action
The frost heave and frost boil cause great difficulties in the maintenance of highways and runways, as discussed above. The following measures are usually taken to mitiagte the ill effects of the frost action.
- The most effective method of prevention of frost action is to replace the frost-susceptible soil by coarse-grained soils such as gravels or coarse sands. In most cases, the method is not economically feasible owning to large quantities of soils involved.
- The frost action can be prevented by providing an insulating blanket between the water table and the ground surface. The insulating blanket consists of gravel and has a thickness of 15 to 30 cm. The blanket reduces the capillary action and hence the migration of water and the formation of the ice lenses.
- A good drainage system prevents the frost action in two ways: i) It lowers the water table and thus increases the distance between the ground level and the water table. ii) The water liberated during thawing is drained away quickly by the drainage system.
- Sometimes additives are used to reduce frost action. Dispersion agents, such as sodium polyphosphate, when mixed with soil, decrease the permeability of the soil.
- Water proofing materials and other chemicals are also used to change the absorbed cations on the clay minerals to reduce the tendency of soils to attract the water dipole.